Luquillo Experimental Forest: 

Research History and Opportunities

U.S. Department 

of Agriculture

Forest Service

EFR-1

May 2012
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1 Winrock International, Ecosystem Services, 2121 Crystal Drive, Suite 

500, Arlington, VA 22202

2 U.S. Department of Agriculture, Forest Service, International Institute of 

Tropical Forestry, 1201 Calle Ceiba, Jardín Botánico Sur, Río Piedras, 

PR 00926–1115

Editors

Nancy L. Harris1, Ariel E. Lugo2, Sandra Brown,1

and Tamara Heartsill Scalley.2

U.S. Department 

of Agriculture

Forest Service

EFR-1

May 2012

Luquillo Experimental Forest: 

Research History and Opportunities

Luquillo Experimental Forest: Research History and Opportunities i

Acknowledgments ii

 1. Introduction (N.L. Harris) 1

 2. General Description of the Research Area (N.L. Harris) 3

 3. Historical Perspective (N.L. Harris) 7

 4. Land Use and Land Cover (N.L. Harris) 9

 5. Climate (N.L. Harris, T. Heartsill Scalley, and F.N. Scatena) 12

 6. Disturbances (N.L. Harris and A.E. Lugo) 21

 7. Hydrology (N.L. Harris and W. Wu) 50

 8. Water Quality of Rivers (N.L.Harris) 56

 9. Geology (N.L. Harris) 60

10. Soils (N.L. Harris) 62

11. Vegetation (T. Heartsill Scalley) 64

12. Fungi (S.A. Cantrell and D.J. Lodge) 73

13. Fauna (N.L. Harris) 77

14. Ecosystem Processes (A.E. Lugo and N.L. Harris) 98

15. Nutrient Cycling in the Luquillo Experimental Forest (A.E. Lugo, N.L. Harris, and W.L. Silver) 107

16. Socioeconomic Indicators (N.L. Harris) 119

17. History and Focus of Research (N.L. Harris and T. Heartsill Scalley) 122

18. Research Opportunities (A.E. Lugo) 124

19. Education and Outreach (E. Meléndez Ackerman) 125

20. Facilities (E. Meléndez Ackerman) 127

Resources 129

Literature Cited 130

Contents
ii Luquillo Experimental Forest: Research History and Opportunities

Acknowledgments

This work was done in cooperation with the University of Puerto Rico as part of the Forest Service, U.S. Department of Agriculture (USDA), 

contribution to the National Science Foundation Long-Term Ecological 

Research Program at the Luquillo Experimental Forest. Work was conducted 

under grants BIR-8811902, DEB-9411973, DEB-9705814, DEB-0080538, 

and DEB-0218039 to the Institute for Tropical Ecosystem Studies of the 

University of Puerto Rico and the International Institute of Tropical Forestry 

(IITF), Forest Service. Special thanks to all contributing authors; special 

thanks to F.N. Scatena, David Murphy, Eda Meléndez Colom, Chelse M. 

Prather, and Hongqing Wang for their collaboration with our data requests; 

Gisel Reyes, Jorge Morales, and other IITF library staff for their assistance in 

literature compilation; Olga Ramos and staff of the Geographic Information 

System Remote Sensing Laboratory of the IITF for their assistance with map 

production; and Mildred Alayón for her assistance in the production process.

Cite this document as: 

Harris, N.L.; Lugo, A.E.; Brown, S.; and Heartsill Scalley, T. (Eds.). 

Luquillo Experimental Forest: Research history and opportunities. EFR-1. 

Washington, DC: U.S. Department of Agriculture. 152 p.
Luquillo Experimental Forest: Research History and Opportunities 1

Tropical forests cover an area of approximately 1.8 billion hectares (ha), or nearly 50 percent of the world’s total 

forest area (Food and Agriculture Organization 2003). Tropical 

forest ecosystems are often introduced to students of ecology 

by dividing them into two types, based solely on rainfall 

patterns—rain forests and dry or seasonal forests. Although 

this representation may provide a simple, approachable view, 

it fails to illustrate the true diversity of all tropical forest types 

and promotes the extrapolation of limited data sets over 

large and heterogeneous regions to produce broad, sweeping 

generalizations about all tropical forests worldwide.

The Holdridge Life Zone System (Holdridge 1967), which is 

based on empirical data and ecophysiological principles, provides 

a more comprehensive picture of tropical forest types: of the 

world’s 112 life zones, more than one-half (66) are tropical 

and 33 include forests (out of 52 forested life zones in the 

world; Lugo and Brown 1991) (fig. 1). Thus, from a climatic 

perspective, tropical forests are more diverse than all other 

forest types combined. Understanding this diversity has posed 

a challenge even to seasoned ecologists, because describing the 

diversity of forest types becomes even more complicated when 

considering local factors, such as geologic formation, soils, 

topography, and aspect, as well as forest function and responses 

to natural and anthropogenic disturbances. 

1. Introduction

N.L. Harris

When using the life zone approach, the guiding principle 

that underlies the diversity of tropical forests is that multiple 

environmental conditions—not just precipitation regime—

dictate the organization, composition, and functioning of 

ecosystems from local to global scales. Therefore, ecological 

comparisons among ecosystems require a clear understanding of 

factors relevant at the various spatial, temporal, and biological 

scales.

The Luquillo Mountains in Puerto Rico provide an excellent 

natural research laboratory in which to study the diversity 

of tropical forest types, because five subtropical life zones are 

represented within an area of just over 11,000 ha: wet forest, 

rain forest, lower montane wet forest, lower montane rain 

forest, and a small tract of land in the southwest portion that 

falls within the moist forest life zone. Puerto Rico falls within 

the subtropical belt of the Holdridge Life Zone System because 

of the temperature regime; frost is absent from all tropical 

and subtropical life zones. The region is also steeped in a rich 

tradition of forestry research that is unique among tropical 

forests; scientists have been active there since the end of the 19th 

century. 

In 1983, Brown et al. summarized major findings of past 

research that once had been scattered among obscure papers and 

Figure 1.—Holdridge Life Zone System. From http://en.wikipedia.org/wiki/Holdridge_life_zones.
2 Luquillo Experimental Forest: Research History and Opportunities

large and inaccessible volumes to produce a succinct yet datarich summary that is still widely cited more than 25 years later. 

Since the publication of Brown et al. (1983), significant new 

research has led to new hypotheses and new conclusions about 

how tropical forests function. Improvements in technology 

and data storage over the past 25 years have resulted in vast new 

data sets, syntheses, and models for the Luquillo Experimental 

Forest (LEF) that were once unimaginable due to data storage 

and processing limitations (e.g., Harris et al. 2008; Harris et al. 

in press; Heartsill Scalley et al. 2007; Wang et al. 2002; Wu et al. 

2006a, 2006b). 

The goals of this Luquillo Experimental Forest: Research History 

and Opportunities are to synthesize the new research that has 

emerged from the LEF since the publication of Brown et al. 

(1983) into a concise summary of key research findings and to 

highlight opportunities for future research that will contribute to 

a greater understanding of the structure and function of tropical 

forested ecosystems in a changing world. The basic structure 

of this publication remains similar to the Brown et al. version, 

but the data presented in each chapter reflect new knowledge 

and new ideas that have arisen through the extensive work of 

modern-day researchers. Although some basic information from 

the Brown et al. (1983) version remains included in this version, 

it is assumed that the interested reader will consult the original 

version for detailed information on data published before 1983.
Luquillo Experimental Forest: Research History and Opportunities 3

Located at 18o N latitude and 66o W longitude in the northeastern Caribbean Sea, Puerto Rico is an archipelago, 

encompassing Vieques, Culebra, Mona, Desecheo, Caja de 

Muertos, and numerous cays. The main island is the smallest 

(8,895 square kilometers (km2)) in the Greater Antilles island 

chain. The island boasts a population of nearly 4 million people, 

however, making Puerto Rico one of the most densely populated 

places on Earth (429 inhabitants km-2; U.S. Census Bureau 

2000). The Luquillo Experimental Forest (LEF) is located in 

the Luquillo Mountains in the northeast corner of the island, 

approximately 30 miles (48.3 km) east of the capital of San Juan 

and southwest of the municipality of Luquillo (fig. 2). Ocean 

and trade winds moderate the island’s climate. Another large 

2. General Description of the Research Area

N.L. Harris

mountain chain in the middle of the island (the Cordillera 

Central, or Central Mountain Range) creates a rain shadow, such 

that annual precipitation across the island spans a gradient of 

almost 5,000 millimeters (mm) from the Luquillo Mountains 

on the windward northeast coast to the Guánica dry forest (800 

mm of annual rainfall) on the leeward southwest coast. One of 

the deepest points of the Atlantic Ocean lies several kilometers 

northwest of the Luquillo Mountains, a factor that, coupled with 

the long wind fetch of the Atlantic, contributes to high-energy 

conditions and thus frequent hurricanes on the north coast of 

the island (Lugo 2008). Of the land surface of Puerto Rico, 85 

percent falls within the subtropical moist and subtropical wet life 

zones (Thomlinson et al. 1996).

Figure 2.—The Luquillo Experimental Forest in Puerto Rico. From Lugo and Heartsill Scalley (in press).
4 Luquillo Experimental Forest: Research History and Opportunities

The Luquillo Mountains rise from the coast to over 1,000 meters 

(m) above sea level, with El Yunque Peak (1,065 m in elevation) 

located 8 km (straight line distance) from the nearest beach (fig. 

3). Other mountain peaks include El Toro (1,075 m), Pico del 

Este (1,050 m), Pico del Oeste (1,020 m), El Cacique (1,020 

m), Los Picachos (968 m), Mount Britton (937 m), and La Mina 

(925 m). Moist air from the trade winds coming off the Atlantic 

Ocean cools and condenses as it rises up the steep, dissected 

mountain slopes, resulting in mountain peaks shrouded in 

cloud cover most of the time. Heavy rainfall at the peaks serves 

as the headwaters for six major rivers: Río Espíritu Santo, Río 

Grande de Loíza, Río Blanco, Río Mameyes, Río Fajardo, and 

Río Sabana (fig. 3). These rivers provide the main water supply 

to approximately 20 percent of the island’s population. Air 

temperature decreases up the mountain, with average monthly 

temperature ranging between 23.5 and 27 oC (74 to 81 oF) at 

the base of the mountain to between 17 and 20 oC (63 to 68 oF) 

on the mountain peaks (García Martinó et al. 1996).

Variation in climatic conditions in the Luquillo Mountains 

results in a shift in vegetation and community structure over a 

relatively small area. Tabonuco trees (Dacryodes excelsa) dominate 

forests at low elevations in the LEF (fig. 4a), while palo colorado 

(Cyrilla racemiflora) with reddish leaves and bark dominates at 

mid-elevations (fig. 4b). Palm forests, dominated by the sierra 

palm (Prestoea montana), occur at all elevations wherever very 

steep slopes and saturated soils prevail (fig. 4c). Forests at the 

mountain peaks are known as elfin forests (fig. 4d) because 

trees are short in stature in response to chronically saturated 

soils that limit transpiration rates and low solar radiation that 

limits photosynthesis (Wang et al. 2003; Harris 2006; Wu et 

al. 2006b; Harris et al. in press). Although these broad forest 

types characterize the vegetation of the LEF, clear boundaries 

between where one forest type stops and another starts are 

nonexistent. Rather, the distribution of vegetation corresponds 

to multiple environmental gradients such as solar radiation, 

temperature, rainfall, soil type, and topographic position that 

Figure 3.—Elevation and rivers of the Luquillo Experimental Forest.
Luquillo Experimental Forest: Research History and Opportunities 5

Figure 4.—Representative forest types within the Luquillo Experimental Forest. (a) Tabonuco forest in the lowlands, dominated by Dacryodes excelsa; (b) 

palo colorado forest at mid-elevations, dominated by Cyrilla racemiflora; (c) palm brake forest in areas with steep slopes and saturated soils, dominated by 

the sierra palm (Prestoea montana); and (d) elfin forest at mountain peaks, with Micropholis garciniifolia as a representative tree species. Photos by G. 

Bauer.

4a. 4b.

4c. 4d.

vary continuously over the landscape rather than as discrete 

categories. Also, many specialized communities occur across 

the landscape, such as floodplain forests, riparian vegetation, 

herbaceous bogs, plantations, and roadside vegetation.

Hurricanes and severe tropical storms are common in the LEF 

and occur mainly during the late summer months. More than 

70 severe storms have occurred in Puerto Rico since the early 

1700s, and the most recent storms to pass over Puerto Rico 

(since 1989) are shown in figure 5. The last two hurricanes to 

hit the LEF were Hurricane Georges in 1998 and Hurricane 

Hugo 9 years earlier in 1989. Hurricanes can be expected to pass 

directly over the forest once every 62 years and pass within 60 

km once every 22 years (Scatena 1989).
6 Luquillo Experimental Forest: Research History and Opportunities

Figure 5.—Weekly rainfall and throughfall at the Bisley Watersheds in the Luquillo Experimental Forest and the names of storms and hurricanes passing 

over Puerto Rico from 1988 to 2003. Weekly means are expressed in mm day-1. The black color line is a running average. Bar colors represent January 

in gray, February in yellow, March in dark yellow, April in orange, May in red, June in dark gray, July in light green, August in dark green, September in light 

blue, October in dark blue, November in cyan, and December in olive. From Heartsill Scalley et al. (2007).

In comparison with tropical forests in the Atlantic lowlands 

of Costa Rica and the lowlands of central Panama—other 

well-known sites of long-term research activity (Gentry 1990)—

the Luquillo Mountains are cooler, wetter, and less seasonal 

(Scatena 1998a). Unlike areas with monsoonal tropical climates 

that receive most of their annual rainfall during a well-defined 

rainy season, rainfall in the Luquillo Mountains is relatively 

evenly distributed throughout the year. Dry periods in these 

mountains last days and weeks rather than months and are only 

moderately seasonal in occurrence. Rainfall in the Luquillo 

Mountains has a nutrient-rich oceanic chemical signature, and 

the precipitation regime is a high frequency of low-intensity 

showers punctuated by periodic high-intensity storms (Heartsill 

Scalley et al. 2007).
Luquillo Experimental Forest: Research History and Opportunities 7

The Origins of Forestry in the Luquillo 

Experimental Forest: 1903 to 1956

When Puerto Rico became part of the United States territory 

in 1898, 5,018 hectares (ha) of forest land in the Luquillo 

Mountains were under the protection of the Spanish Crown. 

The Luquillo Forest Reserve was established by the United States 

in 1903 and renamed the Luquillo National Forest in 1907. 

The Division of Forestry managed the area between 1898 and 

1905. The Forest Service has managed the area continuously 

since 1905, and the first supervisor of the national forest was 

appointed in 1917. The forest was renamed the Caribbean 

National Forest in 1935. In 2007, the name was changed again 

to the El Yunque National Forest to better reflect the cultural 

and historical attitudes of the Puerto Rican people. During 

the past century, the size of the forest under Forest Service 

ownership increased from the original 5,018 ha to 11,300 ha. 

In 1956, this forest area–with the same boundaries as today’s 

El Yunque National Forest–was also designated as the Luquillo 

Experimental Forest (LEF).

Initial Forest Service programs began about 1920 and included 

large-scale trial plantings with both introduced and native 

tree species. With the onset of the Great Depression in the 

early 1930s, the Civilian Conservation Corps program did 

extensive work in reforestation, forest road construction, and 

recreational and administrative improvements to the forest. The 

Forest Service started the first timber management plan for the 

Luquillo Mountains in 1934 (Muñoz 1965). Before this plan, 

management of public lands was minimal, and most privately 

owned areas were either cleared for agriculture or selectively cut 

for desirable species. Sugarcane production was at its peak in 

the Luquillo municipality in the late 1940s (Thomlinson et al. 

1996). The 1956 forest management plan suggested that about 

6,700 ha of the Caribbean National Forest, now El Yunque 

National Forest, could support timber production and were thus 

designated as commercial lands. Close to one-half of these lands 

came under active management, including both the management 

of natural stands and the development of plantations.

The Odum Years: 1963 to 1968

In the 1960s, the Atomic Energy Commission (now the 

U.S. Department of Energy) and the University of Puerto 

Rico provided funding for Howard T. Odum and dozens of 

other scientists and technicians to conduct the first largescale ecosystem study on the effects of disturbance caused 

by radiation in the tabonuco forest. Odum’s experiments 

established the foundation for long-term ecological research 

in Puerto Rico and the tropics in general (Odum and Pigeon 

1970). Through his radiation experiment, Odum found that 

the structure and function of the tabonuco forest ecosystem 

were highly resistant to the effects of radiation (Odum 1970a). 

Odum’s research legacy in the Luquillo Mountains transcends 

the radiation experiment, however. Odum also described in 

detail the climate of the Luquillo Mountains (Odum et al. 

1970) and demonstrated how to measure forest metabolism on 

a large scale by isolating a section of forest within a giant plastic 

cylinder (Odum and Jordan 1970). Scientists involved in the 

Rain Forest Project also made comparisons with other tropical 

forests, both insular and continental (Odum 1970a, 1970b), and 

raised countless questions for future studies. Many of Odum’s 

questions concerned key methodological and monitoring 

approaches of the time, while others emphasized fundamental 

issues for research in tropical forests (see Lugo 2004a).

For Howard Odum, the Luquillo Mountains functioned as 

an integrated ecosystem connected to the rest of the globe by 

regional flows of energy and cycling of materials. He recognized 

the connection between the tabonuco forest and latitudinal wind 

patterns through inputs of water and nutrients. Odum also 

recognized the role of wind and hurricanes in shaping the canopy 

of the forest (Odum 1970b), demonstrated the hierarchical 

nature of forest function, and integrated the functions of 

organisms from microbes to humans (Odum 1970c). Through 

research on fundamental ecosystem structure and function, 

Odum developed models of sustainable land use for the tropics, 

including the design of ecosystems for human uses, such as waste 

recycling and wood production (Odum 1995).

The LTER Network: 1988 to Today

Most research conducted in the Luquillo Mountains until 1988 

was of relatively short duration, from less than a year to a decade. 

Even H.T. Odum’s Rain Forest Project, which in its time was the 

most comprehensive study of a tropical forest ever conducted, 

lasted only 5 years (1963 to 1968). Notable longer term studies 

include (from 1942) monitoring of tree growth and survival 

under natural and managed conditions (Drew et al. 2009; Brown 

et al. 1983), the recovery of vegetation after ionizing radiation 

(Taylor et al. 1995), and the recovery project for the endangered 

Puerto Rican Parrot (Snyder et al. 1987). 

3. Historical Perspective 

N.L. Harris
8 Luquillo Experimental Forest: Research History and Opportunities

The National Science Foundation’s Long Term Ecological 

Research (LTER) Network began with 6 sites in 1980 and has 

grown to 26 sites today, including 2 sites in Antarctica and 1 

coral reef site off the coast of Tahiti (fig. 6). The LEF (known as 

LUQ in the LTER Network) was added to the LTER Network 

in 1988 and remains the only terrestrial tropical site in the 

network. At the time that the LUQ site was established, data 

on tropical forests were limited and basic information on topics 

such as carbon and nutrient dynamics, disturbance effects, and 

trophic interactions was largely lacking; the LUQ site already 

had a strong research basis on which the LTER could build. 

Establishing an LTER program at the LEF initiated a new 

research focus on ecosystem forcing functions of long duration, 

infrequent occurrence, or incremental effect. Now, some 20 

years later, more than 1,000 publications addressing all these 

topics and more have come out of the LUQ LTER.

Figure 6.—The National Science Foundation’s Long Term Ecological 

Research Network consists of 26 sites. The Luquillo Experimental Forest 

is the only terrestrial tropical site in the LTER. 

From http://www.lternet.edu/sites/.
Luquillo Experimental Forest: Research History and Opportunities 9

Most of Puerto Rico was forested at the time of European arrival. Although pre-Columbian Arawak and Taíno 

agriculturalists cleared small tracts of land in the mid-elevation 

forests of Puerto Rico, their activities probably had little effect on 

the vegetation of the Luquillo Experimental Forest (LEF) because 

the coastal plains and the drier intermountain valleys were better 

suited to their needs than the Luquillo range (Wadsworth 1949; 

Walker in press).

Land-clearing activities and the extent of human-induced 

disturbances increased rapidly at the onset of European 

colonization, but then slowed with the development of the 

continental New World. The first two major settlements of 

4. Land Use and Land Cover

N.L. Harris

Puerto Rico, Caparra and San Germán, were established by 

1510, but a third town was not established for another 136 years 

(Scatena 1989). During the first three centuries of colonization, 

the relatively inaccessible nature of the Luquillo Mountains 

limited their use. 

Extensive agricultural activity did not begin in the LEF until 

the onset of government land distribution programs in the 

early 1800s. By 1831, coffee was grown in the region, but 

most of Fajardo and Luquillo were forested and sparsely 

populated (Wadsworth 1949). Between 1830 and 1890, as 

a result of the government land distribution programs, the 

flanks of the mountain became cultivated with coffee, bananas, 

Figure 7.—Land use classes in the municipality of Luquillo, Puerto Rico in (a) 1936, showing the dominance of sugar cane and pasture; (b) 1964, showing 

the increasing area of pasture, largely due to abandonment of sugar cane crops; and (c) 1988, showing the dominance of dense forest. Values in 

parentheses in the legend show the percent canopy closure for each of the three forest classes. From Thomlinson et al. (1996).

7a.

7c.

7b.
10 Luquillo Experimental Forest: Research History and Opportunities

and subsistence crops including upland rice. The forest also 

supplied wood products and water resources to the surrounding 

communities, and local residents made charcoal for personal 

use. The extent of cultivated land in the Luquillo Mountains 

increased steadily from 1800 until a hurricane passed in 1899 

(Wadsworth 1949). After the hurricane hit, many affected 

farms and coffee plantations were abandoned and allowed to 

reforest naturally. By 1934, all original coffee plantations had 

been destroyed, but many original shade trees (Inga fagifolia and 

Inga vera) remained. This decline in agricultural activity and the 

subsequent reforestation predates the general trend of natural 

reforestation in Puerto Rico that began about 1950 (Birdsey and 

Weaver 1982).

Helmer (2004)—who summarized islandwide land cover 

change over the time period between 1977 to 1978 and 1991 

to 1992—found that the largest land cover change on the 

island was a 64-percent (~119,000 ha) decrease in agricultural 

lands, with about one-third of this decrease corresponding to an 

increase in coffee cultivation and secondary forest. In an earlier 

study, Thomlinson et al. (1996) analyzed aerial photographs of 

the municipality of Luquillo, within which part of the LEF is 

located, and found that the pattern of land use between 1936 

and 1988 followed the islandwide trend of a transition from 

high-intensity agriculture to dense forest (fig. 7). Sugar cane 

cropland and pasture were the dominant land uses in Luquillo 

in 1936, occupying about one-third of the study area each, while 

dense forest was rare. By 1988, pasture still occupied about onefourth of the area, mainly because most of the area under sugar 

cane production was transformed to pasture after abandonment. 

Urban areas also increased by more than 2,000 percent between 

1936 and 1988 and continue to encroach on forested areas today. 

Between 1988 and 1993, urbanization around the LEF in the 

municipality of Luquillo increased by 31 percent and represented 

a 5-percent loss in vegetative cover, more than 80 percent 

of which was dense forest (Thomlinson and Rivera 2000). 

Figure 8.—Land cover map of Puerto Rico in 1991-1992. The Luquillo Experimental Forest is located in the northeastern corner. From 

Helmer et al. (2002).
Luquillo Experimental Forest: Research History and Opportunities 11

Suburbanization, or the transition of other land cover types to 

low-density urban areas, continues to encroach on the lowlands 

of the Luquillo Mountains. Thus, the LEF can be viewed as an 

“island within an island” because the forest is embedded within 

a broader mosaic of forested and nonforested land (fig. 8). What 

Figure 9.—Average daily visits to Yokahu Tower, a major visitor center in El Yunque National Forest. Letters represent the day of the week, with 

Sunday being the most popular day. From Scatena (2001).

was once actively managed for timber production, charcoal, 

coffee, and sugar cane is now managed primarily for research and 

recreation. Hundreds of visitors from Puerto Rico and mainland 

United States visit the El Yunque National Forest each day, with 

July and August as peak months for tourism (fig. 9).
12 Luquillo Experimental Forest: Research History and Opportunities

The Luquillo Experimental Forest (LEF) is one of the wettest regions of Puerto Rico. As moisture-laden air masses move 

inland from the Atlantic Ocean and rise more than 1,000 meters 

(m) over a horizontal distance of just 10 to 20 kilometers (km), 

the air cools and condenses, resulting in a wet environment 

with abundant water resources. Hurricanes are a regular part of 

the climate, as are tropical storms and depressions, convective 

storms, northeasterly trade winds, Saharan dust, and winter 

cold fronts (Odum and Pigeon 1970, Prospero and Nees 1986, 

Scatena 1989, McDowell et al. 1990, Larsen 2000). Above the 

lifting condensation level at about 600 m elevation, cloud cover 

reduces solar radiation to about 63 percent of nearby coastal 

areas (Briscoe 1966). Across the forest as a whole, mean annual 

precipitation averages approximately 3,879 mm yr-1 (millimeters 

per year) (García Martinó et al. 1996).

Meteorological Data

Precipitation

Over the past 80 years, more than 18 rain gauges have been 

installed within or adjacent to the LEF. Many stations are part of 

the National Oceanic and Atmospheric Administration (NOAA) 

network. The Forest Service, University of Puerto Rico, U.S. 

Geological Survey (USGS), and U.S. Fish and Wildlife Service 

(USFWS) maintain other stations. Climate data for the five 

NOAA stations can be downloaded from the National Climatic 

Data Center Web site 1. The USGS also maintains several 

weather stations as well as rainfall and streamflow stations within 

the LEF. The earliest rainfall data were recorded for stations at 

152 m and 366 m above sea level at Hacienda Perla (Brown et al. 

1983). Canóvanas, a coastal station northwest of the Luquillo 

Mountains, has one of the longest continuous data sets for the 

Luquillo region with a record of more than 100 years. Three 

other long-term stations within the LEF are Bisley located at 361 

m, El Verde at 400 m, and Pico del Este (East Peak) at 1,041 

m. These stations contain 15 to 30 years of nearly continuous 

climate records. Data from each station are summarized briefly 

below, followed by a section that summarizes elevational trends.

Canóvanas (Coastal)

The Canóvanas station is located just northwest of the LEF in 

the subtropical moist forest life zone. Long-term records depict 

wide oscillations between high and low rainfall years, with 

annual totals ranging from slightly more than 1,300 mm yr-1 in 

the driest years to nearly 3,000 mm yr-1 in the wettest years (fig. 

10a). Although the running average reveals periods of decreasing 

rainfall (from about 1910 to the 1960s) followed by periods 

of increasing rainfall (1960s to 1990), peaks and valleys in the 

running average interrupt these trends. Furthermore, van der 

Molen (2002) showed that six out of the eight stations with the 

longest periods of record on the island had precipitation totals 

that decreased significantly during the past century (fig. 10b).

5. Climate

N.L. Harris, T. Heartsill Scalley, and F.N. Scatena

1

 http://www.ncdc.noaa.gov.

Figure 10.—(a) Running averages for annual rainfall in Canóvanas, a 

coastal station in the subtropical moist forest life zone just northwest of 

the Luquillo Experimental Forest, and two other stations in Puerto Rico. 

Arecibo 3 is west of Canóvanas on the north coast and Aguirre is on the 

south coast. Compiled by Matthew C. Larsen, U.S. Geological Survey. 

(b) Trends in annual precipitation in the 20th century for eight National 

Climatic Data Center stations throughout Puerto Rico. The α represents 

the confidence level. From van der Molen (2002).

10a.

10b.
Luquillo Experimental Forest: Research History and Opportunities 13

El Verde and Bisley (300 to 400 m) 

Heartsill Scalley et al. (2007) reported annual 

and weekly rainfall and throughfall trends 

for a 15-year period (1988 to 2002) for the 

Bisley Experimental Watersheds, which range 

in elevation between 265 and 456 m above 

sea level and fall within the subtropical wetforest life zone. They also report rainfall data 

for the same period at El Verde. Over the 

15-year period analyzed, mean annual rainfall 

and throughfall rates were 3,482 and 2,131 

mm yr-1, respectively, indicating that close to 

40 percent of incoming rainfall is intercepted 

by the forest vegetation and then evaporates. 

The other 60 percent reaches the forest floor, 

enriched with nutrients leached as it passes 

over vegetation surfaces. Only 1 week in the 

15-year analysis period had no measurable 

rainfall (May 23, 1989). Comparison across 

years indicates that the lowest average daily 

throughfall values occurred in 2000 and 

2002 (4.34 and 3.84 mm day-1 (millimeters 

per day), respectively), while the highest 

values occurred in 1988 and 1990 (8.79 and 

8.50 mm day-1, respectively). Comparison 

across monthly data, at both Bisley and El 

Verde, reveals seasonal trends as well; March 

had the lowest average monthly rainfall 

and throughfall, and January through April 

typically had lower rainfall than the rest of 

the year. Comparison of Bisley data with 

El Verde data indicates that rainfall patterns 

across the two sites follow similar monthly 

trends and have statistically similar average 

annual values, but rainfall in the months 

of May, June, and October are significantly 

lower at El Verde than at Bisley (fig. 11). 

These differences are apparently related to 

the greater exposure the Bisley site has to 

the northeast trade winds and the more 

pronounced presence of the afternoon 

adiabatic winds at the El Verde site (Heartsill 

Scalley et al. 2007).

Rainfall declined significantly over the 15-

year period analyzed by Heartsill Scalley et al. 

(2007) (fig. 12a), but after Heartsill Scalley 

et al. (2009a) added the most recent 5 years 

of data to the 15-year time series to produce 

a 20-year time series (1988 to 2008), annual 

mean weekly rainfall appears to increase 

and decrease on approximately 5- to 7-year 

cycles (fig. 12b). If the period of analysis had 

covered only 1988 to 1994 or 2003 to 2008, 

it would have appeared that rainfall in the 

Figure 11.—Per month mean weekly rainfall in mm day-1 for Bisley (gray circles), for El Verde 

(squares) and mean weekly throughfall in mm day-1 for Bisley (gray inverted triangles). Error bars 

represent one standard error of the mean. From Heartsill Scalley et al. (2007).

Figure 12.—(a) Per year weekly rainfall in mm day-1 for Bisley (gray circles) and for El Verde 

(green squares) with regression line for Bisley data. Shows 15 years of data; and error bars 

represent one standard error of the mean. From Heartsill Scalley et al. (2007). (b) Same as 

(a) showing Bisley data, but 20 years of data are shown and regression lines are split by trend 

tendency. Error bars represent one standard error of the mean. From Heartsill Scalley et al. 

(2009a).

12a.

12b.
14 Luquillo Experimental Forest: Research History and Opportunities

LEF had decreased significantly. If the period of analysis had 

covered 1995 to 1999, the reverse trend would have emerged. 

Therefore, figure 12 shows the importance of studying long-term 

trends in key data sets to avoid the misinterpretation of shortterm data. 

Pico del Este (1,051 m)

In addition to receiving rain, mountain peaks of the LEF 

experience an additional component to precipitation: cloud 

water. Not all rain is vertical; much of the rain at Pico del Este 

is wind driven and comes in at an angle. The often stunted, 

epiphyte-laden trees in the elfin forest at the mountain peaks 

receive a considerable amount of their water supply through the 

process of cloud-stripping, or horizontal precipitation, whereby 

water in wind-driven mist and low clouds is intercepted by 

the vegetation (van der Molen 2002). Data collected between 

2002 and 2007 indicate that 15 percent of the 5,168 mm yr-1

of precipitation at Pico del Este is attributable to cloud inputs 

and horizontal rain (fig. 13), which are highest during nighttime 

hours and decrease during the day as clouds burn off (fig. 14).

Elevational Trends in Rainfall

García Martinó et al. (1996) explored correlations between 

rainfall and elevation in the LEF and found that a second order 

polynomial regression model best fit the data available from 

18 rain gauges. Starting at the coast, rainfall rapidly increases 

with elevation for the first 300 m and then increases in a linear 

fashion to the upper summits. In contrast, air temperature 

decreases in a linear fashion from the coast to the uppermost 

elevations. Extensive new data collected at Pico del Este after 

the publication of García Martinó et al. (1996), however, suggest 

that rainfall was underestimated when using rain gauges alone 

(Holwerda et al. 2006); after horizontal cloud inputs and wind 

losses are taken into account, mean annual precipitation at Pico 

del Este is closer to 5,168 mm yr-1. The updated data from Pico 

del Este were incorporated into the rainfall-elevation relationship 

shown in figure 15.

Air Temperature

Long-term temperature data for locations in and adjacent to 

the Luquillo Mountains are more limited than precipitation 

data. The earliest temperature data, like precipitation, were 

measured at Hacienda Perla, where the mean annual temperature 

was approximately 25 oC (Brown et al. 1983). Across the LEF, 

from low- to high-elevation stations, mean annual temperature 

decreases, and the annual variation in temperature is small at 

any point on the landscape. Frost has never been recorded at the 

LEF.

Sabana, Bisley, and Pico del Este 

Average hourly air temperature is lowest during the months of 

February and March and is highest during July and August (fig. 

16).

Average hourly air temperature for Pico del Este (East Peak) 

for the recent 7-year period of 2002 to 2008 is shown in figure 

17. Variation in both daily and annual temperature is apparent, 

with cooler temperatures occurring from January to March and 

warmer temperatures from July to September (fig. 16). Average 

air temperatures at canopy height typically range from about 18 

to 23 oC over the course of a day.

Figure 13.—Total annual precipitation (top figure), horizontal cloud inputs (middle figure), and air temperature (bottom figure) at Pico del Este between 

2002 and 2008. From Scatena (unpublished data are available from Luquillo LTER and Critical Zone Observatory).
Luquillo Experimental Forest: Research History and Opportunities 15

Figure 14.—Hourly variation in climate at Pico del Este. Values represent hourly averages of data collected between 2002 and 2008. Photosynthetic 

Photon Density Flux is PPDF. From Scatena (unpublished data are available from Luquillo LTER and Critical Zone Observatory).

2D Graph 2

mm/hr

0.04

0.06

0.08

0.10

0.12

0.14

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

mm/hr

0.45

0.50

0.55

0.60

0.65

0.70

0.75

Air Temp., avg hourly, C 

18.0

18.4

18.8

19.2

19.6

20.0

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

moles/m2/hr

0

500

1000

1500

2000

2500

3000

m/s

5.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0

Total Precipitation = Vertical Precipitation + Cloud Inputs

Total = 5145 mm/ yr = 4374 Rain + 770 Cloud interception

Cloud Interception, mm/yr

Mean = 6.19 m/s

Median = 6.47 m/s

Total PPDF

Mean = 846 moles/m2/hr

Median = 80 moles/m2/hr

 

Air Temperature, Canopy height

Mean = 18.75

Median = 18.5

Mean = 0.59 mm/hr

Median = 0.59 mm,/hr

Mean = 0.09 mm/hr

Median = 0.11 mm/hr

Wind Speed, m/s

Mean = 846 moles/m2/hr

Median = 80 moles/m2

/hr

moles/m2/hr
16 Luquillo Experimental Forest: Research History and Opportunities

Elevational Trends in Air Temperature

The relationship between mean air temperature and elevation 

is a required parameter for some environmental models and is 

termed the adiabatic lapse rate, or the rate at which temperature 

declines with height as a parcel of dry air is lifted upward 

through the atmosphere with no addition or deletion of heat. 

Air and soil temperature were measured at 10 sites located along 

a windward elevation gradient from 153 to 1,011 m, and data 

were used to develop relationships between mean air and soil 

temperature and elevation. The regression analyses performed 

showed linear relationships between both air and soil mean 

temperature and elevation (fig. 15). The lapse rates calculated 

from these data were 5.58 for air temperature and 5.43 for soil 

temperature (i.e., for each kilometer increase in elevation, air 

and soil temperatures decrease by 5.58 and 5.43 °C, respectively) 

(Meléndez Colom 1999).

Solar Radiation

Solar radiation data currently available from the Sabana Field 

Station (153 m), Bisley Experimental Watersheds (265 to 456 

m), and Pico del Este (1,056 m) indicate that solar radiation 

Figure 15.—Mean annual rainfall (mm yr-1) and air temperature versus elevation, masl (meters above sea level). Updated from García Martinó et al. (1996) 

using data available from Luquillo LTER and Critical Zone Observatory. Values corresponding to the location of the El Verde Field Station, El Yunque, Luquillo 

Mountains, Bisley Lower Tower, Canóvanas, Fajardo, Roosevelt Roads, and East Peak are EV, EY, LM, BLT, CAN, Faj, RR, and EP, respectively.

Figure 16.—Average hourly air temperature by month between 2000 and 

2002 at Sabana, Bisley, and East Peak (unpublished data are available 

from Luquillo LTER and Critical Zone Observatory).
Luquillo Experimental Forest: Research History and Opportunities 17

decreases with elevation (fig. 18). It is evident 

from these data, however, that total incoming solar 

radiation declines with elevation as cloud cover 

increases.

Wind Speed

Average daily wind speeds at El Verde and Bisley are 

between 1 and 2 m s-1 (meters per second) (fig. 19), 

with fairly common events at Bisley of 3 to 4 m s-1

and occasional peaks of 6 to 9 m s-1. Wind speeds 

at the windward Bisley Experimental Watersheds 

are higher than those at the leeward El Verde Field 

Station. During the passage of Hurricane Hugo, 

maximum sustained wind speed southeast of the 

Luquillo Mountains was estimated at 46 m s-1

(Scatena and Larsen 1991).

Climate Models

Data from individual climate stations are useful 

because they have high temporal resolution (e.g., 

hourly to weekly), but installing meteorological 

stations at every point on the landscape is impractical. 

Process-based ecosystem modeling that couples 

Geographic Information Systems (GIS) with remote 

sensing, Global Positioning Systems, and climate 

station data is an effective approach for examining 

spatial and temporal variations in ecosystem processes 

and the climatic factors that influence them. Such 

studies could not be done using conventional field 

experiments and investigations alone. Therefore, 

spatial modeling of climate using empirical data 

from measurement stations is a useful way to scale 

measurements over the landscape. Remote sensing 

can also be a useful tool for climate modeling, because 

the imagery covers large areas and can provide 

estimates at high spatial resolution, although each 

image is still a snapshot of reality and the temporal 

resolution of different satellites varies. Progress 

in both remote sensing and GIS-based modeling 

Figure 17.— A 7-year pattern (2002 to 2008) of the average hourly temperature at Pico del Este, incorporating 55,026 records. From Scatena 

(unpublished data are available from Luquillo LTER and Critical Zone Observatory).

Figure 18.—Mean hourly radiation by month at three elevations in the Luquillo 

Experimental Forest over the course of a day. Bisley is B, S is Sabana, and E is 

East Peak. Hourly data between 2,000 and 2002 are averaged. From Scatena 

(unpublished data are from Luquillo LTER and Critical Zone Observatory).

Figure 19.—Daily maximum (open circles) and mean (closed circles) wind speeds at 

the Bisley Experimental Watersheds (from Heartsill Scalley, unpublished data).
18 Luquillo Experimental Forest: Research History and Opportunities

techniques makes it easier to derive the parameters needed to 

run ecological models. This section summarizes different spatial 

models of climate that have facilitated the analysis of climateforcing functions in the LEF.

Air Temperature, Solar Insolation, and Rainfall

The importance of rainfall to the LEF ecosystem is widely 

recognized. Early studies reviewed some of the available data 

Figure 20.—Simulated air temperature, solar insolation, rainfall, and transpiration in dry and rainy months in the Luquillo Experimental Forest, Puerto 

Rico. From Wang et al. (2003).

(Briscoe 1966, Holben et al. 1979, Brown et al. 1983, Lugo 

1986, McDowell and Estrada Pinto 1988), but a forestwide, 

spatially averaged synthesis of all the long-term rainfall and 

runoff data was not available until García Martinó et al. (1996) 

analyzed long-term rainfall and stream discharge data to develop 

statistically sound relationships among rainfall, stream runoff, 

and elevation. These relationships were then used within a 

GIS to estimate spatially averaged annual water budgets for 

watersheds within the Luquillo Mountains. 
Luquillo Experimental Forest: Research History and Opportunities 19

More recently, Wang et al. (2003) used the TOPOCLIM model 

(TOPOgraphically driven CLIMate model) to simulate climatic 

variables for the LEF using both empirical and mechanistic 

approaches. The model produces estimates of air temperature, 

solar insolation, and rainfall for the Luquillo Mountains 

(Wooster 1989, Everham 1996, Marley 1998, Wang et al. 

2003). Slope, aspect, and elevation values derived from a 30-m 

resolution digital elevation model were used as input data for the 

model and historical climate data compiled by Briscoe (1966), 

Odum et al. (1970), and García Martinó et al. (1996) were used 

for parameterization. Model output generates hourly estimates 

of solar insolation and temperature, as well as daily and monthly 

totals and averages. Rainfall is estimated monthly. For details 

about the equations and algorithms for TOPOCLIM, refer to 

Wang et al. (2003).

In the LEF, simulated monthly temperature and daily solar 

insolation under current climatic conditions decrease as 

elevation increases, with minor topographic variation (fig. 

20). For example, in a rainy month (e.g., October), air 

temperature decreases from 26 oC at low elevations to 20 oC at 

mountain peaks (fig. 20). Solar insolation also decreases from 

approximately 20 MJ m-2 day-1 (megajoule per square meter per 

day) to approximately 8 MJ m-2 day-1 along the same gradient 

(fig. 20). Rainfall, however, increases as elevation increases. 

Rainfall in October increases from 200 mm mo-1 (millimeters 

per month) in the lowlands to 370 mm mo-1 at the peaks. 

Climatic variables also vary with season. During the dry season 

(e.g., March), monthly rainfall in the LEF is between 125 and 

250 mm, while in the rainy season (e.g., October) the range of 

rainfall for the entire LEF is between 200 and 370 mm (fig. 20).

Cloud Cover

Clouds are extremely important to the carbon, water, and energy 

budgets of tropical forests because heavy cloud cover reduces 

solar radiation to the Earth’s surface and limits photosynthesis, 

sap flow, and transpiration of tropical trees (Graham et al. 2003). 

Most models of solar insolation, however, do not account for 

the effects of cloud cover. Wu et al. (2006a) applied two types 

of satellite images (Landsat-7 ETM+ and MODIS—Moderate 

Resolution Imaging Spectroradiometer) to derive the probability 

of cloud cover over the LEF, defined as the proportion of each 

pixel on the map covered by clouds. The probability of cloud 

cover at any hour and any place on the landscape was modeled as 

a function of three topographic variables: aspect, slope, and the 

difference between elevation and lifting condensation level. 

The probability of cloud cover increased with elevation and was 

higher at night in both simulations and observations. Cloud 

cover probability decreased in the morning after the sun rose 

until early afternoon; it then increased again for the rest of the 

day until nighttime, apparently responding to the movement of 

the lifting condensation level (fig. 21). The model was applied 

Figure 21.—Derived probability of cloud cover at 9:00 a.m (a)., 1:00 p.m.(b), 5:00 p.m.(c), and 9:00 p.m.(d) on March 4, 2003, calibrated by a MODIS 

image. From Wu et al. (2006a).

21a.

21b.

21c. 21d.
20 Luquillo Experimental Forest: Research History and Opportunities

Figure 23.—Predicted wind over a 180 x 180 km grid on September 

18, 1989, during the passage of Hurricane Hugo. Cell size = 1 x 1 km. 

(a) Predicted sustained wind velocity and (b) predicted sustained wind 

direction at 1,200 GMT; (c) predicted maximum wind gust velocity and (d) 

predicted total energy of sustained winds of gale strength (17 m s-1) or 

greater over a 24-h period (time step = 2 min). From Boose et al. (1994).

to all months, and the simulations agreed with observed data 

that the probability of cloud cover is lower during the dry season, 

higher during the wet season, and moderate for the rest of the 

year. The cloud cover model used could usually predict the 

probability of cloud cover for each 100-m elevation band at a 

certain time of day with an index of agreement (IoA) of 0.560 to 

0.919 and at a certain location over a day with an IoA of 0.940 

to 0.994, indicating a medium-to-good model simulation at any 

particular time or location (fig. 22).

Wind

Wind, especially hurricane-force wind, serves as an important 

source of landscape-level patterning in forests and is a major 

factor that influences vegetation dynamics (Boose et al. 1994). 

At the ecosystem level, extensive blowdown may factor into the 

regulation of hydrological, energy, and nutrient regimes (Lugo 

and Scatena 1995). Despite an awareness of the importance of 

hurricanes, ecologists have only a rudimentary understanding of 

how a hurricane’s energy is distributed.

Figure 22.—Comparison between an original Landsat image 

(combinations of band 4, 3, and 2 displayed as blue, green, and red, 

respectively) and the simulated probability of cloud cover calibrated by a 

MODIS image at the time when the Landsat image was obtained (cloudy 

areas had a higher probability of cloud cover; white lines on the Landsat 

images are the boundaries of the Luquillo Experimental Forest). (a) 20 

March 20, 2000; (b) 9 January 9, 2001; (c) 20 July 20, 2001; (d) 4 March 

4, 2003. From Wu et al. (2006a).

Boose et al. (1994) developed a simple model (HURRECON) 

of hurricane surface wind fields that was based on meteorological 

studies and calibrated with meteorological data for Puerto 

Rico. Another simple topographic exposure model (EXPOS) 

used wind direction predicted by HURRECON and a digital 

elevation model to predict site exposure to the strongest 

winds on a landscape scale. Simulated regional estimates of 

wind conditions in eastern Puerto Rico during the passage of 

Hurricane Hugo are shown in figure 23. Output from the 

topographic exposure model agreed with actual treefall data; 

most disturbance effects across the LEF were concentrated on 

north-facing slopes, while south-facing slopes that were exposed 

to the southwest or south-southwest winds of the trailing eyewall 

were not severely affected (Scatena and Larsen 1991). The 

actual east-to-west effects gradient across the northern LEF study 

area was even stronger than the model predictions suggested 

(e.g., although disturbance at El Verde was scattered and largely 

confined to defoliation and branch break, much of the Bisley 

Experimental Watershed area sustained much more severe effects, 

including massive uprooting and breakage of most stems).
Luquillo Experimental Forest: Research History and Opportunities 21

Disturbance has been a central focus of ecological research since its inception (Cowles 1899, Cooper 1926, Clements 

1936), but most empirical research has focused on frequent, 

small-scale disturbances because they are easy to study. Before 

the creation of the National Science Foundation’s Long Term 

Ecological Research (LTER) Network, most long-term studies 

relied on retrospective analyses (e.g., Foster 1992, Horn and 

Sanford 1992) or on space-for-time substitution (Pickett 1989, 

Fastie 1995). 

Since 1988, the Luquillo (LUQ) LTER has focused on the 

response of forest structure and dynamics to disturbance 

regimes ranging in size and effect (Guariguata 1990, Lugo and 

Scatena 1995, Walker at al. 1996a). Studies of disturbance in 

the Luquillo Experimental Forest (LEF) challenge the view that 

ecological systems are in a steady state regarding counteracting 

processes such as competitive hierarchies (Connell 1978), species 

immigration and extinction (Whittaker 1996), C (carbon) 

cycling (Lugo 1992a), and nutrient retention and loss (Lodge et 

al. 1994).

Common disturbances that influence ecosystem structure and 

function in the LEF include treefalls, landslides, hurricanes, and 

anthropogenic disturbance (Scatena and Larsen 1991, Scatena 

and Lugo 1995, Zimmerman et al. 1995, Reagan and Waide 

1996, Lugo 2008). Treefalls create small gaps in the forest 

canopy, and landslides can create relatively larger gaps that 

also have exposed mineral soil. Hurricanes and anthropogenic 

disturbances produce larger scale effects that have long-term 

signatures on the structure of the forest landscape (Lugo 2008).

Treefalls

Treefalls are a routine occurrence in mature forests but are also 

a major consequence of windstorms and hurricanes (Brokaw 

1985, Foster 1988). Treefalls create gaps in the forest canopy 

that are critical to the regeneration of forest species. Despite the 

relatively small area of ground surface that is affected, treefalls 

alter the physical environment in ways that affect nutrient 

availability, light, litterfall, and forest regeneration (Richards 

6. Disturbances

N.L. Harris and A.E. Lugo

Landscape

position

Rate of formation

(gaps ha-1 yr-1) Affected trees per gap

Canopy opening

per gap (m2)

Aboveground biomass 

per gap

(Mg gap-1)

Ridges 0.9 1.8 (0.85, 7) 55 (45, 4) 1.37 (0.10, 4)

Slopes and upland valleys 0.5 2.0 (0.48, 12) 57 (21,6) 1.99 (1.11, 6)

Riparian valleys 1.7 1.4 (0.61, 13) 104 (65, 9) 1.81 (0.45, 9)

Total area 0.8 1.8 (0.40, 32) 76 (22, 19) 1.78 (0.38, 19)

Table 1.Standard error and sample size are in parentheses. Data for trees with DBH (diameter at breast height) ≥ 10 cm. From Scatena and Lugo 

(1995).

and Williamson 1975, Denslow 1987, Uhl et al. 1988). In 

boreal or temperate forests, slow decomposition of forest litter 

and cold, wet soils may limit regeneration; thus, exposure of 

mineral soil by an uprooted tree is essential in providing the 

increased nutrients, increased soil temperatures, and decreased 

soil moisture needed for establishing many plant species (Murray 

and Van Veldhuizen 1980, Thompson 1980, Beatty and Stone 

1986). In tropical forests, light is often considered the resource 

that most limits the establishment and growth of plants in the 

understory (Denslow et al. 1990), although Walker et al. (1996b) 

noted dramatic species responses to artificial nutrient additions 

in hurricane-created gaps in the LEF, and Zimmerman et al. 

(1995) demonstrated by adding large amounts of woody debris 

that nutrients can be immobilized in tropical forest gaps.

Background estimates of the percentage of ground surface 

exposed by uprooting are generally low in the tropics (0.09 

percent; Putz 1983) compared with the temperate zone (6 to 

45 percent; Beatty and Stone 1986, Webb 1988). On average, 

the treefall-gap turnover rate is 100 percent per 135 years in the 

LEF (Lugo and Scatena 1995, Scatena and Lugo 1995), and the 

frequency of treefall gaps varies systematically with geomorphic 

setting and increases toward lower landscape positions (table 

1). During the 2 years before Hurricane Hugo hit the LEF, the 

average canopy opening of 21 treefall gaps within a 13 hectares 

study area was 76 m2

 (square meters) which is comparable to the 

60 to 100 m2 openings expected from canopy trees of this size 

that have crown diameters ranging from 8.8 to 11.3 m (Scatena 

and Lugo 1995). When Hurricane Hugo passed directly over 

the LEF on September 18, 1989, 9 to 39 percent of the trees 

were uprooted, depending on distance to the center of the 

hurricane, and 56 percent of the trees in the LEF were severely 

defoliated (Brokaw and Walker 1991, Walker 1991). Subsequent 

uprooting of hurricane-affected trees continued for more than 

3 years (Walker 1995). These continuing effects presented 

a unique experimental situation in which the defoliation of 

the canopy during the passage of Hurricane Hugo caused a 

temporary reduction in shade to both treefall soil pits and the 

undisturbed forest floor, but these two microsites had distinct 

soil conditions. Researchers used the opportunity to tease apart 
22 Luquillo Experimental Forest: Research History and Opportunities

the effects of light and soil nutrient availability on recovery after 

treefall. Walker (2000) compared soil nutrient dynamics in soil 

pits of treefalls and the undisturbed forest floor after Hurricane 

Hugo occurred and found that soil in the pits under uprooted 

trees had lower N (nitrogen), lower phosphorus, and lower 

organic matter; was drier; had higher bulk densities; and was 

more acidic than soils from adjacent forest plots (table 2).

Soil concentrations of Al (aluminum) and Fe (iron) were higher 

and Ca (calcium) was lower in pit soils than forest soils, Mg 

(magnesium) tended to be higher in forest soils than pit soils, 

and K (potassium) and Mn (manganese) did not differ between 

pit and forest soils (table 3). These soil conditions, generally 

unfavorable to plant growth, resulted from the upheaval and 

removal of surface organic matter and exposure of subsurface 

mineral soils (Beatty and Stone 1986, Vitousek and Denslow 

1986).

Canopy gaps resulting from treefall also influence light 

availability and subsequent plant responses (Chazdon and 

Fetcher 1984, Denslow 1987). Brokaw (1987) observed that 

background (nonhurricane) treefall gaps of at least 150 to 250 

m2 are best for the establishment and growth of pioneers such as 

Cecropia schreberiana in the LEF. These species often have viable 

seeds in the soil that germinate in response to an increase in red: 

farred ratios or soil temperature and grow rapidly in high light 

environments (Silander 1979, Vázquez Yanes and Smith 1982, 

Denslow 1987). Upon canopy opening, Cecropia schreberiana

has been observed to grow 2 to more than 4 m in height per 

year in the LEF (Silander 1979, Odum 1970a). Fernández 

and Fetcher (1991) evaluated light levels in a 32-m transect 

within the LEF after Hurricane Hugo occurred. They showed 

that 60 centimeters (cm) above the forest floor, median values 

of total daily photosynthetic photon flux density (PPFD) were 

Tree species Plot type

Organic

matter (%) Moisture (%) Bulk density (g m-2) pH

Ormosia krugii Pit 17.14 ± 0.82 27.91 ± 2.13 0.82 ± 0.02 3.40 ± 0.04

 Forest 28.83 ± 1.61 40.22 ± 3.00 0.59 ± 0.06 3.53 ± 0.06

Casearia arborea Pit 20.22 ± 0.83 30.64 ± 2.71 0.83 ± 0.02 3.40 ± 0.05

 Forest 25.94 ± 0.75 41.95 ± 2.51 0.65 ± 0.04 3.69 ± 0.08

Inga laurina Pit 19.22 ± 1.65 25.11 ± 1.91 0.83 ± 0.06 3.20 ± 0.04

 Forest 26.53 ± 1.81 40.82 ± 2.44 0.63 ± 0.07 3.57 ± 0.13

Two-way ANOVA results:

 Species effect 0.785 NS 0.436 NS 0.955 NS 0.399 NS

 Plot type effect <0.001*** <0.001*** <0.001*** <0.002***

 Species × plot interaction 0.614 NS O.864 NS 0.896 NS 0.575 NS

Table 2. Soil parameters from treefall pit and intact forest plots. Significant Analysis of Variance results are indicated by *** (P < 0.001). 

NS = not significant (P > 0.05). From Walker (2000).

Table 3. Elemental analyses of soils from the pit and forest plots (mg g-1). Significant Analysis of Variance results are indicated by 

* (Probability < 0.05), ** (P < 0.01), *** (P <0.001). NS = not significant (P > 0.05). From Walker (2000).

Tree species Plot type Nitrogen Phosphorus Aluminum Calcium Iron Magnesium Manganese Potassium

Ormosia krugii Pit 1.74 ± 0.10 0.22 ± 0.01 83.44 ± 4.03 0.17 ± .10 78.55 ± 3.76 0.99 ± 0.13 0.38 ± 0.14 0.51 ± 0.04

 Forest 6.36 ± 0.53 0.30 ± 0.01 70.11 ± 4.03 0.86 ± 0.10 62.55 ± 3.76 1.30 ± 0.13 0.68 ± 0.14 0.61 ± 0.04

Casearia arborea Pit 3.06 ± 0.32 0.21 ± 0.01 68.07 ± 3.23 0.36 ± 0.08 62.28 ± 3.01 1.66 ± 0.11 0.35 ± 0.11 0.48 ± 0.03

 Forest 5.41 ± 0.37 0.29 ± 0.01 59.92 ± 3.23 0.79 ± 0.08 54.35 ± 3.01 1.34 ± 0.11 0.52 ± 0.11 0.46 ± 0.03

Inga laurina Pit 2.71 ± 0.84 0.21 ± 0.02 66.25 ± 6.05 0.24 ± 0.15 64.25 ± 5.64 0.96 ± 0.20 0.25 ± 0.21 0.68 ± 0.07

 Forest 6.03 ± 0.15 0.30 ± 0.02 57.00 ± 6.05 0.60 ± 0.15 50.50 ± 5.64 1.17 ± 0.20 0.35 ± 0.21 0.55 ± 0.07

Two-way ANOVA results: 

 Species effect 0.598 NS 0.765 NS 0.002** 0.439 NS 0.002** 0.696 NS 0.416 NS 0.027*

 Plot type effect <0.001*** <0.001*** 0.009** <0.001*** <0.001*** 0.042* 0.157 NS 0.674 NS

 Species × Plot interaction 0.032* 0.986 NS 0.774 NS 0.283 NS 0.479 NS 0.954 NS 0.809 NS 0.134 NS
Luquillo Experimental Forest: Research History and Opportunities 23

similar to values in large treefall gaps (>400 m2) for 10 months 

after the hurricane passed over the forest. By 14 months after 

the hurricane’s passage, regrowth of the forest understory and 

canopy had reestablished a shaded forest environment. Because 

large gap sizes (>150 m2) are not common in the LEF, except 

after hurricanes, widespread disturbance by hurricanes may be 

necessary to explain the current densities of C. schreberiana in the 

LEF (Brokaw 1998, fig. 24).

After Hurricane Hugo hit the LEF, Walker (2000) compared 

seedling and sapling dynamics of treefall pits and found that, 

although seedling densities did not differ between undisturbed 

soil and treefall pits, the number of species present (saplings 

and seedlings combined) was significantly lower in the soil pits 

under each species of uprooted tree than in the undisturbed soil 

(fig. 25). Cecropia schreberiana seedlings grew into canopy trees 

only in the treefall pits, presumably as a consequence of both 

the temporary (ca. 1 yr) increase in light from defoliation of the 

surrounding canopy and the longer (ca. 2 to 4 yr) increase in 

light from the uprooting of the tree. This extended gap duration 

provided by the uprooting of trees and subsequent delay in 

canopy closure was critical for establishing C. schreberiana trees. 

With the additional light following the defoliation of the LEF 

by Hurricane Hugo, treefall pits provided an opportunity for 

C. schreberiana saplings to reach the canopy, many of which 

survived to reproduce and reestablish the seed bank (Brokaw 

1998). The net effect of the hurricane and treefall combination 

resembled multiple or large treefall gaps. In contrast, the 

temporarily higher light conditions in the forest plots due to 

Figure 24.—Cecropia schreberiana sapling that was established due to 

high light levels from treefall in the Luquillo Experimental Forest. Photo by 

N. Harris.

Figure 25.—(a) Species and (b) seedling densities in treefall pits (open 

circles) and forest (closed circles) plots under a total of three uprooted tree 

species for seven sampling dates following Hurricane Hugo (September 

1989; number m-2; ±SE; N=27). From Walker (2000).
24 Luquillo Experimental Forest: Research History and Opportunities

leaf loss during the passage of the hurricane were not sufficient 

to allow for the establishment of C. schreberiana trees or 

the growth of understory seedlings of other species into the 

canopy. Instead, existing canopy trees closed the gaps with 

lateral growth. Therefore, the net effect of treefall gaps on the 

species composition of the LEF was the increased number of C. 

schreberiana trees present during the 45 months after Hurricane 

Hugo came ashore (Walker 2000). 

Landslides

A landslide is a common, natural mass-wasting phenomenon that 

occurs in mountainous areas throughout the world, including in 

the LEF, and represents a net downslope movement of material. 

Depending on the soil type, 6 to 24 percent of soil C stocks 

is exported downslope despite deposition from surrounding 

forest soils, litter from the surrounding landscape, and in situ

successional regrowth (fig. 26). The term “landslide” means 

the downward and outward movement of hillslope-forming 

materials—natural rock, soils, artificial fills, or combinations of 

these materials (Schuster 1978). Landslides can include falls, 

topples, slides, spreads, and flows (Varnes 1978). Vegetation 

on recent landslides is very sparse, consisting mainly of mosses 

and isolated seedlings. Within 25 years, these landslides become 

covered with ferns and eventually with the seedlings and saplings 

that represent the mature forest. Cecropia schreberiana is the 

most abundant woody species growing on LEF landslide sites 

(Myster and Walker 1997) and the tree fern Cyathea arborea is 

also very common (Guariguata 1990, Walker et al. 1996a).

Landslides are part of the process of hillslope erosion and 

contribute to the introduction of sediment into streams, rivers, 

lakes, reservoirs, and finally the ocean. Landsliding triggered 

by earthquakes may be the most significant type of landslide 

in Puerto Rico over the long term (Keefer 1984), but hillslope 

modification for the construction of highways and other 

structures also frequently results in landsliding (Dames and 

Moore 1980, Molinelli 1984). For example, Larsen and Parks 

(1997) determined that within 85 m on either side of roads 

in the Luquillo Mountains, the rate of mass wasting was five 

to eight times higher than in forested areas; however, rainfalltriggered landslides are the most common type of landslide in 

the LEF. In 1989, Hurricane Hugo triggered more than 400 

landslides in northeastern Puerto Rico (Scatena and Larsen 1991, 

Larsen and Torres Sánchez 1992). Guariguata and Larsen (1990) 

prepared a preliminary map of landslide locations in the LEF 

but more recently, Larsen and Torres Sánchez (1998) developed 

a map of 1,859 landslides that occurred in the LEF region 

between 1951 and 1990 (fig. 27) by compiling and analyzing 

approximately 300 sets of stereo aerial photographs (1:20,000 

scale) in combination with a computerized Geographic 

Information Systems. The map was used to determine where 

landslides were common and to devise a method to assess the 

frequency of landslides over the landscape. Landslides were 

relatively evenly distributed among shallow soil slips, debris flows 

and slumps, with only a few debris avalanches. The estimated 

average frequency of landslides increased markedly as landscape 

disturbance increased from forests to agriculture areas to roads 

and structures (table 4). New landslides in the LEF affect 1 

to 3 percent of the forest surface every 100 years (Walker et al. 

1996a).

Slopewash refers to downslope soil sediment loss after a landslide. 

Larsen et al. (1999) measured a 4-year time series of slopewash 

and rainfall at two intact forest sites and two landslide scars in 

the LEF and showed a marked decrease in slopewash over time 

as landslide scars aged (fig. 28). The slopewash rate of a fresh 

landslide scar was equivalent to 2 to 40 times the mean annual 

slopewash rate for undisturbed forest, attesting to the importance 

of landslide scars as localized sediment sources. By the end of the 

study, slopewash rates on both landslide scars were lower than 

rates under the forest canopy, suggesting that the hydrologic and 

biologic characteristics of the soil surface on the landslide scar 

had been substantially altered. The possibility of the decrease 

in slopewash over time on landslide scars as attributable to the 

decline in rainfall during the study period was evaluated by 

calculating slopewash concentration, expressed as slopewash 

divided by surface runoff. Slopewash concentration measured 

on the landslide scars decreased by a factor of 35 to 70, while 

slopewash concentration decreased by no more than a factor of 4 

on the forested hillslopes.

Figure 26.—Distribution of carbon within each soil type (volcaniclastic, 

dioritic, 0 to 10 cm depth) from three measured carbon pools: outputs 

of soil and litter (-) and inputs of litter (+). Proportions of total C within 

each soil type are based on means (g C m-2) of biweekly measurements 

(December 2003 to May 2004) from 1-m2 plots on 8 to 13 month old 

landslides in Puerto Rico. From Walker and Shiels (2008).
Luquillo Experimental Forest: Research History and Opportunities 25

Figure 27.—Shaded relief map of the Blanco study area in Puerto Rico, showing locations of landslides and rivers. From Larsen and 

Torres Sánchez (1998).
26 Luquillo Experimental Forest: Research History and Opportunities

Slopewash rates on landslide scars seem to be controlled strongly 

by the state of vegetative and soil recovery. In the study by 

Larsen et al. (1999), the saprolite (i.e., deposits of clay and 

disintegrated rock found in situ) exposed on the surfaces of the 

landslide scars was denser and less porous compared with that of 

the soils beneath the forest canopy. In addition, a pervasive black 

epiphyll crust composed of mosses and blue-green algae (Nostoc, 

Scytonema, Anabaena and Caolthrix) developed on the landslide 

scar surfaces. Both factors effectively sealed the soil surface and 

reduced the tendency for rainsplash to detach soil particles. As 

the landslide scar surface is revegetated by seedling recruitment 

and encroachment along the perimeter, penetration by roots 

and soil mesofauna gradually returns the soil surface (slopewash 

rates, soil organic material) to the conditions observed under 

the forest canopy (Zarin and Johnson 1995, Larsen et al. 1999). 

Just as soil conditions have returned to predisturbance status, 

however, the local-scale landslide disturbance cycle may be reset 

by hurricanes, which have directly crossed the LEF about once 

every 60 years during the past several hundred years (Scatena 

and Larsen 1991, Scatena 1995). About 60 years is required for 

forest regeneration after landslide disturbance in the uplands of 

the LEF (Guariguata 1990), but regeneration may be faster if 

bird perches are present on the landslide to facilitate forest seed 

inputs through bird dispersal (Shiels and Walker 2003). 

Regeneration of forests on landslide scars affords an opportunity 

to explore how forest vegetation and its associated microbes 

accumulate and manage reservoirs of nutrient capital (Frizano et 

al. 2002). Although oligotrophic soils, with a high dependence 

on biological recycling of mineral nutrients, have been associated 

with tropical forest ecosystems considered to be extremely 

sensitive to disturbance (Uhl et al. 1982), landslide scars in the 

LEF regain predisturbance levels of major nutrients rapidly 

(Zarin 1993). Zarin and Johnson (1995) documented the 

rates at which aboveground and belowground pools of several 

nutrients recovered during afforestation of landslide scars in 

the palo colorado (Cyrilla) forest type by evaluating data from 

Elevation Slope orientation Slope angle Landslides per km2 per decade

Forest land use Agricultural land use Roads and structures

> 400 m Lee of prevailing wind > 12° 0.5 1.0 5.4

≤ 12° 0.9 1.6 3.0

Normal to prevailing wind > 12° 0.7 2.3 6.3

≤ 12° 0.7 1.4 5.5

Facing prevailing wind > 12° 1.2 8.8 15.4

≤ 12° 1.1 2.5 9.0

≤ 400 m Lee of prevailing wind > 12° 0.8 3.3 6.0

≤ 12° 0.3 1.0 0.6

Normal to prevailing wind > 12° 1.0 4.5 4.4

≤ 12° 0.4 1.3 0.4

Facing prevailing wind > 12° 1.7 8.4 8.9

≤ 12° 0.5 2.6 1.1

Averages 0.8 2.5 3.2

Table 4. Matrix showing the estimated frequency of landslides (number of landslides per km2 per decade) in 36 geographic categories in the Río 

Blanco study area. From Larsen and Torres Sánchez (1998).

Values calculated by first dividing the number of landslides per category by area in km2 and then dividing by 5 to account for the five decade span of data.

Figure 28.— Mean monthly slopewash and rainfall on forested hillslopes 

and on landslide scars. Note the logarithmic scale. (a) Site F1 is a 

Dacryodes-dominated forest, and Site LS1 is on a 1991 landslide scar with 

active vegetation regrowth. (b) Site F2 is a Cyrilla-dominated forest, Site 

F3 is elfin forest, and Site LS2 is a 1989 landslide scar. From Larsen et al. 

(1999).
Luquillo Experimental Forest: Research History and Opportunities 27

a chronosequence of landslide scars in the LEF. Landslide 

scars ranged in age from 1 year to more than 55 years. They 

found that major nutrient cation concentrations (neutral-saltextractable Ca2+, Mg2+, K+

, and total soil N pools) recovered to 

prelandslide levels in less than a century (fig. 29), but extractable 

inorganic P (phosphorous) present in the soil of recent landslide 

scars was undetectable. Table 5 summarizes regression equations 

that estimate rates of change during the course of the more-than55-year landslide scar sequence for each variable measured.

Frizano et al. (2002) determined the rate at which labile P capital 

recovers during primary succession on landslides and concluded 

that a net increase in labile P is supplied from at least two 

sources: atmospheric deposition and allochthonous litter input—

and probably from a third source, the pool of inorganic occluded 

P. In the top 10 cm of soil, organic matter, labile and total 

organic P increased with landslide age and labile soil P increased 

to approximately two-thirds of the predisturbance levels in 

the oldest landslide scar (>55 yr) (table 6). Thus, plants, their 

associated microflora and microfauna, and P inputs from offsite 

sources, substantially altered the distribution of soil P fractions 

during forest recovery. Figure 30 illustrates the correlations 

between percent C and resin-Pi (inorganic phosphorus released 

to anion exchange resin) and between percent C and the labile 

P pool. These results correlate with the findings of others 

who have studied acidic, highly weathered soils (Tiessen et al. 

1994). They are also consistent with the idea that turnover of 

easily mineralized organic P through microbial mediation is 

an important controller of plant-available inorganic P in LEF 

forests more than 55 years old (Frizano et al. 2002). The added 

P is captured by forest biota and returned to the soil in organic 

combinations. In the longer term (decades), it appears that the 

most important source of plant-available P shifts to the labile P 

pool whereby vegetation relies on the processing and cycling of 

readily mineralizable organic P.

Concurrent with the increase in exchangeable nutrient cation 

values is a rapid (i.e., within 60 years, Guariguata 1990) 

reestablishment of plant biomass both aboveground and 

belowground (Zarin 1993). Mortality and decomposition of 

biomass provides the developing soil with organic matter, which 

may be of central importance in the provision of exchange sites 

and/or major nutrients (fig. 30). For observed increases in soil 

organic matter (SOM) and nutrient content to occur along with 

the rapid forest regrowth, nutrient sources outside the initial 

exchange pool present in the soils of the new landslide scars must 

be available. In the LEF, after about 30 years after landslide 

disturbance, the supply of labile P in the soil far exceeds the 

annual demand for P (Frizano et al. 2002).

Variable Regression equation R P

Clay 12.7 + 5.03 [log(years)] 0.61 0.036

SOM 0.90 + 1.53 [log(years)] 0.74 0.006

Ca2+ 0.90 + 0.0 (years) 0.92 <0.001

Mg2+ 0.12 + 0.25 [log(years)] 0.76 0.004

K+ 0.08 + 0.05 [log(years)] 0.57 0.051

Al– 2.17 – 0.37 [log(years)] 0.52 0.082

Ca + Mg + K 0.23 + 0.56 [log(years)] 0.83 0.001

Ca = Mg + K + Al 2.45 + 0.01 (years) 0.28 0.410

BSI 0.13 + 0.20 [log(years)] 0.85 <0.001

Table 5. Regression equations calculated based on minimum age 

estimates for the 1 to 55+ year landslide chronosequence in the 

Luquillo Experimental Forest (Zarin and Johnson 1995).

Units are percent for Clay and Soil Organic Matter (SOM), and cmol+ kg-1 for 

the exchangeable cations.

BSI = [Ca + Mg + K]/[Ca + Mg + K = Al]. 

R = a correlation coefficient

P = the probability

Figure 29.—Base saturation index values calculated from surface mineral 

soil characteristics for a chronosequence of landslide scars and two 

late-succession vegetation associations (tabonuco and palo colorado). 

Data and regression calculations for the landslide chronosequence are 

plotted according to minimum plot age estimates of 1, 12, 27, and 55 

years (based on aerial photographs). Estimated ages for palm brake and 

palo colorado forest are more than 100 years and more than 300 years, 

respectively. From Zarin and Johnson (1995).
28 Luquillo Experimental Forest: Research History and Opportunities

Vegetation characteristics on recent (13-month-old) landslides 

appear to vary with soil type (Shiels et al. 2008). Vines, 

trees, shrubs, and ferns with a high proportion of biomass 

aboveground tend to dominate volcaniclastic landslides (high 

soil clay, N, water-holding capacity, and low elevation), while 

herbs and mosses (bryophytes) with a higher proportion of 

belowground biomass dominate the diorite substrate (table 7). 

Fern thickets on low-nutrient landslide soils appear to facilitate 

germination but inhibit growth of tree seedlings, possibly 

delaying forest development on landslides. Walker (1994) 

investigated the effects of fern thickets on landslide succession 

in the LEF and found that soil moisture and total soil N were 

Plot age

Depth 

(cm) N (%) C (%) Resin-Pi Pi Po Pi Po

Dil. 

HCl-Pi Pi Po

Residue 

P Total P

1986-1991 0-10 0.034 0.28 1.11 <0.04 0.22 11.20 4.39 2.60 124 2.97 18.2 162

(0.062) (0.96) (<0.04) (0.53) (2.75) (0.62) (18.9) (2.03) (2.15) (23.1)

10-35 0.026 0.13 0.99 <0.04 <0.04 8.62 6.26 1.71 139 0.240 23.6 165

(0.001) (0.014) (0.48) (<0.04) (<0.04) (0.20) (1.26) (0.86) (23.6) (0.14) (3.20) (15.6)

35-60 0.017 0.07 1.05 <0.04 <0.04 9.90 6.55 2.08 151 8.10 17.0 181

(0.009) (0.015) (0.20) (<0.04) (<0.04) (0.33) (0.99) (0.94) (27.9) (1.44) (1.40) (40.6)

1972-1979 0-10 0.05 0.72 1.45 0.33 1.43 5.83 16.0 4.64 94.6 2.90 20.3 146

(0.006) (0.08) (0.82) (0.33) (1.15) (3.00) (6.90) (1.70) (4.40) (1.10) (0.20) (5.20)

10-35 0.03 0.19 0.86 <0.04 <0.04 13.6 8.62 3.33 82.5 2.10 21.1 119

(0.001) (0.05) (0.86) (<0.04) (<0.04) (1.90) (4.20) (0.20) (11.70) (0.30) (0.90) (11.6)

35-60 0.008 0.07 1.31. <0.04 <0.04 8.72 3.75 4.05 75.6 2.80 17.8 111

(0.008) (0.02) (0.71) (<0.04) (<0.04) (0.24) (1.40) (0.60) (5.00) (0.90) (2.10) (4.10)

1951 0-10 0.13 2.32 9.50 1.40 3.30 17.2 49.0 9.60 147 17.8 42.42 292

(0.01) (0.24) (2.80) (0.69) (1.40) (11.9) (15.5) (3.90) (43.0) (13.60) (6.00) (25.1)

10-35 0.08 1.00 3.10 0.70 0.09 16.9 30.1 4.40 104 20.7 35.5 208

(0.02) (0.25) (0.80) (0.40) (0.06) (7.90) (18.1) (2.00) (26.6) (11.6) (3.00) (21.5)

35-60 0.05 (0.48) 1.54 0.20 0.88 28.3 7.35 4.70 131 16.8 35.1 212

(0.01) (0.16) (0.69) (0.05) (0.50) (6.60) (2.30) (0.80) (34.0) (14.2) (4.60) (22.0)

Pre-1936 0-10 0.16 1.80 2.12 1.12 2.40 16.8 22.8 0.64 26.8 7.85 20.0 100

(0.02) (0.40) (0.30) (0.40) (1.70) (0.40) (3.60) (0.20) (1.50) (2.30) (1.50) (1.70)

10-35 0.12 1.33 2.22 0.52 1.19 28.5 23.2 3.24 33.7 4.76 26.0 123

(0.02) (0.30) (1.20) (0.52) (0.50) (13.8) (6.90) (0.50) (8.50) (0.50) (6.50) (36.0)

35-60 0.05 0.42 0.56 <0.04 1.14 9.70 4.92 3.07 33.9 35.3 18.07 105

(0.02) (0.20) (0.40) (<0.04) (1.14) (0.70) (2.50) (1.10) (11.0) (33.0) (1.80) (22.0)

CDF 0-10 0.27 4.22 15.4 0.91 5.16 4.15 59.8 0.50 26.6 10.9 25.3 149

(0.01) (0.14) (2.40) (0.91) (1.40) (4.15) (3.90) (0.37) (3.30) (1.90) (2.40) (14.3)

10-35 0.12 1.27 3.09 0.14 0.52 18.1 11.9 0.31 22.9 3.09 25.0 83.1

(0.001) (0.03) (0.73) (0.14) (0.52) (2.10) (0.68) (0.27) (1.20) (0.90) (1.20) (4.70)

35-60 0.08 0.82 1.85 0.76 0.57 14.5 4.80 1.22 27.0 2.92 21.0 72.9

(0.006) (0.08) (0.94) (0.76) (0.57) (4.60) (1.30) (0.19) (4.40) (0.50) (0.60) (2.80)

Table 6. Soil nutrient concentrations from a chronosequence of landslide scars (<5 to >55 yr in age) and a mature forest (CDF*; ca 300 

years old) in the Luquillo Experimental Forest. Values presented are means with standard errors in parentheses. For C, N, and the P 

fractions, N=3 samples per depth for all sites, except at the 1986 to 1991 site, N=2 in the 0 to 10 cm depth interval; at the 1951 to 

1964 sites, N=4 in the 0 to 10 and 35 to 60 cm depth intervals and N=5 in the 10 to 35 cm depth interval. P concentration units are in 

μg g-1 (Frizano et al. 2002).

*Cyrilla-dominated forest.

NaHCO3

- NaOH- Concentration HCl-
Luquillo Experimental Forest: Research History and Opportunities 29

higher under fern thickets than in adjacent open areas and soil 

bulk density and soil surface temperatures were lower. Seeds of 

Cecropia schreberiana sown under fern thickets germinated more 

than those sown into adjacent open areas, but tree seedlings of 

Tabebuia heterophylla exhibited a threefold lower photosynthetic 

rate under ferns, probably resulting from a 12-fold reduction of 

PPFD. Although the growth of Tabebuia seedlings was reduced 

under ferns, the distribution of seedlings of naturally occurring 

woody plants was not strongly correlated with the presence of 

fern thickets. 

Figure 30.—Relationship between landslide percent soil carbon and (a) the extractable phosphorus pool and (b) the labile phosphorus pool in the Luquillo 

Experimental Forest. From Frizano et al. (2002).

30a. 30b.

Vegetation measurement

Soil type Significancea

Volcaniclastic Diorite Pb

Biomass (g m-2)

Trees and shrubs 18.07 ± 9.65 1.40 ± 0.99 <0.001

Herbs 2.37 ± 1.88 7.72 ± 4.03 0.698

Vines 22.66 ± 10.19 1.57 ± 0.78 0.002

Graminoids 10.94 ± 5.53 2.94 ± 1.61 0.036

Fernsc 4.55 ± 1.79 0.92 ± 0.38 0.002

Bryophytes 0.12 ± 0.12 0.65 ± 0.43 0.436

Total aboveground 58.71 ± 23.57 15.28 ± 6.13 0.007

Total belowground 17.70 ± 6.48 41.75 ± 22.77 0.997

Total 76.41 ± 28.70 57.03 ± 23.93 0.227

Cover (%)

Trees and shrubs 19.57 ± 6.24 2.20 ± 1.22 <0.001

Herbs 3.50 ± 1.80 13.29 ± 4.70 0.617

Vines 10.75 ± 4.12 1.50 ± 0.93 0.002

Graminoids 8.13 ± 3.31 3.65 ± 0.95 0.082

Ferns 9.71 ± 2.83 3.81 ± 2.07 0.011

Bryophytes 8.86 ± 3.69 9.25 ± 2.62 0.992

Total 40.88 ± 7.26 23.64 ± 5.71 0.031

Table 7. Vegetation characteristics (mean ± SE) on 13-month-old landslides with two different soil types (volcaniclastic, n=8; diorite, n=22) in the 

Luquillo Experimental Forest. From Shiels et al. (2008).

a Results of the Multivariate Analysis of Variance for all variables listed in the biomass category (Wilks’ Lambda = 0.463, P = 0.038, F9.20 = 2.573), and 

the cover category (Wilks’ Lambda = 0.433, P = 0.005, F7.22 = 4.113).

b P = Probability

c The fern category includes fern allies.
30 Luquillo Experimental Forest: Research History and Opportunities

On landslides in the LEF, leaf litter of the Cyathea tree fern 

decomposes significantly faster than Cecropia schreberiana leaf 

litter (Shiels 2006), most likely due to differences in leaf litter 

chemistry between the two species. Decomposition rates on 

landslides are low compared with rates reported in past studies in 

intact forest in the LEF (Zou et al. 1995, González and Seastedt 

2001, Myster and Schaefer 2003), most likely due to fewer 

decomposer organisms and to severe environmental conditions 

on landslides (Calderón González 1993, Walker et al. 1996a). 

Decomposition rates on landslides are also consistent across 

widely ranging landslide conditions, which suggests that the 

species of leaf litter has a greater effect on decomposition rates 

than the location and environmental conditions on a particular 

landslide (Shiels 2006). 

Hurricanes

In recent decades, between 8 and 11 hurricanes have occurred 

in the Atlantic Ocean each year on average (Riehl 1979, Gray 

1990), and an average of 1 to 2 hurricanes per year occur 

throughout the Caribbean. The island of Puerto Rico has 

experienced more than 70 hurricanes since 1700 (Salivia 1950, 

Millas 1968, Neumann et al. 1987, Heartsill Scalley et al. 

2007)—approximately 1 every 21 years—including severe storms 

in 1899, 1928, 1932, 1989, and 1998 (Gifford 1905, Bates 

1930, Weaver 1987, Scatena and Larsen 1991). Hurricanes 

generally approach the island of Puerto Rico from an easterly 

direction (fig. 31).

Hurricanes are classified by their intensity into five wind-speed 

categories according to the Saffir/Sampson Hurricane Scale (table 

Figure 31.—Paths of hurricanes that have crossed Puerto Rico since 1700. Adapted from Salivia (1972), Weaver (1986b), and NWS (1990) ̧ in 

Scatena and Larsen (1991). Note that Hurricane Georges (1998) is missing from the figure.

8, Neumann et al. 1978, Gray et al. 1997). Hurricane Hugo, 

a category 3 hurricane, passed over the extreme eastern tip of 

Puerto Rico on September 18, 1989, with maximum sustained 

winds in a northwesterly direction of 57 m s-1 (meters per 

second). Effects in the LEF were severe, particularly at the Bisley 

study site in the northeastern portion of the LEF, where extensive 

areas of forest were blown down. Nine years later on September 

21, 1988, Hurricane Georges, also a category 3 hurricane, passed 

over the island. 

The occurrence of hurricanes in the LEF has a marked influence 

on the structure and dynamics of plants and animals in the LEF. 

Much has been learned about these effects since the Brown et 

al. (1983) report, due to the extensive research conducted in 

the forest after Hurricanes Hugo and Georges occurred. In 

the 20 years since the passage of Hurricane Hugo, the effects 

of hurricane disturbances on ecosystem processes have been 

an important focus of research for the LUQ LTER Program 

(Waide and Lugo 1992, Lugo and Waide 1993). Lugo (2008) 

reviewed the visible and invisible effects of hurricanes on forest 

ecosystems; much of the information presented in the following 

section is paraphrased from this extensive review.

From an energy perspective, hurricanes have a global role in the 

heat balance of the Earth by transporting excess tropical heat 

to higher latitudes, dissipating approximately 36 x 1010 kWh 

(kilowatt hour) of energy per day, or at a daily dissipation rate 

equivalent to about 10 percent of the annual electrical output 

of the United States in 1999 (DOE 2000). The passage of 

hurricanes is controlled by sea surface temperature, atmospheric 

pressure, and high-altitude wind conditions (Emanuel 1987, 
Luquillo Experimental Forest: Research History and Opportunities 31

2005; Gray et al. 1997). The location of landmasses and local 

topography play a role in determining the direction and speed of 

the hurricane’s passage as well as the effects it has on vegetation 

(Boose et al. 1994, Foster and Boose 1995, Turton 2008, Turton 

and Stork 2008). Hurricanes require warm water to maintain 

strength, which is why the storm strength diminishes when 

landfall is achieved. The rate of forward movement is faster 

for hurricanes at higher latitudes than for hurricanes at lower 

latitudes, which means that hurricanes at lower latitudes have 

longer periods of interaction with tropical forests than they do 

with temperate forests (Foster and Boose 1995). 

Changes in Environmental Conditions

Two environmental factors of hurricanes—rain and wind—have 

strong and independent interactions with forests. Wind causes 

biomass and nutrient transfer to the forest floor as a result of 

treefalls and treesnaps (Frangi and Lugo 1991, Lodge et al. 1991) 

as well as indirect mortality effects (Lugo and Scatena 1996). 

For example, defoliation during the passage of Hurricane Hugo 

deposited a thick layer of litter to the forest floor in the LEF 

equivalent to more than 1 year of normal litterfall (Lodge et 

al. 1991). The second environmental factor—rain—can result 

in landslides, which transport materials away from the stand 

(Larsen and Torres Sánchez 1992, Larsen and Simon 1993). 

These two effects have different consequences. Mass transfer in 

situ (from the canopy to the forest floor) maintains forest stand 

conditions, which can quickly restore a closed canopy forest 

through succession. More importantly, tree uprooting affects 

soil morphology, surface characteristics, nutrient availability, 

regeneration trends, and forest age structure (Schaetzl et al. 

1989). In effect, hurricanes basically shift the ecological space 

available to organisms (sensu Hall et al. 1992) and set in motion 

several biotic responses that range temporally from seconds to 

centuries, eventually culminating in mature stands that can be 

either similar to or different from the prehurricane stand. 

As a result of the wind effects on trees and the removal of the 

canopy, light levels at the forest floor increase dramatically after 

a hurricane has passed (Walker et al. 1992). Median PPFD at 

midday in the LEF rose from 23 μmol m-2 s-1 (micromole per 

Figure 32.—Frequency distribution of photosynthetic photon flux densities 

(PPFD) (1100 to 1300 local time) at the forest floor in the Luquillo 

Experimental Forest before (N=21,660 records) and after (N=34,548 

records) Hurricane Hugo passed over the forest. From Walker et al. 

(1992).

SS scale

Central pressure

(mbar)

Maximum sustained

wind speed (m s-1) Storm surge (m)

Relative potential 

destruction

1 980 33 – 42 1.0 – 1.7 1

2 965 – 979 43 – 49 1.8 – 2.6 10

3 945 – 964 50 – 58 2.7 – 3.8 50

4 920 – 944 59 – 69 3.9 – 5.6 100

5 <920 >69 ≥5.6 250

Table 8. Criteria by which hurricane intensity is defined according to the Saffir/Simpson (SS) scale (Gray et al. 1997, cited in Lugo 2000).

square meter per second) before Hurricane Hugo hit to 404 

μmol m-2 s-1 afterward, and the distribution of PPFD after the 

hurricane occurred showed a high proportion of observations 

more than 1,000 μmol m-2 s-1 (fig. 32), indicating that much of 

the forest was exposed to direct beam solar radiation. 

The passage of Hurricane Hugo over the Bisley Experimental 

Watersheds at the LEF affected the hydrologic cycle by 

decreasing evapotranspiration and increasing throughfall as a 

result of diminished leaf area (Scatena and Larsen 1991, Scatena 

et al. 1996, Heartsill Scalley et al. 2007). During the 2-week 

period before Hurricane Hugo hit the island, rainfall in eastern 

Puerto Rico was 80 to 85 percent below normal (NWS 1990). 

During the storm, however, rainfall totals ranged from 100 to 

339 mm and precipitation intensities ranged from 34 to 39 mm 

hr-1 (millimeters per hour) (U.S. Department of Commerce 

1990). During the weeks after the hurricane came ashore, large 
32 Luquillo Experimental Forest: Research History and Opportunities

Figure 34.—Damage index in relation to various forest or individual tree 

characteristics. The damage index was calculated as the percent of trees 

that uprooted, snapped, or had less than 50 percent canopy branch loss 

(i.e., heavy effect) of all the trees in that class. Damage index in relation 

to (a) tree size; (b) crown class; (c) topography, as determined for each tree 

during a 1992 survey; (d) from Hurricane Georges (1989) in relation to 

tree effects from Hurricane Hugo (1989). Trees were classified into effect 

categories in 1992, and the previous effect is related to the amount of 

effect in 1998 during the second storm. From Ostertag et al. (2005).

Figure 33.—Departure from mean monthly rainfall for long-term rainfall 

stations at Fajardo, eastern Puerto Rico, and the northern slopes of Puerto 

Rico. From NWS (1990), in Scatena and Larsen (1991). 

areas of Puerto Rico experienced below average rainfall (fig. 33). 

Monthly rainfall totals were below long-term averages at Fajardo 

(NWS 1990) and within the LEF at El Verde until the end of the 

calendar year. Within 12 to 24 hours after storm rainfall ended, 

all the streams returned to prestorm discharge rates. Hurricanes 

lower the sediment load of rivers by dislodging and displacing 

vegetation, debris accumulated on hillslopes and small channels, 

blocking bridges, and forming debris dams as shown in Lake 

Loíza, Puerto Rico by Gellis (1993). These debris dams cause 

backwater effects that reduce stream velocities and decrease 

suspended-sediment loads.

Changes in Forest Structure

The most common effect of hurricanes on forest structure is 

defoliation (Bates 1930, Wadsworth and Englerth 1959), which 

is the direct result of wind energy dissipation onto the canopy 

interface. Other effects on forest structure include stem breakage 

(from small limbs to stem boles, which can snap at the base or at 

any height) and uprooting, which can range from various levels 

of leaning to complete blowdown of the tree, thereby exposing 

the complete root system to the atmosphere (Putz et al. 1983).

The ability to resist hurricane effects is a property of both 

individuals and communities and can have strong effects on 

the structure and function of tropical forests. The fraction of 

trees affected by wind is explained by differences in canopy size 

and architecture. In general, trees with larger canopies, with 

higher growth rates, and/or that are present on ridges and valleys 

experience more wind effects than trees with smaller canopies 

in the forest understory, with slower growth rates, and that are 

present on slopes (fig. 34). Trees on ridges, however, exhibited 

less wind effects than trees on slopes in the Bisley Experimental 

Watersheds during the passage of Hurricane Hugo (Scatena 

and Lugo 1995). The inconsistency at Bisley is explained by 

exposure relative to the storm path (windward versus leeward), 

strength of winds, soil types, and vegetation structure. At El 

Verde, which is on the leeward side of the mountain, only 

about 25 percent of all trees had some visible wind effects after 

Hurricane Hugo hit (Zimmerman et al. 1994), while 60 percent 

of the trees were visibly affected in the windward sector of Bisley 

(Basnet et al. 1992). 

Pioneer species, with their low wood density and low shade 

tolerance, experienced larger wind effects after the passage of 

Hurricane Hugo in terms of stem breakage and mortality than 

nonpioneers (Zimmerman et al. 1994, Everham 1996). Also, 

the growth of pioneer species in lower elevation forests and 

graminoid species in elfin woodlands was stimulated more by 

experimental nutrient additions than in mature forest species 

(Walker et al. 1996b). The lower wind effects and lower 

response to fertilization experienced by slow growing trees 

suggest that they can slowly but steadily reclaim and retain 

dominance in a frequent disturbance regime. After Hurricane 

Hugo hit, nonnative herbaceous plants and trees germinated 

in places with an open canopy and lasted 10 to 15 years 

before returning to prehurricane low densities (Chinea 1999, 

Thompson et al. 2007), but the nonnative species ultimately 

failed to survive once the canopy closed and native species 

regained their dominance. Thus, a slow growth rate might 

represent a hurricane response syndrome among tree populations 

that includes architecture, elastic modulus, successional status, 
Luquillo Experimental Forest: Research History and Opportunities 33

and wood density. Larger, more mature trees tend to develop 

on forest areas that escape large hurricanes due to either lower 

exposure or resistance to wind from the presence of tree unions 

through root grafting, which steadies trees during a windstorm 

(Basnet et al. 1992). These refugia are paradoxical in the sense 

that old trees are supported under a disturbance regime that 

generally tends to select for young rather than old trees.

After a hurricane makes landfall, there is also massive seed 

germination and subsequent growth of vines, tree seedlings, 

and herbaceous plants (Lugo and Zimmerman 2002), releafing 

of defoliated plants, abundant sprouting of new branches and 

stems, and epicormic tree growth (Zimmerman et al. 1994). 

In response to high light levels, new leaves produced after the 

passage of the hurricane likely have very high photosynthetic 

rates (Harris 2006, Harris et al. in press) that facilitate initial 

rapid recovery of forest structure. Typically, after a hurricane 

passes, tree density and aboveground biomass decrease (Scatena 

et al. 1996, Frangi and Lugo 1998) but basal area may or may 

not decrease; it all depends on the strength of the hurricane and 

the level of tree mortality. For example, after Hurricane Hugo 

occurred, Fu et al. (1996) reported reductions in basal area and 

tree density in a mahogany plantation and paired secondary 

tabonuco forest at El Verde. After Hurricane Hugo occurred 

in 1989 and Hurricane Georges passed over the island in 1998, 

however, a mature tabonuco forest near the site studied by Fu et 

al. (1996) did not experience basal area reduction (fig. 35b). 

In the years following the passage of strong hurricanes, structural 

parameters (tree density, biomass, and basal area) increased 

rapidly in all forest types in the LEF (fig. 35a-c, Fu et al. 1996, 

Scatena et al. 1996, Frangi and Lugo 1998). About 15 years 

after the hurricane passed, tree density reached a peak and both 

basal area and aboveground biomass stabilized (fig. 35b, c). After 

Figure 35.—Long-term trajectories of change (1943 to 2005) in tree structure of a tabonuco (Dacryodes excelsa) forest plot at El Verde: (a) tree density; 

(b) basal area; (c) aboveground tree biomass; (d) The Shannon-Weiner species diversity index (dots) and Pielou’s J evenness index (squares). These 

data were collected by a variety of people, including F.H. Wadsworth and T. Crow and synthesized by Drew et al. (2009). A major hurricane (San Cipriano) 

passed over the site in 1932. Hurricanes Betsy (1956), Hugo (1989), and Georges (1998) also influenced stand dynamics. From Lugo (2008).

35a.

35c.

35b.

35d.
34 Luquillo Experimental Forest: Research History and Opportunities

15 years, stands entered a thinning phase that increased tree 

mortality and reduced tree density. The reduction in tree density 

made more room for surviving trees and resulted in a spurt in 

basal area and an increase in biomass.

Rapid changes in tree density, basal area, and aboveground 

biomass are accompanied by an equally dramatic change in stand 

composition and species diversity (fig. 35d). The hurricane’s 

disturbance increases the species diversity of stands because 

pioneer and nonpioneer species grow together and compete for 

light and space in the developing forest. More than a decade 

after the last hurricane passed over the LEF (Hurricane Georges 

in 1998), the competition for canopy space is still intense; the 

competitors include surviving upper canopy elements, sprouting 

trees and branches, seed-regenerated trees, and preexisting 

seedlings and saplings that sprint skyward stimulated by the 

high light conditions. Nearly 20 years after Hurricane Hugo 

occurred, canopy morphology has not returned to prehurricane 

conditions, but leaf area index (LAI), litterfall, and hydrologic 

fluxes (interception and throughfall) have (Scatena, unpublished 

data; Heartsill Scalley et al. 2007).

Like animal dispersal in response to habitat changes, plants 

also appear to scatter in ecological space after the passage of 

hurricanes and then reaggregate as prehurricane stand conditions 

return. Heartsill Scalley et al. (2010) showed this with species 

ordinations at the watershed scale (fig. 36). Before the hurricane 

occurred, species were grouped into four distinct species clusters 

with particular structural features. After the hurricane passed, 

new species combinations developed and the location of plots 

expanded into a multidimensional space such that species 

groupings were not detected in ordinations. After 15 years of 

succession, the original species clustering was not reestablished as 

species continued to migrate in multidimensional space.

Animal Responses

Conditions for animals change dramatically with the opening of 

the forest canopy and the transfer of large amounts of biomass 

to the forest floor after a hurricane occurs. Hurricanes not 

only rearrange the structure of the vegetation, they also affect 

many basic processes that influence animal population size and 

composition. Alteration of the canopy structure by hurricanes 

basically compresses the forage space available to organisms into 

fewer vertical (understory and canopy) and horizontal (gap and 

no gap) strata (Wunderle 1995). For example, canopy birds are 

forced to feed on the forest floor or understory, which increases 

the number of interactions among species that normally do 

not interact, owing to their vertical segregation (Wunderle 

et al. 1992). Over longer periods of evolutionary time, the 

frequent occurrence of hurricanes might explain the tendency 

for Caribbean forest-dwelling bird species to segregate into just 

two vertical layers, compared with those in Panama, which 

recognize four layers (Wunderle et al. 1992). Wunderle (1995) 

showed that a lag exists between reestablishing feeding guilds 

and developing the vegetation structure that normally supports 

those guilds. Bird assemblages did not develop immediately after 

vegetation profiles began to define gap areas from no-gap areas 

because fruit production peaked in some locations and not in 

others. Avian populations returned to prehurricane levels within 

6 to10 months, suggesting that the reduction of bird populations 

was mostly due to bird movements in search of food (Waide 

1991), rather than mortality. Examination of avian stomach 

contents showed changes in diets (insectivorous birds feeding on 

different kinds of arthropods) and reductions in consumption, 

suggesting that hurricane disturbances induce feeding plasticity 

in Caribbean birds (Waide 1991). 

In several studies, the capture of birds with mist nets located at 

ground level increased after a hurricane occurred. For example, 

after Hurricane Georges passed over the Maricao State Forest in 

Puerto Rico, counts increased from 26.8 to 57.9 individuals per 

net hour (Tossas 2006). Like plant responses, bird responses to 

hurricanes are species and/or guild specific, with some species 

or guilds showing no net effects, others disappearing from plots 

despite their dominance before the hurricane passed, and still 

others showing various patterns of decline (table 9).

Montane forests lagged behind lowland forests in the recovery 

of canopy conditions, which in turn had a greater effect on 

bird populations than the actual passage of the hurricane 

(Tossas 2006). The effects of a slow canopy recovery rate on 

avifauna include altering foraging patterns and increasing 

chances of species extinction for species restricted to montane 

forests (particularly nectarivores and fruit and seed eaters). In 

short, hurricanes favor the survival of species capable of using a 

broad range of habitats and limit the number of bird species in 

montane habitats through increased endemism and extinction 

rates of these species. 

Strong fliers, such as the Jamaican Fruit Bat (Artibeus 

jamaicensis), left the LEF after Hurricane Hugo hit and did 

not return until 2 years after fruit production had resumed 

in the forest (Gannon et al. 2005). The population of the 

endemic Red Fig Eating Bat (Stenoderma rufum) experienced 

large reductions after Hurricane Hugo disturbed the LEF 

(captures decreased by 80 percent), and the population took 5 

years to reach prehurricane levels. Hurricane Georges passed 

9 years after Hurricane Hugo, decreasing the number of bats 

in this population again; however, no subsequent sign of 

recovery was evident 4 years later. The hurricane also affected 

the reproduction of the species; the representation of young 

bats decreased from 30 to 40 percent of the population before 

Hurricane Hugo occurred to just 10 to 20 percent after the 

hurricane hit. Unlike other frugivorous bats and birds, S. rufrum

remained in the LEF and survived by expanding its home range 

within the forest; it appears these bats might be important in the 

dispersal of seeds of successional plant species. In contrast to the 

above two species (A. jamaicensis and S. rubrum) and Erophylla 
Luquillo Experimental Forest: Research History and Opportunities 35

Figure 36.—Non-metric multidimensional scaling (NMS) of abundance-based species composition. The panels present the output of one NMS ordination, 

using data from the five censuses, as shown in panel (a). Panels (b-g) present the same ordination, on the same axes, but show labels of only two censuses 

at a time for clarity. Panels illustrate the following censuses: (b) 1989 pre-Hugo (labeled 88, color green) and 1989 post-Hugo (89 dark yellow), (c) 1989 

post-Hugo (89 dark yellow) and 1994 (94 gray), (d) 1994 (94 gray) and 1999 (99 light green), (e) 1999 (99 light green) and 2004 (04 blue), (f) 1989 

pre-Hugo (88 green) and 2004 (04 blue), and panel (g) 1989 post-Hugo (89, dark yellow) and 2004 (04, blue). Each panel has its respective analysis of 

similarities (ANOSIM) R statistic value presented with the associated significance value. From Heartsill Scalley et al. 2010.
36 Luquillo Experimental Forest: Research History and Opportunities

sezekorni (the Brown Flower Bat), which also experienced 

declines in populations after hurricanes, the population of 

Monophyllus redmani (the Greater Antillean Long-tongued Bat) 

increased dramatically after Hurricane Hugo passed over the 

forest, after taking advantage of the rapid flowering by plants in 

the open forest understory. 

The loss of canopy cover after a hurricane also creates cover 

problems for predators such as the Puerto Rican Boa, Epicrates 

inornatus, which became more visible to observers in the LEF 

after Hurricane Georges came ashore (Wunderle et al. 2004). 

Before the hurricane hit, observers could see 15.5 percent of 

radio-tagged boas whose fix was known, while the visibility of 

these boas rose to 19.6 percent after the passage of the hurricane 

as the boas moved more frequently and over larger distances to 

look for suitable habitat. 

Although anole abundance was greatest in the forest canopy 

before Hurricane Hugo made landfall, no anoles were observed 

more than 3 m above ground level 1 month after Hugo hit 

(Reagan 1991). Anolis stratulus, the least abundant anole near 

ground level before the passage of Hurricane Hugo, became 

the most abundant species near ground level 1 month after 

the hurricane passed (table 10). The relative abundance of 

A. evermanni, normally present from ground level to canopy, 

also increased at ground level following Hurricane Hugo’s 

disturbance. Anolis gundlachi, a forest interior species, responded 

to the changes in the microenvironment (light and humidity) by 

moving to different heights in search of available suitable habitats 

(table 10). 

Arboreal spiders also adjusted to the effects of the hurricane 

by establishing their webs closer to the ground; those that did, 

increased in numbers, while others that could not attach their 

webs to the resulting forest structure decreased in numbers 

(Pfeiffer 1996). As happened with birds, the effect of the 

hurricane on boas, lizards, and spiders was through its effect on 

habitat location rather than by direct mortality. 

Similar to other animal groups in the LEF, frogs survived 

hurricane winds, but, in contrast to lizards, young frogs of the 

genus Eleutherodactylus were affected by the reduction in relative 

humidity from the drought that followed Hurricane Hugo’s 

disturbance (fig. 37). After the developing canopy restored 

the relative humidity, however, frog populations (adults and 

juveniles) increased rapidly, exceeding prehurricane counts. 

Capture rate

Species Diet and foraging location1

Before

(% of sample)

After

(% of sample)

Ruddy Quail-Dove F, U 2.5 (9.4) 0.8 (1.4)

Puerto Rican Lizard-Cuckoo I, C 0 0.8 (1.4)

Green Mango N, C 0 2.4 (4.1)

Puerto Rican Emerald N, U 1.0 (3.8) 0.8 (1.4)

Puerto Rican Tody I, C 1.0 (3.8) 3.2 (5.5)

Puerto Rican Woodpecker O, C 0 0.8 (1.4)

Puerto Rican Peewee I, U 0.5 (1.9) 0

Gray Kingbird I, C 0 0.8 (1.4)

Puerto Rican Vireo I, C 3.5 (13.2) 4.0 (6.8)

Red-legged Thrush O, C 4.0 (15.1) 1.6 (2.7)

Elfin Woods Warbler I, C 0.5 (1.9) 0

Bananaquit N, C 4.0 (15.1) 18.3 (31.5)

Puerto Rican Tanager O, C 2.0 (7.5) 7.1 (12.3)

Puerto Rican Spindalis F, C 2.0 (7.5) 9.5 (16.4)

Black-faced Grassquit F, U 0.5 (1.9) 0

Puerto Rican Bullfinch F, C 5.1 (18.9) 6.3 (11.0)

Greater Antillean Oriole I, C 0 1.6 (2.7)

Total capture rate 26.8 57.9

Total birds captured 53 73

Table 9. Capture rates (number of individuals captured/100 net hours) and proportion of sample of bird species in Maricao State Forest before 

(July 1998) and after (October 1998) Hurricane Georges with data on diet and foraging location. From Tossas (2006).

1 I = insectivore, O = omnivore, N = nectarivore, F = frugivore, U = understory, C = canopy.
Luquillo Experimental Forest: Research History and Opportunities 37

These increases were attributed to the abundance of retreat sites 

and the reduction in invertebrate predators.

Prehurricane and posthurricane surveys of six invertebrate species 

in the LEF revealed dramatic population decreases in all species. 

In fact, populations of Nenia tridens and Gaeotis nigrolineata

(snails) and Agamemnon iphimedeia (walking stick) were reduced 

to the point that no specimens were detected in posthurricane 

surveys (Willig and Camilo 1991). It is believed that these 

species could not react to, or overcome, the large-scale changes 

in microenvironment induced by the hurricane. Subsequent 

studies in these populations uncovered complex interactions 

among shifts in ecological space, land use legacies, and cross-scale 

responses of biodiversity (Willig et al. 2007).

Hurricanes have significant effects on insects as well (Torres 

1988, 1992). First, hurricanes transport and disperse 

insect species to great distances and account for numerous 

introductions into the Caribbean (Torres 1988). Second, 

hurricanes affect insect population numbers through effects on 

food sources and/or predators, and they cause local extinctions 

or dramatic outbreaks of populations (Torres 1992). After 

Hurricane Hugo passed over the LEF, Torres observed population 

outbreaks of 15 species of Lepidoptera, including larvae of 

Spodoptera eridania (Noctuidae), which were observed feeding 

on 56 plant species belonging to 31 different families. All 

plants were early successional, and many were new host plants 

for the Lepidoptera. The outbreak of these insects ended with 

the decline of the host plants and had measurable influence 

on insect-insect interactions, such as predation and parasitism 

El Verde Bisley

Species

Minimum

density

Relative

abundance

Minimum

density

Relative

abundance

A. gundlachi 560 86% 739 94%

A. evermanni 78 12% 29 4%

A. stratulus 11 2% 17 2%

649 785

Posthurricane

October 1989

A. gundlachi 99 13% 110 17%

A. evermanni 273 35% 247 38%

A. stratulus 389 51% 288 45%

A. krugi 8 1% —

769 645

March 1990

A. gundlachi 188 36% 120 23%

A. evermanni 95 18% 90 17%

A. stratulus 241 46% 319 60%

524 529

October 1990

A. gundlachi 195 38% 212 37%

A. evermanni 133 26% 172 30%

A. stratulus 168 33% 191 33%

A. krugi 17 3% —

513 575

Table 10. Pre- and post-Hugo minimum population densities and 

relative abundances of Anolis lizards at ground level for the El 

Verde and Bisley study sites. Prehurricane estimates are from 

September 1981 (El Verde) and February 1989 (Bisley). From 

Reagan (1991).

Figure 37.—(a) Size distribution of juvenile Eleutherodactylus coqui before 

and after Hurricane Hugo. (b) Maximum number of juvenile E. coqui

counted during each study period between 1987 and 1990 in each of two 

study plots. From Woolbright (1991).

37a.

37b.
38 Luquillo Experimental Forest: Research History and Opportunities

involving other insect groups. Schowalter and Ganio (1999) 

also showed that disturbance from Hurricane Hugo affected 

invertebrate abundances significantly, with sapsuckers and 

mollusks becoming more abundant, and defoliators, detritivores, 

and emergent aquatic insects becoming less abundant in 

recovering treefall gaps than in intact forest. 

Changes in Ecosystem Processes

Hurricane-induced mortality applies not just to tree biomass 

aboveground (Lugo and Scatena 1996) but also to roots and 

animals. Tree mortality can result from almost any hurricane 

effect, including defoliation, shaking of trees, wind throws, 

and stem breakage (Lugo and Scatena 1996). Trees can also 

die owing to sun scalding on the limbs (Wadsworth and 

Englerth 1959). Nevertheless, many trees recover from these 

effects through root or stem sprouts, epicormic growth, and 

reestablishment of bark over wounds.

In the tabonuco forest, fine root biomass decreased to zero at 

the 0 to 10 cm depth over a 3-month period after Hurricane 

Hugo passed over the forest (Parrotta and Lodge 1991). Silver 

and Vogt (1993) found a 70- to 77-percent fine root decline 

occurring with a 6-month delay, and because root decomposition 

was slow, 48 to 65 percent of the fine root necromass remained 

on site, which retained nutrients within the ecosystem. High 

root mortality, however, may result in significant declines of 

nutrient availability to trees if the nutrients are leached out of 

the system rather than retained in the vegetation. Conversely, 

Silver (1992) found that the survival of a stand’s root structure 

results in high nutrient retention by vegetation. Hurricanes can 

also increase populations of some species. For example, after 

the passage of the hurricane, shrimp populations increased from 

the availability of food and the reduced washout of invertebrate 

consumers (Covich et al. 1991).

Ostertag et al. (2003) measured initial litter stocks in six types 

of forests in the LEF before Hurricane Georges made landfall 

in 1998, documented the litter and nutrient transfer from the 

canopy to the forest floor immediately after the hurricane hit, 

and followed the decomposition of the litter for 1 year (fig. 

38). The forest floor standing stocks returned to prehurricane 

values very quickly, within 2 to 10 months. The concentrations 

of N and P were, respectively, 1.1 to 1.5 and were 1.7 to 3.3 

times higher in hurricane leaffall than in nonhurricane leaffall 

(Lodge et al. 1991). It took 30 to 40 months (2.5 to 3.3 years) 

for nutrient concentrations in leaffall to return to prehurricane 

levels after the passage of Hurricane Hugo (Scatena et al. 

1996). Nutrient concentrations of throughfall water decreased 

immediately after the hurricane passed and continued decreasing 

for several weeks resulting from the reduction of leaf surfaces 

(Heartsill Scalley et al. 2007). This rapid disappearance of 

hurricane inputs suggests that such pulses are incorporated 

quickly into nutrient cycles and may be one factor contributing 

to the extraordinary resilience of these forests to hurricanes.

Anthropogenic 

Anthropogenic alteration of the landscape is a long-term 

disturbance. Therefore, it should be considered a part 

of the disturbance regime of an area along with other 

nonanthropogenic disturbances (Thomlinson et al. 1996).

Forest Recovery From Agriculture/Pasture

Over the past century, Puerto Rico has been transformed from 

an agriculturally based economy in a deforested landscape to 

an industrial and service-based economy in a reforested and 

developed landscape (Grau et al. 2003). This increase in forest 

cover is in contrast to the deforestation that is occurring in most 

of the tropics. The economic base of Puerto Rico has changed 

dramatically during the past 60 years, from a largely agricultural 

Figure 38.— Changes in total forest floor mass and leaf mass over time 

in three forest types over 1 year. Values represent plot-level means 

and standard errors. Each value is expressed on an ash-free, dry-mass 

basis and was corrected for incoming litterfall. Points to the left of the 

dotted lines are from samples taken before the hurricane using the same 

methodology. From Ostertag et al. (2003). 
Luquillo Experimental Forest: Research History and Opportunities 39

system to a more urbanized economy, with an emphasis on 

manufacturing (Morales Carrión 1983). In 1934, approximately 

43 percent of the island’s gross national product (GNP) was 

from agricultural products, primarily sugar cane and coffee; by 

1980, that figure had decreased to just 4 percent (Dietz 1986). 

At the same time, the manufacturing sector increased from 7 

to 48 percent of the GNP. Associated with these economic 

changes was the movement of people from rural to urban areas. 

In 1940, 45 percent of employment was rooted in agriculture, 

compared with only 5 percent in 1982. The conversion of land 

from sugar cane cultivation was due mainly to the overall decline 

in competitiveness of Puerto Rican sugar on the world market 

(Morales Carrión 1983). An additional factor was the 1941 

Land Law that restricted corporate land holdings and broke up 

some of the large sugar estates (Koenig 1953, Dietz 1986). 

As agriculture declined in Puerto Rico, forest cover increased. At 

the peak of agricultural activity in the late 1940s, forest cover was 

reduced to 5 percent of the island, with about the same area in 

shade coffee plantations (Koenig 1953). By 1985, however, 34 

percent of the island was forested (Birdsey and Weaver 1987). 

Before agriculture declined and forest regeneration began, most 

of the remaining forest was located in isolated mountainous areas 

such as the Luquillo Mountains. As economic drivers have led 

to the abandonment of agricultural lands, the area in secondary 

forests in Puerto Rico has increased substantially.

Marín Spiotta et al. (2007) examined changes in plant species 

composition and aboveground biomass on a chronosequence of 

abandoned pasturelands that spanned eight decades of tropical 

secondary succession. Tree species composition in all secondary 

forests differed from primary forests, and the highest rates of 

aboveground biomass accumulation occurred in the first 20 years 

(table 11). Reforestation of pastures resulted in an accumulation 

of 125 Mg ha-1 (metric tons per hectare) in aboveground 

standing live biomass over 80 years, a higher value than measured 

in primary forests due to the replacement of woody species by 

palm species in the primary forests. In a related study, Marín 

Spiotta et al. (2008) showed that soil C turnover rates of the 

mineral-associated C in secondary forests recovering from 

abandoned pasture resembled those of primary forests in as little 

as 20 years of succession.

In another study of forest regeneration, Aide et al. (2000) 

combined data from four regions of Puerto Rico to examine the 

recovery of forest structure following pasture abandonment and 

found that structural characteristics (stem density, basal area, 

biomass, number of species) were similar to the oldest sites after 

approximately 40 years of natural regeneration, suggesting that 

natural regeneration can be an effective strategy for tropical forest 

restoration.

In a study of soil C accumulation in reforested pastures, Silver 

et al. (2004) found that significant soil C can accumulate with 

reforestation. They also found strong legacies of pasture use and 

reforestation in plant community structures and rates of plant C 

sequestration. In the reforested ecosystem studied, the total soil 

C pool was greater than the aboveground C pool, and more soil 

C was in the forest (102 ± 10 Mg ha-1

) than was in an adjacent 

pasture of similar age (69± 6 Mg ha-1). Forest soil C increased at 

a rate of 0.9 Mg ha-1 yr-1 (metric tons per hectare per year), but 

residual pasture C was lost at a rate of 0.4 Mg ha-1 yr-1, yielding a 

net gain of 33 Mg ha-1 as a result of 61 years of forest regrowth. 

Few indices of C cycling were correlated with plant community 

composition or structure.

More generally, Silver et al. (2004) showed that aboveground 

biomass production in rehabilitated forests on abandoned land 

in tropical moist, wet, and dry life zones was a function of the 

type of land degradation. Past land use also had an impact on 

the rate of soil C accumulation, although patterns are much 

less clear than for aboveground biomass. Biomass production 

was fastest in abandoned agriculture, slower in abandoned 

pastures, and slowest in cleared land with arrested succession. It 

is possible that crops or pasture were not successful at these sites 

due to nutrient, water, pest, or physical factors that could also 

inhibit forest regeneration. All of these rates are slower, however, 

than those measured in natural successions (Aide et al. 1995, 

Silver at al. 2004). The overall rate of aboveground biomass 

accumulation across forests in all life zones studied was 2.36 Mg 

ha-1 yr-1, but the rate of aboveground biomass accumulation was 

significantly faster during the first 20 years of regrowth (6.17 

Mg ha-1 yr-1). There was no effect of life zone during the first 

20 years of regrowth, but wet forests accumulated C faster than 

moist forests during later phases (years 20 to 80). Overall (80 

years), wet forests accumulated biomass at a rate of 3.24 Mg ha-1

yr-1 and moist forests at a rate of 2.17 Mg ha-1 yr-1, while dry 

forests showed no significant pattern with time.

Age

TAB for trees

≥ 10 cm DBH

(Mg/ha)

TAB for trees

≥ 1.5 cm DBH

(Mg/ha)

Secondary forests

10 yr 6a

 (5) 53a (6)

20 yr 135b

 (25) 135ab (25)

30 yr 89ab (5) 97ab (10)

60 yr 166b

 (4) 181bc (7)

80 yr 269c

 (41) 272c (43)

Primary forests 161b

 (16) 173abc (19)

Table 11. Total aboveground biomass (TAB) estimates for large 

trees (DBH ≥ 10 cm) and all trees (DBH ≥ 1.5 cm) across a 

secondary forest chronosequence in Puerto Rico. Biomass 

calculated from general allometric biomass equations in Weaver 

and Gillespie (1992). From Marin Spiotta et al. (2007).

Notes: Values are means with standard error in parentheses. Different letters 

following values represent significant differences at P < 0.05 within columns. 

DBH = diameter at breast height.
40 Luquillo Experimental Forest: Research History and Opportunities

Climate Effects of Urbanization

Much of the freshwater supply for the San Juan Metropolitan 

Area (SJMA) originates in the Luquillo Mountains. This water 

supply is barely adequate in dry years, and there is new concern 

that overall precipitation trends in Puerto Rico during the 20th 

century have decreased by about 16 percent (van der Molen 

2002), and that 3 of the top 10 driest years in the past century 

were recorded in the 1990s (Larsen 2000). A growing body of 

evidence further suggests that urbanization may affect regional 

climates even more than increasing global temperatures (Hulme 

and Viner 1995, Scatena 1998b, Brazel et al. 2000). Therefore, 

research efforts should focus not only on global climate change, 

but also on the regional climatic effects of urbanization, 

especially the effects of the urban temperature phenomenon 

known as urban heat islands (UHIs). 

As urban development continues in Puerto Rico, forests and 

grasslands are being converted to nonvegetated surfaces, 

changing the magnitude and geographic range of the UHI. 

Murphy et al. (2010) quantified the spatial and temporal 

variations of the UHI in and around the SJMA and found that 

San Juan exhibits an average urban heat island of 1.75 °C during 

nighttime hours and 0.93 °C during daytime hours (table 12), 

with the UHI during the early evening reaching at least to the 

foothills of the Luquillo Mountains. Only trees and shading 

were successful agents in reducing daytime warming; green space 

was insufficient for abating the warming trend during the day. 

Predictions of future development and temperatures suggest that 

if the present pattern of development continues in Puerto Rico, 

more than 140 square kilometers (km2

) of land, that showed no 

signs of UHI in 2000, will have an average UHI between +0.4 

°C and +1.55 °C by 2050 (fig. 39). Furthermore, more than 130 

UHI (°C), ∆TCBD-HB

Wet season

Hobo Land local cover

Early night UHI

(1900 – 2359)

Late night UHI

(0000 – 559)

Day UHI

(0600 – 1859)

1S Abandoned agricultural fields 1.25 1.47 0.97

2S Urban center 0.71 (min) 0.83 (min) 0.55 (min)

3S Industrial 1.32 1.61 0.90

4S Residential 0.31 0.64 0.25

5S Major road crossing 0.78 0,93 0.55

6S Mowed-grassland 2.44 2.43 0.91

2A Old-growth forest 2.14 1.86 2.23 (max)

3A Grassland mix 1.47 1.44 1.10

4A Residential limits 2.01 1.76 1.81

1F Residential recreation park 2.05 2.28 0.80

2F Residential church 1.47 1.74 0.55

3F Suburban mix 1.69 1.90 0.51

4F Rural road 2.09 2.42 0.65

5F Major road 3.01 (max) 2.80 1.02

6F Rural dirt road 2.79 3.02 (max) 1.20

Average ± Standard Error 1.70 ± 0.19 1.81 ± 0.17 0.93 ± 0.13

Dry season

2A Old-growth forest 1.78 2.02 1.77 (max)

3F Suburban mix 0.86 (min) 1.32 (min) 0.40

4F Rural road 1.21 1.65 0.34 (min)

5F Major road 1.95 (max) 2.27 (max) 0.77

6F Rural dirt road 1.48 2.22 1.00

Average ± Standard error 1.46 ± 0.19 1.96 ± 0.15 0.86 ± 0.26

Table 12. Average temperature difference for all Hobo temperature sensors along an urban to rural gradient in northern Puerto Rico, calculated 

as Turban reference –Tindividaual Hobos. Maximum and minimum values for each time period indicated in parentheses. Urban Heat Island is UHI. From 

Murphy et al. (2007).
Luquillo Experimental Forest: Research History and Opportunities 41

Figure 39.—Maps of the San Juan Metropolitan Area showing the current (a) and projected (b–f) urban heat island effect in northeastern Puerto 

Rico. From Murphy et al. (2010).
42 Luquillo Experimental Forest: Research History and Opportunities

km2 of land area with a UHI between +0.4 °C and +1.4 °C in 

2000 will have an average UHI greater than +1.55 °C by 2050. 

Future research in this area should focus on how urbanization 

around the LEF may affect lifting condensation levels and hence 

the average elevation at which precipitation occurs.

Infrastructure (Roads and Dams)

Roads can have both direct and indirect effects on surrounding 

vegetation (Auerbach et al. 1997, Guariguata and Dupuy 1997), 

can be major sources of landslides and sediment deposits (Larsen 

and Torres Sánchez 1992, 1996; Megahan and Ketcheson 1996; 

Larsen and Parks 1997), and can increase forest fragmentation 

by creating barriers to migration and gene flow (Andrews 1990, 

Mader 1984). The presence of roads often increases the extent 

of anthropogenic activities in remote areas, providing a conduit 

for the introduction of nonnative flora and fauna (Mader 

1984, Andrews 1990, Wein et al. 1992, Hamilton et al. 1995, 

Raynor 1995, Whistler 1995). Roads can also be considered 

as ecosystems—albeit human-dominated—in their own right, 

with a unique ecology to be considered before making policy or 

management decisions (Lugo and Gucinski 2000).

The first study of the effects of road construction by Olander et 

al. (1998) was conducted in the elfin forest along the road that 

the U.S. Navy built in 1960 to provide access to communication 

facilities at Pico del Este (Sowers 1971, Scatena 1995). Results 

indicated that recent roadfills have higher light levels, soil 

temperatures, bulk densities, large pools of exchangeable soil 

nutrients, and higher soil oxygen concentrations but have lower 

seedling and tree densities, basal area, woody biomass, soil 

moisture, SOM, and total soil N than a mature forest (tables 13 

and 14). The total aboveground biomass of the 6-month-old 

roadfill was about 2 Mg ha-1 and was dominated by a variety 

of monocot and herbaceous species (no woody biomass), while 

the 35-year-old roadfill areas had a higher biomass of 10.5 Mg 

ha-1, 77 percent of which was nonwoody. Seedling density, tree 

density, and total woody biomass in roadfill areas were 12, 28, 

and 2 percent of mature forest sites, respectively. In these areas 

where soils were disturbed during construction, accumulation 

of biomass was the slowest known for the LEF (Olander et 

al. 1998). The time for biomass reaccumulation after road 

construction is likely to be faster for forests growing at low to 

mid elevations not limited by the climatic conditions of high 

rainfall and high wind found at upper elevations in the LEF 

(Heyne 2000). 

The later study by Heyne (2000) broadened the Olander et 

al. (1998) study by presenting successional trends in soil and 

vegetation characteristics along a chronosequence of abandoned 

paved roads at lower elevations (80 to 530 m elevation) that 

ranged in age from 4 to 60 years. After 11 years, abandoned 

road site litterfall and soil characteristics were found to be 

similar to those at forest sites of similar age, despite the presence 

of residual pavement. Net nitrogen mineralization (g g-1 dry 

soil) and percent SOM were similar on abandoned roads and 

adjacent forest sites (fig. 40), while soil pH required 60 years of 

abandonment for full recovery to forest conditions (fig.40). The 

paved surfaces of roads must first accumulate soils before any 

forest recovery process can begin, and, judging from the results 

of Heyne (2000), it seems that the physical accumulation of soil 

above the pavement was the delaying factor to the full recovery 

of soil properties. 

Site type n

Seedling

density (ha-1)

Tree

density (ha-1)

Basal area

(m2 ha-1)

Woody biomass

(Mg ha-1)a

New road (6 months) 5 0 a 0 a 0.00a 0.00 a

Old road (35 years) 9 4,185 b 3,148 a 2.07a 2.17 a

(1,167) (747) (0.46) (0.52)

Mature forest 12 34,014 c 11,257A 37.56b 91.67 b

(4,202) (1,079) (3.85) (11.65)

Mature cloud forest 3 37,236A 13,722A 32.70A 73.05A

(6,098) (821) (3.02) (13.36)

Forest with 3 28,222A 12,028 b 15.79A 33.13A

grass understory (3,192) (2,816) (6.67) (15.80)

Table 13. Effects of road construction and management on vegetative structure by plot type along the East Peak road, Luquillo Experimental 

Forest. Totals of individuals <1.5 m height for each plot type with standard error of the mean in parentheses. The number of 120 m2 plots 

measured is specified by n. Means with the same letter are not significantly different at the 0.01 level according to Tukey’s range test on log 

transformed data. Means for grass dominated plots were compared to the means for the 6 mature forest plots located in the cloud forest. For 

this comparison significance is denoted with capital letters. From Olander et al. (1998).

* Woody biomass was calculated using equations for dicots (Weaver 1990) and palms (Frangi and Lugo 1985).
Luquillo Experimental Forest: Research History and Opportunities 43

Litter mass on abandoned roads also reached quantities similar 

to those at forest sites after 11 years (fig. 40); at one site, litter 

recovered after only 4 years. This fast recovery of litter mass 

highlights that the localized canopy removal that is often 

associated with roadbuilding—and the presence of adjacent 

forest—facilitates the rapid return of organic matter inputs 

to the system and thus supports the soil formation process. 

Furthermore, litter on these newly deposited soils above roads 

likely alters environmental conditions by minimizing soil 

temperatures, providing physical structure for retaining seeds, 

creating soil fauna microhabitats, and minimizing erosive soil 

loss due to rainfall (Heyne 2000).

Measured differences in understory vegetation structure between 

abandoned roads and adjacent forest areas were very clear (fig. 

41a). After 30 years, stem densities and the number of species 

reached the same values as those observed in adjacent forests (fig. 

41b-c). Canopy structure was similar between abandoned roads 

and adjacent forests after 40 years. Basal areas on abandoned 

road sites were similar to those found in adjacent forests only 

after 60 years (fig. 41d), and variation in floristic composition 

began to approximate, but was never attained, at any of the 

measured sites (Heyne 2000). From the work of Heyne (2000), 

we can conclude that the ecosystem dynamics of primary 

succession on abandoned roads in the LEF are rapid; the noted 

exception to this pattern of recovery is the elfin cloud forest sites 

on the mountain peaks, which have longer biomass recovery 

times (Olander et al. 1998). 

In addition to building roads, dam building is another primary 

means by which humans alter ecosystems. Compared with other 

regions, the rivers of Puerto Rico historically have had a high 

level of damming (fig. 42a). Over the past century, the number 

of large dams built per unit area in Puerto Rico has outpaced, 

by an average of 2 to 1, the number built in California, which is 

well known for its extensive hydrological engineering (fig. 42b).

Dams change the physical, chemical, and biological structure and 

function of rivers and streams dramatically. When considering 

the effects of dams on tropical island streams, it is instructive to 

separate dams into two size classes (large, defined as those with 

a spillway height more than 15 m, and small, with a spillway 

height less than 15 m) and streams into low- and high-elevation/

gradient streams.

Large dams can alter the distribution and abundance of 

island faunas significantly by blocking migratory pathways 

(Holmquist et al. 1998), although the extent of alteration 

depends on characteristics of dam design, dam operation, 

and fauna. The first major Puerto Rican study of large dams 

and diadromous fauna, which focused on the loss of native 

migratory fauna from low-gradient streams (Holmquist et 

al. 1998), found a complete extirpation of all native fish and 

shrimp from the upstream habitat. In contrast, large dams with 

spillways (i.e., structures that allow water to flow over the face 

of a dam) allow the passage of some native fish and shrimp. 

Greathouse et al. (2006b) extended the findings of Holmquist 

et al. (1998) to high-gradient streams of the LEF, where native 

New road

Mature tall stature

(Cloud forest)

Mature tall stature

(Grass understory) Old road Fertilized Unfertilized

Bulk density g cm-3 0.39 a 0.24 (0.01) a 0.6 (0.04) ab 1.03 1.04 bc

% Soil moisture 0.59 (0.02) a 0.67 (0.05) b 0.45 (0.01) c na 0.32 (0.02) d

% Soil O2 8 (0.8) a 11 (0.065) b 15 (0.4) c na na

pH (KCl) 3.8 (0.04) a 3.7 (0.07) a 4.1 (0.07) a 53 (0.3) b 5.4 (0.4) b

C:N 22.1 (0.35) a 24 (0.4) a 19 (0.5) b 31 (3) c 34 (5.8) c

C Mg ha-1 55.7 (2.4) a 57 (4.6) a 19 (1.2) b 1.2 (2.5) c 13 (1.9) c

N Mg ha-1 2.5 (0.1) a 24 (0.2) a 1 (0.06) b 0.4 (0.1) c 0.5 (1.9) c

P kg ha-1 1.18 (0.26) ab 0.85 (0.2) a 0.75 (0.06) b 1.3 (0.1) b 1.7 (0.14) b

Ca kg ha-1 138 (22) a 114 (-11) a 529 (100) b 2,152 (434) c 2,784 (615) c

Mg kg ha-1 34 (6.6) a 20 (5.8) a 74 (14) b 84 (12) b 49 (6) b

K kg ha-1 35 (2.6) a 14 (2.1) b 65 (2.3) c 1.8 (9) d 88 (11) d

Mn kg ha-1 6.1 (1.5) a 3.9 (0.6) a 84 (113) b 81 (11) b 1.9 (16) b

Al kg ha-1 753 (47) a 430 (24) ab 510 (58) b 524 (194) c 310 (66) c

Table 14. Impacts of road construction and management on soil nutrient properties for the 0 to 10 cm horizon by plot type along the East Peak 

road, Luquillo Experimental Forest. Standard error in parentheses. Different lowercase letters identify statistically significant differences among 

sites. From Olander et al. (1998).

na = not applicable
44 Luquillo Experimental Forest: Research History and Opportunities

Figure 40.—Soil and litter properties of a chronosequence of abandoned paved roads versus forest plots of a similar age. From Heyne (2000).
Luquillo Experimental Forest: Research History and Opportunities 45

Figure 41.—Vegetation characteristics of a chronosequence of abandoned paved road plots versus forest plots of a similar age; (a) relative cover of 

vegetation less than 2 meters (m) in height in plots on roads versus forest cover, (b) stem count (± SE ) by form for vegetation greater than or equal to 2 m 

on roads and in adjacent plots, (c) species richness blox plots, and (d) mean basal areas by vegetation form greater than or equal to 2 m tall (± SE of total 

basal area). * indicates no significant differences between road and forest treatments. See Heyne (2000) for sample sizes. From Heyne (2000).

41a. 41b.

41c. 41d.

ROAD

ROAD

ROAD

FOREST

FOREST

FOREST

Time Since Road Abandonment (yr)

Time Since Road Abandonment (yr)

Time Since Road Abandonment (yr) Time Since Road Abandonment (yr) Stem Count (m-2) Basal Area (cm2 m-2)

Species Richness (m-2) Small Plot Relative Cover (%)
46 Luquillo Experimental Forest: Research History and Opportunities

Figure 42.—Numbers of large dams (height >15 meters) per unit land 

area in Puerto Rico and selected regions of North America and the tropics. 

(a) Number per unit land area over time in Puerto Rico, California, and the 

Dominican Republic. (b) Estimates of current number of dams (completed 

and under construction) in selected regions: bars are numbers per unit 

land area; numbers above bars are total number of large dams. From 

Greathouse et al. (2006a).

predatory fish are naturally absent (i.e., above waterfalls). The 

studies showed that populations of migratory fauna differed 

dramatically among sites upstream from large dams compared 

with sites without large dams; numbers of migratory shrimp 

and Sicydium at undammed sites were more than 300 times 

higher than in dammed sites (fig. 43). While conducting field 

work at three streams above dams without regular spillway 

discharge, however, Greathouse et al. (2006b) visually observed 

extremely rare occurrences of migratory shrimp and fish that 

were undetectable by electroshocking and snorkeling methods. 

The studies of Holmquist et al. (1998) and Greathouse et al. 

(2006b) together indicate that migratory shrimp and fish are 

nearly—but not completely—extirpated from sites above large 

dams that lack regular spillway discharge. Many native fish 

and shrimp of tropical islands have evolved to migrate over 

high waterfalls. Shrimp simply walk over dams with spillways, 

sometimes leaving the water but remaining in the wetted area 

near the stream. Fish such as gobies and some eleotrids use 

modified pelvic fins as suction cups to climb vertical rocks and 

waterfalls. Although these native fish and shrimp are able to 

climb, they need flowing water as a cue to direct them (Hamano 

and Hayashi 1992; Benstead et al. 1999). Without it, they are 

unable to determine the direction of waterflow, and they remain 

below the dam. Large dams and associated reservoirs may also 

disrupt the downstream migration of fish and shrimp larvae by 

reducing water flow, thereby lengthening the time water takes 

to reach the estuary. First-stage larvae of amphidromous shrimp 

and gobies are typically nonfeeding, and must reach saltwater to 

metamorphose into more advanced feeding stages. Laboratory 

studies have shown that amphidromous shrimp larvae die 

after only a few days in fresh water (e.g., Lewis and Ward 

1965). Similarly, gobiid larvae have limited food resources, and 

increased retention time in freshwater can lead to starvation 

(Moriyama et al. 1998, Iguchi and Mizuno 1990). Furthermore, 

reduced flows through reservoirs may also increase predation on 

larvae.

Small, low-head dams also interfere with the migration of 

tropical island faunas (fig. 44). The effects of low-head dams 

on the upstream migration of faunas appear to be similar to 

those of large dams with spillways; small low-head dams do 

not act as a complete barrier to upstream migration. Shrimp, 

fish, and snails are able to scale the dam and were found to be 

abundant in upstream reaches in the study by Benstead et al. 

1999. The low-head dam, however, acted as a bottleneck that 

increased the densities of upstream migrating animals below the 

dam. The bottleneck effect observed at this dam may have been 

exacerbated by the absence of a working shrimp or fish ladder. 

Upstream migrating animals at this site experienced increased 

difficulty during periods of low river flow, when no water was 

coming over the dam. During these periods, migrating shrimp 

lacked the directional cue provided by flowing water and became 

disoriented and experienced 100-percent mortality (Benstead 

et al. 1999). In addition to impeding the upstream migration 

of migratory faunas, water withdrawal resulted in significant 

Figure 43.—Number of migratory and nonmigratory macroconsumers 

captured per point sample (mean and SE; n = 7 dammed and 10 

undammed sites) at high-gradient sites without large dams or upstream 

from large dams. Migratory fauna are native adult shrimp (Xiphocaris 

elongate, atyids, Macrobrachium spp.) and the native green stream goby, 

Sicydium plumier. Nonmigratory fauna are the native crab, Epilobocera

sinuatifrons, nonnative poeciliid fish, and the native tadpole, Leptodactylus 

albilabris. From Greathouse et al. (2006b).
Luquillo Experimental Forest: Research History and Opportunities 47

mortality of downstream migrating shrimp larvae (Benstead et al. 

1999). During periods of low river flow, all the water and thus 

all the shrimp larvae were entrained into the intake. Over the 

course of the study, 42 percent of the migrating shrimp larvae 

were entrained into the water intake (Benstead et al. 1999). 

Long-term estimates of larval shrimp mortality were between 34 

and 62 percent, depending on the amount of water withdrawn.

Effects of both large and small dams on migratory faunas can 

have consequences that reach far beyond simply altering the 

relative abundance and distribution of fish and shrimp. For 

example, in many tropical island streams, freshwater shrimp 

account for the great majority of secondary production 

(Bright 1982) and significantly affect many aspects of benthic 

community composition and ecosystem processes (Pringle et 

al. 1999). Small-scale experimental exclusions of shrimp result 

in increased quantity and quality of organic matter (Pringle et 

al. 1999), increased sediment cover, shifts in insect and algal 

assemblage composition and biomass (March et al. 2002), 

and decreased rates of litter processing (March et al. 2001). 

Large-scale “experiments” caused by anthropogenic disturbance 

support the results of these small-scale exclusions. For example, 

Figure 44.—The number of (a) fish and (b) shrimp found above and below 

a low-head dam (Río Espíritu Santo) and in-channel water intake (Río 

Mameyes). Figure from March et al. (2003); data reported originally in 

Benstead et al. (1999).

elimination of shrimp from an entire reach due to chlorine 

poisoning resulted in dramatic increases in sediment cover and 

algal biomass (Greathouse et al. 2005).

Introduced Species 

Human activities that alter land use can change the species 

composition of residual forest stands and cause new forests to 

form (Lugo and Helmer 2004) because a proportion of the 

species that are successful in anthropogenic landscapes are not 

native to the area (Horvitz et al. 1998). Ecologists have not 

reached a consensus on the implications of human-mediated 

changes in species composition of ecosystems, because many 

nonnative naturalized tree species may have benefits such as 

reforesting abandoned pasture (e.g., Spathodea campanulata in 

Puerto Rico; Lugo 2004b, Lugo and Helmer 2004), while others 

may have negative effects through changing ecosystem structure 

and function, causing the loss of native plants and animals or 

changing the genetic structure of native populations through 

hybridization (Richardson et al. 2000b, Nyoka 2003). Some 

introduced species in the 16-ha Luquillo Forest Dynamics Plot 

(LFDP) in the tabonuco forest of the LEF are listed in table 15.

Thompson et al. (2007) found that the total number of 

individuals per hectare of most introduced species in table 15 

were relatively small and changed little over the 15-year period 

analyzed (fig. 45a) and that the maximum basal area (m2

 ha-1) 

of introduced species was also very low (<1 percent of the total 

stand basal area) and changed little over the period analyzed 

(fig. 45b). Thus, the introduced species survived over the 15-

year study and remained small components of the forest. Some 

species spread, but only over relatively small distances with only a 

few individuals establishing in the mature forest.

Bamboo, another nonnative tree species that was introduced 

to montane wet forests of the LEF in the 1930s and 1940s to 

stabilize downhill slopes along roads (White and Childers 1945), 

has led to present-day monocultures in numerous riparian areas 

in the LEF, spreading downstream at a rate of 8 m yr-1, with 

culms growing an average of 15.3 cm day-1 (O’Connor et al. 

2000). If this expansion rate continued, colonies of bamboo 

would extend only 1 km downstream over 125 years, suggesting 

that the rate of spread is slow. Over a 10-month study, leaffall 

from bamboo stands exceeded that of native mixed species forest 

by about 30 percent (fig. 46), and concentrations of nutrients 

important to stream communities (e.g., Ca, P, K, Mg, N) were 

lower in bamboo than in native forest leaffall (O’Connor et 

al. 2000). Elemental losses in streams were rapid, suggesting 

a change in riparian zone/stream dynamics following bamboo 

invasion. As nonnative bamboo spreads along streams in Puerto 

Rico, this species could potentially alter aquatic communities 

that are dependent on leaf input.
48 Luquillo Experimental Forest: Research History and Opportunities

Species Status Seed dispersal Vegetative growth Shade response

Artocarpus altilis (Parkinson, Fosberg) A Animal Root sproutingd Intolerant

Calophyllum calaba (Jacq.) N/P Animal Intolerant

Citrus paradis (Macfad.) A Animal Intolerant

Coffea arabica (L.) A Animal Tolerant

Genipa americana (L.) N Animal Intolerant

Hibiscus pernambucensis (Arruda) I/Na Animal Roots from fallen stems Intolerant

Mangifera indica (L.) A Animal Intolerant

Musa sp. A Culmsb Intolerant

Simarouba glauca (DC) P Animal Intolerant

Spathodea campanulata (Beauv.) E Windc Intolerant

Swietenia macrophylla (King) E/P Wind Intolerant

Syzygium jambos ((L.) Alston) E Animal Tolerante

Table 15. Characteristics of introduced species in the Long-Term Forest Dynamics Plot (LFDP) including status of introduction (Little and Woodbury 

1976; Francis and Logier 1991), mode of dispersal, and response to shade. From Thompson et al. (2007).

A = Introduced planted for agricultural use, E = Naturalised (sensu Richardson et al. 2000) introduced, N = native species planted for agricultural or ornamental use, P 

= plantation species for timber.

a

 I/N = Either introduced (Little and Woodbury 1976) or native (Logier 1994).

b

 Spread by sprouting new culms.

c

 Francis (2000a).

d Francis (2000b).

e

 Parrotta (1994).

Nomenclature follows Logier (1985, 1988, 1994, 1995, 1997).
Luquillo Experimental Forest: Research History and Opportunities 49

Figure 45.—(a) Number per hectare of live individual plants and (b) total 

basal area (square meters per hectare) of introduced trees with stems 

greater than or equal to 1 cm DBH (diameter at breast height) in at least 

one in three censuses over 15 years of the Luquillo Forest Dynamics 

Plot, Luquillo Experimental Forest, Puerto Rico. Species codes are: 

ARTALT Artocarpus altilis, CALBRA Calophyllum calaba, GENAME Genipa 

rabicaa, CITPAR Citrus paradis, COFARA Coffea rabica, HIBTIL Hibiscus 

pernambucensis, MANIND Mangifera indica, MUSSPP Musa sp., SIMGLA

Simarouba glauca, SWIMAC Swietenia macrophylla, SYZJAM Syzygium 

jambos, SPACAM Spathodea campanulata. From Thompson et al. (2007).

Figure 46.—(a) Mean daily leaffall rates under bamboo (hatched bars) and 

mixed native forest stands (open bars) from August 1996 to May 1997 

and (b) Input rates of selected elements in leaffall within bamboo (hatched 

bars) and native mixed (open bars) forest stands in the Bisley Experimental 

Watersheds, Luquillo Experimental Forest, Puerto Rico. 

* denotes native forest leaffall N from nonhurricane leaffall from Lodge et 

al. (1991). From O’Connor et al. (2000).

46a.

46b.
50 Luquillo Experimental Forest: Research History and Opportunities

The Luquillo Mountains are a major source of water for the island and an important recreation site. The mountains 

supply more than 20 percent of the island’s municipal water 

each year, with El Yunque National Forest providing an 

average of 276 hm3 (cubic hectometer) of water per year for 

municipal uses (Scatena and Johnson 2001). The nine rivers 

that drain the Luquillo Experimental Forest (LEF) all have steep 

gradients, boulder- and bedrock-lined channels with steep-pool 

morphology, and waterfalls (Ahmad et al. 1993). The watersheds 

draining the LEF and the locations of stream gauges and water 

intakes as of 2004 are shown in figures 47 and 48, respectively 

(Crook et al. 2007).

Puerto Rico receives precipitation from at least five major 

weather systems: the northeast trade winds, tropical depressions 

and hurricanes, northern cold fronts, systems originating in the 

Pacific, and systems originating in the Amazon basin. Headwater 

basins in the LEF have complex hydrologic regimes dominated 

by variability at two scales: daily and multiyear. The daily scale 

responses are attributable to short precipitation inputs, including 

daily orographic storms, whose inputs are transmitted on the 

scale of hours to stream channels by shallow saturated flow 

through numerous macropores in the upper 0.5 meters (m) of 

7. Hydrology

N.L. Harris and W. Wu

soil. Annual and interannual scale responses are attributable 

to moisture storage in the very deep (9 m) clay soils, which 

appears to be released slowly into stream channels as base flow. 

Approximately 70 percent of total streamflow goes out to sea 

during storms. Municipal uses of this water are impossible due 

to the large, short-term fluctuations in streamflow and because 

even the largest reservoirs on the island are small and can be 

filled several times during one large storm.

Rainfall and Throughfall

Rainfall and runoff occur every month of the year and increase 

with elevation (García Martinó et al. 1996). Annual precipitation 

ranges from approximately 1,500 mm yr-1 (millimeters per year) 

near the coast to more than 5,000 mm yr-1 in the uppermost cloud 

forests. These rainfall data are summarized in the previous section 

on climate. Throughfall levels depend on the balance among 

precipitation, evaporation, and canopy storage (Horton 1919, 

Leonard 1967, Rutter et al. 1972). 

Scatena (1990) estimated annual canopy throughfall and 

stemflow in the Bisley Experimental Watersheds as 59 percent 

and 2.3 percent of annual rainfall, respectively. Canopy 

throughfall was highest in gaps and along stream channels, with 

91 percent falling over 75 percent of the watershed area; however, 

50 percent of the total stemflow came from less than 12 percent 

of the total stems. More recently, Heartsill Scalley et al. (2007) 

summarized canopy rainfall and throughfall data in the Bisley 

Experimental Watersheds for a 15-year time series (fig. 5). Over 

the 15-year study period, mean annual rainfall and throughfall 

was 3,482 and 2,131 mm yr-1, respectively, indicating that close 

to 40 percent of incoming rainfall is intercepted and absorbed 

by the forest vegetation and the other 60 percent reaches the 

forest floor. These data are summarized in the previous section 

on climate and are not repeated here. The forest’s relatively high 

annual interception and low throughfall are attributed to the 

high frequency of low-intensity, short-duration rainfall events; 

net upward transport of evaporated moisture associated with 

heat from condensation; advected energy; and a relatively low 

aerodynamic resistance of the canopy (Scatena 1990, Schellekens 

et al. 2000). 

Streamflow 

In addition to developing a regression equation between rainfall 

and elevation, García Martinó et al. (1996) also developed a 

regression equation between runoff (i.e., stream discharge) and 

elevation for the LEF (fig. 49). Although both relationships are 

strong, the regression that uses the weighted average elevation 

(WAE) (fig. 49a) is a better predictor than the relationship 

that uses elevation of the gauge outlet itself (fig. 49b). Higher 

confidence should be placed on figure 49a because the WAE 

accounts for the spatial distribution of elevation within the 

watersheds, while figure 49b has a large concentration of data 

points at lower elevations.

Figure 47.—Watersheds draining the Luquillo Experimental Forest 

(identified as El Yunque National Forest). The Río Grande and Río Espíritu 

Santo join near the estuary. The Río Grande is located on the west side of 

the Espíritu Santo watershed. The Río Canovanas and Canovanillas join 

the Río Grande de Loiza watershed. The Río Canovanas is east of the Río 

Canovanillas. From Crook et al. (2007).
Luquillo Experimental Forest: Research History and Opportunities 51

Wu et al. (2007) used Landsat imagery, hydrological modeling, 

and simulation analysis to investigate the influence of recent 

and future land-cover changes on streamflow of the Fajardo 

watershed, including part of the LEF. Monthly and annual 

streamflows were compared between an agricultural period 

(1973 to 1980) and an urbanized/reforested period (1988 to 

1995) using a revised Generalized Watershed Loading Function 

model. Validated results of streamflow (fig. 50) show that a 

lower proportion of rainfall became streamflow in the urbanized/

forested period compared with the agricultural period (fig. 51), 

apparently due to a higher effect from reforestation than from 

urbanization on streamflow. Simulations of projected land-cover 

scenarios indicate that annual streamflows would increase by 9.6 

percent in a total urbanization scenario, decrease by 3.6 percent 

in a total reforestation scenario, and decrease by 1.1 percent if 

both reforestation and urbanization continue at their current 

rates to the year 2020. An imposed hurricane event that was 

similar in scale to the largest recent event on the three land-cover 

scenarios would increase the daily streamflow by 62.1, 68.4, and 

67.1 percent, respectively (fig. 52). 

An accurate forecast of future streamflow is vital for water 

resource managers, especially during droughts. In the past 40 

years, seven severe islandwide droughts have occurred, between 

1956 and 1957, 1964 and 1965, and 1967 and 1968; in 1976, 

1994, and 1996 (Scatena 1996); and, most recently, in 2002. It 

is thus desirable to be able to calculate a watershed parameter 

that describes how streamflow diminishes in the absence of 

rainfall. This parameter is called the recession constant and 

describes the slope of the streamflow decline following a recharge 

event (Arnold et al. 1995).

The most common mathematical function to describe 

how streamflow diminishes after a rainfall event is a simple 

exponential decay function (Hall 1968, Nathan and McMahon 

Figure 48.—U.S. Geological Survey (USGS) stream gauges. Water intakes are shown in relation to stream gauges for reference. Watersheds, clockwise 

from left, bottom corner are Gurabo, Canóvanas (Loíza), Espíritu Santo, Mameyes, Sabana, Fajardo, Blanco. From Crook et al. (2007).
52 Luquillo Experimental Forest: Research History and Opportunities

1990, Arnold et al. 1995, Tallaksen 1995, Rutledge and Mesko 

1996):

 Qt = Q0kt

where Q0 is the initial discharge at any time; Qt is the discharge 

t time units after Q0; and k is the recession constant. A large k

value indicates that drainage is very slow, while a smaller k value 

indicates a rapid drainage and little storage. Rivera Ramírez et 

al. (2002) developed master recession curves (and k values) for 

two LEF watersheds (Río Espíritu Santo and Río Fajardo) shown 

in figure 53.

Evapotranspiration

Evapotranspiration and related processes in tropical forests can 

explain 70 percent of the lateral global energy transport through 

latent heat, making them very important in the redistribution 

of water on the Earth’s surface (Mauser and Schädlich 1998). 

Few spatial studies of evapotranspiration in these redistribution 

processes for tropical forests exist, but several point estimates are 

available for the LEF (table 16) and range from 1.93 to 5 mm d-1

(millimeters per day), depending on forest type and method of 

estimation.

Wu et al. (2006b) integrated one Landsat Thematic Mapper 

(TM) image and three MODIS images with a hydrological 

model (Granger and Gray 1989) to estimate the spatial pattern 

of actual evapotranspiration (aET) in the LEF for the month 

of January (fig. 54), when these remote sensing images were 

acquired. The derived January aETs from this study ranged 

from 0 to 7.22 mm day-1 with a mean of 3.08± 1.35 mm day-1, 

which were comparable to other estimates (table 16). Simulated 

aET was highest in the low-elevation forest and decreased 

progressively toward higher elevations. Because of differences in 

Figure 49.—(a) Relationship between the weighted average elevation 

(WAE) of the watershed and average runoff and (b) the elevation at each 

stream gauge and gauge runoff for nine-gauge stations draining the 

Luquillo Experimental Forest. From García Martinó et al. (1996).

49a.

49b.

Figure 50.—Average daily streamflow for each month of each 

simulation year (top: 1974 to 1980; bottom: 1988 to 1995). From 

Wu et al. (2007).
Luquillo Experimental Forest: Research History and Opportunities 53

Figure 51.—The proportion of rainfall that contributed to streamflow for 

each month over two simulation time periods: 1973 to 1980, when the 

area was under agriculture, and 1988 to 1995, when the area was under 

urbanization/reforestation (upper graph: model simulation; lower graph: 

empirical observations). From Wu et al. (2007).

Figure 52.—Simulated daily streamflows for every month under the three 

projected land cover scenarios with or without hurricane disturbance in 

September 2020. From Wu et al. (2007).

Figure 53.—Master recession curves for Río Espíritu Santo and Río 

Fajardo watersheds in the Luquillo Experimental Forest; correlation 

method. Dry Season: January through April and June through July; average 

rainfall 194 to 311 mm mo-1 (millimeters per month). Wet Season: May 

and August through December; average rainfall, 289 to 378 mm mo-1. 

From Rivera Ramírez et al. (2002).

solar radiation at different elevations, aspects and topographic 

positions, aET tended to be higher on south slopes and along 

ridges than on north slopes and in valleys. In addition, the 

Bowen ratio (the ratio of sensible heat to latent heat) varied 

across different vegetation types and increased with elevation, 

thus reflecting differences in the distribution of net solar 

radiation incident on the Earth’s surface. Over a 24-hour period, 

the highest simulated aET occurred around noon. 

Wu et al. (2007) investigated the influence of recent and future 

land-cover changes on evapotranspiration (ET) in the Fajardo 

watershed, including part of the LEF. From the 1970s to 1990s, 

the average simulated ET decreased 15.2 percent between May 

and August, when it was warm and rainy. The average simulated 

ET stayed almost the same the rest of the year (fig. 55). This 

pattern could result from urbanization in the watershed. The 

highest ET occurred in February or March, when few clouds 

were present, and the lowest occurred in May, when the 

probability of cloud cover is high (Wu et al. 2006a). In general, 

closed forests had a higher predicted ET than urban areas. 

During the relatively wet season (September to January), ET was 

higher for the forests than for agricultural lands and pastures. 

During the March-to-August growing season, ET varied little 

among forests, agricultural lands, and pastures. The calculated 

ET in urban areas was generally lower than in other land cover 

types except during the months of January and May 1990, 

when ET calculated for urban areas was slightly greater than ET 

in agricultural areas and pastures, respectively. This seeming 

inconsistency may be due to the uncertainties associated with the 

large area (56.9 percent) excluded from the analysis in the 1991 

Landsat imagery due to cloud cover.
54 Luquillo Experimental Forest: Research History and Opportunities

Water Budgets for the LEF

A catchment’s water balance can be calculated as

 P = Q + ET + ∆S + ∆G + L

where P is the precipitation input, Q is the amount of streamflow 

leaving the catchment, ET is evapotranspiration, ∆S and ∆G are 

the changes in soil moisture and ground water storages over the 

period of measurement, respectively, and L is the leakage into or 

out of the catchment (all expressed as millimeters of water). It is 

typically assumed that when completing water budgets based on 

long-term data, soil moisture (∆S) should be relatively constant 

and this term is eliminated. Also, groundwater contributions 

(∆G) to streamflow are small in the LEF (Larsen 1997, Ortiz 

Zayas 1998)

Naumann (1994) developed a water use budget for the LEF 

using estimates of rainfall and ET for each forest type and life 

Wu et al. 

(2006)

(mm/day)

Other 

estimates

(mm/day) Reference

2.06 Wang (2001)

Whole 

forest

3.15 4.8 Brown et al. (1983)

4.7 García Martinó et al. (1996)

2.92 Van der Molen (2002)

Tabonuco 3.43 3.5 – 5.0 Odum (1970)

2.0 – 2.4 Schellekens et al. (2000)

Colorado 2.93 2.86 Van der Molen (2002)

Palm 2.88 1.99 Van der Molen (2002)

Elfin 1.62 1.93 Van der Molen (2002)

Table 16. Comparison between modeled estimates of 

evapotranspiration (Wu et al. 2006) and other point estimates of 

evapotranspiration in the Luquillo Experimental Forest. From Wu 

et al. (2006).

Figure 54.—Derived daily average actual evapotranspiration (aET) in January displayed in 3-D (unit: mm day-1). From Wu et al. (2006b).

Figure 55.—Average daily evapotranspiration (ET) for each month 

for different land cover types (upper: between 1973 and 1980; lower: 

between 1988 and 1995). From Wu et al. (2007).
Luquillo Experimental Forest: Research History and Opportunities 55

zone found in the LEF, based on an earlier study by Lugo (1986). 

Runoff was calculated as the difference between estimates of 

rainfall and ET and estimates of streamflow from Hansen et al. 

(1985) were used. Water withdrawals were based on available 

information from Puerto Rico Aqueduct and Sewer Authority 

and existing water permits. Naumann’s water budget estimated 

an average annual rainfall of 3,390 mm yr-1, ET of 1,320 mm 

yr-1 and stream runoff of 2,070 mm yr-1 for the entire forest. 

In 1996, García Martinó et al. developed hydrologic budgets by 

forest type in the LEF based on relationships between rainfall 

and elevation and the U.S. Geological Survey (USGS) long-term 

streamflow data and elevation (table 17). On an average annual 

basis, results indicate that the forest receives 3,869 mm yr-1 of 

rainfall, 65 percent of which is converted to stream runoff and 

35 percent is recycled by evapotranspiration or lost by other 

abstractions. Unlike the Naumann (1994) budget, García 

Martinó et al. (1996) assumed that the withdrawal of water 

for municipal uses did not affect stream discharge significantly. 

Although this assumption is incorrect, most large water intakes 

are located downstream of the gauging stations used in their 

analysis (Naumann 1994).

Naumann’s 1994 water budget for the LEF indicated that more 

than one-half of all water flowing from the LEF was extracted 

for municipal use. Between 1994 and 2004, urbanization and 

water withdrawals from the forest increased, with at least six new 

points of water withdrawal (i.e., intakes) added on rivers draining 

the LEF to meet present and projected municipal water demand. 

Four intakes were added within the forest and two intakes were 

added outside the forest on the Río Fajardo, Río Mameyes, Río 

Espíritu Santo, and Río Blanco. Two intakes draw particularly 

large amounts of water: the intake at Río Mameyes (outside the 

forest) is permitted to extract 5 million gallons per day (mgd) 

(18,939 m3 day-1) and the intake at Río Fajardo is permitted 

to extract 12 mgd (45,455 m3 day-1) (Ortiz Zayas and Scatena 

2004). The Mameyes intake was discontinued after the 

construction of a reservoir elsewhere in the region.

Crook et al. (2007) updated the water budget for the LEF for the 

year 2004 to incorporate these new intakes. The rainfall-elevation 

regression developed by García Martinó et al. (1996), used to 

estimate average annual rainfall and average monthly runoff for 

each watershed, was calculated from long-term daily runoff rates 

from 17 USGS stream gauges. The water budget of Crook et al. 

estimated an average annual rainfall of 3,580 mm yr-1, ET of 1,300 

mm yr-1, and stream runoff of 2,280 mm yr-1 for the entire forest. 

The amount of water extracted from the LEF is estimated at 250 

mm yr-1, or 7 percent of the average annual rainfall and 11 percent 

of the average annual runoff. The daily percentage of water 

typically generated within the forest but diverted before reaching 

the ocean increased from 54 percent in 1994 to 70 percent in 

2004 (Naumann 1994, Crook et al. 2007). Watersheds with 

large water intakes have the most dramatic decrease in streamflow, 

particularly the Río Espíritu Santo watershed, where 82 percent of 

the median flow is diverted.

Table 18 compares the various water budgets for the LEF. In 

general, about 40 percent of total precipitation is recycled 

through evapotranspiration and the other 60 percent ends up in 

streamflow.

Forest type

Drainage

area (ha)

Weighted average

elevation (m)

Average rain

(mm yr-1)

Average runoff

(mm yr-1)

ET=Rainfallrunoff

Tabonuco 5,787 402 3,537 1,830 1,707

Colorado 3,476 720 4,191 3,197 994

Palm 1,856 711 4,167 3,158 1,009

Elfin 372 897 4,849 3,958 1,144

Weighted average 3,879 2,526 1,245

Table 17. Water inputs and outputs from four forest types inside the Luquillo Experimental Forest (García Martinó et al. 1996).

Study Precipitation (mm yr-1) Runoff (mm yr-1)

Evapotranpiration

(mm yr-1)

Lugo (1986) (42%)

Naumann (1994) 3,390 2,070 (61%) 1,320 (39%)

García Martinó et al. (1996) 3,869 2,526 (65%) 1,338 (35%)

Crook et al. (2007) 3,580 2,280 (64%) 1,300 (36%)

Wu et al. (2006)* 1,021

Table 18. Comparison of water budgets for the Luquillo Experimental Forest. Numbers in parentheses indicate the percentage of total precipitation 

in each component.

*Based on model of evapotranspiration by Granger and Gray (1989).
56 Luquillo Experimental Forest: Research History and Opportunities

Water quality reflects the composition of water as affected by natural processes and by humans’ cultural activities, 

expressed as measurable quantities and related to intended 

water use (Novotny and Chesters 1981). Surface waters contain 

many chemical constituents in the dissolved state that play an 

important role in the survival of both terrestrial and aquatic 

ecosystems, and these chemical constituents are related to the 

geology, vegetation, precipitation, topography, and human 

activities within a watershed. Although necessary to sustain 

ecosystems, excessive amounts of some dissolved species and 

microorganisms can degrade water quality and be detrimental to 

the health of many organisms, including humans. 

Puerto Rico has approximately 1,300 streams, 17 of which are 

large enough to be classified as rivers (Pringle and Scatena 1999), 

but has no natural lakes, and locations for reservoirs are few. The 

contamination of groundwater from accidental spills, industrial 

wastes, and leaking septic tanks has limited the groundwater and 

surface water that can be developed without treatment in Puerto 

Rico (Pringle and Scatena 1999).

On the other hand, streamwater quality within the El Yunque 

National Forest is excellent, and these streams are considered to 

be the cleanest and most natural on the island (Santos Roman et 

al. 2003). Water temperature in headwater streams is relatively 

consistent throughout the year and ranges between 18 and 24 oC 

(Covich and McDowell 1996). The average pH of streamwater 

is 7.2. Stream discharge is highly variable and high flows can 

occur at any time of the year; 10-fold increases in discharge have 

been recorded within hours (Scatena and Johnson 2001).

Although the headwaters of these rivers have been protected 

for more than a century, and some areas have been protected 

since Columbus landed on the island, the lowlands support a 

mixture of urban, suburban, and agricultural land uses. In recent 

decades, land use in the periphery of the Luquillo Mountains has 

changed drastically as agricultural lands have been abandoned 

and changed to forests or developed areas (Grau et al. 2003; 

land cover maps in Lugo et al. 2000 and, 2004). Increases in 

population, combined with increases in the number of second 

homes and resorts, have also increased the demands for the 

region’s natural and aquatic resources.

Because Puerto Rico is a Commonwealth Territory of the 

United States, island residents are subject to the environmental 

regulations of the U.S. Environmental Protection Agency and 

other Federal agencies. Many Federal agencies are involved in 

the island’s water management, including the U.S. Geological 

Survey, which monitors the island’s water quality and quantity; 

the U.S. Army Corps of Engineers, which is responsible for 

navigable waters and wetlands; the U.S. Fish and Wildlife 

Service, which manages threatened and endangered species 

and wetlands; the Natural Resources Conservation Service, 

which helps farmers implement best land and soil conservation 

8. Water Quality of Rivers 

N.L. Harris

practices; and the Forest Service and National Park Service, 

which manage national forests and national parks, respectively.

By the early 1900s, it was estimated that on a typical day 

more than 50 percent of the water draining the El Yunque 

National Forest was appropriated for municipal uses before 

it reached the ocean (Scatena and Johnson 2001). In 1995, 

the area’s per capita water use was among the highest in the 

world at 732 liters per person per day (Ortiz Zayas and Scatena 

2004). Unfortunately, water use efficiency is low and nearly 

42 percent of the water processes in water treatment plants are 

unaccounted for and lost to leaks either in the water distribution 

systems, illegal connections, or in accounting errors. Today, 

the region faces nearly all the management issues associated 

commonly with urban hydrology, including those related to 

stormwater management, drinking water supply, and wastewater 

management. The complexity of these issues is increased due to 

differences in the quality of water supply infrastructure between 

the new developments and modern resort complexes and the 

aging water supply infrastructure of the older, and generally 

poorer, communities.

Although the Luquillo region has abundant rainfall and 

numerous surface water bodies, it has limited opportunities for 

developing additional municipal water sources. Moreover, few 

areas exist where dams can be built (Ortiz Zayas and Scatena 

2004). Most municipal water in the region is abstracted directly 

from streams, and is thus constrained by the temporal variation 

in streamflows and chemistry. Although demand for municipal 

water supplies has increased, so have instream uses for recreation 

and the consequent need to maintain instream flows and water 

quality.

The following section reviews the chemistry of rainfall, 

throughfall, and cloud water because a portion of all these water 

sources ends up ultimately as streamwater. 

Rainfall and Throughfall Chemistry

Rainfall and throughfall are major sources of nutrients in tropical 

forests and streams (Proctor 2005). Heartsill Scalley et al. (2007) 

explored the seasonal and interannual variations in rainfall 

and throughfall constituent fluxes in the Bisley Experimental 

Watersheds by analyzing weekly rainfall and throughfall data over 

a 15-year period that included the effects of 10 named tropical 

storms, several prolonged dry periods, and volcanic activity in the 

region. Mean weekly fluxes and volume-weighted concentrations 

during the 15-year study period for rainfall and throughfall are 

presented in table 19. Average annual rainfall and throughfall 

fluxes of K+

 (potassium), Ca2+ (calcium), Mg2+ (magnesium), 

Cl- (chlorine), Na+ (Sodium), and SO4-S (sulfate) were similar 

but somewhat larger than those reported for most tropical 

forests. Rainfall inputs of N (nitrogen) were comparatively low 

and reflect the relative isolation of the air shed. More chemical 
Luquillo Experimental Forest: Research History and Opportunities 57

constituents had seasonal differences in rainfall fluxes (6 out of 

12) than throughfall fluxes (4 out of 12). The term “enrichment 

ratio” is a useful metric for representing the difference in 

chemical concentration between rainfall and throughfall of a 

given chemical constituent and can be calculated as the ratio of 

a given throughfall constituent flux to the corresponding rainfall 

constituent flux. In the Bisley Experimental Watersheds, all 

enrichment ratios calculated for the 15-year period were greater 

than 1, indicating that, over the long term, water that passes 

through the canopy was enriched such that the total flux of 

nutrients to the forest floor from throughfall was greater than the 

rainfall inputs into the canopy. Canopy-level biological processes 

that enrich throughfall were most pronounced for NH4-N 

(ammonium-nitrogen), whose weekly throughfall fluxes were not 

even correlated to the amount of weekly rainfall or throughfall. 

Median weekly enrichment ratios, however, were less than 1 for 

sea salts and dissolved organic carbon and greater than 10 for 

NH4

-N, PO4-P (phosphate), and K.

Cloud Water Chemistry

In addition to rainfall, deposition of cloud water onto vegetation 

is another important hydrologic and chemical input in montane 

ecosystems. Clouds have ionic concentrations 2 to 5 times 

higher than concentrations in rain (Weathers et al. 1988), and 

Scatena et al. (unpublished data) measured cloud inputs as 15 

percent of total precipitation at Pico del Este. Therefore, cloud 

inputs are an important source of nutrient elements to cloud 

forests in the Luquillo Experimental Forest (LEF). 

Asbury et al. (1994) published data on atmospheric deposition 

of nutrients to Pico del Este in the LEF and found that, although 

liquid water content of sampled clouds was low (0.016 g m-3

[grams per cubic meter]), the deposition of water (1.3 mm d-1

[millimeters per day]) was comparable to other sites, apparently 

due to efficient capture of clouds by epiphyte-laden vegetation. 

Elemental deposition by cloud water was only 8 to 30 percent of 

total deposition (cloud-only minus rain) due to the high rainfall 

Rainfall Throughfall Enrichment ratio

yr mn Average flux

(kg ha-1 d-1)

Volume-weighted

[] (mg L-1)

yr mn n Average flux 

(kg ha-1 d-1)

Volume-weighted

[] (mg L-1)

n Median SD Volume-weighted

[] (mg L-1)

NH4–N * * 0.002 0.025 * 422 0.018 0.302 307 12.4 492 12.08

NO3–N * * 0.004 0.0451 * 553 0.006 0.1085 372 1.79 271 2.41

TDN * 0.013 0.135 * * 276 0.048 0.758 223 4.39 40.2 5.62

PO4–P * 0.0003 0.004 * * 418 0.003 0.055 340 13.2 133 13.75

K+ 0.022 0.219 * * 666 0.170 2.815 539 10.0 17.6 12.85

Ca2+ * 0.044 0.439 * 664 0.057 0.942 541 1.50 2.8 2.15

Mg2+ 0.037 0.367 * 666 0.036 0.604 543 1.17 1.5 1.65

DOC * * 0.332 3.569 * 232 0.361 6.301 188 0.80 2.4 1.77

Cl– * * 0.386 4.022 * 579 0.331 5.785 505 0.80 2.4 1.44

Na+ * 0.243 2.430 * * 666 0.181 3.009 548 0.80 1.6 1.24

SO4–S * 0.126 1.300 * 613 0.149 2.576 511 1.49 10.3 1.98

SlO2 * 0.101 1.130 * 436 0.102 1.778 220 1.37 30.0 1.57

pH * 5.17 * 629 6.14 553 1.17 0.14

Table 19. Mean weekly fluxes and volume weighted concentration for rainfall and throughfall during a 15-year study period (1988 to 2002) in the 

Bisley watersheds of the Luquillo Experimental Forest. From Heartsill Scalley et al. (2007).

Asterisks represent significant differences in flux among years (yr) and/or months (mn) for each constituent. Enrichment ratio is throughfall flux over rainfall flux, SD is 

the standard deviation of sample, and [] is concentration. Median values for pH. n = number of samples.
58 Luquillo Experimental Forest: Research History and Opportunities

at the site. Solute concentrations in throughfall and stemflow 

were greater than in cloud water for all measured constituents 

except H+ (Hydrogen ion), NO3

-

 (nitrate ion), NH4

+

, and total 

N (table 20). Na and Cl from marine aerosols dominated cloud 

chemistry, reflecting the proximity of Pico del Este to the ocean 

and consequent high concentrations of these ions in cloud 

water. Deposition rates of SO4

2- (sulfate ion) and NO3-

 were 

surprisingly high at this remote site, and estimated deposition 

rates of other constituents were in the ranges found at other 

sites. After passage through the canopy, concentrations of base 

cations in deposited cloud water increased and concentrations of 

N decreased. 

Streamwater Chemistry

Schaefer et al. (2000) present a long-term (2.5 to 11 yr) weekly 

record of streamwater chemistry on eight forested watersheds 

(catchment basins) in the Luquillo Mountains. NO3

–

 (nitrate), 

K, and NH4

+ concentrations increased after the passage of 

Hurricane Hugo in 1989 and remained elevated for up to 2 

years. Sulphate, chloride, Na+, Mg2+, and Ca2+ showed smaller 

relative significant changes. Average streamwater exports of K+, 

NO3

–

, and NH4

+ increased by 13.1, 3.6, and 0.54 kg ha-1 yr-1

(kilograms per hectare per year) in the first posthurricane year 

across all watersheds (fig. 56). These represent increases of 119, 

182, and 102 percent, respectively, compared with the other 

years of record. The increased stream outputs of K+ and N in the 

first 2 years after the hurricane are equivalent to 3 percent (K) 

and 1 percent (N) of the hurricane-derived plant litter. 

For both El Verde and the Bisley Experimental Watersheds, the 

streamwater ionic concentrations most affected by disturbance 

included K+

 (fig. 57a and fig. 58a), NO3

–

 (fig. 57b and fig. 58b), 

and NH4

+

 (fig. 57e and fig. 58e). The posthurricane NO3

–

peak was of unprecedented magnitude and duration regarding 

long-term sampling and was observed in all eight watersheds. 

NO3

–

 concentrations returned to their prehurricane values in all 

watersheds by March 1991. The posthurricane K+ concentration 

peak (fig. 57a and fig. 58a) was of similar relative magnitude to 

the NO3

–

 peak (i.e., both approximately tripled prehurricane 

concentrations). Concentrations of K+ declined to prehurricane 

levels in seven of eight watersheds by October 1990. The 

posthurricane K+ peak was higher in the Bisley Experimental 

Watersheds than in the El Verde watersheds.

Hurricane disturbance appeared to have only minor effects on 

Mg2+ (fig. 57c and fig. 58c) and Ca2+ (fig. 57d and fig. 58d) 

concentrations. For these two ions, the annual periodicity in 

concentrations result from inverse concentration/discharge 

relationships and seasonality in streamflow. Sodium, sulphate, 

and chloride concentrations were the least affected by hurricane 

disturbance.

Figure 56.—Comparison of stream chemical fluxes in the first year after 

Hurricane Hugo (1989) in the Luquillo Experimental Forest, Puerto Rico, 

with those averaged over all other years of record. Bars show mean 

values across watersheds ± 95-percent confidence intervals (n=6). From 

Schaefer et al. (2000).

Solute Events Cloud water Stemflow Throughfall Rainfall

H+ 12 27.6 8.3 4.1 6.4

SO4

2– 12 181 219 166 47.0

NO3

– 12 63.6 32.8 14.6 7.9

NH4

+ 3 31.5 4.5 5.4 9.9

Cl– 12 384 550 498 175

Na+ 12 397 450 442 215

K+ 12 13 23.6 29.3 13.6

Ca2+ 12 62.3 98.0 78.2 47.1

Mg2+ 12 87.6 122 109 49.3

PO4

3– 3 0.12 0.32 0.34 ----

Total N 3 65 52 74 ----

Total P 3 0.13 0.64 1.2 ----

Table 20. Volume-weighted mean solute concentrations in cloud 

water, stemflow, and throughfall during cloud events at Pico 

del Este. Units are μeq L-1, except total N and total P (μm L-1). 

Values for concentrations in rainfall at Pico del Este included for 

comparision. From Asbury et al. (1994).

N = Nitrogen.

P = Phosphorus.
Luquillo Experimental Forest: Research History and Opportunities 59

Figure 57.—Chemical concentrations in El Verde, Puerto Rico, streams 

(QS, QT, QPA, and QPB) before and after the passage of Hurricane Hugo, 

9-week moving averages; (a) potassium, (b) nitrate-N, (c) magnesium, (d) 

calcium, and (e) ammonium-N. The vertical lines mark the time of the 

hurricane disturbance. From Schaefer et al. (2000).

Figure 58.—Chemical concentrations in Bisley, Puerto Rico, streams 

(PRM, BQ1, BQ2, and BQ3) before and after the passage of Hurricane 

Hugo, 9-week moving averages; (a) potassium, (b) nitrate-N, (c) 

magnesium, (d) calcium, and (e) ammonium-N. The vertical lines mark the 

time of the hurricane disturbance. From Schaefer et al. (2000).
60 Luquillo Experimental Forest: Research History and Opportunities

Puerto Rico has been shaped by its history as a volcanic oceanic island and has been described as a geologic “heap of 

volcanic debris” (Hodge 1920, Mitchell 1954). Like mountains 

on other high-elevation Caribbean islands, the Luquillo 

Mountains have a central igneous core, formed in association 

with Cretaceous and Tertiary volcanoes, surrounded by an apron 

of sedimentary rocks that become progressively younger as they 

spread toward the coast. 

Like other islands in the Greater Antilles, Puerto Rico is part of 

a large volcanic island-arc complex that rests along the junction 

between the American and Caribbean crustal plates. The 

region’s geologic history is a complex sequence of accretionary 

and fragmentary events that occurred during the separation of 

North and South America. During its development, Puerto Rico 

underwent a full cycle of mountain development and is now 

relatively stable (King 1977).

Compared with the geology on the islands of Hispaniola and 

Cuba, fault and valley formation have played relatively minor 

roles in the geologic history of Puerto Rico, yet two distinct 

northwest trending fault zones divide the island into three major 

structural blocks (fig. 59). The Luquillo Mountains form the 

core of the northeastern structural block. A fault-line scarp that 

separates the northeastern and central structural bodies forms the 

southern margin of the asymmetric Luquillo range. Along the 

9. Geology

N.L. Harris

northern and eastern flanks of the range, radiating ridges extend 

from the mountain into the lower lying coastal regions.

Volcanoclastic sediments or dioritic intrusions underlie most of 

the Luquillo Experimental Forest (LEF). Both types of rock were 

derived from a similar andesitic magma that was active during 

the Cretaceous and lower Tertiary periods (Seiders 1971a). 

Following the accumulation of the volcanoclastic sediments, 

late Eocene or early Oligocene tectonic activity produced the 

dominant structural features of the mountains. The subsequent 

intrusion of the quartz-rich dioritic Río Blanco complex marks 

the last phase of igneous activity in the area (Seiders 1971a). 

Because earlier intrusions were quartz poor, this last episode of 

igneous activity probably contained differentiated and reworked 

magma.

Early Oligocene tectonic activity was followed by a period of 

quiescence, stability, and degradation. This period lasted from 

the Upper Oligocene to Middle Miocene, during which time 

the Caribbean plate drifted eastward and the Antilles began to 

take on their current configuration (Rosen 1985). Partial base 

leveling of Puerto Rico is thought to have occurred during this 

period and an upper or a ‘St. John’ peneplain developed (Lobeck 

1922, Meyerhoff 1933, Beinroth 1982). The St. John period 

was terminated in the Upper Miocene by a series of uplifts that 

raised the island to its present elevation (Beinroth 1982). A 

Figure 59.—The principal geological elements of Puerto Rico. From Glover (1971), reprinted in Scatena (1989).
Luquillo Experimental Forest: Research History and Opportunities 61

second period of prolonged stability during the Pliocene period 

is believed to have formed a second and lower erosion surface, 

the Caguana Peneplain. Since the end of the Tertiary period, 

Puerto Rico has been tectonically stable, yet Pleistocene sea levels 

at various times have reached 25 to 65 meters (m) above present 

levels (Monroe 1968). 

The Luquillo Mountains have a structurally complex terrain 

dominated by northwest-trending faults and associated 

northeast-trending folds. The major structural features in the 

northern section of the mountains are two broad, northeast 

trending folds: the Río Canóvanas syncline and the Luquillo 

anticline (Seiders 1971a). Northwest dipping beds cut by faults 

of small to medium displacement largely underlie the area 

between the fold axes. 

The Río Blanco complex dominates the southern section of the 

Luquillo Mountains. The rectilinear outline of this intrusion 

suggests that its emplacement was influenced by northwesttrending fractures and northeast-trending bedding (Seiders 

1971a). Although the emplacement of the stock produced 

little or no folding in the adjacent host rocks, a zone of contact 

metamorphisms surrounds the stock. Many of the large 

landslides in the LEF are associated with this contact zone.

Bedded volcanoclastic rocks underlie most of the northern 

Luquillo Mountains. In the Bisley area, these sedimentary 

rocks are members of the lower Cretaceous, Albian age, Fajardo 

Formation (Briggs and Aguilar Cortés 1980). The source of 

clastic sediment for these rocks was an active volcanic complex 

standing at or near sea level (Seiders 1971b). Volcanic debris was 

deposited into moderately deep water after it was transported 

and reworked by submarine slides, turbidity currents, ash flows, 

and ash falls. Alternating episodes of rapid volcanoclastic 

deposition and slow pelagic sedimentation resulted in a net 

accumulation of approximately 7,600 m of sediment (Seiders 

1971b).

The bedrock underlying the Bisley Experimental Watersheds 

has been mapped as the upper thick-bedded tuff unit of the 

Fajardo Formation (Briggs and Aguilar Cortés 1980). This unit 

consists of thick-bedded, very dark- to light-bluish-gray and 

dark-greenish-gray tuff that is interbedded with thick-bedded 

to massive tuff breccias and thin- to thick-bedded tuffaceous 

sandstone. The typical thick-bedded tuff weathers to an olive 

gray color, whereas the tuffaceous sandstone weathers to a pale 

brown color. The total thickness of this unit ranges from 800 to 

1,100 m.

Within the Bisley Experimental Watersheds, outcrops of bedrock 

are generally limited to the headwaters of the mainstream 

channels. The dominant exposures are thick-bedded tuffaceous 

sandstones that are saprolitic and riddled with clay-lined 

faults, and joints. Primary sedimentary structures are difficult 

to distinguish, but conglomeritic lenses and well-indurated 

siltstone beds are definable. Relic grains of plagioclase and mafic 

minerals are recognizable in some hand specimens. Weathering 

of these mafic, quartz-poor rocks produces a clay-rich, sandpoor residuum; however, where intersecting joint planes breaks 

outcrops, resistant angular boulders can be formed. The size and 

shape of these boulders are similar to those found along stream 

channels and drainages.

Regarding chemical rock weathering, a compilation of solute 

fluxes from a worldwide distribution of watersheds (White 

and Blum 1995) indicates that the effects of high temperature 

and precipitation combine to make solute fluxes from the Río 

Icacos watershed in the LEF the fastest documented chemical 

weathering rate of granitoid rocks on the Earth’s surface 

(McDowell and Asbury 1994).

Advances in our understanding of geology in the LEF will be 

obtained from the newly developed Luquillo Critical Zone 

Observatory (LCZO), part of a National Critical Observatory 

program, which focuses on landforms and watersheds that have 

different bedrock but similar climate and land use (http://www.

sas.upenn.edu/lczo). Although bedrock lithology and chemistry 

have been considered primary state factors in landscape and soil 

development for over a century, the influences of lithology on 

denudation, hydrologic routing, and geochemical processing are 

poorly constrained in most studies. To address this challenge, 

the LCZO will use the natural laboratory of the LEF to quantify 

and contrast how critical zone processes (those processes between 

rock, water, soils and atmosphere) in watersheds underlain by 

different bedrock are coupled and decoupled with climatic 

conditions and hydrologic, geochemical and biogeochemical 

cycles.

Differences in weathering patterns have a profound influence on 

landslide frequency, chemical denudation, and the morphology 

and longitudinal profiles of streams and hillslopes within the 

two watersheds. The LCZO will focus on two main study 

watersheds, the Río Mameyes and the Río Blanco, which have 

similar climatic and environmental histories but different 

lithology. The Río Mamayes watershed is primarily volcaniclastic 

bedrock that weathers to produce clays and boulders with a wide 

range of grainsizes, while the Río Blanco watershed is underlain 

by granodiorite which weathers into saprolite comprised of 

sand and large granodiorite corestones. Some of the planned 

research includes laboratory and field studies that will examine 

the sediment mobility in these watersheds over two very 

different timescales: event-based response to individual floods, 

and millenial-scale estimates of bed material transport through 

streams.
62 Luquillo Experimental Forest: Research History and Opportunities

Roberts (1942) was the first to map the soils of the Luquillo Experimental Forest (LEF) at a scale of 1:50,000. The U.S. 

Department of Agriculture, Soil Conservation Service, now the 

Natural Resources Conservation Service, updated the soil survey 

of eastern Puerto Rico (scale 1:20,000), including the LEF 

(Boccheciamp 1977). The Forest Service compiled a new soil 

map for the LEF in 2000 (fig. 60). The main soil orders of the 

LEF are Ultisols and Inceptisols, which occupy approximately 50 

and 20 percent, respectively, of the LEF (Brown et al. 1983). 

Soil characteristics in the LEF are strongly related to local 

topography and drainage. Humatus soils are moderately well 

drained and occupy stable upland surfaces, while sideslope soils 

are the most common in the Bisley Experimental Watersheds 

and have been classified as Tropohumults (Boccheciamp 1977). 

These are generally clayey soils with a moderately developed, 

coarse, subangular, blocky structure. The soils have a high 

available water capacity and well-developed mottles. Based on 

distinct variations in mottles and soil development, these soils 

can be subdivided into two associations: Zarzal and Cristal soils. 

Depressions and other saturated areas have gray, gley-type soils. 

Stony soils occur in headwater areas and along drainages. The 

characteristics of some individual soil types are discussed below 

and were summarized originally in Scatena (1989).

Zarzal Soils. The driest of the sideslope soils and the dominant 

pedon, Zarzal soils occur on steep to very steep, convex, 

and middle sideslopes. The solums are well-drained, typic 

10. Soils

N.L. Harris

Tropohumults that form on residuum and colluviums of 

volcanoclastic sandstones. The subsoil is dark, yellowish-brown 

clay, whereas the substratum is a strong, stony, brown, clay 

loam. Red mottles and small black iron-manganese concretions 

are common in the lower portions of the subsoil, but the low 

chroma mottles that characterize the Cristal soils are absent.

Cristal Soils. These soils occupy lower sideslopes and are in 

topographic positions that receive additional water through 

subsurface flow and upland runoff. They are somewhat poorly 

drained and have well-developed low chroma mottles (white and 

gray) in addition to the red mottles common to the Zarzal soils. 

The upper part of the subsoil is typically 80 centimeters (cm) 

thick, mottled pale brown clay that grades with depth to mottled 

strong brown clay. 

Humatus Soils. Moderately well-drained, stable landscape 

positions on upland surfaces have small patches of Humatus 

soils. These profiles typically have a yellowish-brown surface 

soil and yellowish-red, friable clay at depth. The well-developed 

mottles that characterize the Cristal and Zarzal soils are absent in 

this pedon.

Rough Stony Land. Areas with greater than 20 percent stone 

rubble surfaces are also common in the LEF. These cobble- and 

boulder-covered areas are typically found at heads of drainage 

areas or along watercourses. The highly variable soils in these 

areas are found between and beneath the rubble surface. The 

Figure 60.—Soil map of the Luquillo Experimental Forest. Colors and numbers correspond to 1: Zarzal-Cristal-Humatas Association; 2: SonadoraCaguabo-Prieto Association; 3: Yunque-Los Guineos-Moeado Association; 4: Picacho-Utuado-Ciales Association; 5: Dwarf. White areas are private land. 

Map from http://soils.usda.gov/survey/online_surveys/caribbean/cnf/maps/gsm.pdf.
Luquillo Experimental Forest: Research History and Opportunities 63

soil’s surface layer is commonly thin with dark-grayish-brown, 

plastic clay containing abundant leaf litter. The subsoil consists 

of either well-mottled clays of the Cristal series or gley-type soils.

Gley Soils. In areas with excess soil moisture, gley structureless 

soils are common. These soils are typically found at the heads 

of drainages, adjacent to stream channels, and in footslopes 

below eroding hillslope scarps. Gley soils are commonly, but 

not exclusively, associated with stony land. The typical subsoil is 

structureless, gray, plastic, heavy clay, 1 to 20 cm thick. Beneath 

the gley surface, horizons range from bluish gray to mottled 

rusty brown. In some areas, the gley soils cap relatively unaltered 

Cristal soils.

Soil erosion is a complex and interactive process between eroding 

forces and the resistance of the surface to them. The universal 

soil loss equation (Wischmeier and Smith 1978) indicates that 

erosion increases with increased rainfall and rainfall intensity. 

This relationship is confounded, however, by many interrelated 

factors. In the Bisley Experimental Watersheds of the LEF, 

Basnet (1992) used erosion pins to assess soil movement 

rates before and after the passage of Hurricane Hugo at three 

topographic positions: ridges, slopes, and valleys. Ridges had a 

significantly lower rate of erosion than slopes, most likely because 

ridges are underlain by stable geologic substrates and support 

a dense tree canopy that protects the ground from the direct 

impact of rainfall. Valleys experienced high sedimentation due 

to low slope angles and high runoff from both ridges and slopes. 

After this large-scale disturbance hurricane hit, the rate of soil 

movement increased 10-fold, masking any effects of topography 

(fig. 61).

Research currently being conducted by the newly developed 

Luquillo Critical Zone Observatory will advance our 

understanding soils in relation to landforms and bedrock 

(http://www.sas.upenn.edu/lczo).

Figure 61.—Rate of soil erosion in the Bisley Experimental Watersheds 

before and after the passage of Hurricane Hugo in (a) forest and (b) 

nonforest areas. From Basnet (1992).

61a.

61b.
64 Luquillo Experimental Forest: Research History and Opportunities

The first forest supervisor and chief forester of the then Luquillo National Forest and eventual Luquillo 

Experimental Forest (LEF) recognized four forest types in the 

national forest. These forest types were based on the “nature, 

circumstance, and elevation of the vegetation” for “convenience 

in description and to aid in preparation of management plans” 

(Bruner 1919). The four forest types are riparian, slope, sierra 

palm, and elfin forests. Bruner’s classification formed the 

basis for the forest “sites” or “areas of distinct productivity,” as 

later renamed by Wadsworth (1951), and this nomenclature is 

presently in common usage: tabonuco, palo colorado, palm, and 

elfin (also termed dwarf, cloud, or mossy forest) forest types. We 

don’t recommend the use of dwarf or cloud forest to identify 

elfin forests because these elfin or mossy forests (sensu Howard 

1968) are not dwarfed sensu stricto nor are they uniquely cloud 

forests. All forests above the cloud condensation level at the LEF 

are cloud forests, including the elfin forests. Elfin forests have 

the highest concentration of epiphytic mosses and other plants 

along the elevation gradient of the LEF, thus the name mossy 

forest. The height of elfin forests ranges from 1 to 5 meters (m) 

depending on their aspect or local hydrologic conditions.

Brown et al. (1983) described Wadsworth’s four forest types 

according to their spatial distribution, species composition, 

and structural attributes. Since the publication of Brown et al. 

(1983), a key realization of many researchers has been that the 

vegetation of the LEF is distributed along both longitudinal 

(elevational) and lateral (topographical) gradients. Although 

the four-forest-types model has served well to describe general 

patterns at a coarse scale, the tradeoff is the simplification of 

vegetation patterns observed at finer scales. Advances in the 

way researchers analyze and understand data on vegetation 

distribution and patterns signal a need to develop a new 

paradigm of vegetation associations in the LEF. Therefore, 

the gradient concept has replaced the four-forest-types model 

and presents a more thorough view of vegetation associations 

within the LEF. The section below organizes vegetation types 

by elevation range and by the topographic gradients that occur 

within the elevation ranges. Finally, the section reviews some 

species of concern within the LEF. This section does not address 

plantation forests or the effects on vegetation of past land use or 

hurricanes or other disturbances because these topics are covered 

elsewhere in the text.

Factors Influencing Species Distribution 

Patterns

A panoramic view of the forested LEF landscape reveals a 

continuous canopy (fig. 62), but, upon closer examination, 

the codominance of various tree species at different elevations 

begins to emerge. Although specks of silvery white (Cecropia 

schreberiana) and dark yellow (Schefflera morototoni) can be seen 

on days with strong winds, most apparent are the conspicuous 

patches of light-green fronds that comprise the palm forest, 

breaking up an otherwise darker green canopy. In fact, the most 

widespread and abundant canopy-forming species—the sierra 

palm, Prestoea montana—occurs from the lowest elevations of the 

LEF to elevations of more than 1,000 m above sea level.

What factors control the distribution of plant species in the LEF? 

Vegetation patterns along environmental gradients historically 

have been described using individualistic-continuum models in 

which the vegetation present at any particular site is the result of 

the co-occurrence of species with similar habitat requirements 

(Gleason 1926, Austin and Smith 1989, Collins et al. 1993, 

Hoagland and Collins 1997). Most studies of tropical montane 

vegetation along elevation gradients, however, tend to describe 

vegetation as discrete (sensu Clements 1936) community types 

(Gleason and Cook 1927, Weaver and Murphy 1990, Waide et 

al. 1998). A recent community-level study (Barone et al. 2008) 

focused on species distribution and species turnover variance 

along the LEF’s elevation gradient. The study’s results indicate 

a more complex framework of vegetation communities and 

metacommunities, forest types, associations, and individual 

species distribution dynamics than the Clementian model or 

the four-forest type paradigm would suggest. As described 

previously (Brown et al. 1983, Weaver and Murphy 1990, 

Waide et al. 1998), a pattern of decreasing species richness with 

increasing elevation is apparent, but individual tree species 

also respond to environmental conditions such as rainfall, soil 

moisture, and geomorphic position (Frangi 1983, Weaver 1991, 

Lugo et al. 1995, Lugo and Scatena 1995, Scatena and Lugo 

1995, Heartsill Scalley 2005, Barone et al. 2008). The Barone 

et al. (2008) data set clearly shows that species occurrence and 

distribution conform to the continuum models, but also shows 

that species groups tend to form discrete community units 

(fig. 63). Tree species along the Sonadora stream elevation 

gradient in the LEF have various distribution patterns (fig. 64), 

with species such as Prestoea montana and Psychotria berteriana 

occurring in great abundance at all elevations, while others such 

11. Vegetation 

T. Heartsill Scalley

Figure 62.—Landscape view from the west of the Luquillo Experimental 

Forest. Note Cyathea arborea and Cecropia schreberiana in the 

foreground. Photo by T. Heartsill Scalley.
Luquillo Experimental Forest: Research History and Opportunities 65

as Dacryodes excelsa and Manilkara bidentata occur exclusively 

at lower elevations. From mid-elevations to higher elevations, 

Rhedia portoricensis and Magnolia splendens are more abundant, 

while Cyathea bryophylla and Lobelia portoricensis are found only 

in the highest elevation sites. The intensively sampled elevation 

transects of Barone et al. (2008) demonstrate that boundaries 

of vegetation associations are not as clear as boundaries in 

surrounding palm forests.

Community types are influenced by environmental factors 

that vary along the elevation gradient (referred to here as 

“longitudinal” gradients), but riparian to upland gradients (i.e., 

“lateral” gradients) also influence vegetation characteristics 

as a result of fine-scale variations in soil moisture and the 

accumulation of nutrients and organic materials from litterfall 

and woody debris. Changes in most species associations are 

subtle and gradual, with each species honing in on specific 

habitat requirements, such as the documented differences in tree 

species distributions in ridges, slopes, upland areas, and riparian 

valleys within sites at various elevations in the LEF (Weaver 

Figure 64.—Total number of stems of dominant tree species measured in 0.1 hectare plots across an elevation gradient in the Sonadora watershed. 

Luquillo Long Term Ecological Research data set lterdb123, compiled by T. Heartsill Scalley.

Figure 63.—Total number of stems of all tree species measured in 0.1 

hectare plots across an elevation gradient in the Sonadora watershed. 

Luquillo Long Term Ecological Research data set lterdb123, compiled by T. 

Heartsill Scalley.
66 Luquillo Experimental Forest: Research History and Opportunities

1991; Scatena and Lugo 1995; Lugo and Scatena 1995; Weaver 

2000; Heartsill Scalley et al. 2009b). 

The vegetation communities in the LEF are also associated 

with periodic disturbances, including treefalls, landslides, and 

hurricanes (Scatena and Larsen 1991, Weaver 1991, Scatena and 

Lugo 1995, Zimmerman et al. 1995, Reagan and Waide 1996, 

Lugo 2008). An additional factor that influences vegetation 

community structure is the massenerhebung, or mountain 

mass elevation, effect. The mountainous terrain of the LEF 

rises steeply and abruptly to above 1,000 m in elevation over 

a linear distance of 8 kilometer (km) from the coast, resulting 

in distinctive changes in vegetation structure (fig. 65a) and 

composition (such as the presence of the cloud forest types) at 

a relatively low elevation (Grubb 1971, Weaver 1991). The 

observations of Barone et al. (2008) suggest that the extent 

of cloud cover is an important factor that influences species 

distribution along the LEF elevation gradient. Although no 

experimental evidence is currently available, they proposed that 

the observed decrease in species richness at mid-elevations of 

the LEF (fig. 65b) could be related to the lower boundary of the 

cloud cover base in the LEF, as other studies have connected the 

frequency of fog with the transitions among different forest types 

on tropical mountains (Grubb 1971).

Silver et al. (1999) documented a soil O2

 (oxygen) concentration 

gradient with elevation and along topography at the LEF (see 

section on nutrient fluxes) and found that tree species density 

decreased as the average soil O2

 concentration declined along 

a ridge, slope, valley catena (their table 1). It appears from 

this study and the observations of Frangi (1983) that all plant 

communities above the cloud condensation level are wetland 

communities (fig. 66a). Above this level, soil O2 concentrations 

decrease, soil saturation increases, and plant species density and 

distributions are affected.

Figure 65.—Stem (a) and species (b) density versus elevation in the 

Mameyes and Sonadora watersheds of the Luquillo Experimental Forest. 

From Luquillo Long Term Ecological Research data set lterdb123, compiled 

by T. Heartsill Scalley.

65a.

65b.

Figure 66.—(a) Conceptual model of wetland ecosystems in the Luquillo Experimental Forest. Arrows indicate movement of water flow. From Frangi 

(1983). (b) Pterocarpus forest, one of the wetland ecosystems of the Luquillo Experimental Forest. Photos by Neftalí Ríos and Samuel Moya.

66a. 66b.
Luquillo Experimental Forest: Research History and Opportunities 67

Low-Elevation Vegetation Below the Cloud 

Condensation Level 

Tall trees with canopy heights extending up to ~30 m along 

with lianas (i.e., woody vines) dominate low elevations (<600 

m) of the LEF and are abundant and important structural 

elements of these forest communities. Dominant tree species 

at low elevations—Dacryodes excelsa (tabonuco) and Sloanea 

berteriana (motillo)—are distributed primarily as a function of 

the geomorphology of the underlying terrain (lateral gradients), 

as these species tend to occupy ridges and ridge tops (Basnet 

et al. 1993). Other common emergent trees at low elevations 

in the LEF include Manilkara bidentata (ausubo), Guarea 

guidonia (guaraguao), Buchenavia capitata (granadillo), and 

Ocotea leucoxylon (laurel geo) (table 21). This tree community, 

referred to commonly as the tabonuco forest, ranges in elevation 

from approximately 200 to 600 m above sea level. Younger 

successional forest stands are observed commonly at the 

periphery of the LEF, ranging from low to mid elevations, and 

include tree species such as Tabebuia heterophylla, C. schreberiana, 

S. morototoni, and the shrub Miconia prasina. Animals disperse 

the seeds of most of these tree species—M. prasina and S. 

morototoni by birds and C. schreberiana by bats—while only 

the T. heterophylla seeds are dispersed by wind. Landscape 

views of the western periphery of the LEF at low elevations 

are particularly identifiable by Tabebuia heterophylla during its 

flowering periods (fig. 67).

The most abundant lianas in the low-elevation forests are 

Marcgravia spp., Rourea surinamensis, Schlegia brachyata, and 

Paullinia pinnata (Rice et al. 2004). Two of these lianas, R. 

surinamensis and S. brachyata, contribute nearly 5 percent of 

annual litterfall (Zalamea and González 2008). Epiphytes 

that rely on other vegetation for structural support—such 

as bromeliads and ferns—are also present in both upland 

and riparian areas, but are much more abundant in riparian 

areas (Sharpe 1997). Ferns also tend to dominate the forest 

a. Stems ha-1

Pre Hugo 1989 Post Hugo 1989 1994 1999 2004

Sloanea berteriana 92.87 Sloanea berteriana 76.72 Cecropia schreberiana 104.4 Cecropia schreberiana 107.3 Cecropia schreberiana 103.3

Prestoea montana 71.90 Prestoea montana 55.50 Prestoea montana 61.52 Psychotria berteriana 95.85 Prestoea montana 100.4

Dacroydes excelsa 61.41 Dacroydes excelsa 45.70 Sloanea berteriana 60.09 Prestoea montana 94.42 Sloanea berteriana 76.39

Inga laurina 40.44 Inga laurina 26.11 Dacroydes excelsa 37.2 Sloanea berteriana 84.41 Psychotria berteriana 76.39

Ocotea leucoxylon 38.94 Casearea arborea 19.58 Inga laurina 27.18 Dacroydes excels 48.64 Dacroydes excelsa 52.34

C. portorricensis 35.92 C. portorricensis 17.95 Casearea arborea 24.32 Sapium laurocerasus 41.49 C. portorricensis 42.44

Guarea glabra 28.46 Guarea guidonia 16.32 Guarea guidonia 18.6 C. portorricensis 37.2 Ocotea leucoxylon 38.2

Guarea guidonia 25.46 Cordia borinquensis 14.69 Sapium laurocerasus 18.6 Inga laurina 34.33 Sapium laurocerasus 35.37

Casearea arborea 25.46 Ocotea leucoxylon 14.69 C. portorricensis 17.17 Ocotea leucoxylon 34.33 Inga laurina 32.54

Psychotria berteriana 25.46 Guarea glabra 13.05 Cordia borinquensis 12.88 Casearea arborea 32.9 Casearea arborea 32.54

b. Percent Biomass

Pre Hugo 1989 Post Hugo 1989 1994 1999 2004

Dacroydes excelsa 39.87 Dacroydes excelsa 43.88 Dacroydes excelsa 39.39 Dacroydes excelsa 34.99 Dacroydes excelsa 33.06

Guarea guidonia 16.38 Guarea guidonia 11.39 Cecropia schreberiana 14.14 Cecropia schreberiana 18.68 Cecropia schreberiana 18.82

Sloanea berteriana 8.85 A. floribunda 8.26 Guarea guidonia 12.53 Guarea guidonia 9.23 Guarea guidonia 13.94

A. floribunda 4.27 Buchenavia capitata 6.96 Guarea guidonia 5.48 Prestoea montana 7.76 Prestoea montana 8.10

Ormosia krugii 3.77 Ormosia krugii 4.21 Prestoea montana 4.72 Sloanea berteriana 5.64 Sloanea berteriana 3.60

Buchenavia capitata 3.48 Inga laurina 3.86 Inga laurina 4.58 Inga laurina 4.04 Alchornea latifolia 2.63

Schefflera morototoni 2.62 Prestoea montana 2.79 A. floribunda 3.11 Buchenavia capitata 2.64 Buchenavia capitata 2.22

Cecropia schreberiana 2.49 Sloanea berteriana 2.71 Buchenavia capitata 2.72 A. floribunda 2.13 Manilkara bidentata 1.78

Prestoea montana 2.44 Alchornea latifolia 2.61 Ormosia krugii 2.26 Laetia procera 1.81 Laetia procera 1.70

Sapium laurocerasus 2.09 Laetia procera 1.93 Alchornea latifolia 2.02 Alchornea latifolia 1.67 Inga laurina 1.47

Table 21. Stem density and biomass of various species of the tabonuco forest in the Bisley watersheds. Data from (Heartsill Scalley et al. 2010). 

In bold low light/primary forest species, underlined high light/secondary forest species, other species are intermediary in their successional 

status and light requirements. C. portorricensis = Cyathea portorricensis, A. floribunda = Alchorneopsis floribunda.
68 Luquillo Experimental Forest: Research History and Opportunities

understory, however, and have been shown to constitute up to 34 

percent of the total vegetation (Chinea 1999). Another epiphyte 

seen commonly on large trees in open canopy areas of the lowelevation forest is the cactus known as the “tree beard” (Rhipsalis 

baccifera). This species has long, thornless stems that hang from 

tree trunks and branches, and birds readily consume its fruits 

(Santiago 2008). It is the only member of the cactus family that 

grows in the LEF (fig. 68). 

Orchids (both epiphytic and terrestrial) are also present in 

low-elevation forests, with epiphytic orchids present typically 

above the first branch of larger diameter trees (>16.5 cm 

DBH [diameter at breast height]) (Migenis and Ackerman 

1993). Guarea guidonia and Dacryodes excelsa are the two tree 

species that have been observed to host the most epiphytic 

orchids (such as Maxillaria coccinea, Pleurothallis ruscifolia, and 

Jaquiniella globosa) within the tabonuco forest type (Migenis and 

Ackerman 1993). The spatial distribution of terrestrial orchids 

in the tabonuco forest type has been related to disturbances 

and past land use (Bergman et al. 2006). This is the case 

of Wullschlaegelia calcarata, the aclorophyllous, saprophytic 

(nonphotosynthetic) orchid found in primary, closed canopy 

forests and absent from areas with past land use history and 

natural disturbances such as canopy gaps. There were positive 

associations of W. calcarata and D. excelsa and a negative 

association of W. calcarata and C. schreberiana, a light-demander 

pioneer (early successional) tree. Another positive relationship 

was found with W. calcarata and leaf litter abundance (but not 

basal area) of Buchenavia tetraphylla, a deciduous tree species 

with a unimodal (March and April) leaffall pattern. Previous 

land use effects on the soils and the fungal leaf litter symbionts 

of W. calcarata may be part of the complex environmental 

requirements of this orchid’s distribution. Other easily observed 

orchids are those occurring along riparian zones on streamside 

boulders and stems, in particular the various species of the 

endemic genus Lepanthes (fig. 69).

In addition to the tabonuco-dominated, low-elevation forests, 

wetlands in the LEF found below the cloud condensation level 

include riparian forests (fig. 66a). In the lower elevations of the 

Mameyes river watershed, the wetland tree P. officinalis (Álvarez 

López 1990) forms stands along riparian areas. This tree, with 

its characteristic massive buttress roots and floating seeds, has 

leaves that are readily consumed by stream fauna (Crowl et al. 

2006, Welsh et al. unpublished data). Buttress roots up to 8 m 

in width and 5 m in height have been recorded, and root color 

varies with age such that younger roots are yellow and change 

to reddish and dark gray as they age. A P. officinalis stand of 

approximately 3.7 hectares occurs at 500 m above sea level in 

the LEF; it is described as a montane riverine forest system 

with organic soil over clay (Álvarez López 1990). Other species 

within this rare wetland forest type include Casearia arborea, 

Cordia borinquensis, Inga laurina, Manilkara bidentata, the tree 

fern Nephelea portoricensis, and the palm Prestoea montana. The 

Figure 67.—Tabebuia heterophylla, a common tree species that inhabits 

low elevations of the Luquillo Experimental Forest, during a flowering 

period. Photo by T. Heartsill Scalley. 

Figure 68.—Rhipsalis baccifera, or “tree beard,” a common epiphyte of 

the tabonuco forest type. Photo by Eugenio Santiago Valentín.

Figure 69.—Orchid of the genus Lepanthes, which is seen commonly on 

streamside boulders and in adjacent riparian areas. Photo by T. Heartsill 

Scalley.
Luquillo Experimental Forest: Research History and Opportunities 69

upper canopy is 25 to 30 m in height, and a second canopy 

layer is found at 15 m, with abundant epiphytes and woody 

vines. Pterocarpus officinalis, a species with a wide Caribbean 

distribution (Rivera Ocasio et al. 2006), also occurs in small 

groups of individuals along streamsides at elevations below the 

LEF boundary (fig. 66b).

More diverse riparian communities dominate other lowelevation wetland areas. In the Luquillo Forest Dynamics Plot 

along the Prieta and Toronja streams, Heartsill Scalley et al. 

(2009b) calculated the minimum distance of individual stems 

(>10 cm DBH) to the nearest of the two streams. The species 

with stems in close proximity to streams were Clusia rosea, 

Coccoloba diversifolia, Margaritaria nobilis, Ixora ferrea, Trichilia 

pallida, and Guettarda valenzuelana, with a median distance 

to streams between 21 and 29 m. Margaritaria nobilis and C. 

diversifolia, however, were most abundant closer to the streams, 

with 75 percent of their stems within 30 to 40 m of the stream. 

Other species had 75 percent of their stems within 40 to 50 m 

from the stream and included C. rosea, Guatteria caribaea, G. 

valenzuelana, Byrsonima wadsworthii, and Eugenia stahlii (fig. 

70). 

Cloud and Wetland Forests Above the Cloud 

Condensation Level

As rainfall increases toward higher elevations of the LEF as a 

result of adiabatic cooling, forest structure shifts to an increased 

presence of epiphytes and to a higher density of shorter and 

smaller trees and shrubs. The lifting condensation level, which 

occurs typically around 600 m above sea level, determines where 

clouds will form and, thus, where the cloud forest community 

begins. The lower elevation cloud forest (~600 to 800 m) is 

characterized by the late successional species Cyrilla racemiflora

(palo colorado) but is dominated on a stem density basis by 

Prestoea montana, Micropholis garciniifolia, and Calycogonium 

squamulosum. Bromeliads are common on the forest floor 

in the palo colorado forest particularly those of the genus 

Guzmania. Weaver (1991, 2000) has described the tree species 

in this vegetation association in great detail and discussed 

lateral gradients in relation to aspect (leeward versus windward) 

observed regarding both composition and structure. Lateral 

gradients in vegetation structure and community characteristics 

described for forests below the cloud condensation level are also 

observed in higher elevation cloud forests, with higher stem 

density, species richness, and aboveground biomass on ridges 

versus slopes and riparian valleys. 

Figure 70.—Tree species distribution in relation to distance from streams. From Heartsill Scalley et al. (2009b).
70 Luquillo Experimental Forest: Research History and Opportunities

Large stands of the sierra palm (Prestoea montana) begin to 

occur at approximately 500-m elevation—at the intersection 

of premontane wet and rain forest life zones—and extend up 

to the mountain peaks on steep slopes and ravines adjacent to 

elfin cloud forests (Brown et al. 1983, Weaver and Murphy 

1990, Lugo et al. 1995). Palm forests are generally classified as 

either palm brake (i.e., palm slope) or palm floodplain forest, 

depending on the specific environmental and geomorphic 

conditions present. Palm brake forests are found on steep slopes 

where soils undergo long periods of saturation, while floodplain 

forests are periodically inundated with water (Brown et al. 1983, 

Frangi and Lugo 1985, Lugo et al. 1995). Common tree species 

in the palm brake forest include Daphnopsis philippiana and 

Calycogonium squamulosum in addition to the palm P. montana, a 

species that is found in all LEF elevation gradients (Barone et al. 

2008).

Lugo et al. (1995) found that the species composition and 

species density of palm brakes varied with aspect. More species 

occur on leeward slops than windward slopes, presumably 

due to greater rainfall and soil saturation in the windward 

slopes. Long-term records of species density changes in these 

palm brakes also showed that after the 1932 hurricane and the 

initial enrichment of sites due to pioneer species, the number 

of species decreased over the next 40 years in the windward 

slopes, while in the leeward slopes species continued to 

accumulate. Apparently differences in the long-term patterns of 

soil saturation determined the number of tree species capable of 

surviving in these contrasting palm brakes. Lugo and Scatena 

(1995) observed that the length of successional sequences after 

disturbances were shorter above the cloud condensation level 

when compared to communities below the cloud condensation 

level (their fig. 4.8).

The most common trees based on stem density in the palm 

floodplain forest include Croton poecilanthus, Micropholis 

crysophylloides, and Eugenia eggersii, but the trees with the highest 

aboveground biomass are M. splendens and S. berteriana. In 

contrast to the adjacent palo colorado and elfin forest types, 

the most abundant bromeliad in the palm floodplain forest, 

Guzmania berteriana, is found more commonly along canopy 

tree trunks than on the forest floor, whereas Selaginella spp. and 

other bryophytes cover the ground at various distances from the 

river floodplain (Frangi and Lugo 1985). In a study relating 

P. montana fecundity to hurricane disturbance, Gregory and 

Sabat (1996) found that palms with more fronds in exposed 

areas produced more fruit than palms in less disturbed sites, 

and that hurricane effects on the canopy opening resulted in 

increased palm seed production. Therefore, one can conclude 

that palm forest areas subjected to hurricane effects will grow 

and reproduce more than those in undisturbed areas. The peak 

timing time for the palm fruit to fall in the palm floodplain 

forest occurs between May and June (Lugo and Frangi 1993), 

and fruit production of P. montana individuals in the palm 

floodplain forest is an order of magnitude higher than that in the 

tabonuco forest (Bannister 1970, Lugo and Frangi 1993).

All vegetation above the cloud condensation level is classified 

as wetlands (Frangi 1983, Scatena 1993, Heartsill Scalley 

2005). Many environmental gradients reach their extreme 

values at the mountain peaks (e.g., highest wind speeds, lowest 

air temperatures, lowest incoming sunlight, highest rainfall, 

lowest soil O2

 concentrations, etc.). The vegetation responds in 

unique ways including unique vegetation structures, unique even 

between the forests of East Peak and West Peak (Howard 1968). 

Most notable is the woody vegetation of the elfin forest with an 

abundance of vascular flowering plants that adopt an epiphytic 

habit, particularly in the seedling and immature stages. At 

elevations above ~800 m, the elfin cloud forest is characterized 

by abundant stems of the tree fern Cyathea bryophylla, stunted 

individuals of the tree species Ocotea spathulata and Tabebuia 

rigida, and an abundance of epiphytes, of which the most 

striking and abundant is the small red bromeliad Werahuia 

sintenisii (fig. 71). This bromeliad is so common that it can 

be found in densities of up to 32,000 individuals per hectare 

(Lasso 2001) and contributes 12 percent of the net primary 

productivity of this forest type. It can also hold up to 3.3 tons of 

water per hectare and accumulate 25 percent of the phosphorus 

and K rainfall inputs (Richardson et al. 2000a).

Figure 71.—The most abundant epiphyte of the elfin cloud forest in the 

Luquillo Experimental Forest, the bromeliad Werahuia sintenisii. Photo by 

Alexis Molinares.
Luquillo Experimental Forest: Research History and Opportunities 71

The elfin cloud forest’s narrow ranges of temperature, 

precipitation, and cloud cover make this forest particularly 

vulnerable to changes in environmental factors, such as those 

changes proposed to occur as a result of global climate change. 

In particular, the epiphytic vegetation in the cloud forests 

may serve as indicators of climate change. For example, Lasso 

and Ackerman (2003 and 2004) observed an earlier flowering 

period for the elfin forest bromeliad W. sintenisii and showed 

that monthly mean minimum temperatures and monthly 

mean irradiance in the months leading up to flowering events 

explained 66 percent of the variation in the number of open 

flowers per month. It seems that W. sintenisii is responding to 

changes in the elfin cloud forests’ environment by timing its 

flowering correspond to the cues of the nocturnal minimum 

temperatures and to the changes in cloud cover as measured 

indirectly by increases in solar irradiance.

Another distinction of the cloud forests when compared with 

the lower elevation forest types is the abundance of epiphytes, 

including liverworts and mosses (Bryophyta). The LEF contains 

more than one-half of Puerto Rico’s moss flora (Sastre De Jesús 

and Tan 1995). One characteristic bryophyte is Sphagnum 

portoricense (fig. 72), whose presence has been identified as an 

indicator of the striking differences in ecosystem conditions 

existing between cloud forests and lower elevation tabonuco 

forests (Wadsworth 1951, Wadsworth and Bonnet 1951). With 

a very low tolerance for desiccation, S. portoricense is found next 

to open water in coastal areas, rivers, or wet banks throughout 

the rest of its geographical range (from the eastern coast of the 

United States to Venezuela). Throughout most of the LEF, S. 

portoricense is present as forest floor mats and surrounding the 

base of trees and shrubs where stem flow accumulates. However, 

it is also abundant next to disturbed roadsides and trails in 

saturated cloud forest soils. This species occasionally forms 

small bogs over exposed soil surfaces of uprooted trees or other 

small soil surface depressions (Karlin 2006). Within the LEF 

elfin cloud forests, Karlin (2006) describes the distribution of S. 

portoricense as “patchy”; it has been observed more commonly on 

the easternmost mountain summits (e.g., along the road to and 

at Pico del Este) while not observed along the trails to El Yunque 

and El Toro Peaks. The ecology of S. portoricense has not been 

studied extensively, however, and limited information is available 

regarding the threshold conditions that limit its distribution, 

growth, reproduction, survival, and roles in succession and 

competition in the forest types where it occurs. Also, very 

few notes exist on the distribution of S. portoricense before 

anthropogenic disturbances occurred in the cloud forests of the 

LEF; one hypothesis is that its present distribution may reflect its 

ability to colonize after disturbance.

Herbaceous and sphagnum bogs, riparian vegetation, and palm 

floodplain forests (Frangi 1983, Frangi and Lugo 1985, Lugo 

et al. 1990) also occur above the cloud condensation level. The 

functioning of floodplain palm forests is discussed below in the 

nutrient cycling section. In environments with high light levels, 

one can observe herbaceous bog areas that include clumps of 

Carex polystachya, Eleocharis spp., large masses of Sphagnum, and 

other wetland vegetation (Howard 1968, Karlin 2006).

Although the LEF supports most of the moss flora found across 

the island of Puerto Rico, 19 percent of the moss species found 

in the LEF are endemic. It is estimated, however, that the cloud 

forests have lost many more moss species than the lowland and 

mid-elevation forests; one-half of the moss species originally 

observed in cloud forests have not been observed again since 

their initial reporting (Sastre De Jesús and Tan 1995, Sastre 

De Jesús and Santiago Valentín 1996). One hypothesis for the 

loss of species, such as Thamniopsis incurve, Bryoerythrophyllum 

recurvirostre, Squamidium isocladum, Cyrtohypnum minitulum,

Schoenobryum concavifolium, and Breutelia scoparia, from 

the cloud forests is the loss of suitable habitat resulting from 

disturbances related to the establishment of a communications 

infrastructure and road construction in the elfin cloud forests 

(Sastre De Jesús and Tan 1995). Although the opening 

of canopy structure and a resulting loss of forest cover are 

considered to be the main factors leading to the decline of most 

moss species, the role of climate change, changes in air quality, 

and other factors associated with land use change and pollution 

may also contribute to the decline. One species, S. portoricense,

seems to be able to occupy disturbed areas in the elfin cloud 

forest. The distribution, physiology, and ecology of bryophytes 

in the LEF remain to be further described and studied 

particularly in the elfin cloud forest.

Figure 72.—Sphagnum portoricense mat surrounding a terrestrial 

bromeliad in cloud forest near the summit of Pico del Este, Puerto Rico. 

Photo by E.F. Karlin, from Karlin (2006).
72 Luquillo Experimental Forest: Research History and Opportunities

Species of Concern

Tree species found in the LEF that are listed as endangered 

include Ilex sintenisii (Sintenis’ holly), Styrax portoricensis (jazmin 

tree), Ternstroemia luquillensis (colorado), and Ternstroemia 

subsessilis (yunque colorado). All four species are endemic to 

Puerto Rico and exist only in the Luquillo Mountains (Santiago 

Valentín and Rivera 1993). In 2009, the U.S. Fish and Wildlife 

Service initiated a 5-year review of the state of these species to 

consider programs that propagate and re-introduce some of these 

species into their native ranges. Another endangered tree species, 

Callicarpa ampla (capá rosa), which previously had distribution 

in the U.S. Virgin Islands, is now restricted to the Luquillo 

Mountains. Other rare and threatened species of concern 

include the tree Pleodendron macranthum (chupacallos) and 

Lepanthes eltoroensis, the Luquillo Mountain babies’ boots orchid 

(fig. 73).

Figure 73.—Lepanthes 

eltoroensis, the Luquillo 

Mountain babies’ boots 

orchid. Photo by Raymond 

Tremblay.
Luquillo Experimental Forest: Research History and Opportunities 73

Of the approximately 1.5 million fungal species in the world, about 95 percent are undescribed (Hawksworth 

1993). Therefore, the fungi of Puerto Rico and the Luquillo 

Experimental Forest (LEF) are relatively well known by 

comparison. In Puerto Rico, Stevenson (1975) found 27 percent 

of the 11,268 species of fungi reported for the Caribbean Basin 

(Minter et al. 2001). The highest numbers of fungal species in 

the Caribbean have been reported from Cuba and Puerto Rico, 

where many resident and visiting mycologists have conducted 

research. Lodge (1996a) compiled a checklist with 400 of the 

approximately 1,000 fungal species identified in the LEF. These 

were primarily higher basidiomycetes and large ascomycetes 

found at the El Verde Field Station. Lodge’s checklist represents 

about 30 percent of the total number of macromycetes, and 10 

to 20 percent of those listed were new to science at the time the 

checklist was published. In a given field expedition to any site 

in the LEF, approximately 15 to 30 percent of the fungal species 

collected are new records or species. Table 22 summarizes the 

number of plant species (and endemics), the estimated number 

of fungal species based on fungi: plant ratios, and the number of 

known fungal species for various islands in the Caribbean Basin. 

Some of the earliest records of fungi from the LEF were based 

on opportunistic collections of durable fruit bodies (such as 

polypores and shelf fungi and xylariaceous ascomycetes) by 

botanists such as Britton (Stevenson 1975, Lodge 1996b). The 

El Verde Research Area has been studied most intensively, with 

fungal surveys focused primarily on habitat partitioning (Lodge 

and Cantrell 1995, Lodge 1997). For example, Padgett (1976) 

found a low diversity of aquatic hyphomycetous fungi, within 

six-leaf litter species, submerged in a stream at El Verde, possibly 

due to fungal host specificity. The earliest fungal surveys at El 

Verde included a study by Cowley (1970), who showed that 

microfungal decomposer communities differed more among leaf 

species than among sites, but that canopy disturbance had the 

strongest effect in driving microbial community patterns. In 

addition, Holler and Cowley (1970) showed almost complete 

nonoverlap (i.e., high complementarity or species turnover) in 

12. Fungi

S.A. Cantrell and D.J. Lodge

fungal species composition among decomposing leaf litter, soil, 

and roots, but they identified only a few of the microfungi to the 

species level. 

The ecological studies by Cowley (1970) and Holler and 

Cowley (1970) were important, but were based primarily on 

morpho-species. The checklist of fungi at El Verde published 

by Lodge (1996a) was the first taxonomically based ecological 

survey in the LEF organized by substratum that revealed strong 

habitat partitioning. Huhndorf and collaborators surveyed 

ascomycetes occurring on large and small woody debris in the 

Luquillo Forest Dynamics Plot (Huhndorf and Lodge 1997; 

Huhndorf and Fernández 1998; Huhndorf et al. 1999, 2003, 

2004, 2005; Fernández and Huhndorf 2005; Fernández et al. 

2006), described new species and genera, and confirmed fungal 

habitat partitioning based primarily on substratum size (but 

rarely by host). Læssøe and Lodge (1994) demonstrated host 

specificity in three species of Xylaria at El Verde, and Lodge et 

al. (2008) used a meta-analysis of Xylaria records from the LEF 

and elsewhere to show specificity of some Neotropical species for 

cloud forest habitat. Schmit (2005) found a positive relationship 

between initial C (carbon) density and macrofungal diversity on 

large woody debris at El Verde. Samuels and his collaborators 

contributed to knowledge of hypocreaceous ascomycetes in the 

LEF–a group that includes plant, animal, and fungal parasites 

and also decomposers (Lu and Samuels 2003; Mantiri et al. 

2001; Poldma et al. 1997; Samuels and Chaverri 2004; Samuels 

and Lodge 1996a, 1996b), while Liu and collaborators (Liu 

and Hodge 2005, Liu et al. 2006) described new species of 

clavicipitaceous parasites on scale insects. Surveys of microfungi 

included those by Lodge et al. (1996) who showed partitioning 

of endophytic fungi (fungi growing in plants without causing 

disease) among co-occurring tree species. Polishook et al. (1996) 

showed strong partitioning of leaf decomposer fungi among leaf 

species that occurred together on the forest floor (58 percent 

nonoverlap), confirming Cowley’s (1970) earlier work based 

on morphospecies. Furthermore, Lodge (1997) observed that 

decomposers rapidly replaced 95 percent of endophytic fungi 

Island/region

Native seed plant

species (endemics)a

Estimated number of

fungal speciesb

Number of known 

fungal speciesc

Caribbean Region 12,000 (7,000) 60,000-72,000 11,268

West Indies 10,948 (7,868) 54,740-65,688 10,329

Cuba 5,991 (3,187) 29, 955-35, 946 5, 668

Hispaniola 4, 612 23, 060-27,672 2,468

Puerto Rico 2,221 (305) 11,105-13,326 3,315

Jamaica 2,540 (874) 12,700-15,240 322

Table 22. Floristic and fungal diversity in the Caribbean Basin.

a The number of endemic species is shown in parentheses. From Acevedo Rodríguez and Strong (2008).

b Hawksworth (1993) used a 6:1 ration of plant to fungal species while Mueller et al. (2007) used a 5:1 ratio.

c From Minter et al. (2001).
74 Luquillo Experimental Forest: Research History and Opportunities

when the leaves fell to the forest floor. El Verde was included 

by Stephenson and his collaborators (Stephenson et al. 1999, 

Rojas et al. 2008) in two studies of dictyostelids, protestelids, 

and myxomycete slime molds that showed vertical stratification 

from the forest floor to the canopy and changes with elevation in 

the LEF. Cantrell and Lodge (2008) summarized the diversity 

of fungi for Puerto Rico and presented an updated checklist in 

a chapter in a book series on the natural history of the island. A 

complete list of fungi of Puerto Rico can be found at http://luq.

lternet.edu/data/lterdb86/metadata/lterdb86.html (Lodge 2003) 

and http://www.cybertruffle.org.uk/puerfung (Cantrell et al. 

2006). 

Degrees of Endemism

Most fungi are endemic (i.e., prevalent in only one locality) at 

regional or local scales and, thus, show strong biogeographic 

patterns (Mueller et al. 2007, Kubicek et al. 2008). Relatively 

few fungal species found in Puerto Rico are found in the nearby 

Lesser Antilles. For example, only one-half of the known Agaric 

fungi from Puerto Rico are documented in Pegler’s (1983) Agaric 

Flora of the Lesser Antilles, and most shared species are generally 

widespread throughout the Caribbean Basin. Puerto Rico shares 

fungal species with other islands in the Greater Antilles and 

South America (Cantrell et al. 2001, 2004; Baroni et al. 1997; 

Lodge et al. 2002), and also has some fungal species in common 

with North America, Africa, and Madagascar (Baroni et al. 1997; 

Cantrell et al. 2001, 2004; Lodge et al. 2002). 

On average, about 23 percent of all potential fungal endemics 

are unique to the Greater Antilles (table 23). Some families of 

basidiomycetes that are restricted to wet forests in the Greater 

Antilles, such as the Hygrophoraceae and the Entolomataceae 

(Baroni et al. 2008; Baroni and Lodge 1998; Cantrell and Lodge 

2000, 2001, 2004), have higher degrees of endemism (60 to 75 

percent), including some that may be restricted to Puerto Rico. 

For example, of the approximately 50 species of Hygrophoraceae 

in Puerto Rico, 17 (34 percent) were described as new and were 

found nowhere else but in Puerto Rico. Most of these species 

are found in the LEF and several new species are known to be 

from only one or two localities in the LEF, such as Hygrocybe 

bruneosquamosa, H. miniatofirma, H. laboyi, and H. olivaceofirma 

(fig. 74).

Fungal Roles in Ecosystem Processes 

In the LEF, fungi provide many ecosystem services such as 

recycling nutrients through decomposition, reducing soil erosion 

and stream sediment loads by holding litter in place on steep 

slopes, regulating plant and animal populations, facilitating 

nutrient uptake through symbiotic relationships, and defending 

plants from pathogens and herbivores (Lodge 1996a). Fungi 

are the dominant decomposers of fallen leaves and wood. 

Decomposition releases nutrients from the wood and litter into 

forms available for plant and microbial growth. Some fungi 

Figure 74. —Some species of Hygrocybe that have been described only for certain localities of the Luquillo Experimental Forest. (a) 

Hygrocybe bruneosquamosa; (b) H. miniatifirma; (c) H. laboyi; and (d) H. olivaceofirma.

74a. 74b. 74c. 74d.

Country

Endemics (unique 

records)a Endemism (%)

Venezuela 1,334 34 

Cuba 2,200 39 

Dominican Republic 699 29 

Haiti 6 3 

Jamaica 322 20 

Puerto Rico 789 24 

Table 23. Number of endemic fungi (unique records) for the 

countries of the Greater Antilles and Venezuela.

a Minter et al. (2001).
Luquillo Experimental Forest: Research History and Opportunities 75

form symbioses with algae or cyanobacteria to form lichens, 

many of which are important in nitrogen fixation in the LEF. 

Other types of fungi form symbiotic relationships with tree 

roots known as mycorrhizae. In the LEF, 98 percent of all the 

trees depend on mycorrhizal fungi for nutrient uptake from 

the soil (Lodge 1996a). Some leaf decomposer mushrooms are 

important because they bind the litter together; preventing litter 

and soil export from steep slopes during heavy rains (Lodge and 

Asbury 1998, Lodge et al. 2008). The percentage of ground 

covered by litter fungal mats generally increases with slope, 

indicating their importance in preventing erosive soil-nutrient 

loss on steep terrain in the LEF (fig. 75) (Lodge et al. 2008). 

The abundance of fungal litter mats (fig. 75) and the fatty 

acid fungal marker (18:2w6) (fig. 76) decrease with elevation, 

although not consistently; there is a strong dip at 600 meters 

above sea level at the lower boundary of the palo colorado forest 

type, which corresponds with the cloud base level and with 

abrupt changes in plant communities. 

Fungal diversity in decomposing leaves in tabonuco forest at 

the LEF is high (Polishook et al. 1996). Microfungal species 

found in decomposing leaves recur on particular host plants 

and to decomposition state (Polishook et al. 1996, Lodge 1997, 

Santana et al. 2005). Patterns of host-exclusivity and hostrecurrence among different leaf species might be explained by 

responses of decomposers to differences in physical structure, 

differences in nutrient and lignin concentrations, and different 

responses to the presence of secondary plant compounds among 

host plants. Santana et al. (2005) used a replicated complete 

factorial microcosm experiment to study the mass loss of 10 

dominant early stage decomposer microfungi obtained from 

5 leaf species (50 source-substratum combinations) and found 

a highly significant interaction between microfungi and leaf 

species. Decomposition was faster when fungal source leaves 

were of the same species or family as the substratum, or when 

the fungal source resembled the substratum in quality than when 

the fungal source and substratum leaves were mismatched (i.e., 

unrelated and of dissimilar quality) (fig. 77). A basidiomycete 

contaminant was present in some microcosms and had a strong 

additive effect on decomposition of Croton poecilanthus leaves, 

increasing rates by 22 percent above that of microfungi alone 

(fig. 77). Agaric basidiomycetes produce enzymes that degrade 

lignin, thereby providing greater microbial access to labile C. 

Decomposition by agaric fungi accelerated the mass loss of leaf 

litter by 16.3 percent in the field at Sabana in the LEF (Lodge et 

al. 2008).

Effects of Disturbance on Fungi 

The effects of natural disturbances on leaf and soil fungi have 

been studied at El Verde Field Station by Cowley (1970), 

Lodge (1996a), Lodge and Cantrell (1995), and Stephenson 

et al. (1999). Lodge (1997) and Miller and Lodge (1997, 

2007) reviewed the earlier studies of disturbance effects on 

decomposer fungi. Stephensen et al. (1999) found that diversity 

of protestelids, dictyostelids, and myxomycetes in the litter layer 

was highest at El Verde, in the most disturbed part of the LFDP, 

corresponding to areas with a higher functional diversity and 

to areas with an abundance of bacteria, which are their primary 

food source (Willig et al. 1996). One main environmental factor 

that negatively affects the diversity and abundance of fungi in 

wet tropical forests, such as the LEF, is desiccation (Lodge 1993, 

Lodge et al. 1994, Lodge and Cantrell 1995). After the passage 

of Hurricane Hugo at El Verde in the LEF, some species of 

Figure 75.—Percent of land area covered by decomposer basidiomycete 

litter mats as a function of slope (percent) and elevation class in the 

Luquillo Experimental Forest. Elevation classes: 1- 150 to 300 meters (m); 

2-301 to 450 m; 3- 451 to 600 m; 4-601 to 750 m; 5-751 to 1,000 m. 

Slope classes: 1-0 to 10 percent; 2-11 to 20 percent; 3-21 to 30 percent; 

4-31 to 40 percent; 5- 41 to 50 percent. From Lodge et al. (2008).

Figure 76.—Concentration (Mol percent) of the fungal fatty acid marker 

(18:2w6) along an elevation gradient and across different forest types. 

From García Orta (unpublished data).
76 Luquillo Experimental Forest: Research History and Opportunities

agaric leaf decomposer fungi completely disappeared from sites 

on ridges that were exposed to greater sun and wind, including 

Collybia johnstonii (Lodge and Cantrell 1995). Data on changes 

in fungal biovolumes and nutrient stores in response to hurricane 

disturbance and drying in the LEF were used to illustrate fungal 

importance in controlling nutrient availability to trees in wet 

tropical forests through pulsed nutrient dynamics (Lodge et 

al. 1994, Miller and Lodge 1997). An ongoing simulation of 

the effects of canopy opening and the subsequent addition of 

Figure 79.—Concentration of the fungal fatty acid marker (18:2w6) in soil 

before and after canopy opening and debris addition (T = Canopy Trim, NT 

= No Trim, D = Debris added). From Rivera Figueroa (2008).

Figure 78.—Diversity of fungi based on ITS TRFLP phylotypes in (a) 

decomposing green leaves and (b) decomposing senesced leaves in 

relation to canopy opening and debris addition (T = Canopy Trim, NT = No 

Trim, D = Debris added). From Ortiz Hernández (2008).

78a.

78b.

Figure 77.—Percent mass remaining of two leaf species decomposed by 

microfungi in microcosms when the fungal source (plant and substratum) 

were matched by leaf quality and phylogenetic relatedness. Open bars 

represent a basidiomycete contaminant that appeared after gamma 

radiation in addition to microfungi. From Santana et al. (2005).

debris monitors how such disturbances affect the biota. Ortiz 

Hernández (2008) demonstrated that canopy opening decreased 

the diversity of fungi in green and senesced leaves while debris 

addition increased the diversity (fig. 78a). The diversity of fungi 

declined as the canopy closed and decomposition progressed (fig. 

78b). Canopy opening had a delayed effect on the abundance 

of the fatty acid fungal marker (18:2w6) in soil, which declined 

a few months after canopy was trimmed (Rivera Figueroa 2008, 

fig. 79).
Luquillo Experimental Forest: Research History and Opportunities 77

Early studies on fauna in the Luquillo Experimental Forest (LEF) represented an effort to understand the population 

and community dynamics of animals in the forest and 

understand their importance to forest function. More recent 

research on faunal communities has focused heavily on the effect 

of disturbance. Reagan and Waide (1996) summarized the 

natural history and trophic dynamics of the tropical rain forest 

community at El Verde; much of the information summarized 

below is taken from this source.

Terrestrial Fauna

Termites

The chief role of termites in the LEF is the consumption of 

dead wood and litter, which recycles organic matter through 

the decomposer community. Compared with other tropical 

ecosystems, however, the LEF has few termite species, relatively 

low termite densities, and no termite species that feed primarily 

on soil and fungus gardens, suggesting that termites play a less 

dominant role in the LEF ecosystem than in many other tropical 

forests. Only four species of termites (Order Isoptera) have 

been reported from the tabonuco forest of the LEF: Nasutitermes 

costalis (Holmgren), N. nigriceps (Haldeman), Parvitermes discolor 

(Banks), and Glyptotermes pubescens (Snyder). These four species 

represent about one-third of the termite species reported from 

Puerto Rico. Wiegert (1970) carried out one of the first studies 

of wet forest termite population dynamics on N. costalis in the 

El Verde tabonuco forest. Most research on termites in the LEF 

has concentrated on this species.

All the El Verde termites appear to seek out relatively firm, 

standing dead wood as food, although they may also consume 

twigs and leaf litter on the forest floor. None of the four species 

is known to consume living wood. Two of the four species (N. 

costalis and N. nigriceps) build carton nests (the dark, ovoid nests 

that dot the landscape at El Verde), P. discolor excavates stumps 

for its nests, and G. pubescens is a drywood species that lives in 

chambers and tunnels within the sound wood on which it feeds. 

Wiegert (1970) made the only quantitative evaluation so far 

of the role of N. costalis in the total decomposer energy budget 

of the tabonuco forest. He estimated total numbers of the 

various termite castes and the number of developmental stages 

from harvested nests, obtained dry weights for the various 

castes, measured oxygen consumption of individual termites 

and carbon dioxide (CO2

) production of entire nests, and 

determined calorific content per gram of each caste and stage. 

He also measured the calorific content and ash percentage of 

the C (carbon) material from termite nests. From these data, 

Wiegert estimated a total energy flow of 20.5 to 24.5 kJ m-2

y-1 (kilojoules per meter squared per year) which represents 

such a small fraction of the total energy flow into the detritusdecomposer food chain that N. costalis may not be of much 

importance in the total decomposer energy budget of the LEF. 

13. Fauna

N.L. Harris

Of course, this study did not include the other three termite 

species, and together their effect probably exceeds that of N. 

costalis, but even when biomass figures for N. costalis, P. discolor

and N. nigriceps are added (table 24), the biomass value becomes 

1.01 g m-2 (gram per meter squared) wet weight or about 0.22 

g m-2 dry weight. Even if G. pubescens data were available, the 

biomass figure for the termites of El Verde would still represent 

a relatively low energy flow. A significant route of energy return 

from termite colony to ecosystem is via predation. As food for 

predators, termites probably make their greatest contribution 

during the swarming season, when alates may provide a large 

part of the diet of arthropods, lizards, bats, and possibly frogs.

Insects

Insect herbivores are a prevalent component of forest ecosystems 

and a key factor in regulating canopy processes (Schowalter et al. 

1986, Huntly 1991, Rinker et al. 2001) due to their high level 

of mobility and rapid rate of reproduction that enable them to 

respond quickly to alterations in their environment and exploit 

emerging resources. Selective feeding by insect herbivores 

exerts a strong influence on several forest ecosystem properties 

(Schowalter 2000). In addition to the well-documented and 

conspicuous effects they can have on plant growth and survival, 

herbivores can substantially alter plant species composition 

(Ritchie et al. 1998) and the flow of nutrients within forest 

systems (Schowalter et al. 1991, Hunter 2001). 

Despite the important role insect herbivores play in ecosystem 

processes, very little is known about their true number and 

diversity. Several new species have been discovered in the LEF 

in recent years, including Omicrus ingens, a hydrophilid beetle 

(Hansen and Richardson 1998) and Tanytarsus bromelicola, a 

chironomid with tube-constructing larvae (Cranston 2007). A 

potential new psychodid species endemic to Puerto Rico—Alepia 

zavortinki spec. nov.—was reported by Wagner et al. (2008) after 

sampling bromeliad plants in an ongoing study recording the 

diversity of their invertebrate fauna at different elevations in the 

LEF. 

Herbivorous insects modify nutrient flows directly by altering 

the timing, quantity, and quality of canopy inputs to the forest 

floor. Herbivore-derived inputs are commonly in the form of 

insect feces (frass), green leaf fragments (greenfall), and modified 

Species Density 

(individuals m-2)

Biomass 

(g m-2)

Nasutitermes costalis 75 0.24

Parvitermes discolor 374 0.71

Nasutitermes nigriceps 9.4 0.06

Total 458.4 1.01

Table 24. Density and biomass (wet weight) for termites in El 

Verde.
78 Luquillo Experimental Forest: Research History and Opportunities

throughfall, all of which form a chemically distinct component 

of canopy inputs to the forest floor. These herbivore-induced 

alterations generally increase the availability, and often the 

quantity, of nutrients that fall on the forest floor and could 

further modify ecosystem nutrient flow by altering patterns of 

nutrient input to the soil subsystem. After Willig et al. (1993) 

demonstrated that walking sticks (Lamponius portoricensis

(Phasmatidea)) occur on the prevalent understory plant Piper 

treleaseanum almost twice as often as expected, based on plant 

“apparency” values (i.e., the number of times a leaf of a given 

plant species in a particular site is hit by a string at any height 

along measured transects), Fonte and Schowalter (2005) 

manipulated herbivory levels in the LEF using walking sticks on 

Piper glabrescens and measured the effects on nutrient input to 

the forest floor and rates of litter decomposition. A significant 

positive relationship was found between all measures of herbivory 

and the concentration of NO3

-

 (nitrate ion) in ion exchange resin 

bags located in the litter layer. Rates of litter decomposition 

were significantly affected by frass-related herbivore inputs. This 

study demonstrated a modest but direct relationship between 

herbivory and both litter decomposition and NO3

-

 transfer to 

the forest floor, with results that suggest insect herbivores can 

influence forest floor nutrient dynamics.

Richardson and Hull (2000) studied insect larval communities 

in Heliconia caribaea (fig. 80) and found that, although the 

composition, relative abundance, and populations of species 

were consistent from year to year, most species were specific 

to Heliconia and different species utilized different bracts (i.e., 

modified leaves), with populations peaking at different stages 

of bract development. Ceratopogonid larvae were the earliest 

colonizers, followed by psychodids, syrphids, and culicids. 

Tipulid larvae appeared much later in the cycle of bract 

development and aging.

Recent research in the LEF has focused on changes in 

forest processes over the elevation gradient. Richardson 

et al. (2000a and 2005) shed light on the trends in litter 

invertebrate communities with elevation and showed that 

mineral concentrations in litter are significantly lower at 

higher elevations, with invertebrate abundance and species 

richness mirroring this pattern, showing populations declined 

significantly in mid- and high-elevation forests. Elfin forest 

bromeliads contained the lowest amount of debris and the lowest 

concentrations of nutrients in plant tissues. Richardson et al. 

(2000a) report a mid-elevational peak in species richness while 

abundance showed no apparent pattern with elevation, but after 

sierra palm (P. montana), which occurs along the extent of the 

elevation gradient, was used as a control in Richardson et al. 

(2005), neither insect abundance nor species richness changed 

significantly over the gradient. Richardson et al. (2005) suggest 

that the differences in abundance, species richness, and the 

uniformity of insect communities observed from the lower slopes 

to the summits are better explained by the contribution of forest 

composition to the chemical and physical nature of litter and 

forest heterogeneity, rather than to direct effects of temperature 

and rainfall differences. 

Earthworms and Soil Fauna

Many organisms spend all or part of their lives in the soil, with 

earthworms being one conspicuous group of soil dwellers that 

are often viewed as the most important animals influencing 

soil ecosystem functioning (Hendrix and Bohlen 2002). In the 

tabonuco forest of the LEF, earthworms dominate the soil food 

web in terms of biomass (Moore and Burns 1970), and their 

density and community structure is affected by land use changes 

(González et al. 1999, Zou and González 1997). 

Earthworms are generally classified into endogeic, anecic, 

and epigeic species to represent soil, soil and litter, and litter 

feeders, respectively (Bouché 1977). Earthworms alter a soil’s 

physical properties and biogeochemical processes according to 

their functionality. Endogeic (soil feeding) earthworms (e.g., 

Pontoscolex corethrurus Müller) alter a soil’s properties primarily 

through changing soil physical and chemical properties, and 

epigeic (litter feeding) earthworms affect microbial inoculation 

and associated decomposition processes. Anecic (soil and litter 

feeding) earthworms (e.g., native species Estherella gatesi Borges 

and Moreno and exotic species Amynthas rodericensis Grube) 

influence soils through both burrowing and inoculation activities 

(González et al. 2006). 

Earthworms, through their burrowing activities, may alter 

soil structure and water infiltration rates and thus affect 

the environmental conditions for microbial growth and 

biogeochemical fluxes (González et al. 2006). Earthworms also 

act as inoculators of soil microbes for the freshly fallen plant 

materials, and consequently accelerate the decomposition of 

plant materials (fig. 81, Liu and Zou 2002, Dechaine et al. 

Figure 80.—Heliconia

plants are a common 

sight in the Luquillo 

Experimental 

Forest and support 

diverse invertebrate 

communities. Heliconia 

caribaea is shown here. 

Photo by Jerry Bauer. 
Luquillo Experimental Forest: Research History and Opportunities 79

2005, Ruan et al. 2005). In addition to direct consumption, 

earthworms accelerate plant litter decomposition through 

elevating soil microbial activity and possibly enhancing the 

inoculation of soil microbes in worm casts to the fresh plant 

leaf materials (Liu and Zou 2002). This acceleration of organic 

matter mineralization provides a reliable flow of nutrients for 

continuous growth and maintenance of ecosystem production.

Previous research on earthworms in the Luquillo Mountains was 

centered in the tabonuco forest. Recently, González et al. (2007) 

expanded this research to include three other forest types within 

the LEF and four at lower elevations in northeastern Puerto 

Rico; sites were situated along an elevation gradient that spanned 

more than 1,000 meters (m). The study quantified both native 

and nonnative earthworm abundance and species diversity 

and found significant differences in the mean total density and 

biomass of earthworms among forest types. The Pterocarpus

forest had the highest density of total earthworms (1,024 worms 

m-2), and the mangrove forest the lowest (2.7 worms m-2) (fig. 

82a). In terms of biomass, the elfin forest and the Pterocarpus

forest had the highest (168.2 and 180.6 g m-2, respectively) and 

the dry and mangrove forests had the lowest (<1.2 g m-2). The 

palm, palo colorado, tabonuco, and lowland moist forests had 

values of earthworm biomass ranging from 27.8 to 49.4 g m-2. 

A total of 14 species belonging to 6 families were sampled along 

the elevation gradient. The elfin forest had the highest total 

number of earthworm species found at a given forest type (12 

species); the second highest was the palo colorado forest with 

6 earthworm species (fig. 82b). Earthworm biomass, density, 

and species numbers were consistently higher in the upper soil 

horizons than in the lower soil horizons.

In addition to changes in earthworm populations with elevation, 

Zou and González (1997) also demonstrated changes in 

earthworm density and community structure during secondary 

succession in abandoned tropical pastures. The chronological 

sequence of plant communities was pasture, grass-vine-fern, 

shrub-small tree, and forest. Earthworm density was highest in 

the active pasture (831 worms m-2 in top 0.25 m soil), decreased 

as secondary succession proceeded, and reached the lowest 

(32 worms m-2) in the forest (table 25). Furthermore, worm 

biomass measured in the pasture was nine times higher than 

worm biomass in the mature forest. Whereas P. corethrurus

(nonnative) was the only worm species present in the pasture and 

grass-vine-fern communities, both anecic species A. rodericensis

and E. gatesi were also present in the shrub-small tree and forest 

communities (table 25). Zou and González (1997) suggest 

that the decrease in earthworm density and increase in worm 

Figure 81.—Correlation between the natural logarithm (ln) values of 

earthworm abundance and rates of plant litter decomposition. From 

Dechaine et al. (2005).

Figure 82.—(a) Mean total density (per square meter ± S.E.; n = 9) of 

earthworms and (b) mean number of earthworm species in upper (0 to 10 

cm) soil depths and lower (11 to 30 cm) soil depths in eight forest types 

along an elevation gradient in northeastern Puerto Rico. Asterisks indicate 

a significant effect of soil depth within a forest type (one-way Analysis of 

Variance; α = 0.05). From González et al. (2007).

82a.

82b.
80 Luquillo Experimental Forest: Research History and Opportunities

community diversity during secondary succession may result 

from changes in the chemistry of organic inputs rather than in 

soil properties and litter quantity. For example, González et 

al. (1999) found that soil water content, which ranged from 

40 to more than 100 percent, had no influence on earthworm 

communities and P. corethrurus densities in wet (tabonuco) 

forests. With the conversion of pastures to secondary forests, 

the richness of earthworm species increases due to the presence 

of native species (Zou and González 1997), yet the nonnative 

earthworms still dominate in terms of biomass and density (Zou 

and González 1997, Sánchez de Leon et al. 2003); González et 

al. (1996) found that P. corethrurus dominated both plantations 

and secondary forests but native earthworms occurred only in 

secondary forests. 

Yang et al. (2007) investigated effects of nutrient addition 

through fertilization (applied quarterly since 1989) on litter 

arthropods and earthworm populations in the tabonuco and 

elfin cloud forests in the LEF. Fertilization significantly increased 

monthly litterfall production and litter quality, and this higher 

litter production led to significant litter accumulation on the 

forest floor. Litter arthropod abundance increased in the wet 

forest fertilized plots due to the accumulated litter, but higher 

quality of litter did not affect litter arthropod density in either 

forest. A significant reduction in earthworm populations in 

fertilized plots may have offset the positive effect of litter quality 

on litter turnover in the wet forest. The Shannon biodiversity 

index for litter arthropod taxa was similar among treatments in 

the wet forest, and was higher in the wet forest than cloud forest. 

Litter arthropod abundance and dominance in both forest types 

are shown in table 26.

Snails

In terrestrial ecosystems, land mollusks such as snails rank second 

in species richness; only arthropods have more species (Russell 

Hunter 1983). The terrestrial snail fauna of the tabonuco forest 

include 34 species (Álvarez 1991, Garrison and Willig 1996), 

the most abundant being Caracolus caracolla. Álvarez (1991) 

and Álvarez and Willig (1993) identified seven species of snails 

at El Verde that were not previously recorded for the tabonuco 

forest: Lamellaxis micra, Opeas pumilum, Nesovitrea subhyalina, 

Guppya gundlachi, Habroconus ernsti, Striatura meridionalis, and 

Chondropoma riisei. These taxa may have been absent from 

earlier inventories at El Verde due to their small size (diameter 

or length less than 5 millimeters (mm)) and due to their soil 

or litter microhabitat associations during the day (Garrison 

and Willig 1996). Table 27 (Willig et al. 1998) shows a list of 

terrestrial snail species captured in the LEF from 1990 to 1995 

after the passage of Hurricane Hugo. Snail activity in the LEF 

is primarily nocturnal or associated with high relative humidity 

during rain showers. 

Recent research on terrestrial snails in the LEF has evaluated the 

response of snail populations and communities to disturbances 

from treefall gaps (Álvarez 1991, Álvarez and Willig 1993) and 

hurricanes (Willig and Camilo 1991, Secrest et al. 1996). Willig 

et al. (1998) report data from the long-term (1990 to 1995) 

monitoring of snail populations after the passage of Hurricane 

Hugo at the 16-hectares (ha) grid (Hurricane Recovery Plot) at 

El Verde and a 13-ha plot at the Bisley Experimental Watersheds. 

Two species of snails sampled at Bisley, C. caracolla and N. 

tridens, suffered significant reductions in density in the aftermath 

of Hurricane Hugo, to the point that no specimens of N. tridens

were detected in posthurricane surveys (Willig and Camilo 

1991) but increased to densities three- and six-fold higher than 

prehurricane estimates (fig. 83). Willig et al. (1998) also showed 

that historical land use at El Verde had no significant effect 

on the abundance of most snail species. Trends in population 

density of each snail species at the El Verde Hurricane Recovery 

Plot are shown in figures 84 and 85. Bloch and Willig (2006) 

extended this data set and report population density trends for 

seven gastropods from 1991 to 2004 (fig. 86).

Puerto Rico has a low diversity of freshwater snails (van der 

Schalie 1948) that have not been studied extensively. Pyron 

and Covich (2003) measured snail-size frequency distributions 

in Ríos Espíritu Santo and Mameyes and showed that Neritina 

punctulata has similar migration and life history patterns to other 

tropical coastal streams, with snails migrating upstream at a rate 

of 0.08 to 0.17 cm s-1 (centimeters per second). Snails with 

Successional

community Density (no. m-2)

Biomass (g m-2)

Soil H2O% Soil pH

Ground litter 

(g m-2 Total % Anecic worm )

Pasture 831 a 175 a 0 a 73 a 4.8 a 14 a

Grass-vine-fern 403 a 110 ab 0 a 83 a 5.1 a 26 a

Shrub-small tree 141 b 41 ab 15 ab 61 b 4.7 a 524 b

Forest 32 b 19 b 52 b 70 ab 5.0 a 335 b

Table 25. Changes in earthworm density, biomass, soil pH and water content, and ground litter biomass during secondary succession in 

abandoned tropical pastures in Puerto Rico. Common letters within a column indicate no significant difference (Scheffe’s multi-range test, 

α = 0.10) between successional communities. From Zou and González (1997).
Luquillo Experimental Forest: Research History and Opportunities 81

Lowland wet forest Cloud forest

Abundance (individuals m-2) % Abundance (individuals m-2) %

Fertilization Control Fertilization Control Fertilization Control Fertilization Control 

Mean SE Mean SE Mean SE Mean SE

Acari 7504.381A 2848.52 4031.619B 667.58 63.79 64.33 2496.91A 709.84 2496.00A 928.30 78.68 86.31

Collembola 1055.248A 324.11 723.429A 342.99 8.97 11.54 263.314A 67.94 123.657B 61.75 8.30 4.28

Ants (Hymenoptera) 1299.429A 361.55 427.429B 80.51 11.04 6.82 159.77A 122.80 132.57A 129.02 5.03 4.58

Homoptera 495.238A 506.52 173.714A 38.80 4.21 2.77 1.37A 1.68 8.00A 12.07 0.04 0.28

Isoptera 79.62A 96.89 171.810A 197.10 0.68 2.74 – – – – – 0.–

Pseudoscorpiones 169.524A 116.79 120.000A 103.10 1.44 1.91 4.11A 6.65 10.971B 7.39 0.13 0.38

Aranene 155.810A 51.87 118.857A 54.70 1.32 1.90 10.286A 6.00 4.342B 4.30 0.32 1.15

Diptera 199.238A 68.22 92.19A 54.89 1.69 1.47 108.8A 48.45 32.914B 14.80 3.43 1.14

Coleoptera (adult) 219.048A 98.31 89.905B 19.93 1.86 1.43 48.91A 21.70 41.14A 19.50 1.54 0.42

Diplopoda 134.476A 59.36 52.571B 17.80 1.14 0.84 14.63A 7.76 8.00A 7.48 0.46 0.28

Coleoptera (larvae) 94.095A 11.05 49.905B 16.73 0.80 0.80 10.971A 5.63 1.143B 0.72 0.35 0.04

Hemiptera 104.762A 59.12 40.762B 23.18 0.89 0.65 16.229A 6.32 7.314B 4.00 0.51 0.25

Isopoda 58.667A 37.14 38.476A 13.66 0.50 0.61 16.91A 7.76 14.17A 2.85 0.53 0.49

Psocoptera 27.81A 14.13 37.333A 33.78 0.24 0.60 14.17A 16.46 8.23A 7.24 0.45 0.28

Thysanoptera 40.381A 26.17 30.095A 25.34 0.34 0.48 0.23A 0.46 0.91A 1.83 0.01 0.03

Blattodea 25.905A 7.83 12.952B 10.28 0.22 0.21 – – – – – –

Opiliones 14.476A 2.35 9.524A 7.94 0.12 0.15 1.60A 2.12 1.37A 2.22 0.05 0.05

Lepidoptera 20.952A 15.08 8.381A 8.67 0.18 0.13 0.23 0.46 – – 0.01 –

Trichoptera 9.143A 1.87 8.000A 5.20 0.08 0.13 0.23 0.46 – – 0.01 –

Symphyia 13.333A 7.00 8.000A 6.12 0.11 0.13 – – – – – –

Protura 6.48A 5.93 6.476A 6.75 0.06 0.10 – – – – – 0.02

Chilopoda 5.714A 2.80 4.952A 2.69 0.05 0.08 2.514A 1.83 0.457B 0.56 0.08 0.02

Diplura 28.95A 14.65 3.429B 0.93 0.25 0.05 0.46A 0.91 0.23A 0.46 0.01 0.01

Neuroptera 1.524A 1.43 1.905A 1.94 0.01 0.03 – – – – – –

Orthoptera – – 0.762 1.08 – 0.01 1.37A 1.68 0.46A 0.91 0.04 0.02

Demaptera 0.38 0.54 – – – – – – – – – –

Microcoryphia 0.381A 0.54 – – – – – – – – – –

Total 11,746.95A 4209.82 6267.05B 1015.59 3173.49A 752.20 2891.89A 1019.52

Diversity index 1.36A 0.05 1.33A 0.08 0.87A 0.06 0.63B 0.04

Table 26. Litter arthropod abundance and dominance (%) in control and fertilization treatments in lowland wet and cloud forests in the Luquillo 

Experimental Forest. From Yang et al. (2007).

Different upper case letters indicate statistical significance between treatments (repeated measures Analysis of Variance).
82 Luquillo Experimental Forest: Research History and Opportunities

Alcadia alta Nenia tridens

Alcadia striata Obeliscus terebrasier

Austroselenites alticola Oleacina glabra

Caracolus caracolla Oleacina playa

Caracolus marginella Platysuccinea portorricensis

Cepolis squamosa Polydones acutangula

Gaeotis nigrolineata Sublina octana

Megatomastoma croceum Vaginulus occidentalis

Table 27. List of snails captured in the Luquillo Experimental Forest 

(El Verde and Bisley watersheds) from 1990 to 1995. From Willig 

et al. (1998).

Figure 83.—Temporal sequence (numbers on x-axis refer to years, letters 

refer to seasons; W = wet, D = dry) of density estimates (mean ± SE) 

for common snails (Caracolus caracolla, shaded bars; Nenia tridens, 

unshaded bars) on the 13-hectare grid at the Bisley site of the Luquillo 

Experimental Forest. From Willig et al. (1998).

Figure 84.—Temporal trend of density estimates (mean ± SE) for nine common snails at the El Verde Hurricane Recovery Plot in the Luquillo Experimental 

Forest from 1990 to 1995. Numbers on the x-axis refer to years, letters refer to seasons: W = wet, D = dry. From Willig et al. (1998).
Luquillo Experimental Forest: Research History and Opportunities 83

Figure 85.—Temporal trend of density estimates (mean ± SE) for seven less common snails at the El Verde Hurricane Recovery Plot in the Luquillo 

Experimental Forest from 1990 to 1995. Numbers on the x-axis refer to years, letters refer to seasons: W = wet, D = dry. From Willig et al. (1998).

Alcadia alta Caracolus marginella

Obeliscus terebraster

Sublina octana

Megalomastoma croceum

Platysuccinea portoricensis

Vaginulus occidentalis

Density (per hectare) Density (per hectare)

Density (per hectare) Density (per hectare) Density (per hectare)

Density (per hectare)

Density (per hectare)

91W 92D 92W 93D 93W 94D 94W 95D

Season

91W 92D 92W 93D 93W 94D 94W 95D

Season

91W 92D 92W 93D 93W 94D 94W 95D

Season

91W 92D 92W 93D 93W 94D 94W 95D

Season

91W 92D 92W 93D 93W 94D 94W 95D

Season

91W 92D 92W 93D 93W 94D 94W 95D

Season

91W 92D 92W 93D 93W 94D 94W 95D

Season

80

60

40

20

0

30

20

10

0

30

25

20

15

10

5

0

100

80

60

40

20

0

200

160

120

80

40

0

150

120

90

60

30

0

120

90

60

30

0
84 Luquillo Experimental Forest: Research History and Opportunities

Figure 86.—Terrestrial gastropod population densities through time in the Luquillo Experimental Forest. From Bloch and Willig (2006).
Luquillo Experimental Forest: Research History and Opportunities 85

shell lengths greater than 30 mm were the most abundant size 

class at upstream sites, while snails with shorter shells were more 

abundant at downstream sites (fig. 87). Smaller individuals also 

grow at approximately twice the rate of larger individuals, with 

growth rates of 0.015 mm day-1 (millimeters per day) for a large 

cohort (~25 mm shell length) and 0.035 mm day-1 for a small 

(~15 mm shell length). These growth data suggest a minimum 

longevity between 3 and 7 years.

Blanco and Scatena (2006, 2007) studied another dominant 

diadromous snail, Neritina virginea. The density of this snail 

species increases with water depth (fig. 88), and no populations 

of N. virginea were found in rivers that were disconnected from 

the ocean for most of the year. Likely reasons for the absence of 

N. virginea in these rivers include channel dewatering, formation 

of sediment bars at their mouths, and low mean monthly 

discharge. 

Coquis

Eleutherodactylus coqui is the most widespread and abundant 

of the frog species of Puerto Rico (Rivero 1978, Stewart and 

Woolbright 1996, Beard et al. 2003), and its mating call is the 

most distinct sound heard in the forest at night. The coqui 

develops directly from terrestrial deposition of eggs without an 

aquatic tadpole stage (Townsend and Stewart 1985), so this frog 

can develop independent of bodies of water. Stewart and Pough 

(1983) increased the population density of coquis in study 

plots by supplementing retreats with artificial “frog houses” that 

protect frogs from predators, and results suggest that the primary 

factor limiting population density is not related to predation, but 

to the availability of suitable retreat and nest sites. Woolbright 

(1996) also reported substantial increases of E. coqui populations 

associated with increases in retreat and nest sites created by 

the debris from fallen trees in the wake of Hugo. Although 

Woolbright et al. (2006) reported census results for the LEF at 

1,846 frogs ha-1 (per hectare) with a male to female sex ratio of 

2.5 for a 4-plot sample in 2002 (table 28), the density of the 

Puerto Rican population of coquis has been estimated over the 

long term using the sum of estimated population numbers for 

adult and juvenile frogs, which yielded a figure of 20,570 frogs 

ha-1 (Stewart and Woolbright 1996).

Beard et al. (2002) recorded large, observable, and significant 

effects of E. coqui on nutrient concentrations and fluxes in the 

LEF, presumed to result from the conversion of insects into 

nutrient forms that are more readily available for microbes 

and plants. In a manipulative experiment, coquis at natural 

densities were contained or excluded from 1 m3 enclosures for 

4 months, and the chemistry of leaf wash (throughfall), foliage, 

and decomposed leaf litter in the enclosures was measured as 

indicators of coqui effects on nutrient cycling. Coqui exclusion 

decreased elemental concentrations in leaf washes by 83 percent 

for dissolved organic C, 71 percent for NH4

+ (ammonium) 33 

Indicator Value

Total adults marked over 4 nights 42.3 ± 4.9

Jolly-Seber estimate of adult abundance 

(frogs ha-1)

1846 ± 323

Sex ratio male/female 2.5 ± 0.5

Maximum juvenile count over 4 nights 136.3 ± 58.0

Ratio daily juvenile count to adult count 9.2 ± 1.9

Table 28. The 2002 census results for adult and juvenile 

Eleutherodactylus coqui (mean ± SE) in 20 x 20 m study plots in 

the Luquillo Experimental Forest. N = 4 plots. From Woolbright et 

al. (2006).

Figure 87.—Regressions of mean snail size against distance from the 

ocean for the sites of Ríos Mameyes and Espíritu Santo in the Luquillo 

Experimental Forest. Equations are Y = 5.89 + 2.04X (R2 = 0.92) and Y = 

-13.9 + 3.24X (R2 = 0.98), respectively. Elevations are listed next to each 

site. From Pyron and Covich (2003). Mean Size (mm)

Distance from ocean (km)

Figure 88.—Variation in Neritina virginea density relative to water depth 

in two pool types in the Luquillo Experimental Forest. From Blanco and 

Scatena (2006). Snail density (ind./m2

)

Water depth (cm)
86 Luquillo Experimental Forest: Research History and Opportunities

percent for NO3

-

, 60 percent for dissolved organic N (nitrogen) 

and between 60 and 100 percent for Ca (calcium), Fe (iron), 

Mg (magnesium), Mn (manganese), P (phosphorous), K 

(potassium), and Zn (table 29). Although coqui exclusion 

also had no effect on foliar chemistry of plants transplanted 

into the enclosures, exclusion decreased nutrient availability in 

decomposing mixed leaf litter by 12 percent and 14 percent for 

K and P, respectively, and increased C:N ratios by 13 percent. 

A model of nutrient cycling in coqui populations in the LEF 

is displayed in figure 89. In a later study, Beard et al. (2003) 

also showed that E. coqui populations reduce populations of 

aerial invertebrates and reduce leaf herbivory rates and increase 

foliage production and decomposition rates, most likely through 

elimination and excretion of nutrients rather than through 

controlling herbivorous prey.

Anoline Lizards

The lizard genus Anolis is a common and conspicuous 

component of the fauna of all terrestrial environments 

found in the Caribbean islands. The Anolis genus comprises 

approximately 300 species and is among the largest vertebrate 

genera. Rand (1964) first described the patterns of distribution 

in Puerto Rican anoles and explained differences based on 

variations in structural habitat (perch height and perch 

diameter) and climatic habitat (exposure to the sun). Reagan 

Element

Piper Manilkara

Coquís No Coquís Effect Coquís No coquís Effect

DOC 12.62±7.55 1.52±0.34 -88% 6.02±3.23 1.35±0.34 -78%

DON 0.61±0.90 0.35±0.16* -43% 1.50±0.47 0.34±0.09* -77%

NH4

+ 1.50±0.47 0.34±0.09* -75% 0.58±0.23 0.19±0.09* -67%

NO3

- 0.17±0.11 0.04±0.01 -76% 0.09±0.03 0.10±0.08 +11%

Al 0.017±0.01 0.014±0.00 -18% 0.017±0.00 0.021±0.01 +24%

Ca 1.87±0.77 0.70±0.14* -63% 0.78±0.34 0.33±0.03* -58%

Fe 0.28±0.27 0.010±0.00* -96% 0.14±0.13 0.02±0.01 -88%

Mg 0.20±0.08 0.086±0.02* -58% 0.078±0.02 0.04±0.01* -49%

Mn 0.12±0.11 0.003±0.00* -97% 0.055±0.05 0.005±0.00* -90%

P 8.62±8.52 0.55±0.54* -94% 0.98±0.67 0.065±0.03* -93%

K 22.22±21.67 0.34±0.06* -98% 10.38±10.13 0.89±0.74* -91%

Zn 0.12±0.12 0.00±0.00 -100% 0.052±0.05 0.00±0.00 -100%

Table 29. Mean elemental concentrations in mg L-1 (± SE) in leaf washes collected biweekly from June to October 1999 in enclosures with coquis at 

post-disturbance densities and without coquis in the Bisley watersheds in the Luquillo Experimental Forest. Effect was determined as the percent 

change in concentrations in leaf washes collected as a result of coqui exclusion and was calculated as the percent in the variable: (without 

coquis – with coquis) / (with coquis); DOC = dissolved organic carbon and DON = dissolved organic nitrogen. From Beard et al. (2002).

* There were significant differences when making comparisons by element between treatments for that plant species using a t = test at p=0.05.

Figure 89.— Model of nutrients cycling in coqui populations at average 

densities in the Luquillo Experimental Forest. Values for ingestion, 

egestion, and biomass are independent estimates. Coqui respiration 

and production are dependent estimates and were estimated using 

parameters from Stewart and Woolbright (1996). The missing calcium 

source may be from prey items, such as snails, that were not included in 

the analysis. From Beard et al. (2002).
Luquillo Experimental Forest: Research History and Opportunities 87

(1996) reported the vertical distribution patterns of the three 

anoles found in the LEF—A. gundlachi, A. evermanni, and 

A. stratulus—and these patterns are presented in figure 90. 

Although A. stratulus is relatively uncommon near ground level 

in the forest interior, it is extremely abundant in the forest 

canopy 8 to 24 m above ground level. Of the three common 

species, only A. stratulus appears to prefer perches of a particular 

size (fig. 91). By comparing the availability of perches in 

different size categories with the observed vertical distributions 

of the three common anole species, data suggest that most 

A. stratulus inhabit the canopy where small perches are more 

abundant.

Like coquis, anolis lizards have been shown to have significant 

effects on invertebrate populations and herbivory rates (Dial and 

Roughgarden 1995). When Anolis lizards were excluded for 6 

months from tree crowns in the LEF, lizard removal had strong, 

significant, and positive effects on arthropods greater than 2 

mm in length and weak negative effects on arthropods less than 

2 mm in length (fig. 92). Herbivory effects were significantly 

greater in lizard removal crowns than in controls, indicating an 

indirect effect of anoles on plants.

Birds

Puerto Rico has 269 species of birds, 106 of which are breeding 

residents and 126 are nonbreeding migrants or visitors (Raffaele 

1983). The tabonuco forest is home to 49 of the 66 species 

of land birds found in the Luquillo Mountains (Wiley and 

Bauer 1985), and 40 of these species have been recorded at El 

Verde. Of these 40 species, 30 are found regularly in the forest 

surrounding the El Verde Field Station and are considered to 

constitute the avian component of the tabonuco forest food web.

The 30 common species at El Verde occur in 8 orders, each 

represented by a single family, and the Passeriformes, which 

comprises 4 families (1 of which has 5 subfamilies). Of the 30 

species, 8 are endemic to Puerto Rico, 2 others are found only 

in Puerto Rico and the Virgin Islands (Raffaele 1983), and 

one-half (15 of 30) of the avifauna at El Verde and two-thirds 

of the breeding avifauna are West Indian endemics. Both the 

endemic parakeet (Aratinga maugei) and the White-Necked 

Crow (Corvus leucognaphalus) became extinct. The endangered 

Puerto Rican parrot, once reduced to only 13 birds in the wild, 

reached a population of about 49 individuals before the passage 

Figure 90.—The vertical distribution of Anolis gundlachi, A. evermanni, and A. stratulus in tabonuco forest near El Verde in the Luquillo Experimental 

Forest. From Reagan (1996), originally presented in Reagan (1992). Height (m)

Figure 91.—Frequencies of perch diameters selected by the three 

common anole species in tabonuco forest. From Reagan (1996), originally 

presented in Reagan (1992). Percent Percent

Percent
88 Luquillo Experimental Forest: Research History and Opportunities

Figure 92.—Responses of small (less than 2 millimeters) and large (larger 

than 2 millimeters) arthropods to lizard removal compared with controls: 

(a) large arthropods caught in sticky traps; (b) large arthropods observed 

on leaflets; (c) small arthropods caught in sticky-traps; (d) small arthropods 

observed on leaflets. Closed circles represent data from lizard removals, 

open circles represent data from controls. Data points are mean ± 1 SE. 

The dotted line shows the mean date of lizard removal. From Dial and 

Roughgarden (1995).

of Hurricane Hugo. The parrot, however, has not been seen in 

the El Verde area since 1981. 

The avifauna of El Verde experience both annual and seasonal 

population density fluctuations (fig. 93) related to patterns of 

reproduction and mortality and to effects from long-distance 

migration. Annual changes in avifauna population density 

Figure 93.—Breeding seasons of birds in the Luquillo Experimental 

Forest. Principal nesting periods are indicated by a solid line, sporadic 

nesting by a dotted line, and species for which there are no data by a 

dashed line. From Waide (1996).

Figure 94.—Relative abundance of species caught in mist nets at El 

Verde. Data are from Reagan et al. (1982) and Wunderle et al. (1987); 

figure from Waide (1996).

estimates may reflect long-term trends, but few tropical sites 

have sufficiently long records to detect such tendencies. Seasonal 

changes in population density estimates may reflect fluctuations 

in actual numbers or seasonal cycles in behavior such as breeding, 

dispersal, or migration; a thorough understanding of temporal 

changes in bird populations is needed.

The rank abundance distribution of mist net captures from 

El Verde (fig. 94) shows clearly the dominance of the Ruddy 

Quail-Dove, followed in rank by the Bananaquit, Puerto Rican 

Emerald, Puerto Rican Tody, Puerto Rican Tanager, and PearlyEyed Thrasher. The remaining eight species in the sample (57 

percent) were rare (less than 2 percent of the sample). Mist 

net samples from mainland tropical forests reveal a much larger 

proportion of rare species (85 percent from Pipe Line road 

in Panama, Karr 1990; 90 percent from the central Amazon, 

Bierregaard 1990). The relatively small sample size from the 

El Verde mist net samples may partially explain the relatively 

low proportion of rare species, indicating that as the sample 

size increases, more rare species are likely to be found. If all the 

species at El Verde with the potential to be captured by mist nets 
Luquillo Experimental Forest: Research History and Opportunities 89

were included in the count, 82 percent of the species would be 

considered rare.

The largest avian predator in the forest is the Red-Tailed Hawk, 

which feeds on vertebrates and large invertebrates such as rats, 

small birds, and centipedes. The diets of other birds found at El 

Verde are shown in table 30. In Puerto Rico, Red-Tailed Hawks 

reach their highest population densities (higher than at most sites 

in the North American continent) on the eastern slopes of the 

Luquillo Mountains, where they are an important predator of 

the endangered Puerto Rican Parrot (Santana and Temple 1988). 

These hawks also prey on the introduced small Indian mongoose 

(Herpestes auropunctatus), which makes them the top predator in 

the forest.

Bats 

Compared with mainland areas of similar size and habitat 

diversity, Puerto Rico harbors few mammal species, and 

population numbers for these mammals are generally low. Bats 

compose the major portion of the Puerto Rican mammal fauna 

in terms of species richness and density (Willig and Bauman 

1984, Gannon and Willig 1994), with 13 species representing 5 

feeding guilds currently found on the island. Within the LEF, 

four species of bats dominate, although some are common in 

certain habitats and rare or absent in others. Three of these 

species, Stenoderma rufum, Artibeus jamaicensis, and Brachyphylla 

carvernarum, are primarily frugivorous (i.e., fruit eating). The 

fourth, Monophyllus redmani, feeds on flower nectar. Bats have 

been implicated as major seed dispersers of important species 

of trees in the tabonuco forest (Devoe 1990), and for some 

species, such as Manilkara bidentata, they appear to be the only 

seed dispersal agent (You 1991). In fact, S. rufum is the only 

frugivorous bat known to consume Manilkara in appreciable 

amounts, comprising approximately 23 percent of its diet 

(Gannon and Willig 1994).

Stenoderma rufum, the Red Fig-Eating Bat, has been found at 

only two localities on Puerto Rico and on the nearby islands of 

St. John and St. Thomas (Genoways and Baker 1972). Until 

recently, the Red Fig-Eating Bat was thought to be extinct and 

was known only from fossil records. As a result, this poorly 

known species is rare in scientific collections. The status of other 

Red Fig-Eating Bat populations on Puerto Rico or the Virgin 

Islands is unknown. Stenoderma rufum consumes a variety 

of fruits, exhibits asynchronous biomodal polyestry, roosts in 

canopy foliage, and has a relatively small home range (mean = 

2.1 ha) to which it exhibits high site fidelity for several months 

during the rainy season (Gannon 1991, Gannon and Willig 

1992, Gannon and Willig 1994, Willig and Bauman 1984). 

Artibeus jamaicensis, the Jamaican Fruit Bat, has a wide 

geographic distribution in tropical and sub-tropical America. 

It is a much-studied species and is known to consume a wide 

variety of fruits as well as some flowers and insects. Although 

extensive work has examined aspects of the ecology of several 

mainland Jamaican Fruit Bat populations (Morrison 1975, 

1978a, 1978b, 1979; Handley et al. 1991), little has been 

studied regarding island populations in general, or specifically 

regarding Puerto Rican populations. Previous research indicates 

A. jamaicensis comprises at least 60 percent of LEF bat fauna 

(Willig and Bauman 1984) and exhibits aseasonal polyestry 

(Willig and Bauman 1984, Gannon and Willig 1994).

Monophyllus redmani, the Greater Antillean Long-Tongued 

Bat, feeds primarily on flower nectar. It has a distributional 

range restricted to the Greater Antilles and several islands in 

the Bahamas (Homan and Jones 1975). Little is known of its 

ecology other than anecdotal observations. It is common on 

Puerto Rico, where it comprises a substantial portion of the LEF 

bat fauna (Willig and Bauman 1984, Gannon and Willig 1994).

Current distribution patterns of bats in the LEF (fig. 95) suggest 

that the tabonuco forest is the most diverse zone because it 

contains a larger number of bat species and those species have 

a relatively even distribution. Moreover, the tabonuco forest 

is unique in harboring a population of S. rufum. The palo 

colorado forest is almost as diverse in bat species as the tabonuco 

forest but does not contain S. rufum. In the palo colorado forest, 

B. cavernarum occurs at a much higher population density than 

elsewhere in the LEF. The elfin forest is the least diverse zone 

regarding bat populations; bat fauna comprises only three species 

and the nectarivorous bat, M. redmani (fig. 95) dominates bat 

distribution.

Gannon and Willig (1994) estimated the effect of recent 

hurricanes on the demographic parameters of S. rufrum. The 

proportion of captured juvenile individuals, a measure of 

recent reproductive success and an index of the potential of the 

population to increase in density in the near future, decreased 

after the passage of Hurricane Hugo. Juveniles represented 

Figure 95.—Proportional representation of species in the bat fauna within 

each of three life zones (tabonuco rain forest, palo colorado forest, and 

elfin forest [labeled as dwarf]) of the Luquillo Experimental Forest. From 

Gannon and Willig (1994).
90 Luquillo Experimental Forest: Research History and Opportunities

Taxon

Trophic 

leveld Principal foode

Foraging 

location

Order: Falconiformes

Family: Accipitridae

red-tailed hawka

 (Buteo jamaicensis) 3 rats, birds canopy

broad-winged hawk (Buteo platypterus) 2, 3 reptiles, birds, invertebrates, canopy

sharp-shinned hawk (Accipeter striatus) 3 birds canopy

Order: Charadriiformes 

Family: Scolopacidae

spotted sandpiper (Actitis mascularia) 2 crustaceans, insects streams

Order: Columbiformes

Family: Columbidae

scaly-naped pigeona

 (Columba squamosa) 1 fruit canopy

zenaida dovea (Zenaida aurita) 1 seeds litter

ruddy quail-dovea (Geotrygon montana) 1 seeds litter

Order: Psittaciformes

Family: Psittacidae

Puerto Rican parrota

 (Amazona vittata) 1 fruit canopy

Order: Cuculiformes

Family: Cuculidae

Puerto Rican lizard-cuckooa

 (Saurothera vielloti) 2, 3 lizards, spiders, Lepidoptera, insects understory

Order: Strigiformes

Family: Strigidae

Puerto Rican screech-owla

 (Otus nudipes) 2, 3 insects, lizards, birds understory

Order: Apodiformes

Family: Apodidae

black swift (Cypseloides niger) 2 flies, beteles open air

Family: Trochilidae

Puerto Rican emeralda

 (Chlorostilbon maugaeus) 1, 2 nectar, spiders, insects understory

green mangoa (Anthracothorax viridis) 1, 2 nectar, Diptera, spiders understory

Order: Coraciiformes

Family: Alcedinidae

belted kingfisher (Ceryle alcyonb

) 3 fish, shrimp streams

Family: Todidae

Puerto Rican todya

 (Todus mexicanus) 2, 3 Diptera, Coleoptera understory

Order: Piciformes

Family: Picidae

Puerto Rican woodpeckera, d (Melanerpes portoricensis) 1, 2 insects, fruits canopy

Order: Passeriformes

Family: Tyrannidae

Puerto Rican flycatchera

 (Myiarchus antillarum) 1, 2 Hemiptera, coleoptera, caterpillars, fruit canopy

loggerhead kingbird (Tyrannus caudifasciatus) 1, 2, 3 fruit, insects, reptiles, amphibians clearings

gray kingbird (Tyrannus dominicensis) 1, 2 fruit, insects clearings

Family: Muscicapidae

Table 30. Diets of birds at El Verde. From Waide (1996).
Luquillo Experimental Forest: Research History and Opportunities 91

red-legged thrusha (Turdus plumbeus) 1, 2, 3 fruit, caterpillars, lizards litter

Family: Mimidae

pearly-eyed thrashera

 (Margarops fuscatus) 1 fruit, vertebrates, invertebrates canopy

Family: Vireonidae

Puerto Rican vireoa, c (Vireo latimeri) 1, 2 Orthoptera, Homoptera, Lepidoptera, fruit canopy

black-whiskered vireod (V. alltiloquus) 1, 2 fruit, insects

Family: Emberizidae

 Sub family: Parulinae

black-and-white warblera, b (Mniotilta varia) 2 Coleoptera, Orthoptera understory

parula warblera, b (Parula Americana) 2, 3 spiders, Homoptera, Coleoptera understory

Cape May warblera, b (Dendroica tigrina) 1, 2 nectar, insects understory

black-throated blue warblera, b (D. caerulescens) 1, 2, 3 insects, fruit, spiders understory

black-throated green warblera, b (D. virens) 2 insects canopy

prairie warblera, b (D. discolor) 2, 3 fulgoroids, spiders understory

elfin woods warbler (D. angelae) 2 insects canopy

ovenbirdb (Seiurus aurocapillus) 1, 2 snails, insects, seeds litter

Louisiana waterthrusha, b (S. motacilla) 2 insects litter

American redstarta, b (Setophaga ruticilla) 2 fulgoroids, Diptera understory

 Sub family: Coerebinae

bananaquita

 (Coereba flaveola) 1, 2 nectar, caterpillars, spiders understory

 Sub family: Thraupinae

Antillean euphoniaa

 (Euphonia música) 1 mistletoe canopy

stripe-headed tanagera (Spindalis zena) 1 fruit canopy

Puerto Rican tanagera (Nesospingus speculiferus) 1, 2 caterpillers, fruit understory, canopy

 Sub family: Icterinae

black-cowled oriole (Icterus dominicensis) 2, 3 Coleoptera, spiders, earwigs canopy

 Sub family: Emberizinae

Puerto Rican bullfincha

 (Loxigilla portorricensis) 1, 2 fruit, insects understory, canopy

black-faced grassquita

 (Tiaris bicolor) 1 seeds openings

Sources: Names correspond to nomenclature used in the Checklist of American Birds (AOU 1983). Data on principal foods from Wetmore (1916) modified by results from 

Waide (unpublished), Wiley (pers. comm.), and Arendt (pers. comm.). Foraging locations are from Raffaele (1983), Wunderle et al. (1987), Waide (unpublished), Wiley 

(pers. comm.), and Arendt (pers. comm.).

a Species regularly found in El Verde.

b Breeds in North America, winters in Puerto Rico.

c Breeds in Puerto Rico, winters in South America.

d 1, 2, 3 = Primary, secondary, and tertiary consumers, respectively. Species are included in a trophic level if at least 10 percent of their diet comes from the next lowest 

trophic level.

e In order of importance. Major invertebrate groups in the diet are listed when possible.

Taxon

Trophic 

leveld Principal foode

Foraging 

location
92 Luquillo Experimental Forest: Research History and Opportunities

Figure 97.—Number of bats (per 10 net hours) in six habitat sites 

sampled in July before and after the passage of Hurricane Georges over 

Puerto Rico. Continuous lines join habitat sites. From Jones et al. (2001).

between 30 and 40 percent of the population before the 

hurricane came ashore, with a substantial decline in juvenile 

representation (to about 10 percent) after the hurricane made 

landfall (fig. 96a). Changes in female reproductive patterns 

occurred as well (fig. 96b). Before Hurricane Hugo hit, at 

least 55 percent of the adult female S. rufrum bats sampled 

were reproductively active (pregnant or lactating). After the 

hurricane passed over the forest, the proportion of females 

that were breeding declined, with relatively few individuals 

producing young. This decline persisted for at least 3 years (i.e., 

until 1992), when less than 5 percent of the females exhibited 

reproductive activity.

Jones et al. (2001) also studied bat recovery rates in a broader 

variety of habitat types across Puerto Rico after the passage 

of Hurricane Georges. At all sites sampled, bat abundance 

decreased after the hurricane (fig. 97), and some evidence 

showed that frugivorous and nectarivorous species may be more 

affected by hurricane disturbance than insectivorous species.

The Stream Community

The benthic invertebrate community in the LEF is 

overwhelmingly dominated by three genera of freshwater 

shrimp—Atya, Xiphocaris, and Macrobrachium (table 31)—and a 

relatively low abundance of benthic insects (Pringle et al. 1993). 

Ten species of omnivorous shrimp inhabit rivers of the LEF and 

represent most of the functional feeding groups found in stream 

macroinvertebrate communities. Xiphocaris is best characterized 

as a predator, shredder, and particle feeder (Crowl et al. 1996, 

2001); Atya is both a filter-feeder and a scraper-grazer (Covich 

1988); and Macrobrachium is a predator (Crowl and Covich 

1994). The palaemonid shrimp Macrobrachium carcinus (L), 

M. faustinum (De Saussure), M. crenulatum Holthuis, M. 

acanthurus (Wiegmann), and M. heterochirus (Wigmann) are 

thought to consume decomposing leaf litter, fine particulate 

Figure 96.—Histograms representing (a) the demographic structure, and 

(b) the proportion of breeding females of the population of Stenoderma 

rufum in the tabonuco forest at El Verde. An arrow represents the 

occurrence of Hurricane Hugo. From Willig and Gannon (1996). Percent Percent
Luquillo Experimental Forest: Research History and Opportunities 93

Table 31. Common aquatic fauna of the Luquillo Experimental Forest’s streams. Provided by Omar Pérez Reyes.

Species Description Picture (arrow shows detail) Species Description Picture (arrow shows detail)

Claws equal in 

shape but unequal 

in size; but both 

are narrow. Body 

likely to be heavily 

banded. Fingers 

2/3 as long as 

palm.

Lateral lobes 

of rostrum 

subacute and 

directed anteriorly. 

Periopods 

extremely robust

with tubercles.

Claws of equal 

size and shape, 

not hairy, tends 

towards dark blue 

longitudinal stripes 

down sides and 

back.

Lateral lobes of 

rostrum obtuse. 

Periopods robust 

with tubercles.

Claws unequal in 

size and shape with 

spines between 

fingers, telson 

orange, fourth 

and fifth terga 

purplish red each 

with narrow yellow 

transverse band.

Periopods with 

long hair at end of 

fingers. Rostrum 

with dorsal teeth. 

Body with vertical 

bands or median 

stripe.

Claws unequal in 

size and shape, 

palm of larger 

claw is inflated 

and claws thickly 

pubescent, fingers 

banded.

Rostrum armed 

with teeth in the 

basal part of the 

dorsal margin; 

ventral margin 

serrate. Body color 

translucent with 

internal organs 

visible.

Claws of equal size 

and shape, thin 

and narrow, claw 

thickly pubescent, 

yellow articulation of 

podomeres, dorsally 

banded.

Freshwater crab. 

Body color brown.

Absence of 

horizontal lobe 

or tooth on either 

side of the rostrum. 

Periopods without 

tubercles or 

scales, and thin.

Macrobrachium 

heterochirus

Macrobrachium 

carcinus

Macrobrachium 

crenulatum

Macrobrachium 

faustinum

Macrobrachium 

acanthurus

Atya lanipes

Atya scabra

Atya innocous

Micratya poeyi

Xiphocaris 

elongata

Epilobocera 

sinuatifrons
94 Luquillo Experimental Forest: Research History and Opportunities

organic matter, algae, macrophytes, insects, mollusks, small fish, 

and other shrimp (Covich and McDowell 1996). The atyid 

shrimp, represented by Atya lanipes Holthuis, A. scabra (Leach), 

A. innocuous (Herbst), and Micratya poeyi (Guérin Méneville), 

consume fine particulate organic matter, leaf material, algae, and 

small sessile insects by brushing with their cheliped fans (Pringle 

et al. 1993). They use the same cheliped fans to filter feed when 

flow conditions are suitable (Covich 1988). Xiphocaris elongata

(Guérin Méneville) consumes leaf matter, periphyton, fine 

particulate organic matter, insects, small flowers, and fruit, using 

tiny pincers (Covich and McDowell 1996, Pringle 1996). 

One endemic freshwater crab (Epilobocera sinuatifrons: 

Pseudothelphusidae) also inhabits LEF rivers and streams, as 

do predatory fish including the freshwater mullet (Agonostomus 

monticolus: Mugilidae), the freshwater eel (Anguilla rostrata: 

Anguillidae), and three species of omnivorous gobies (Awaous 

taiasica, Bathygobius soporator, and Gobiomorus dormitor: 

Gobiidae). At high elevations, the only fish species present is the 

algivorous goby Sycidium plumier (Gobiidae) that can negotiate 

waterfalls and rapids with its modified pelvic fins (Townsend 

et al. 2002). Two genera of freshwater snails, Niritina spp. and 

Thiara granifera (Lamarck), inhabit mid- and low-elevation sites. 

Although the stream insect community is represented by most 

major taxa (Masteller and Buzby 1993), the mayflies Baetidae 

and Leptophlebiidae comprise the majority of individuals 

(Pescador et al. 1993). Despite the presence of these other fauna, 

shrimp represent as much as 95 percent of the standing stock 

biomass of total stream organisms (Covich et al. 1991, 1996; 

Crowl and Covich 1994) and shrimp density is approximately 20 

individuals m-2 (Covich 1988).

All the LEF’s shrimp species are amphidromous—spending 

most of their life cycle in freshwater where they grow from 

postlarvae into reproductive adults (Covich and McDowell 1996, 

Johnson et al. 1998). Gravid females release planktonic larvae 

into flowing waters, where the larvae are carried downstream to 

coastal lagoons and estuaries where they continue to develop in 

brackish water. In stream reaches that contain predatory fish, 

most migratory drift by shrimp occurs at night, with a nocturnal 

peak in drift occurring from 7 to 10 p.m. Diel pattern in drift, 

however, does not occur where predatory fish are absent (March 

et al. 1998). Larvae spend 50 to 110 days in the estuary before 

migrating back upstream as metamorphosed postlarvae (Chase 

and Hobbs 1969, Hunte 1978, Benstead et al. 2000) where 

they occupy pools at different elevations and distances from 

the coast. High elevation sites are dominated by atyid shrimp 

(mostly A. lanipes) and X. elongatea, while Macrobrachium spp. 

and X. elongata comprise most of the shrimp assemblage at 

lower elevation sites (March et al. 2001, 2002). With increased 

elevation and travel distances, these migratory shrimp confront 

time and energy tradeoffs between avoiding predators and 

avoiding habitats where physical washout of food and potential 

injury during stormflows may occur (Covich et al. 1996).

Until recently, shrimp population dynamics had not been studied 

extensively within the LEF. An initial study by Covich et al. 

(1991) reported atyid shrimp populations following hurricaneassociated stormflows in LEF streams. Although declines in 

shrimp population densities were predicted, results indicated that 

neither extensive shrimp removal nor washout of organic detritus 

(food resources) was observed after the passage of Hurricane 

Hugo; overall average atyid shrimp density increased from 4.7 to 

6.5 shrimp m-2. However, the effect of the hurricane on atyid 

population density varied by the pool type in which shrimp 

lived. Pool type was classified by pool size, pool elevation, and 

pool gradient. In headwater pools, Atya densities were reduced 

by an average of 20 percent, apparently as a result of washout 

from flooding associated with the hurricane, while densities 

increased by 60 percent in the mid-elevation pools and by more 

than 100 percent in the low-elevation pools, presumably as a 

result of accumulation of displaced individuals from headwater 

reaches and minor tributaries. Large leaf piles found in debris 

dams served to slow the washout of invertebrate consumers while 

also providing more crevice structure and increasing detrital 

food supplies (Covich and Crowl 1990, O’Connor 1991). Over 

the 8 months that followed Hurricane Hugo’s landfall, debris 

dams were slowly undercut by the stream’s base flow so that 

microbially conditioned leaf detritus was carried downstream in 

a relatively continuous manner. High sunlight conditions after 

the hurricane occurred resulted in abundant growth of algae, 

so that between scraping conditioned leaf detritus and filtering 

periphyton, shrimp had a superabundance of food resources. 

After studying longer term effects of hydrological disturbance 

on shrimp populations at multiple elevations in the LEF (50 

months post-Hugo and a 12-month low-flow period), Covich 

et al. (1996) found that Atya densities had increased beyond 

pre-Hugo levels at all elevations, even in the headwater pools 

that initially experienced reductions in population density. The 

distribution of the three shrimp genera studied (Atya, Xiphocaris, 

Macrobrachium) varied both along an elevational gradient and in 

response to hydrological disturbance (fig. 98). Atya density and 

abundance along the elevational gradient generally increased in 

both post-Hugo and dry periods, while Xiphocaris densities and 

abundance increased with elevation during pre- and post-Hugo 

periods but decreased with elevation during the dry period (fig. 

98). Macrobrachium density and abundance decreased with 

elevation and were unaffected by hydrologic disturbance. Several 

potential mechanisms may explain these shrimp distributions. 

For example, because Macrobrachium is predatory, prey 

species Atya and Xiphocaris may obtain a spatial refuge from 

predation by occupying high-elevation pools (Crowl and Covich 

1994). The coexistence of Atya and Xiphocaris with predatory 

Macrobrachium in the lower elevation pools possibly occurs 

because low-elevation pools have greater depths and greater 

spatial complexity.
Luquillo Experimental Forest: Research History and Opportunities 95

In another study of Atya population dynamics, Heartsill Scalley 

et al. (2001) compared the size-class structure of shrimp in 

pools with different physical conditions along an elevation 

gradient over a 4-year sampling period in the first order tributary 

Quebrada Prieta, part of the Río Espíritu Santo watershed. The 

pools were sampled every 2 months, from January 1996 to 

December 1999. For all captured individuals carapace length 

Figure 98.—Elevational gradient and least squares analysis of log mean 

density distributions of Atya (top graph), Xiphocaris (middle graph), and 

Macrobrachium (bottom graph) for three periods: Prehurricane Hugo 

(black circle for mean densities per pool, solid line for regression); postHurricane Hugo (open circle, dashed line); and dry (open triangle, dotted 

line). From Covich et al. (1996).

was measured, and three size class categories were established. 

Although the size-class structure for juveniles was not 

significantly different among pools and did not show seasonal 

patterns, there were clear seasonal trends in the proportion of 

gravid females (fig. 99). The low abundance of juveniles in 

the population (fig. 100) is thought to be a result of low adult 

mortality, high fecundity, and a relatively long life span. Large 

recruitment events may not be important to the stability of this 

population; it seems that enough adults are always present in 

the population to produce eggs and release large numbers of 

potential new recruits.

Covich et al. (2006) extended the data set shown in figure 

98 and reported long-term trends in Macrobrachium shrimp 

populations to determine their distributions along an elevational 

gradient during a series of disturbances (Hurricane Hugo in 

1989, a drought in 1994, and Hurricane Georges in 1998) that 

occurred over a 15-year period (1988 to 2002). In general, 

Macrobrachium abundance declined with elevation during most 

years (fig. 101). The lowest mean abundance of Macrobrachium

occurred during the 1994 drought, the driest year in 28 years 

on record in the Espíritu Santo drainage. Macrobrachium

increased in abundance for 6 years following the 1994 drought. 

In contrast, hurricane and storm flow disturbances had relatively 

little effect on Macrobrachium abundance (fig. 102).

Given their high population densities, relatively high biomass 

per individual, and high mobility, shrimp play an important 

functional role in organizing stream community structure 

(Pringle et al. 1999) and detrital processing (Crowl et al. 2001, 

March et al. 2001, Wright and Covich 2005, Crowl et al. 

2006). For example, atyid shrimp reduce sediment cover and 

algal standing crop on the substratum and alter insect and algal 

community composition (Pringle et al. 1993; Pringle and Blake 

Figure 99.—The percent of gravid females in Atya shrimp populations 

within different pools of the Quebrada Prieta, a f﻿﻿irst order tributary of the 

Espíritu Santo River. From Heartsill Scalley et al. (2001).
96 Luquillo Experimental Forest: Research History and Opportunities

Figure 100.—Size class structure of Atya shrimp populations across pools 

in the Quebrada Prieta, a first order tributary of the Espíritu Santo River in 

the Luquillo Experimental Forest. Individuals smaller than 10 millimeters 

(mm) carapace length were classified as juveniles, individuals between 10 

and 19.9 mm were considered to be reproductive females, and individuals 

larger than 19.9 mm were considered males. From Heartsill Scalley et al. 

(2001).

Figure 101.—Relationship between total abundance of Macrobrachium 

(1988 to 2002) and elevation in Quebrada Prieta. The outlier point 

represents shrimp abundance in a pool that was consistently larger and 

deeper relative to other middle-elevation pools. From Covich et al. (2006).

Figure 102.—Mean (± 1 SE) total abundance of Macrobrachium in 20 

pools in Quebrada Prieta from 1988 to 2002. Years with the same letter 

are not significantly different. The arrows indicate flow-based events: 

Hurricane Hugo (1989), Hurricane Georges (1998), and the most severe 

drought in 28 years (1994). From Covich et al. (2006).

1994; Pringle 1996). Xiphocaris shred intact, large leaves and 

convert them into fine, medium, and coarse particulates, thereby 

increasing the concentration and rate of downstream transport 

of suspended particulate organic matter and increasing the 

concentration of both total dissolved N and dissolved organic 

C (Crowl et al. 2001). March and Pringle (2003) used stable 

isotope analysis to determine the relative importance of algae 

and leaves as a food source to stream consumers and found that 

shrimp in the LEF rely more on algal-based C resources than 

terrestrially derived resources at all three sites measured, although 

there were variations in dependence on algal versus leaf based 

detritus among shrimp taxa and size classes within taxa (table 

32). Insect shredders obtained more C from terrestrially derived 

detritus than from algae, collector-gathering leptophlebiid 

and baetid mayflies relied almost equally on both algal and 

leaf-derived C, and the collector-filtering caddisfly Chimarra 

albomaculata was found only at the low-elevation site and 

depended more on algal-based than leaf-based C (table 32). 

Snail species, Neritina spp. and T. granifera, were abundant only 

at the low-elevation site and relied on 69 and 76 percent algal 

resources, respectively, while the omnivorous crab Epilobocera 

sinuatifrons was found only at the high elevation site and relied 

on slightly more leaf-based than algal-based C.
Luquillo Experimental Forest: Research History and Opportunities 97

Percent algal dependence

δ13C δ15N Mean

Consumers/organic matter sources N x SD x SD (% linear model) SE (%)

High-elevation reach 9 -22.23 0.61 6.97 1.19 58 3

Machrobrachium crenulatum (28-37) 7 -19.67 0.90 6.81 0.24 79 5

Machrobrachium spp. Small (14-18) 6 -23.07 0.56 5.74 0.28 52 3

Atya lanipes large (17-19) 6 -21.52 2.02 5.29 0.49 65 8

A. lanipes medium (11-13) 8 -22.19 0.87 5.41 0.54 60 4

A. lanipes small (6-10) 3 -22.97 0.06 5.71 0.30 53 3

A. innocuous large (17-19) 3 -23.10 0.23 5.73 0.24 52 3

A. scabra (9-11) 2 -23.14 0.13 5.69 0.24 52 3

Xiphocaris elongate large (11-13) 5 -22.31 0.87 4.94 0.17 59 4

X. elongate medium (8-9) 4 -19.07 1.72 5.26 0.29 86 9

X. elongate small (6-7) 5 -17.56 1.19 6.64 0.33 99 7

Sicydium plumier 3 -17.77 2.92 6.49 0.64 97 15

Epilobocera sinuatifrons 1 -23.78 4.91 46 9

Leptophlebiidae 3 -24.25 0.72 2.51 0.20 46 4

Stream leaves 5 -30.01 0.14 0.13 0.68

Algae 5 -18.24 1.32 1.40 0.63

Biofilm 10 -22.96 1.51 5.12 1.34

Mid-elevation reach

Machrobrachium carcinus (40-51) 2 -20.13 1.03 7.31 0.37 65 14

M crenulatum (31) 1 -23.75 5.73 34 11

Machrobrachium spp. small (13-16) 5 -19.44 0.81 7.11 0.11 71 14

Xiphocaris elongate medium (9-10) 5 -15.43 0.53 6.06 0.31 106 21

X. elongate small (6) 5 -17.60 1.02 7.84 0.17 88 18

Agonostomus monticola 5 -18.92 0.88 8.93 0.56 75 15

Anguila rostrata 1 -19.41 8.68 71 16

Gobiomorus dormitor 1 -17.56 8.02 87 19

Sicydium plumier 4 -16.69 1.70 7.57 1.77 96 20

Leptophlebiidae, Baetidae 2 -22.11 0.69 4.35 0.57 53 11

Phanocerus spp. 2 -25.88 0.04 1.28 0.03 21 4

Stream Leaves 5 -28.79 0.40 0.62 0.12

Algae 2 -17.03 2.16 2.16 0.30

Biofilm 11 -20.15 3.38 3.38 0.43

Low-elevation reach

Machrobrachium acanthurus (27) 1 -19.16 9.44 82 25

M. heterochirus (not measured) 1 -21.53 7.03 59 21

Machrobrachium spp. medium (10-11) 5 -20.99 3.16 9.88 2.35 71 17

Machrobrachium small (3-6) 5 -20.27 1.30 9.34 0.65 64 20

Xiphocaris elongata medium (8-10) 5 -19.85 2.20 8.11 0.32 78 18

X. elongata small (5) 4 -19.80 0.90 7.42 0.12 78 20

Eleotris pisonis 1 -20.46 9.53 70 23

Gobiomorus dormitor 5 -19.28 0.42 9.03 0.84 81 18

Sicydium plumieri 1 -21.55 8.72 61 21

Neritina spp. 5 -21.48 2.94 8.55 0.35 62 19

Thiara granifera 5 -20.84 0.92 7.29 0.19 68 16

Leptophlebiidae, Baetidae 1 -23.30 6.13 48 20

Chimarra albomaculata 1 -21.78 6.42 63 22

Phanocerus spp. 1 -25.30 3.66 28 18

Stream Leaves 4 -28.55 0.29 1.47 0.75

Algae 1 -18.36 5.96

Biofilm 1 -24.11 6.42

Macrophytes 4 -32.79 0.55 7.99 0.89

Table 32. Index of algal reliance for consumers at each reach measured in the Luquillo Experimental Forest. The range of carapace length for 

shrimps is in parentheses. The index of algal reliance was based on carbon and nitrogen isotope values. From March and Pringle (2003). 
98 Luquillo Experimental Forest: Research History and Opportunities

Carbon Cycling in the Luquillo Experimental 

Forest

Carbon (C) in forests resides in various pools: aboveground 

biomass, belowground biomass, litter, deadwood, and soil. 

At the most fundamental level, C fluxes through a forest 

ecosystem include CO2 uptake by leaves through the process of 

photosynthesis and CO2 release to the atmosphere through the 

respiration of leaves, stems, roots (autotrophs), and heterotrophs 

(soil microorganisms and forest fauna). Net primary 

productivity (NPP) is the difference between total photosynthesis 

(gross primary productivity—GPP) and total plant respiration 

in an ecosystem. Due to past difficulties quantifying the gas 

exchange through an ecosystem, NPP has also been measured 

in the field as the total new organic matter produced during a 

specified interval (Clark et al. 2001, fig. 103).

Several studies of NPP—and individual components of NPP 

such as litterfall and herbivory rates—have been conducted in 

the Luquillo Experimental Forest (LEF). Early efforts focused 

on measuring the aboveground biomass increment of forests 

at various elevations, while later research focused on long-term 

rates of leaf litterfall and decomposition. Technological advances 

such as the use of infrared gas analyzers and computer models 

have shaped recent studies of NPP, resulting in a more holistic 

approach in quantifying primary productivity (Wang et al. 2002, 

14. Ecosystem Processes

A.E. Lugo and N.L. Harris

2003; Harris 2006; Harris et al. 2008; Harris et al., in press). In 

the following sections, note that forest biomass is approximately 

50 percent C.

Biomass Studies

Brown et al. (1983) reported a range of biomass measured for 

mature forests in the LEF from about 100 Mg ha-1 (metric 

tons per hectare) in the elfin forest to about 400 Mg ha-1 in the 

tabonuco forest. Later, Lugo and Scatena (1995) observed that 

biomass measurements vary within the tabonuco forest, such 

that the same biomass range observed from the bottom to the 

top of the mountains (400 to 100 Mg ha-1) existed within El 

Verde sector of the tabonuco forest. Brown et al. (1983) also 

reported data on biomass distribution within tabonuco and elfin 

forests, and some preliminary information on animal biomass 

and temporal changes in aboveground biomass. Since that data 

synthesis, significant progress has been made on the following 

aspects, which we summarize below: biomass along the elevation 

gradient, biomass distribution with topography, biomass levels 

and distribution in other forest types, biomass distribution by 

tree species, and biomass accumulation after disturbance events. 

This section is illustrative rather than exhaustive of all the 

biomass data for LEF forests.

Biomass Along the Elevation Gradient

Recent data along the elevation gradient do not change the 

range reported by Brown et al. (1983) but they provide 

information about forest types at mid-elevations. As discussed 

in the vegetation section, however, data collection at the LEF 

has used the forest type designation, as a surrogate for elevation 

and it is possible that the variability within each forest type 

masks elevation gradients found in biomass. For example, the 

total biomass for mature stands of the four forest types shows 

that the tabonuco forest at about 400 meters (m) elevation, the 

palm floodplain forest at about 800 m elevation and the palo 

colorado forest above 600 m elevation have higher values than 

the elfin forest at about 1,000 m elevation (fig. 104). These 

data do not reflect a smooth gradient with elevation but suggest 

that more data need to be systematically collected to see if 

there are gradients or, alternatively, if biomass peaks at low and 

intermediate elevations as reflected through other structural 

parameters observed in the LEF (Brown et al. 1983).

Biomass Distribution With Topography

Within the Bisley Experimental Watersheds of the LEF, 

researchers observed that mean aboveground biomass changes 

with topographic position such that biomass is highest on ridges, 

intermediate on slopes, and lowest on upland and riparian valleys 

(fig. 105); however, only the mean aboveground biomass on 

ridges is statistically different from the aboveground biomass 

on the other topographic positions. As already observed by 

Lugo and Scatena (1995), aboveground biomass is spatially 

variable in the LEF, with the highest variability observed on 

ridges and in riparian valleys. Scatena and Lugo (1995) found 

Figure 103.—Components of net primary productivity. 

From Clark et al. (2001).
Luquillo Experimental Forest: Research History and Opportunities 99

that other structural parameters of forests and forest species 

richness varied with topographic position. Weaver (1995) found 

similar topographic effects on structural parameters in both palo 

colorado and elfin forests. He also reported that aboveground 

biomass of elfin forests ranged from 48 Mg ha-1 on exposed 

ridges to 110 Mg ha-1 on the leeward side of the summit. These 

and other data led to the conclusion that topographic position 

is a strong determinant of forest structure and composition in 

Puerto Rican forests (Lugo 2005).

Biomass Distribution Among and Within Forests

Age and land use also influence biomass accumulation in 

forests of the LEF. Aboveground biomass increases with age in 

secondary forests and plantations, which collectively have lower 

aboveground biomass than mature forests, with the exception 

being elfin forests. Plantation forests had higher aboveground 

biomass than secondary forests of a similar age.

Biomass distribution in native LEF forests is characterized by 

a high proportion of root biomass versus a low proportion of 

root biomass in plantations (fig. 106). Plantation forests had a 

higher proportion of their biomass in wood than native forests 

had; however, the elfin forest, with a high proportion of woody 

biomass, was an exception. Pine plantations had a higher 

proportion of biomass in leaves and the highest proportional 

biomass in litter when compared with native forests.

Animals, microbes, bryophytes, and epiphytes constitute a small 

fraction of total biomass found in the LEF, except in the elfin 

forest where epiphytes can exceed 5 percent of the total biomass. 

The range of biomass in organisms other than trees at the LEF 

extends six orders of magnitude (fig. 107). The highest biomass 

values are those of epiphytes at high elevation forests, followed by 

earthworms that represent the highest biomass values for animal 

species, and fungi and bacteria found in the top 10 centimeters 

(cm) of soil. On the lower extreme of biomass value are spiders, 

with the lowest estimated biomass in El Verde tabonuco forest. 

Birds and frogs share similar biomass values. Lugo and Scatena 

(1992, 1995), Frangi and Lugo (1992), Reagan and Waide 

(1996), and Beard et al. (2002, 2003) showed that although 

Figure 104.—Total biomass (aboveground and roots) for different forests 

in the Luquillo Experimental Forest. Data are from Weaver and Murphy 

(1990) for the bottom four mature forests and from Lugo (1992) for 

paired plantations and secondary forests. The age of secondary forests 

is approximately similar to that of its paired plantation. The paired 

plantation-secondary forests are presented in the same order from oldest 

to youngest.

Figure 105.—Aboveground biomass for forest stands on different 

topographic positions in the Bisley Experimental Watersheds, Luquillo 

Experimental Forest. Data are from Scatena and Lugo (1995) and include 

mean and standard error. The number of sample plots was 22, 40, 12, 

and 9 for ridge, slope, upland valley, and riparian valley, respectively.

Figure 106.—Distribution of total biomass in several forest stands in the 

Luquillo Experimental Forest. The percentages are based on total stand 

data in figure 104.
100 Luquillo Experimental Forest: Research History and Opportunities

these organisms constitute a relatively low proportion of total 

biomass, they play disproportionate and critical roles in nutrient 

and C cycling of forests at all elevations.

Biomass of Tree Species in Relation to Importance Value

The Importance Value (IV) of a tree species is the combination 

of its basal area, tree density, and frequency in relation to other 

species in a community. It is assumed that the IV of a species is 

proportional to its functional importance in terms of uptake and 

storage of biomass and nutrients. For example, Heartsill Scalley 

et al. (2010) found that after Hurricane Hugo occurred, the 

high IV of pioneer species was reflected in an increment of the 

cycling and accumulation of Mg (magnesium) in forest stands, 

due to the high proportion of Mg-rich leaves of pioneer species. 

Scatena and Lugo (1995) found that the relative biomass of a 

tree species in the Bisley Experimental Watersheds was always 

higher than its IV and generally declined as the IV declined (fig. 

108). Some species, however, had relative biomass values that 

were higher than expected based on their IV in the community 

(species 4, 10, 12, and 15 in fig. 108). This disproportion is 

probably related to larger diameter trees than expected found 

in the population, as biomass accumulation is calculated as a 

cubed power of the tree diameter. If this observation is true, the 

functional importance of an arboreal species, while proportional 

to its IV, appears to be influenced by the size of its trees, which 

in turn might be a function of the quality of the site those trees 

occupy or the age of the trees.

Biomass After Disturbance

Figure 109 contains four temporal aboveground biomass records 

from the LEF following three types of disturbance events: 

hurricanes, radiation, and land degradation. All these records 

are from low-elevation forests (below 400 m above sea level). 

The steeper rates of aboveground biomass accumulation are 

associated with mature vegetation during the first two decades 

after a hurricane occurred. In comparison, gamma radiation 

exposure decreased both the level and rate of aboveground 

biomass accumulation in a mature tabonuco forest. The slowest 

rate of biomass accumulation was observed after deforestation, 

agricultural activity, and land abandonment, interpreted as 

Figure 107.—Levels of dry weight biomass of spiders, birds, all animals, 

fungi, earthworms, and epiphytes in the Luquillo Experimental Forest 

(LEF). Data and the original sources are found in Moore and Burns 

(1970), Brown et al. (1983), Frangi and Lugo (1992), Lugo and Scatena 

(1992), and Reagan and Waide (1996). The epiphyte data are for high 

elevation forests and all the rest are for the El Verde sector of the Luquillo 

Experimental Forest.

Figure 108.—Importance Value (open circles) and relative aboveground 

biomass of tree species in the Bisley Experimental Watersheds of the 

Luquillo Experimental Forest. From Scatena and Lugo (1995).

Figure 109.—Temporal trends of aboveground biomass after disturbance 

events at low elevations of the Luquillo Experimental Forest. Data are from 

Drew et al. (2009) and Scatena et al. (1996) for hurricane responses at El 

Verde-3 (EV-3) and Bisley, respectively; Aide et al. (1995) for degradation; 

and Taylor et al. (1995) for radiation disturbances.
Luquillo Experimental Forest: Research History and Opportunities 101

land degradation. This chronic disturbance event reduced 

woody aboveground biomass to zero for almost a decade after 

abandonment, and after 60 years of growth, aboveground 

biomass was still about half way from that of mature native 

forests.

Litter Studies

Leaf litterfall has been studied extensively in the tabonuco and 

elfin forests of the LEF (at El Verde and Pico del Este), initially 

as part of the Minority Research Centers of Excellence (MRCE) 

experiment and then monitored continuously as part of the 

National Science Foundation’s Long Term Ecological Research 

data collection starting in 1998. The MRCE decomposition 

experiment was designed to examine short-term disappearance 

of foliage from three important species: Prestoea montana, 

Dacryodes excelsa, and Cyrilla racemiflora. Hurricane Hugo 

provided a unique opportunity to study effects of this type of 

natural disturbance on decomposition rates. The MRCE study 

was repeated in 1990 with some design modifications to allow 

for comparisons between short-term foliar litter biomass and N 

(nitrogen) dynamics.

Production of new leaves in the tree species D. excelsa, M. 

bidentata, G. guidonia, Palicourea riparia, S. berteriana, 

Tabebuia heterophylla, Casearia arborea, and Piper glabrescens 

was not correlated to rainfall during a 2-year study by Angulo 

Sandoval and Aide (2000). They followed leaf production 

in 20 individuals of 8 species (fig. 110) and found a positive 

relationship between leaf production and the photosynthetic 

photon flux density of the previous 2 months. In addition, they 

observed that more than 30 percent of annual leaf production 

during the study period occurred between May and June (fig. 

111). Although synchronous leaf production was observed in 

this forest type, differences in herbivory rates were not observed 

when comparing peak production periods with the rest of 

the year. In terms of leaffall patterns, Zalamea and González 

(2008) found two community-level peaks of leaf litter fall, one 

from April to June and another from August to September (fig. 

112). These peaks occurred in the LEF during periods with 

higher solar radiation levels. Patterns of leaffall were correlated 

with various factors, such as solar radiation, photon flux, day 

length, and temperature. Of the 32 studied species, 16 had a 

unimodal phenological pattern. Two dominant species in the 

tabonuco forest, D. excelsa and G. guidonia, along with Matayba 

dominguensis, were observed to have bimodal leaf-fall patterns, 

while the tree Drypetes glauca and the vine R. surinamensis had 

multimodal patterns (fig. 113). These findings highlight the 

need for continued research on individual species phenology 

patterns. Further, Zalamea and González (2008) demonstrate 

that leaf litter is a spatially and temporally heterogeneous 

resource whose species composition changes throughout the year, 

and thus changes in its quality and quantity can have potentially 

localized effects on organic matter processing and nutrient 

cycling. Using a 10-year data set patterns were established for 

Figure 110.— Annual leaf production for eight common species in the 

Luquillo Experimental Forest from year 1 (November 1994 to October 

1995) and year 2 (November 1995 to October 1996). In year 1, only 2 of 

20 individuals of Tabebuia heterophylla produced leaves. From Angulo 

Sandoval and Aide (2000). 

Figure 111.—Annual leaf production in the Luquillo Experimental Forest. 

Data were collected between November 1994 and October 1996. Error 

bars represent standard deviation. From Angulo Sandoval and Aide 

(2000).
102 Luquillo Experimental Forest: Research History and Opportunities

peak flowering and fruiting (fig. 114) that also correlated with 

monthly mean irradiance (Zimmerman et al. 2007); however, 

fruit development times varied during the year. Species in which 

fruits established in sunnier months took an average of 3 months 

to develop, while those species that fruited during October and 

February took nearly 6 months to develop.

Leaf litter decomposition has also been a long-term focus of 

research in the LEF. Rates of leaf litter decomposition depend on 

the particular interactions between vegetation and decomposer 

food webs. Prather (unpublished data) recently compiled and 

standardized leaf litter decay constants, which are summarized in 

table 33.

Zou et al. (1995) compared litterfall and litter decomposition 

rates between a mid-successional forest and an adjacent mature 

forest in the LEF and found that basal area, litterfall, and leaf 

decomposition were not significantly different, even though 

tree species composition in the mid-successional forest had not 

yet returned to that of mature forest. These results suggest that 

ecosystem function in secondary forests recovers more quickly 

than ecosystem structure. 

Several models assume that climate and environmental 

factors are the primary controllers of leaf litter decomposition 

(Meentemeyer 1977, 1978), and similar patterns may or 

may not occur with root decomposition, a process which is 

somewhat buffered from extremes in climate by its location in 

the soil. Silver and Miya (2001) reviewed global patterns of root 

decomposition and found that the parameters that explained the 

largest amount of variability in root decay were root Ca (calcium) 

concentrations and root C to N ratios, with a smaller proportion 

explained by latitude, mean annual temperature, mean annual 

precipitation and actual evapotranspiration. Bloomfield et 

al. (1993) studied decomposition rates of both fine roots and 

foliage in the LEF and found that at all locations studied in the 

tabonuco forest, fine roots decayed significantly more slowly than 

foliage, most likely due to higher levels of nondigestible cell wall 

fiber and lignin (table 34).

Dead Wood 

Dead wood as a C pool has not been studied as extensively 

as other pools in the LEF. Under nonhurricane conditions, 

Odum (1970b) found that the decomposition rate of about 10 

percent per year for coarse woody debris in tabonuco forests 

balances with rates of woodfall. In general, wood decomposition 

in the LEF is rapid, with most—but not all—woody debris 

produced by a hurricane decomposing before the forest reaches 

prehurricane biomass (Frangi and Lugo 1998, Beard et al. 

2005). Coarse wood (>10 cm in diameter) of species such as 

Manilkara bidentata, Dacryodes excelsa, and Cyrilla racemiflora

decompose at rates that require more than 20 years to complete 

the process (Odum 1970b, Torres 1994, Beard et al. 2005, 

Torres and González 2005, Zalamea et al. 2007). This rate 

of decomposition is due to wood characteristics and to the 

reduction of decomposition rates as decomposers encounter 

the heartwood. For example, Zalamea et al. (2007) found that 

the decomposition rate of D. excelsa declines from 3.2 percent 

yr-1 the first 6 years after the hurricane occurred to 1.3 percent 

yr-1 after 15 years. Torres (1994) and Torres and González 

(2005) observed that the initial decomposition of coarse wood 

of C. racemiflora is due to the loss of sapwood and that, after 

15 years, the heartwood of this species was almost intact. The 

decomposition process can also be retarded by water saturation 

of partially decomposed logs, which reduces the supply of oxygen 

to decomposers (Torres 1994). Even if constant decomposition 

rates were applied to M. bidentata, using the initial average rates 

of 4 percent yr-1 (Odum 1970a, Beard et al. 2005), it would 

still require more than 20 years to decompose all the material. 

Moreover, Zalamea et al. (2007) found that, as the wood 

decomposes, soil organic matter increases, thus contributing to a 

long-term C sink. 

Figure 112.—Monthly rates of total litterfall (dashed line), leaffall 

(solid line), reproductive parts (crosses), wood (closed circles), and 

miscellaneous components (open circles). From Zalamea and González 

(2008).

Figure 113.—Leaffall pattern for bimodal species; represented as 

percentage of the annual leaffall for each species. The genera are 

Dacryodes (D), Guarea (G), and Matayba (M). From Zalamea and González 

(2008).
Luquillo Experimental Forest: Research History and Opportunities 103

Figure 114.—Patterns of peak flowering and fruiting in the Luquillo Experimental Forest. From Zimmerman et al. (2007).
104 Luquillo Experimental Forest: Research History and Opportunities

Soil Carbon

Soil C stocks were sampled throughout the LEF by Wang et al. 

(2002) and used to validate a spatially explicit version of the 

CENTURY soil model applied to the LEF landscape (Parton et 

al. 1987, 1988, 1989). The model simulated spatial patterns of 

storage and flux of soil organic carbon (SOC) stocks in the top 

30 cm of soil (0 to 30 cm depth) over 100 years as functions 

of climate, vegetation, and soil type. The model was driven by 

monthly estimates of average air temperature, precipitation, 

and potential evapotranspiration (see the previous section on 

climate models) and estimates of GPP and the distribution of 

aboveground and belowground biomass production. Simulated 

storage of SOC in the LEF is highly variable, ranging from 

approximately 20 to 230 Mg ha-1 and varies with elevation 

Decay constant

(k)

(years-1) r2 n

Leaves

 P. montana 1.6 0.92 58

 D. excelsa 1.3 0.91 58

Roots

 P. montana 0.60 0.85 56

 D. excelsa 0.83 0.85 57

Table 34. Decay constant (k) values for a single exponential model. 

In (X) = -kt, where X = Xt/Xo, X = weight remaining and 

t = time in years for leaf and root tissues of Prestoea montana

and Dacroydes excelsa. From Bloomfield et al. 1993.

r

2 = Coefficient of determination.

n = Number of observations.

Table 33. Leaf litter decay constants (k) for various species of leaves in the LEF. Compiled by C. Prather.

Species Family

Litter

Type Life form

Monthly

k value

Annual 

K value Reference

Cecropia schreberiana Cecropiaceae Brown Tree -0.122 -1.465 González and Seastedt, 2001

Cecropia schreberiana Cecropiaceae Brown Tree -0.098 -1.181 Harris, unpublished

Cecropia schreberiana Cecropiaceae Brown Tree -0.070 -0.838 Sheils and Yang, unpublished

Cecropia schreberiana Cecropiaceae Brown Tree -0.057 -0.680 Sheils, 2006

Cecropia schreberiana Cecropiaceae Brown Tree -0.057 -0.680 La Caro, 1985

Cyathea arborea Cyatheaceae Brown Tree -0.078 -0.930 Sheils, 2006

Cyrilla racemiflora Cyrillaceae Brown Tree -0.071 -0.850 Sullivan et al., 1999

Dacryodes excelsa Burseraceae Brown Tree -0.164 -1.963 Sullivan et al., 1999

Dacryodes excelsa Burseraceae Brown Tree -0.108 -1.300 Bloomfield and Vogt, 1993

Dacryodes excelsa Burseraceae Brown Tree -0.104 -1.253 Harris, unpublished

Dacryodes excelsa Burseraceae Brown Tree -0.096 -1.150 La Caro, 1985

Dacryodes excelsa Burseraceae Green Tree -0.296 -3.550 Fonte and Schowalter, 2004

Drypetes glauca Euphobiaceae Brown Tree -0.128 -1.540 La Caro, 195

Guarea guidonia Meliaceae Brown Tree -0.097 -1.160 Fonte and Schowalter, 2006

Guarea guidonia Meliaceae Green Tree -0.225 -2.700 Fonte and Schowalter, 2006

Inga vera Fabaceae Brown Tree -0.053 -0.640 La Caro, 1985

Manilkara bidentata Sapotaceae Brown Tree -0.099 -1.190 Fonte and Schowalter, 2004

Manilkara bidentata Sapotaceae Brown Tree -0.063 -0.756 Harris, unpublished

Manilkara bidentata Sapotaceae Green Tree -0.171 -2.050 Fonte and Schowalter, 2004

Microphyllis garciniifolia Sapotaceae Brown Tree -0.043 -0.511 Harris, unpublished

Prestoea montana Arecaceae Brown Palm -0.123 -1.479 Sheils and Yang, unpublished

Prestoea montana Arecaceae Brown Palm -0.082 -0.986 Harris, unpublished

Prestoea montana Arecaceae Green Palm -0.133 -1.600 Bloomfield et al., 1993

Sloanea berteriana Elaeocarpaceae Brown Tree -0.083 -0.900 La Caro, 1985
Luquillo Experimental Forest: Research History and Opportunities 105

(fig. 115). This variability in SOC reflects the heterogeneity 

in vegetation, soil, and climate resulting from the complex 

topography of the Luquillo Mountains. For example, rainfall, 

relative humidity, soil moisture, and wind velocity increase 

with elevation, whereas air temperature, evapotranspiration, 

and solar insolation decrease with elevation. The interaction of 

these climatic factors with vegetation and soils determines the 

distribution of SOC across the gradients of physical conditions 

within the Luquillo landscape. Furthermore, Zalamea et al. 

(2007) showed that decaying wood can also contribute to 

variability in soil organic matter (SOM) dynamics by changing 

the relative quantity and quality of different SOM fractions 

depending on the species and the stage of decomposition. On 

the other hand, Vogt et al. (1996) reviewed global patterns of 

SOM accumulation and found that the chemistry of the soil 

(i.e., soil order) was more important in determining SOM 

accumulation than climate. Soil orders that were high in 

aluminum, iron, and clay (e.g., Ultisols, Oxisols) had high 

total living and dead organic matter accumulations, especially 

in the tropics. Although continuous fertilization over 7 years 

in the tabonuco forest of the LEF resulted in increased rates of 

both litterfall and leaf litter decomposition, fertilization did not 

affect the SOC pool (Li et al. 2006a). Results suggest, however, 

that fertilization may enhance long-term C sequestration by 

enhancing the proportion of the heavy-fraction organic C pool.

In general, soil respiration rates in the LEF decrease with 

elevation in response to cooler temperatures and more saturated 

soils (McGroddy and Silver 2000; Harris 2006; Harris et al., 

in press). The proportion of soil respiration attributable to 

autotrophic respiration (RA) versus heterotrophic respiration, 

however, has not been well studied in wet tropical forests. In 

the LEF, Li et al. (2006b) used root exclusion experiments to 

separate root respiration from heterotrophic respiration in a 

pine plantation and adjacent secondary forest. The results 

suggest that the proportion of total soil respiration from root 

respiration is higher in secondary forest than in pine plantation 

(69 percent versus 56 percent, respectively). These percentages 

are generally higher than those reported in broad-leaved forests 

in the same temperate zone and much higher than those 

percentages found in a similar study conducted by Silver et al. 

(2005) in tropical lowland forest soils in the Amazon, where the 

proportion of root respiration to soil respiration varied from 29 

to 35 percent. Nakane et al. (1996) suggest that the proportion 

of root respiration to soil surface CO2

 efflux may converge to 

a proportion of about 50 percent irrespective of forest types, 

when the cycle of soil C is near a dynamic equilibrium in a 

forest ecosystem. The fact that root contribution to total soil 

respiration in the LEF was higher than average values might be 

explained by analyzing the aggrading stage of both the plantation 

and the secondary forest.

Figure 115.—Simulated distribution of soil organic carbon storage (0–30 cm depth, (unit: Mg ha-1) in the Luquillo Experimental Forest, Puerto Rico. From 

Wang et al. (2002).
106 Luquillo Experimental Forest: Research History and Opportunities

Primary Productivity

After the pioneer work of Odum (1970a), 

primary productivity (C fluxes) as an 

ecosystem process has been studied in the 

LEF by several authors (Weaver 1986a; 

Weaver and Murphy 1990; Scatena et al. 

1996; Frangi and Lugo 1998; Lugo 1992b, 

Wang et al. 2003; Harris 2006; Harris et al. 

2008; Harris et al., in review) using different 

methods. Frangi and Lugo (1998) measured 

aboveground NPP (ANPP) in palm forests 

before and after the passage of Hurricanes 

Hugo and Georges and found the ANPP 

increased from 1.5 Mg C ha-1 (metric tons of 

C per hectare) before the hurricane passed to 

4.6 Mg C ha-1 after the hurricane occurred. 

Scatena et al. (1996) report ANPP values as 

high as 20 Mg C ha-1 yr-1 in tabonuco forest 

with a 5-year mean of 10.8 Mg C ha-1 yr-1. 

Wang et al. (2003) simulated NPP across the 

whole LEF using the FOREST-BGC model 

(Running and Coughlan 1988, Running 

and Gower 1991). Simulated values ranged 

from 0.5 to 24 Mg C ha-1 yr-1, with a mean 

of 9.4 Mg C ha-1 yr-1 (Wang et al. 2003), 

but very few empirical data were available to 

validate the model. Harris et al. (in review) 

measured the photosynthesis and respiration 

of trees and soil respiration from the lowlands 

to the top of the Luquillo Mountains and 

scaled measurements to the landscape level 

using empirically based modeling techniques. 

Forest GPP in the LEF ranged from 38 Mg C 

ha-1 yr-1 at low elevations to 11 Mg C ha-1 yr-1

at high elevations, with a forestwide mean of 

29 Mg C ha-1 yr-1 (fig. 116a). Total RA also 

decreased along the elevation gradient, from 

27 Mg C ha-1 yr-1 at the lowest elevations 

to 4 Mg C ha-1 yr-1 at the highest elevations 

(fig. 116b), and varied slightly over the year 

in response to changing temperatures. Soil 

respiration was the highest component of 

total respiration per unit ground area, and 

woody stem respiration (Harris et al. 2008) 

was typically the lowest component. Annual 

NPP in the LEF ranged from 8 Mg C ha-1

yr-1 at the lowest elevations to a maximum 

value of 24 Mg C ha-1 yr-1 at the intermediate 

elevation of 450 m to 6.5 Mg C ha-1 yr-1 at the 

highest elevations (fig. 116c). Results from 

all data collected on NPP suggest that forests 

have the capacity to accumulate biomass 

after disturbances such as hurricanes at rates 

sufficient to overcome soil respiration.

Figure 116.—Annual (a) gross primary production, (b) autotrophic respiration, and (c) net 

primary productivity (metric tons of carbon per hectare per year) for the Luquillo Experimental 

Forest, Puerto Rico, extrapolated from field measurements. The green areas on each map 

reflect the four peaks of this mountain system. From Harris et al. (in press).

116a.

116b.

116c.
Luquillo Experimental Forest: Research History and Opportunities 107

I

n general, studies of nutrient cycling in tropical forests have 

shifted and broadened over the past few decades. Early 

observations made in the 1930s (as summarized by Proctor 1983 

and Whitmore 1989) suggested that most nutrients in tropical 

forests were held in aboveground biomass and that nutrient 

cycles were particularly “tight” with efficient recycling from fallen 

litter to growing vegetation (McDowell 2001). More recent 

literature, however, emphasizes that for most elements and at 

most sites, soil is a larger nutrient pool than the aboveground 

biomass (e.g., Edwards and Grubb 1982; Jordan et al. 1983; 

Proctor 1983, 1987; Singh 1989; Scatena et al. 1993; Silver et 

al. 1994). Brown et al. (1983) summarized available data on 

nutrient cycling for the Luquillo Experimental Forest (LEF) up 

to the end of the 1970s, and these data were focused mainly on 

the tabonuco forest at El Verde, with some additional work in 

the elfin forest. The establishment of a chemistry laboratory at 

the International Institute of Tropical Forestry in 1981 (Sánchez 

et al. 1997), coupled with ongoing research at National Science 

Foundation’s Long Term Ecological Research and Forest Service, 

has resulted in a significant expansion of nutrient cycling 

research at the LEF. Biogeochemical research has focused on key 

questions and concepts related to nutrient limitation, ecosystem 

nutrient retention, and controls of nutrient transformations. 

This section highlights some advances made in recent research 

with the objective of illustrating the types of studies conducted. 

The review of data is illustrative rather than exhaustive and, 

although most previous studies included multiple element 

data, this review focuses primarily on N (nitrogen) and P 

(phosphorous) data.

Nutrient Pools in Soils and Plants

Assessing the paradigm of tight nutrient cycling in the LEF has 

been problematic, as small losses of N in streams draining the 

LEF suggest very efficient recycling and conservation of N, and 

only a very small fraction of the N in soil or biota is exported 

in streamwater. On the other hand, high concentrations 

of N in deep soil water suggest that the forest floor is not 

particularly effective at retaining N and that low N losses at 

the watershed level are achieved through mechanisms other 

than the tight recycling from decomposing litter to the root 

mat, originally thought to be common in humid tropical 

forests (McDowell 2001). The relative distribution of nutrients 

between live vegetation and soil in the LEF is depicted in table 

35. For the LEF, soil nutrient storage is consistently greater 

than aboveground storage in live vegetation. For N and Mg 

(magnesium) the ratio is well above 5 times, while for P and K 

(K) the ratio is closer to 1 and in some locations such as ridges, 

where leaching might be high, the ratio is below 1 for K. 

15. Nutrient Cycling in the Luquillo Experimental Forest

A.E. Lugo, N.L. Harris and W.L. Silver

One potential mechanism for low N losses in upland tropical 

forest soils such as those found in the LEF is a novel reductive 

pathway for N cycling, called dissimilatory NO3

– (nitrate) 

reduction to NH4

+ (ammonium) (DNRA) (Silver et al. 2001). 

This pathway decreases the amount of NO3

–

 susceptible to 

leaching and denitrification, thus conserving N in the ecosystem. 

Rates of DNRA in upland humid tropical forest soils were 

three times higher than the combined N2O and N2 fluxes from 

nitrification and denitrification (DNRA averaged approximately 

0.6 mg/g.day) and accounted for 75 percent of the turnover of 

the NO3

–

 pool. Tests of sterile soils further indicated that the 

dominant reductive pathway for both NO2 and NO3

–

 was biotic 

and not abiotic, and that the availability of NO3

–

, as opposed to 

C (carbon) or O2, apparently limited the process. Because NH4

+

is the form of N generally favored for assimilation by plants and 

microbes, and NO3

–

 is easily lost from the ecosystem, this rapid 

and direct transformation of NO3

–

 to NH4

+

 via DNRA has the 

potential to play an important role in ecosystem N conservation 

and the reduction of N2O losses (Silver et al. 2001, Templer et al. 

2008).

The range and distribution of nutrient pools in the LEF are 

demonstrated by comparing plantations and secondary forests 

of similar age. Plantation forests store more total N (fig. 117a) 

and P (fig. 117b) than do secondary forests of similar age (Lugo 

1992b), but secondary forests had a higher proportion of their 

Stand Location N P K Ca Mg

Ridge 8.0 1.0 0.5 1.3 5.0

Slope 11.5 1.3 1.0 3.8 10.7

Valley 13.7 2.2 1.3 9.5 11.7

Riparian Valley 12.0 3.4 2.1 11.0 14.3

El Verde mahogany 5.5 38.0 44.0

El Verde old 

secondary forest

18.5 76.0 57.0

El Verde mature 9.9

Palm forest 7.6

Table 35. Ratio of belowground to aboveground nutrient storage 

in various forest stands of the Luquillo Experimental Forest. Data 

for different topographic positions correspond to tabonuco forest 

in the Bisley sector where soils were sampled to 60 cm depth 

(Silver et al. 1994). Nutrients in litter were included with soil. Soil 

data to 1 m depth are from Lugo (1992) for the El Verde sector 

data and Frangi and Lugo (1985) for the palm forest. All nutrient 

data correspond to total extractions. Silver et al. (1994) contains 

similar information for extractable nutrients. Empty cells indicate 

no data.
108 Luquillo Experimental Forest: Research History and Opportunities

N and P stored in roots (fig. 118a,b) than did plantation forests 

of similar age (fig. 119a,b). Plantation forests stored most of 

their N and P in woody parts, and pine plantations had a higher 

proportion of their N and P stored in leaves than did secondary 

forests and mahogany plantations (fig. 119a,b).

Ridge vegetation also contains higher nutrient levels than 

vegetation found at other topographic positions. Silver et al. 

(1994) found that soil nutrient storage was generally linearly 

correlated to soil organic matter (SOM), although the level 

of correlation changed with topographic position. They 

compared soils in the Bisley Experimental Watersheds with other 

tropical montane forests and found that for Ca (calcium), Mg 

(magnesium), and K the soil storage was within the range for 

other forests, while P tended to be higher and N tended to be 

lower than in other tropical montane forest soils.

The importance of understory palms, bark, and branches as sinks 

of N and P after the passage of Hurricane Hugo is illustrated 

by the distribution of N and P in aboveground biomass of a 

tabonuco forest at Bisley (fig. 120). Bacteria and fungi are 

also important N and P sinks in the tabonuco forest, as are 

bryophytes in palm forests (fig. 121a,b). Animals, saplings, 

and herbaceous plants are minor sinks of N and P in tabonuco 

forests.

Figure 117.—Total nitrogen (a) and phosphorus (b) in paired plantations 

and secondary forests of similar age in the Luquillo Experimental Forest 

(LEF). Data include aboveground and belowground biomass. Forest 

location refers to different sectors of the LEF. Data are from Lugo (1992).

117a.

117b.

Figure 118.—Distribution of total nitrogen (a) and total phosphorus (b) in 

secondary forests in the Luquillo Experimental Forest (LEF) (Lugo 1992b). 

Forest location refers to different sectors of the LEF. The actual size of 

the nitrogen or phosphorous pool is given inside each bar in kilograms per 

hectare.

118a.

118b.
Luquillo Experimental Forest: Research History and Opportunities 109

Figure 119.—Distribution of total nitrogen (a) and total phosphorus (b) 

in plantation forests in the Luquillo Experimental Forest (Lugo 1992b). 

The plantation species are Pinus caribaea (pine), Swietenia macrophylla

(mahogany, age 17 yr), and Swietenia macrophylla X S. mahagoni

(mahogany, age 49 yr). The actual size of the N or P pool is given inside 

each bar in kilograms per hectare.

119a.

119b.

Figure 120.—Distribution of nitrogen and phosphorus in the vegetation of 

Bisley Experimental Watersheds in the Luquillo Experimental Forest after 

the passage of Hurricane Hugo in 1989. Data are from Scatena et al. 

(1993). These data reflect the mean for two watersheds covering 13 ha.

Figure 121.—Nitrogen (a) and phosphorus (b) pools in organisms other 

than trees in tabonuco and palm floodplain (bryophytes) forests in the 

Luquillo Experimental Forest. Data are from Lugo and Scatena (1995), 

Lodge (1996a), Frangi and Lugo (1992), Scatena et al. (1993), and Beard 

et al. (2002).

121a.

121b.

An extensive study of belowground nutrient pools and soil 

physical properties was conducted in the Bisley Experimental 

Watersheds before and after Hurricane Hugo occurred. Two 

watersheds covering 13 hectares (ha) were sampled every 40 

meters (m) using small quantitative pits, and soils were analyzed 

using multiple extraction procedures. These data, together 

with detailed studies of plant species composition, tree diameter 

and height, allometry, and chemical composition showed 

how nutrients were distributed spatially across the landscape, 

within and across species, and aboveground and belowground 

(Scatena et al. 1993, Silver and Vogt 1993, Silver et al. 1994). 

The spatial patterns of nutrient distribution showed that most 

soil nutrients pools increased along a gradient from ridges to 
110 Luquillo Experimental Forest: Research History and Opportunities

valley bottoms, while plant nutrient pools decreased along the 

topographic gradient. The change in plant nutrient content was 

due to a decrease in biomass from ridges to valleys. These studies 

demonstrated that there is a considerable plant-available capital 

and total nutrient capital of most elements in Bisley soils, with 

the possible exception of K and P (table 35). In the case of P, 

the most commonly limiting nutrient to plant growth in highly 

weathered tropical soils, the size of extractable soil pools and 

aboveground plant pools were approximately the same (Silver et 

al. 1994). The total N and P pools in soil were larger than their 

corresponding pool in aboveground vegetation (table 35).

Although nutrient pools have been studied extensively at specific 

sites within the LEF (e.g., the Bisley Experimental Watersheds), 

spatial models have the advantage of scaling point-specific data 

over the landscape. Wang et al. (2002) developed a spatially 

explicit version of the CENTURY soil model (Parton et al., 

1987, 1988, 1989) to characterize the spatial patterns of soil 

N pools and N mineralization rates in the LEF as a function 

of climate, vegetation, and soils. Model results indicate large 

variations in soil N pools (fig. 122) and N mineralization rates 

along elevational and topographic gradients over the LEF. As 

elevation increases, pools of soil total N increase. On the other 

hand, N mineralization rates peak in middle elevations and then 

decrease as elevation increases. Total N pools decrease along a 

topographic gradient from ridge to valleys (e.g., near riparian 

zones). Model results suggest, however, that N mineralization 

rates have no significant topographic variation over the LEF. 

Precipitation and clay content appear to be the major factors that 

determine the elevational and topographic gradients of soil N 

pools and N mineralization rates in the LEF.

Nutrient Fluxes

The range of N and P flux rates at the LEF is illustrated in figure 

123. The fastest fluxes are associated with primary productivity, 

uptake, and return of nutrients to the forest floor. Fluxes 

associated with water (throughfall and enriched throughfall) and 

retranslocation had intermediate rates, while fluxes associated 

with normal rainfall and N gas exports had the lowest rates. 

Ingestion by the common frog Eleutherodactylus coqui had the 

same N flux rate as watershed export, but watershed export was 

higher in P export at high elevations. 

Figure 122.—Distribution of simulated soil total nitrogen (0 to 30 cm) in 

the Luquillo Experimental Forest, Puerto Rico (Unit: t ha-1). From Wang and 

Hall (2004).

Figure 123.—Nitrogen (a) and phosphorus (b) fluxes in various locations 

in the Luquillo Experimental Forest. Rainfall and throughfall data are from 

Heartsill Scalley et al. (2007); watershed output data and fruitfall data 

are from Frangi and Lugo (1985) and Lewis et al. (1999); coqui data are 

from Beard et al. (2002); retranslocation, uptake, and NPP (net primary 

productivity) data are from Lugo (1992); return to forest floor data are for 

Hernandia sonora in Cuevas and Lugo (1998).

123a.

123b.
Luquillo Experimental Forest: Research History and Opportunities 111

A tabonuco stand at El Verde was continuously fertilized for 

7 years with quarterly additions of nutrients at an annual 

equivalence rate of kg ha-1 (kilograms per hectare) 300 N, 100 P, 

100 K, and (kg ha-1) 8 boron, 15.4 copper, 2.2 Fe (iron), 25 Mn 

(manganese), 26 zinc, and 19 Mg. Li et al. (2006a) measured 

several ecosystem attributes, found that fertilization may enhance 

long-term C sequestration in forest soils, and reported three 

significant responses among the measured attributes (fig. 124).

Nutrient Budgets

Input-output budgets for the Luquillo Mountains show that 

the site is a net exporter of most elements, with both high 

weathering rates and relatively large elemental inputs from 

precipitation due to the influence of marine aerosols (McDowell 

and Asbury 1994). 

Chestnut et al. (1999) quantified inputs, outputs, and internal 

fluxes of N for a hillslope in the tabonuco forest type of the LEF 

(table 36), improving the one reported in Brown et al. (1983). 

The budget highlights the effects of disturbance, gaseous and 

groundwater losses, and atmospheric deposition. Because data 

Figure 124.—Response of annual litterfall, leaf decomposition constant, 

and fungal biomass in fertilized and control plots in a wet forest located 

in the El Verde sector of the Luquillo Experimental Forest (Li et al. 2006a). 

Differences are all significant. Fungal biomass was to 25 cm depth.

External

input

Outflows Biomass

accumulation

Internal 

flux

Precipitation1 4

Dry deposition 0-2a

Geological weathering 0b

Nitrogen fixation ?

Total inputs 4-6 + ?

Groundwater2, 3 5-11

Particulate organic N losses3 0.3-0.7

Gaseous N losses4, 5 1-4

Total outputs 6.3-15.7

Aboveground biomass accumulation6, 7, 8 6

Belowground biomass accumulation6, 7, 8 2

Total biomass accumulation 8

Whole tree mortality5, 9 18

Coarse woody debris (branchfall) 6, 10 6

Coarse root turnover5, 10 1

Fine litterfall7, 11 101-103

Fine root turnover6, 12, 13 17

Net throughfall1, 14, 15 3

Total N Uptake Requirement 154-156c

Table 36. Fluxes of nitrogen (kg ha-1yr-1) in a hillslope tabonuco forest in the Luquillo Experimental Forest. Data reported as ranges of published 

values. From Chestnut et al. (1999).

1McDowell et al. 1990; 2McDowell et al. 1992; 3McDowell and Asbury 1994; 4Erickson in prep.; 5Chestnut et al. 1999; 6Scatena et al. 1993; 7Weaver and Murphy 1990; 8Crow 1980; 9Lugo & Scatena 1996; 10Vogt et al.; 11Lodge et al.; 12Silver and Vogt 1993; 13Vogt et al. 1995; 14McDowell 1998; 15Scatena 1990.

a

Estimates potential range based on dry deposition rates at other sites.

b

Estimate based on basin geologic characteristics.

c

Total N uptake requirement equals biomass accumulation and internal flow.
112 Luquillo Experimental Forest: Research History and Opportunities

for aboveground and belowground N fixation are lacking, the 

budget is unbalanced by 8.3 to 19.7 kg N ha-1 yr-1 (kilograms of 

N per hectare per year) suggesting that inputs from N fixation 

may be a large fraction of total N inputs to this forest type. They 

also found that N storage in soils and vegetation is very large 

relative to inputs and outputs and that the soil N pool to 0.6 m 

depth is an order of magnitude larger than the vegetation pool 

(7,488 versus 754 kg ha-1 in belowground versus aboveground 

pools, respectively). Internal fluxes of N in throughfall and soil 

solution are also large relative to inputs and outputs, suggesting 

that this forest type is a N-rich system with considerable control 

over N losses in streamwater. 

Cusack et al. (2009) added values for N-fixation for soil, forest 

floor, and surface and canopy lichens, epiphylls, and mosses for 

both the tabonuco and palo colorado forest zones. The relative 

rates of biological N-fixation were highest for the mosses (up 

to 26 ± 14 μg N m-2 h-1 (micrograms of N per square meter 

per hour)), but most of N fixed on a mass basis came from 

soils due to the large volume of potential N fixing sites (7.2 ± 

1.7 kg N ha-1 yr-1 (kilograms of N per hectare per year) in the 

tabonuco forest and 3.4 ± 1.2 kg N ha-1 y-1 in the palo colorado 

forest). Overall, N inputs via biological N fixation amounted 

to approximately 12.3 ± 2.7 kg N ha-1 y-1 in the tabonuco forest 

and 8.4 ± 1.4 kg N ha-1 y-1 in the palo colorado forest. In the 

tabonuco forest the inputs via N fixation are within the range 

of the missing N in the Chestnut et al. (1999) budget, but 

insufficient to balance the N budget in the upper elevation forest.

The K pools and fluxes illustrate the opposite end of the 

spectrum, with large K stores in aboveground biomass relative 

to the exchangeable pool in soils, and little change in K flux 

as water moves through the soil profile and is lost from the 

ecosystem as streamflow.

The P budget of a palm floodplain forest at 750 m elevation 

in the LEF (fig. 125) shows that despite internal conservation 

mechanisms (Frangi and Lugo 1985), the forest exported about 

5 kg P ha-1 yr-1 (kilograms of Phosphorus per hectare per year) 

in excess of inputs from rainfall and overland litter input. 

Export rates were about nine times higher than rainfall input 

rates. Soil up to 1 m depth contained seven times the amount 

of P in vegetation, and, with only 0.5 percent of the soil P 

accumulation, it was still enough to supply the annual growth 

requirements of the forest. An estimated 0.58 g m-2 yr-1 of P 

inputs to the forest would be required to maintain a steady state 

(Frangi and Lugo 1985).

High deposition rates of mercury (Hg) were shown for the 

LEF by Shanley et al. (2008), who reported on 2 years of wet 

Hg deposition at Río Icacos in the LEF. Despite receiving 

unpolluted air masses off the Atlantic Ocean from northeasterly 

trade winds, the site averaged 27.9 μg m-2 yr-1 wet Hg deposition, 

or about 30 percent more than Florida and the Gulf Coast, 

the highest deposition areas of the United States. These 

high Hg deposition rates are driven in part by high rainfall, 

which averaged 2,855 mm yr-1 (millimeters per year). The 

volume-weighted mean Hg concentration was 9.8 ng L-1, 

somewhat lower than the Southeastern United States, but high 

concentrations occurred even in high-rainfall weeks; there was no 

significant relation between Hg concentration and weekly rainfall 

depth. During a 2-month period, they measured atmospheric 

concentration and speciation of Hg at sea level and on a 

mountain ridge (750 m elevation) above the deposition station 

(476 m elevation). Particulate Hg and reactive gaseous mercury 

(RGM) concentrations were low, consistently less than 12 pg 

m-3. Given this apparent lack of RGM at ground level to support 

the high wet Hg deposition, they suggest that RGM is being 

scavenged by precipitation and cloud water either after formation 

in the mixing layer, or by high cloud tops scavenging RGM from 

above the mixing layer. Photo oxidation of Hg0 from the global 

pool is likely more intense in the tropics and helps to maintain 

a pool of RGM. Data showed a low fraction of methylmercury 

(MeHg) in streamwater (0.7 percent). Total mercury (THg) 

(MeHg) concentrations and export were dominated by 

particulate fraction in association with DOC at high flows, with 

maximum concentrations of 112 (75) ng L-1. Mercury export 

was episodic in response to discharge increase. Bromeliad water, 

which has a best-case scenario for methylation, was collinear with 

stream samples on the filtered THg versus DOC plot. There 

did not appear to be any Hg, C, sulfur, or bacterial limitations 

for methylation in bromeliads. The low percentage of MeHg in 

streamwater is probably due to high rates of demethylation.

Figure 125.—Phosphorus budget for a palm (Prestoea montana) 

floodplain forest at 750 m elevation in the Luquillo Experimental Forest 

(Frangi and Lugo 1985).
Luquillo Experimental Forest: Research History and Opportunities 113

Soil O2 (oxygen) is another critical resource for plant and 

microbial processes and plays an important role in ecosystemlevel biogeochemical cycling (Gambrell and Patrick 1978, 

Crawford 1992). Reduced O2 availability can change the 

composition and activity of soil microbial communities (Morris 

1984, Magnusson 1992, 1994), lower redox potential (Gambrell 

and Patrick 1978, Hook 1984), and alter nutrient availability 

and utilization (Drew 1991, Crawford 1992, Silver et al. 1994). 

Studies in the LEF have shown that surface soils in wet forests 

experience very low soil C availability for all or part of the year 

(Silver et al. 1999, McGroddy and Silver 2000). These studies 

identified high clay soils, perhumid conditions, high microbial 

activity, and warm temperatures as mechanisms that lead to rates 

of O2 consumption that exceed diffusional resupply, creating 

anaerobic conditions that lead to nitrous oxide (N2O) and 

methane (CH4) emissions.

Soil O2 concentrations along a rainfall and elevation gradient 

and along topographic gradients in the LEF were measured by 

Silver et al. (1999). Along the rainfall gradients at the LEF, soil 

O2

 availability decreases significantly with increasing annual 

rainfall and reached very low levels (<3 percent) in individual 

chambers for up to 25 consecutive weeks over the 82 weeks of 

the study (fig. 126). Along localized topographic gradients, soil 

O2 concentrations are more variable and decrease significantly 

from ridges to valleys (fig. 127). In the valleys, soil O2 was 

frequently less than 3 percent (ambient O2 is 21 percent). Cross 

correlation analyses showed that soil O2 concentrations have 

significant positive correlation along the topographic gradient, 

and are sensitive to rainfall and hydrologic output. Soil O2

concentrations in valley soils are correlated with rainfall from the 

previous day, while ridge sites were correlated with cumulative 

rainfall inputs over 4 weeks. 

Because methane (CH4) is so readily oxidized in the presence of 

O2, above-ambient CH4 concentrations and efflux are perhaps 

the best indicators of strongly reducing conditions in soils 

(Gambrell and Patrick 1978). Methane concentrations in the 

elfin and palo colorado forests averaged above ambient levels in 

the study by Silver et al. (1999), and exceeded ambient levels by 

Figure 126.—Mean soil O2 concentrations (±1 standard error) over time 

at (A) 10 cm depth and (B) 35 cm depth in the upper elevation cloud 

forest, mid-elevation palo colorado forest and lower elevation tabonuco 

forest in the Luquillo Experimental Forest, Puerto Rico. From Silver et al. 

(1999).

Figure 127.—Average soil O2 concentrations at (a) 10 cm depth and 

(b) 35 cm depth on ridges (open circles), slopes (triangles), and valleys 

(squares) in the Bisley Research Watersheds. Error bars were excluded 

for clarity; soil O2 content decreases significantly from ridges to slopes to 

valleys. From Silver et al. (1999).
114 Luquillo Experimental Forest: Research History and Opportunities

more than four orders of magnitude in the elfin forest, indicating 

a strong influence of anaerobic processes at these elevations. In 

fact, methane emissions from sites with low O2 concentrations 

in the LEF were shown to be equal in magnitude to methane 

emissions from natural wetlands (Teh et al. 2005).

Controls on CH4 dynamics have been studied extensively in 

the LEF (Teh et al. 2005, Teh and Silver 2006, Teh et al. 2006, 

Teh et al. 2008) and have shown that CH4 oxidation through 

anaerobic respiration is the dominant control on CH4 fluxes 

from these forests. Methane production occurred even in 

relatively well-aerated soils, due to the high degree of aggregation 

typical of these iron and aluminum oxide clays. The dominance 

of iron reducers in these soils has the potential to inhibit 

methanogenesis through competition for acetate as the primary 

substrate for iron or C reduction, but soil structure is likely to 

physically separate these two groups of microbes, allowing both 

processes to occur under natural conditions (Teh and Silver 

2006, Teh et al. 2008). Other measures of biogeochemical 

function such as SOM content and P availability are sensitive to 

chronic O2

 depletion along the rainfall gradient, but less sensitive 

to the variable soil O2 environment exhibited at lower elevations 

along topographic gradients (Silver et al. 1999). 

Stream O2 concentrations provide an indication of ecosystem 

metabolism, and they were measured in a stream within the 

Bisley Experimental Watersheds by Mulholland et al. (2001) as 

part of a larger study across streams of North America. Rates of 

gross primary productivity (GPP) and R in streams (in units of 

g O2 m-2 d-1) were established by measuring diurnal changes in 

dissolved O2 in open systems. The Bisley site showed no evidence 

of GPP, and the rate of net ecosystem production (NEP) was 

highly negative and somewhat variable, with no diurnal trend 

(fig. 128). These highly negative NEP values emphasize the 

importance of allochthonous (external to the stream) sources of 

C in fueling heterotrophic metabolism.

Nutrient Use Efficiency

The ecosystems of the LEF are generally nutrient rich and 

exhibit low N use efficiency (Lugo 1992b, Scatena et al. 1996). 

These ecosystems exhibit numerous nutrient conservation 

mechanisms, especially considering the heavy rainfall to which 

they are exposed (Lugo and Scatena 1995). These conservation 

mechanisms are illustrated here (fig. 129) with ratios of organic 

matter to P, known as nutrient use efficiencies (NUE). Three 

different estimates of NUE are illustrated: ratios of mass fall to 

P in mass for leaf and litterfall, ratios of aboveground biomass 

to aboveground P, and ratios of P stored by net primary 

productivity to P taken up by roots. For illustrative purposes, 

data are shown for tabonuco (successional and mature stands), 

palm, and plantation forests of many species. For comparison, 

we show in red the mean NUE for litterfall in tropical forests. 

Most values in figure 129 are larger than the mean values for 

Figure 128.—Daily rates of gross primary productivity (GPP), total 

respiration (R), and net ecosystem production (NEP) in eight streams 

across North America. Streams are ordered from lowest (left) to highest 

latitude (right); Quebrada Bisley (QBPR) is located in the Luquillo 

Experimental Forest and is the only tropical site, see inset map. Values on 

bars in (c) are GPP:R ratios. From Mulholland et al. (2001).
Luquillo Experimental Forest: Research History and Opportunities 115

many tropical forests, indicating higher use efficiency. Low NUE 

is associated with fruitfall; leaffall after the passage of Hurricane 

Hugo; leaffall of Hernandia sonora, a planted native tree species; 

and biomass accumulation in tabonuco forests’ riparian and 

valley locations at the Bisley Experimental Watersheds. Some 

species such as plantations of mahogany, eucalyptus, and pines 

(two species) exhibit high NUE, as does the fall of highly 

senesced brown palm fronds at high elevation in the LEF. 

Secondary forests had lower NUE than paired plantation species. 

High fine root biomass and rapid root regrowth after Hurricane 

Hugo occurred have also been cited as important nutrient 

conservation mechanisms in this forest (Silver and Vogt 1993, 

Silver et al. 1996).

Changes in Carbon and Nutrients With 

Elevation

Less work has been done on the elevation gradient regarding 

nutrient pools compared with, for example, variations in 

community structure or C fluxes. As mentioned in an earlier 

paragraph, phosphorus is thought to be the most commonly 

limiting element to net primary productivity in tropical forests 

on highly weathered soils such as those in the LEF. McGroddy 

and Silver (2000) measured soil P and C fractions at five sites 

along the elevation gradient and found that total P was generally 

higher in low- to mid-elevation soils than in upper elevations 

where high rainfall is likely to augment leaching losses. Soil 

P at low- to mid-elevations was associated predominantly 

with iron and aluminum oxides, which are generally thought 

to be relatively unavailable to plants, while P in soil at higher 

elevations was more likely to be associated with organic matter. 

The variable redox dynamics of wet tropical forest soils, however, 

have the potential to release P bound to iron oxides and 

contribute to plant and microbial nutrient cycling. Work in the 

LEF has documented significant P release during intermediate 

and low redox events (Chacón et al. 2006, Liptzin and Silver 

2009). The reduction of iron oxides and hydroxides during these 

events is also likely to be responsible for significant CO2

 efflux 

from soils (Chacón et al. 2006, Liptzin and Silver 2009). 

Richardson et al. (2000a) constructed preliminary budgets of 

tank bromeliads along an elevation gradient and found that they 

Figure 129.—Nutrient use efficiency (NUE; ratio of organic matter to P) for tabonuco (successional and mature stands), palm, and plantation forests of 

many species. For comparison, the mean NUE for litterfall in tropical forests is shown in red. Compiled by A. Lugo.

0 2000 4000 6000 8000 10000 

Fruit fall palm forest 

Litterfall Hernandia sonora 

Flower fall palm forest 

Leaffall post-hurricane Bisley 

NPP/Uptake Sabana secondary 

Litterfall Hibiscus elatus 

Leaffall pre-hurricane Bisley 

Green-yellow palm leaves leaffall 

Litterfall Terminalia ivorensis 

Mean tropical forest litterfall 

Litterfall Anthocephalus chinensis 

NPP/Uptake Cubuy secondary 

Aboveground biomass valleys 

NPP/Uptake El Verde secondary 

All litter fall palm forest 

Litterfall Khaya nyasica 

Branchwood fall palm forest 

Litterfall Pinus caribaea 

Litterfall Swietenia macrophylla 

NPP/Uyptake pine 4-yr 

NPP/Uptake Guzman secondary 

Litterfall Eucalyptus patentinervis 

Litterfall Pinus elliottii 

NPP/Uptake pine 18.5-yr 

Dicotyledonous leaffall palm forest 

Aboveground biomass riparian valleys 

Litterfall Eucalyptus saligna 

NPP/Uptake mahogany 17-yr 

Watershed-level aboveground biomass Bisley 

NPP/Uptake mahogany 49-yr 

Aboveground biomass slopes 

Aboveground biomass ridges 

Miscellaneous fall palm forest 

Brown palm leaves leaffall 

kg Organic Matter / kg Phosphorus 
116 Luquillo Experimental Forest: Research History and Opportunities

exported between 93 and 96 percent of the N and between 74 

and 97 percent of the P entering the system. The elfin forest 

bromeliads retained the most P of the three forests studied. 

Richardson et al. (2000a) studied an elevation gradient of 

nutrient inputs to tank bromeliads, the concentration, storage, 

and turnover of nutrients and biomass of these bromeliads, 

and the diversity, nutrient, and biomass content of animal 

inhabitants of tank bromeliads. They found that the nutrient 

concentration of canopy-derived debris declined with elevation 

and that at the elfin forest bromeliads had the lowest amounts of 

debris per plant and lowest concentrations of nutrients in these 

debris components. The elfin forest had the highest turnover 

rate and density of bromeliads, however, and thus accounted for 

12.8 percent of forest net primary productivity (0.48 Mg ha-1 yr-1

(metric tons per hectare per year)) and contained 3.3 Mg ha-1 of 

water. Total N and P storage in bromeliads was low in tabonuco 

forest and increased above the cloud condensation level in palo 

colorado and elfin forests (fig. 130). The bromeliad microcosm 

is nutrient abundant and accumulates less than 5 percent of 

most nutrients passing through, with the exception of elfin 

forest bromeliads, which accumulated about 25 percent of the 

inputs. Within this microcosm, most of the N and P were stored 

in plant tissue and water, with lower amounts in animals and 

debris, and generally higher values at elfin forests (fig. 131).

Tropical elevation gradients are a convenient surrogate for 

climate change studies. The humid elevation gradient in the 

LEF has been used to examine relationships among climate, 

edaphic conditions, belowground C storage, and soil respiration 

rates (McGroddy and Silver 2000). Total soil C, the light 

C fraction, and all component fractions of the P pool were 

significantly related to soil moisture, and all but total soil C 

Figure 130.—Total nitrogen (a) and phosphorus (b) storage in tank 

bromeliads along an elevation gradient in the Luquillo Experimental 

Forest. Forests are arranged from low elevation to the top of the mountain 

(Richardson et al. 2000a). The cloud condensation level is above the 

tabonuco forest.

130a.

130b.

Figure 131.—Nitrogen (a) and phosphorus (b) storage of compartments 

of tank bromeliads along an elevation gradient in the Luquillo Experimental 

Forest. Forests are arranged from low elevation to the top of the mountain 

(Richardson et al. 2000a). The cloud condensation level is above the 

tabonuco forest.

131a.

131b.
Luquillo Experimental Forest: Research History and Opportunities 117

were also significantly related to temperature along the gradient, 

but few patterns followed an expected linear trend. Both labile 

and recalcitrant soil P fractions were negatively correlated to the 

labile C pool (light C fraction), while the dilute hydrochlorideextractable P pool, generally thought of as intermediate in 

availability, was positively correlated to light C suggesting that P 

may play an important role in C cycling in these systems. Total 

fine root biomass was greatest at 1,000 m elevation and lowest 

at 150 m elevation, and was strongly positively correlated with 

soil moisture content. Soil respiration rates showed significant 

negative correlation to fine root biomass and to the light C 

fraction. In forested sites, soil respiration rates had a strong 

negative correlation to total belowground C pools (soils + roots + 

forest floor). Belowground C pools did not follow the expected 

increasing trend with decreases in temperature along the 

gradient. The results indicate relationships in wet forests among 

soil C and nutrient pools, soil respiration rates, and climate are 

complex. Frequent and prolonged anaerobic events may be 

important features of these environments that help explain the 

observed trends (McGroddy and Silver 2000).

Effects of Disturbance on Nutrient Pools and 

Fluxes

Biogeochemical response to hurricanes was rapid but short lived. 

Losses of most nutrients declined to predisturbance levels in 1.5 

to 2.5 years (McDowell et al. 1996, Schaefer et al. 2000), most 

likely associated with forest regrowth (McDowell et al. 1996). 

The elements most affected by hurricane disturbance were those 

with significant biotic control over concentrations: N and K 

were most responsive to disturbance, and sodium and silica the 

least responsive (McDowell et al. 1996). NO3 concentrations 

increased approximately four-fold over background 

concentrations, but the peak was short lived (fig. 132). 

In a palm floodplain forest after the passage of Hurricane Hugo, 

the combined instantaneous and delayed hurricane-induced P 

fluxes to the forest floor were 3.3 times higher than the normal 

flux (compare fig. 125 with fig. 133b). The high nutrient flux 

to the forest floor was due to wind-induced mass transfer to the 

ground (fig. 133), and the high concentration of nutrients in 

Figure 132.—Monthly average nitrate-N concentrations in Quebrada 

Prieta before and after Hurricane Hugo occurred (September 1989; 

marked by an arrow). Data are originally from Schaefer et al. (2000) but 

replotted in McDowell (2001).

Figure 133.—Transfer by Hurricane Hugo of mass (a) nitrogen (b) and 

phosphorus (c) from aboveground and upland forests to the forest floor 

of a palm (Prestoea montana) floodplain forest at 750-meter elevation in 

the Luquillo Experimental Forest. Tanks represent the accumulation in 

Mg ha-1 (metric tons per year) for mass and kg ha-1 (kilograms per acre) for 

nutrients. Dotted lines connect aboveground live accumulation before the 

hurricane in 1989 to tanks depicting the corresponding accumulation after 

the hurricane in 1990. Solid lines depict transfer of mass or nutrients to 

necromass and as import or export to and from the forest floor. Fluxes 

after the disturbance event are reported in Mg ha-1 and kg ha-1 for 

nutrients (Frangi and Lugo 1991).
118 Luquillo Experimental Forest: Research History and Opportunities

that mass was because it was mostly live mass as opposed to the 

senesced biomass that normally falls. Frangi and Lugo (1991) 

found that 10 percent of the aboveground biomass and 12 to 

16 percent of the nutrients (depending on which nutrient) were 

transferred to the ground after the event. About 18 percent of 

the nutrient pool was found as necromass 7 months after the 

hurricane passed (table 37). This was 10.7 times greater than the 

prehurricane ratio.

The disturbance effects caused by harvesting aboveground 

biomass and Hurricane Hugo on soil chemical and physical 

properties and live and dead root biomass have been studied 

for more than 13 years in the Bisley Experimental Watersheds. 

The goal of these studies was to (1) quantify nutrient loss and 

retention following aboveground disturbance, (2) identify 

nutrient conservation mechanisms, and (3) determine how 

aboveground and belowground processes changed at different 

temporal scales including the period immediately before 

revegetation (9 weeks), the intermediate period of initial 

regrowth (9 months), and the longer term reorganization of the 

vegetation and biogeochemical cycling (>1 year). Harvesting 

aboveground biomass resulted in temporary increases in the 

availability of exchangeable nutrients, but forest floor and soil 

nutrient pools had generally returned to preharvest values over 

a 9-week period. Significant amounts of K moved through the 

soil over this time period, amounting to 29 to 46 kg/ha, and 

resulted in a size reduction of the exchangeable soil K pool. The 

hurricane deposited approximately 845 kg ha-1 of forest floor 

mass and considerable nutrients on the soil surface, and increased 

soil NO3-N (nitrate-nitrogen) and exchangeable K pools, but 

Component

Total aboveground 

(kg ha-1)

Necromass 

(% of total)

Biomass 198,100 18

Nitrogen 1047 19

Phosphorus 67 18

Potassium 817 17

Calcium 711 23

Magnesium 192 16

Table 37. Fraction of aboveground nutrients and mass in 

necromass 7 months after the passage of Hurricane Hugo in a 

floodplain forest in the Luquillo Experimental Forest (Frangi and 

Lugo 1991).

pool sizes always returned to prehurricane values within 9 

months. Examination of the data on an annual time step over 

a 6-year period revealed an increase in soil cation pools and a 

significant decrease in soil pH. No change in SOM was detected 

at any time step following the disturbances. Live fine root 

biomass was dramatically reduced as a result of the hurricane’s 

disturbance, and was only beginning to show signs of recovery 

at the 6-year point of the experiment (Silver et al. 1996). In 

general, root biomass, relatively slow root decomposition, and 

maintenance of organic matter pools function to conserve 

nutrients following disturbance in this ecosystem.

The effects of road disturbance and subsequent succession 

on nutrient pools were studied in the upper montane forests 

of the LEF (Olander et al. 1998). They found that recently 

disturbed sites had higher Al (aluminum) and soil O2 than older 

successional sites and intact forest. Forest vegetation appeared 

to be uniquely adapted to low redox conditions of the older 

sites. Abandoned roads, which ranged in age since abandonment 

from 4 to 60 years, had litter and soil characteristics similar to 

their adjacent forest areas after 11 years (Heyne 2000). Net 

N mineralization (g g-1 dry soil) and percent SOM were not 

different between abandoned roads and adjacent forest, while soil 

pH required more than 60 years of abandonment for recovery to 

forest conditions.

Epiphytes generally occupy arboreal perches, which are 

inherently unstable environments due to periodic windstorms, 

branch falls, and treefalls. During high wind events, arboreal 

bromeliads are often knocked from the canopy and deposited 

on the forest floor. Pett Ridge and Silver (2002) tested the 

effects of displacement on the common bromeliad Guzmania 

berteroniana using a reciprocal transplant experiment to and 

from trees and the forest floor. Although survival rates were 

relatively high in all habitats, bromeliads transplanted to trees 

grew significantly longer roots (189 ± 43 cm) than those moved 

to the forest floor (53 ± 15 cm), and bromeliads transplanted 

to trees also experienced lower rates of leaf area loss. All 

transplanted bromeliads rapidly altered the substrate they 

occupied. Individuals transplanted to and among trees rapidly 

decreased base cation concentrations, but significantly increased 

P concentrations of their underlying substrate. On the ground, 

bromeliads increased C, N, and P concentrations within 9 

months of placement. These results suggest that in this montane 

forest, bromeliads respond rapidly to displacement, can locally 

modify their substrates, and can access the resources needed for 

survival regardless of habitat.
Luquillo Experimental Forest: Research History and Opportunities 119

Historical Economy in Puerto Rico

After the arrival of the Spanish in 1493, the economy of Puerto 

Rico was dominated by gold mining. At the end of the 16th 

century, the human population was less than 4,000. By 1776, 

the population had grown to more than 80,000, and agriculture 

was still just a minor activity in a landscape dominated by 

forests (Dietz 1986). During the 19th century, however, 

changes accelerated. Puerto Rico’s population exhibited a 10-

fold increase (fig. 134a), and agriculture (sugarcane, coffee, 

plantain, corn, cotton, and rice) spread throughout the island 

(Dietz 1986). Even though political control of Puerto Rico 

shifted from Spain to the United States in 1898, socioeconomic 

conditions did not change during the ensuing decades. Until 

1940, Puerto Rico’s economy resembled that of most Neotropical 

countries. Pastureland, the most common type of land use in 

the Neotropics, covered 55 percent of the island in 1899; in 

1934, agriculture (including cattle production) represented about 

45 percent of the gross national product, while manufacturing 

accounted for only 7 percent (fig. 134b). Population growth 

before 1940 was accompanied by a consistent decrease in forest 

cover (fig. 134a).

Shifts From Agriculture to Industry

In the late 1940s, the political status of Puerto Rico changed, 

and the postwar efforts to promote industrialization (Operation 

Bootstrap; Dietz 1986) shifted the economy from agriculture 

to light industry. Between 1940 and 1980, the percentage 

of farmland in Puerto Rico decreased from 85 to 37 percent 

(Cruz Baez and Boswell 1997); by 1989, agriculture represented 

less than 5 percent of the gross national product, while 

manufacturing had grown to almost 50 percent (fig. 134b; 

Dietz 1986). The human population of the island continued to 

increase, from slightly less than 1 million inhabitants in 1900 to 

more than 3.5 million in 1990 (fig. 134c) to just under 4 million 

today. Furthermore, the spatial distribution of the population 

changed (fig. 135) because economic changes promoted 

migration from rural to urban areas and to continental urban 

centers such as New York, Philadelphia, Miami, and Chicago 

(Cruz Baez and Boswell 1997, Rudel et al. 2000). Since the 

1940s, while out-migration dominated the central to western 

mountains, immigration and population growth occurred mainly 

in the urban areas and along the northeast coast. Between 1940 

and 1990, the rural population did not change significantly (fig. 

134c), but the rural working population (between 15 and 64 

years old) decreased (Cruz Baez and Boswell 1997), reflecting the 

decrease in agricultural activities.

Puerto Rico and the Luquillo Experimental 

Forest in the 21st Century

Urban sprawl has been part of the Puerto Rican environment 

for the past 40 years, but has become an increasingly dominant 

land use pattern. The Transportation Research Board (2002) 

defines urban sprawl as “peripheral growth that expands in 

an unlimited and noncontiguous way outward from the solid 

built-up core of a metropolitan area.” It is characterized by lowdensity development (Edwin 1997), significant per capita land 

16. Socioeconomic Indicators

N.L. Harris

Figure 134.—(a) Population growth (as a percentage of current population) 

and forest cover (as a percentage of total land area) since 1800, (b) 

percentage of gross national product attributed to agriculture and 

manufacturing since 1930, and (c) rural and urban population during the 

20th century. From Grau et al. (2003).
120 Luquillo Experimental Forest: Research History and Opportunities

consumption of exurban lands, and almost total reliance on the 

automobile for transportation. Principal consequences include 

traffic congestion, increased energy consumption, increased 

costs for community services, reduction of open spaces, the 

fragmentation of habitats, and the pollution of airways and 

waterways.

Today, Puerto Ricans are suffering the consequences and paying 

the costs of sprawl. On the island, traffic jams are common, 

enhanced by a high proportion of cars per person equal to 0.54 

(U.S. DOT 2002). Industrial electricity costs are 73 percent 

higher than for developed economies and are dependent upon 

imported oil. Forty percent of water bodies are affected and 

do not meet quality standards (Puerto Rico Vision 20252

). 

Productive agricultural lands have been transformed into urban 

areas (Helmer 2004), and even forest reserves like the Luquillo 

Experimental Forest (LEF) have come under increasing human 

pressure (Lugo et al. 2000, 2004).

Martinuzzi et al. (2007) integrated geospatial technology and 

population census data to understand how Puerto Ricans are 

using and developing land according to imagery obtained 

between 1999 and 2003. Rather than using a dichotomous 

urban/rural classification to map land use, three new regions for 

Puerto Rico were defined: urban, densely populated rural, and 

sparsely populated rural. Their interpretation defined 11 percent 

of the island (95,342 hectares) as being composed of urban 

built-up surfaces (fig. 136a), with approximately 40 percent 

corresponding to low-density developments expanding outward 

from urban centers in linear patterns that follow the road 

network and other isolated construction. As seen from, figure 

136a developed pixels are distributed throughout the island as a 

major “urban ring” around the island and another minor urban 

ring that encircles the LEF. 

Figure 135.—Human population migration in and out of each municipality of Puerto Rico between 1940 and 1990. From Grau et al. (2003).

2 Puerto Rico Vision 2025 is a governmental long-term vision for the island, 

encompassing all aspects of economic, social, environmental, and infrastructure 

development.
Luquillo Experimental Forest: Research History and Opportunities 121

The results of Martinuzzi et al. (2007) show a clear increase 

in tendency toward urban sprawl in people moving away from 

urban centers to the periphery, to exurban agglomerations, and 

to developments along rural roads (fig. 136b). Most of the 78 

urban centers of Puerto Rico are relatively poorly populated (as 

compared with urbanized areas of similar population in different 

countries) and are surrounded by sprawl developments. Nearly 

one-half of the total development covers one-fourth of the best 

lands for agriculture, thereby affecting watersheds and reducing 

open spaces. This study reinforces the necessity of an effective 

and efficient land use plan for Puerto Rico and emphasizes the 

importance of protected areas such as the LEF for maintaining 

ecosystem services such as water quality and quantity amidst 

increasing pressure from urbanization.

Figure 136.—(a) Patterns of urban development of Puerto Rico: high-density developments (in black) and low-density developments (in red). (b) 

Map of Puerto Rico’s developed lands, showing different tendencies toward urban sprawl. From Martinuzzi et al. (2007).

136a.

136b.
122 Luquillo Experimental Forest: Research History and Opportunities

Figure 137.—Traditional single use of data model and new model with 

metadata and data archives modified from J. Porter, University of Virginia.

Level Benefits

Individual 

scientist

Facilitates collaboration and publication, wider 

recognition among peers, higher number of 

citations

Research 

institutions

Larger role in science, increased potential for 

additional research funding

Broader 

scientific 

community

Catalyzes new analyses and syntheses, integrates 

data resources

Society Better science leads to better policy

Table 38. Benefits of sound information management systems and 

data archival policies. Based on Costello (2009).

Data Archival and Long-Term Data Sets

“Advances in science, engineering, and all fields of research 

depend on the reliability of the research record. Sustained 

public trust in the research enterprise also requires confidence 

in the research record and in the processes involved in its 

ongoing development.” 

Forest Service Manual, 4086 Scientific Ethics

Sound information management systems and data archival 

policies facilitate the publication and accessibility of data sets 

and deliver benefits to individual scientists, research institutions, 

and the general scientific community (table 38). Data archival 

allows for an easy connection to be made between past and 

present research in terms of where data were collected, when, 

by whom, and which products, reports, and publications were 

generated from a given study so that scientists’ ability to interpret 

and understand data in the future is maintained. In this way, the 

useful lifespan of data is much longer due to increased collective 

data access and data accumulation over time. This approach 

represents a new model for conducting research and managing 

data (Fig. 137).

Since its establishment in 1988, the Luquillo (LUQ) Long 

Term Ecological Research (LTER) has had a formal Information 

and Data Management System (IMS), part of which is the 

publication of data generated by ongoing research activities. 

All long-term data sets for the Luquillo Experimental Forest 

(LEF) are available at the LTER Web site3

 and currently include 

data sets on meteorology, hydrology, disturbance, nutrient and 

carbon dynamics, vegetation dynamics, animal population 

dynamics, and biological diversity. Information Management 

System guidelines for the LUQ LTER are in the Handbook to 

17. History and Focus of Research

N.L. Harris and T. Heartsill Scalley

Data Management Policies, Data Archival, and Data Requests 

(Meléndez Colom 2009), which includes instructions and best 

practices for filing a new data set4

. One fundamental aspect of 

this data archival process is metadata, or “data about the data.” 

Metadata provide users with critical details about the data set 

under consideration, and documentation standards for metadata 

are also available at the LUQ LTER Web site5. 

The LUQ LTER follows Ecological Metadata Language (EML) 

standards because these comply with current archival mandates 

for both the LTER and Forest Service, which include the Federal 

Geographic Data Committee for both geographic and spatial 

data and the National Biological Information Infrastructure 

for biological and nongeospatial data. Furthermore, EML 

can be cross-walked to other standards, which facilitates 

correspondence to other data sets if needed and guarantees 

future accessibility and minimizes loss of information. Because 

the data management and archival policies of the LEF are stated 

explicitly, adequate data usage and interpretation and proper 

acknowledgment of data and resources are possible. 

Another advantage to having a centralized site from which to 

access data and metadata for the LEF is that potential data users 

can request particular data sets from the information manager. 

In this way, the requests and usage of the data are recorded can 

thus be used to gauge a given data set’s influence and value to 

scientists over time. These records are an important part of 

assessing the greater effect and influence of LEF research. 

3 http://luq.lternet.edu/data

4 http://luq.lternet.edu/datamng/imdocs/division.html

5 http://luq.lternet.edu/datamng/imdocs/MetadataGuidelines.htm
Luquillo Experimental Forest: Research History and Opportunities 123

Major Research Findings

The LEF has been an LTER site since 1988, but has been a hub 

for ecological research for more than a century. The list below 

highlights some major research findings that have emerged since 

the inception of the LUQ LTER site:

n	 Forests are resilient. Research in the LEF has shown that 

forests are much more resilient in the face of disturbance 

than previously assumed. Forests are not particularly 

susceptible to long-term degradation following largescale natural disturbances. Nutrient cycles return to 

predisturbance levels within 1.5 to 2.5 years, litterfall 

rates return to predisturbance levels within 5 years, and 

biomass accumulation exceeds soil respiration resulting in 

a net carbon sink in as few as 15 years after a large-scale 

disturbance.

n	 Climate change is upon us. Although a 20-year rainfall 

record for the Bisley Experimental Watersheds indicates no 

significant change in mean annual rainfall, the dry months 

(February and March) are getting drier, and intense rainfall 

events of more than 60 mm day-1 (millimeters per day) are 

without precedent. Other weather stations in the region 

suggest a long-term decrease in precipitation, potentially in 

response to global climate change. Urban heat islands with 

net temperatures of up to 1.75 °C radiate heat to at least 

the foothills of the Luquillo Mountains during the early 

evenings.

n	 Consumers can have major effects. Single groups of 

consumers (shrimp, coquis, walking sticks), although a 

minor component of total forest biomass, can affect key 

ecosystem processes such as nutrient cycling, decomposition, 

and primary productivity.

n	 The “four-forest-types” paradigm is dead. The four-foresttypes paradigm within the LEF has shifted to a recognition 

that the distribution of vegetation across a landscape results 

from plant responses to multiple environmental gradients 

that vary over space and time. Although this paradigm 

has heuristic value and has been convenient for displaying 

data, as we do in this review, we now recognize that discrete 

boundaries between one forest type and another do not exist; 

species assemblages occur as a continuum across a landscape.

n	 Most nutrients are in the soil. Modern studies of nutrient 

cycling in tropical forests have shifted and broadened 

compared with earlier research. There has been a paradigm 

shift away from the notion that most nutrients in tropical 

forests are held in aboveground biomass; recent literature 

emphasizes that for most elements and at most sites, soil is a 

larger nutrient pool than aboveground biomass.

n	 Energy transfer occurs through food webs, not food 

chains. The classic concept of a “food chain” is too 

simplistic for describing energy flow and trophic interactions 

in the LEF; recent studies have supported the concept of a 

much more complex “food web.” 

n	 The cloud condensation level is likely a determinant of 

forest structure and function. The point at which clouds 

begin to form in the LEF, at approximately 600-meter 

elevation, may induce changes in climatic conditions and, 

therefore, changes in forest structure and function. Net 

primary productivity peaks at mid-elevations, stem density 

exhibits a mid-elevation dip, and species richness starts to 

decline at mid-elevations.
124 Luquillo Experimental Forest: Research History and Opportunities

The document by Brown et al. (1983) and this document both contain comprehensive reviews of available 

information for the Luquillo Experimental Forest (LEF), in 

particular, and the Luquillo Mountains, in general. The richness 

of information available in publications, the National Science 

Foundation’s Long Term Ecological Research (LTER) Web site 

(http://luq.lternet.edu), research files of the Forest Service, and 

other agencies (Federal and State) and universities with research 

programs in the Luquillo Mountains makes the LEF an attractive 

site for research collaboration. Research facilities discussed 

below in this document and the cadre of local scientists and 

students with active research programs in the LEF complete 

the case for the attractiveness of the LEF as a place to conduct 

research on tropical systems of all kinds (ecological, hydrological, 

geological, urban, atmospheric, etc.). Research opportunities in 

the Luquillo Mountains are unlimited, but some areas outlined 

below are not well represented in current research programs, 

yet are of particular interest to scientists and forest managers. 

Other research opportunities will emerge as knowledge about the 

Luquillo Mountains expands.

Research opportunities are emerging in the following areas:

n	 The interaction between visitors to the forest and the 

natural ecosystems of the LEF. As discussed earlier, the 

LEF and El Yunque National Forest receive hundreds 

of thousands of visitors annually. These visitors have an 

effect on the biota and ecological systems of the Luquillo 

Mountains, for example, in the propagation of introduced 

species, on the water quality of streams, conditions of trails, 

and other demands placed on facilities and on natural 

ecosystems. An opportunity exists to conduct socioecological 

research that will begin to unravel the complex interactions 

between people (both visitors and people in communities 

surrounding the LEF) and ecosystems in the LEF. This 

research is a priority for forest managers responsible for 

managing visitors, interactions with local communities, and 

ecological systems of the national forest.

n	 How climate change will affect the elevational gradients 

of the Luquillo Mountains. This review has established 

the many elevation gradients of the LEF, including climate, 

vegetation, soils, and hydrology. One critical gradient in 

the LEF is the adiabatic lapse rate and the critical cloud 

condensation point at about 600-meter elevation. These 

two physical indicators of environmental conditions have 

particular influence on vegetation, and vegetation extends 

its influence to many other components of the biota along 

the elevation gradient. Climate change scenarios for the 

Caribbean, coupled with the influence that urbanization has 

on climatic gradients on the mountain, anticipate increases 

in air temperature and an upward movement of the critical 

cloud condensation level. These dramatic scenarios provide 

impetus for unique long-term research opportunities in the 

LEF. 

n	 The global connectivity of the Luquillo Mountains. 

The Luquillo Mountains are exposed to five global airsheds 

discussed previously in the Climate section above. Each 

airshed, whether originating over the Atlantic or in Africa, 

the Northeastern United States, or Central America, is 

characterized by its own chemical and particulate signature, 

which presumably affects local ecosystems in different ways. 

Research opportunities exist to continue the characterization 

of these airsheds and establish the effects of these global 

connections on the biota. Puerto Rico’s effects on the quality 

of the air masses as they pass over the island also require 

further characterization.

n	 The interface between urban and natural ecosystems. 

The LEF is a terminus of the urban to pristine forest 

gradient in Puerto Rico. Although much research has been 

conducted on the pristine forests of the Luquillo Mountains, 

very little research attention has been given to the urban 

and suburban portions of the gradient or to the interactions 

among the systems found along the gradient. With the 

urban population of the world increasing everywhere, there 

is a research need and an opportunity to further understand 

how ecosystem services from different parts of the gradient 

are delivered and sustained as human pressure for those 

services increases. The research opportunity is focused on 

socioecological research, an emerging new integrative science 

with a focus on people and ecosystems.

In addition to the opportunities listed above, there is an 

opportunity to connect the LEF to local and global networks 

such as the National Environmental Observatory Network 

(NEON) of the National Science Foundation. Puerto Rico will 

have a NEON site at the Guánica dry forest, and this brings 

an opportunity to compare the moist, wet, and rain forests of 

Luquillo with the dry ones of Guánica. Already Lugo et al. 

(2002) compared these forests and identified the similarities and 

differences that need further comprehensive documentation and 

analysis.

18. Research Opportunities

A.E. Lugo
Luquillo Experimental Forest: Research History and Opportunities 125

Concerted efforts by scientists affiliated with the Institute for Tropical Ecosystems Studies [ITES; formerly the Terrestrial 

Ecology Division of the Center for Energy and Environmental 

Research, University of Puerto Rico-Río Piedras] and the Forest 

Service’s International Institute for Tropical Forestry (IITF), 

Luquillo (LUQ) Long Term Ecological Research (LTER) 

Program, and El Yunque National Forest have led to developing 

an array of venues for environmental education and public 

outreach. Current education and outreach strategies range from 

the training of future professionals and researchers in ecological 

and environmental sciences to activities aimed at educating the 

public on issues related to water and to ecosystem conservation 

and management strategies in the Luquillo Mountains.

Graduate Education—The LUQ LTER Program has been a 

magnet for graduate students seeking to conduct research in 

areas related to tropical ecology. Between 2000 and 2007, more 

than 30 students working under the sponsorship of the LUQ 

LTER have earned doctoral or master’s degrees. Three current 

LUQ LTER senior investigators began their careers as graduate 

students with LUQ LTER senior investigator advisors. The ITES 

contributes to graduate education in the Luquillo Mountains 

through the management of the El Verde Field Station (EVFS), 

which provides logistical support and lodging to ecology course 

participants who visit the station regularly and on- and offisland graduate students who conduct research for their degree 

programs. 

Summer Internships—Accessible, hands-on research experiences 

have been proven as an effective strategy for the recruitment 

and retention of students into the field of ecology because 

these experiences get them excited about science in general. 

Each summer, the EVFS, home base of the LUQ LTER 

Program, hosts the Research Experience for Undergraduates 

in Ecology and Evolution (REU) summer program (http://

ites.upr.edu/REU/). This program is part of a nationwide 

initiative sponsored by the National Science Foundation’s 

(NSF’s) Research Experience for Undergraduate Program that 

was designed to develop and enhance the knowledge and skills 

needed for students to conduct their own independent research. 

Students become familiar with ongoing research at the Luquillo 

Experimental Forest (LEF) and are paired with research mentors 

who are actively conducting research in various disciplines 

(e.g., plant ecology, community ecology, ecosystem research, 

nutrient cycling, and plant-animal interactions), in different 

systems (e.g., plant, soils, streams) and levels of organization 

(from microorganisms to ecosystems). Students are expected to 

attend workshops and research seminars and conduct scientific 

research that they will write and publish an article about after 

completing the internship. The 8-week REU program has been 

active at the EVFS since 2000 and has supported full fellowships 

for undergraduate students. The program has become very 

popular among students; more than 130 applications are 

received each year from students from all over the country to fill 

the 8 fellowships offered each summer. Since 2000, the REU 

program has sponsored research experiences for 76 students 

from Puerto Rico and the continental United States. Additional 

undergraduate student research training has also been possible 

in part through supplemental NSF-REU funds and individual 

grants from LTER scientists working in the LEF. 

Kindergarten to Grade 12 Outreach—Researchers from ITES and 

IITF have been key contributors in developing science education 

and training activities that target both teachers and students at 

the Kindergarten to Grade 12 (K-12) level. Some of the most 

successful activities incorporate data and lessons learned from the 

LUQ LTER. Some of the programs are described below.

The Journey to El Yunque (http://elyunque.net/journey.html) 

program is a joint effort between ITES (LTER scientist J.K. 

Zimmerman) and The Learning Partnership (educational 

researcher S. McGee), intended to replace 4 weeks of the 

standard biology curriculum devoted to ecology at the 6th grade 

level. The Puerto Rico Department of Education has held 

workshops for more than 500 teachers to share with them this 

educational resource. In Journey to El Yunque, students are able 

to work with predictive models and compare predictions with 

LUQ LTER-generated data. 

The mission of the LUQ LTER Schoolyard (http://luq.lternet.

edu/outreach/schoolyard/index.html) program is to increase 

ecological literacy and promote a scientific culture in K-12 

environments and disseminate workshops, internships, and 

seminars to K-12 teachers and students. This program has 

leveraged funds from NSF’s Instructional Materials Development 

program and the Puerto Rico Department of Education to 

develop programs that have reached some 900 Puerto Rico high 

school teachers. Workshops and internships led by ITES and 

IITF have been provided to students and teachers at the EVFS, 

the official research site of LUQ LTER. 

Ecoplexity is an NSF-funded Web site created by the Teaching 

Ecological Complexity program (http://ecoplexity.org/home; 

LTER-ITES scientist Jorge Ortiz Co-PI). This Web site is 

designed to train high school science teachers to conduct field 

ecology research and use qualitative modeling to broaden their 

understanding of complexity, diversity, and ecology as a science. 

The Web site incorporates products from five LTER Schoolyard 

sites (including LUQ LTER) and contains a variety of resources 

(protocols, databases, field guides, links) to help teachers design 

experiments.

The Hydrology for the Environment, Life, and Policy (HELP)

program is a joint global initiative between the United 

Nations Scientific Organization (UNESCO) and the World 

Metereological Organization aimed at improving the linkage 

between hydrology and society through facilitating the 

development of global watershed networks where scientists, 

19. Education and Outreach

E. Meléndez Ackerman
126 Luquillo Experimental Forest: Research History and Opportunities

policymakers, and stakeholders can form alliances to maximize 

economic and social welfare without compromising the 

sustainability of watershed ecosystems (Ortiz Zayas and Scatena, 

2004). In 2001, this UNESCO and Forest Service Washington 

Office partnership initiative established an international network 

of watersheds and incorporated seven watersheds in Eastern 

Puerto Rico (Río Grande de Loíza, Río Espíritu Santo, Río 

Mameyes, Río Sabana, Río Fajardo, Río Santiago, Río Blanco) 

that all originate in the Luquillo Mountains. Guided by the 

goals of the international principle of integrated watershed 

resource management, HELP’s goal is to promote the concept 

of integrated management of water resources in the Luquillo 

Mountains (http://luquillohelp.upr.edu/). With that guiding 

principle, the Luquillo-HELP program seeks to foster the 

effective dissemination of information related to the water 

resources of the Luquillo Mountains among all the social sectors 

of the eastern region of Puerto Rico accountable for managing 

water resources. Targeted groups include the communities that 

depend on the water supply from the Luquillo Mountains, water 

resource managers, scientists focused on hydrology research, 

government agencies in charge of administering water resources, 

and government officials in charge of developing public policy 

on water use. The program is sustained using external funds that 

support annual workshops and panel discussions to educate the 

public on issues related to water use, management, and research 

and includes sponsors from Federal and State agencies including 

the Forest Service’s El Yunque National Forest and IITF, U.S. 

Geological Survey, U.S. Army Corps of Engineers, Puerto Rico 

Aqueduct and Sewer Authority, Water Resources Division, 

Water Plan Office of Department of Natural and Environmental 

Resources, and The Environmental Quality Board of the 

Commonwealth of Puerto Rico. 

The El Portal Visitor Center for the El Yunque National Forest 

is located on Highway PR 191 at the entrance to the El Yunque 

National Forest. El Portal offers interactive exhibits designed 

to inform the visiting public about important aspects related 

to tropical rain forest diversity and function. Visitors can also 

request guided tours through the forest. The El Portal Visitor 

Center is one of several visitor information centers located 

throughout the forest. The El Yunque receives an estimated 1.2 

million visitors annually from Puerto Rico and the continental 

United States combined. Given the large influx of people into 

the Luquillo Mountains, El Portal, and associated facilities at El 

Yunque National Forest, these facilities provide an important 

outlet for scientific information generated by research and 

academic institutions about the LEF. 
Luquillo Experimental Forest: Research History and Opportunities 127

Years Activity

1940s The U.S. Department of Agriculture, Forest Service 

starts ecological research in four vegetation zones.

1960s Rain Forest Project, sponsored by the Atomic Energy 

Commission, studied ecosystem processes at large 

scales.

1970s Terrestrial Ecology Division of the Center for Energy and 

Environment Research conducted research on nutrient 

cycling and energy flow.

1980s Rain Forest Cycling and Transport Project initiated by 

the U.S. Department of Energy and the University of 

Puerto Rico.

1988 to 

today

Large-scale, long-term monitoring projects and experiments implemented, many based out of the El Verde 

Research Area. Some of the largest and most recent 

include the nutrient addition plots (NSF-MRCE, NSFCREST), population and community monitoring projects 

of The Luquillo Long-Term Ecological Research Program 

(NSF-LTER), The Forest Dynamics Plot (NSF, Andrew 

Mellon Foundation), and, the most recent, the Canopy 

Trimming Experiment, established in 2006 (NSF-LTER).

Table 39. Activities associated with the El Verde Research Station 

over the past seven decades.

NSF is National Science Foundation

MRCE is Minority Research Centers of Excellence

CREST is Center for Research Excellence in Science and Technology

Building Facilities

The El Yunque National Forest, also the Luquillo Experimental 

Forest (LEF), is home to several research facilities that provide 

logistical support to researchers, students, technicians, and 

managers from local and off-island institutions. The living 

and working facilities throughout the forest are described 

below. The establishment and maintenance of these facilities 

have contributed greatly to developing cutting-edge research 

in ecosystem function at the LEF and its position as one of the 

most well studied tropical forests in the world.

The El Verde Field Station (EVFS), which is situated at 

350-meter (m) elevation on the northeastern slope of the 

Luquillo Mountains, has been a vital location for research and 

training efforts since the middle of the past century (table 39). 

It has a rich history, both culturally and scientifically, with 

some facilities dating back to the late 1930s and some longterm research sites dating back to the 1940s. The station is 

surrounded by tabonuco forest, the most well-studied vegetation 

type in the LEF. In 1988, the National Science Foundation 

established the Luquillo (LUQ) Long Term Ecological Research 

(LTER) Program and the EVFS has been the principal site for 

LTER research ever since. The EVFS has provided logistical 

support to scientists from more than 50 institutions in Puerto 

Rico and overseas. It is probably the oldest terrestrial ecology 

field station in Puerto Rico and has increased in usage over time.

The Institute for Tropical Ecosystem Studies (ITES) administers 

the EVFS and surrounding research area (80 acres) under an 

agreement with the Forest Service. Core facilities include two 

dormitory buildings and laboratory facilities. An original 

building with historic value was constructed in 1937 and 

renovated in the early 1960s at the start of the Howard Odum’s 

Rain Forest Project. A second dormitory-style apartment 

building was inaugurated in 2007 and has improved the quality 

of the accommodations at El Verde significantly. Laboratory 

facilities at EVFS include offices, wet and dry laboratories, 

storage rooms, a locker room, a carpentry shop, a biodiversity 

collections room, and a combined conference and computer 

room. Laboratories at the field station are supplemented by 

modern analytical facilities operated by ITES in San Juan. 

Biodiversity collections at the EVFS contain complete collections 

of the fungi, ferns and dicots, amphibians, and reptiles of the 

LEF. The herbarium collection will be digitized to incorporate 

El Verde specimens to the University of Puerto Rico-Río Piedras 

Virtual Herbarium.

In the forest, researchers have access to a permanent walkup 

tower that supports environmental monitoring equipment and 

provides access to canopy vegetation and fauna. This tower 

also houses a station for the National Atmospheric Deposition 

Program. A footbridge within the research area allows passage 

over the Quebrada Sonadora to experimental field plots. 

Sabana Field Research Station and Bisley Experimental 

Watersheds—The Sabana Field Research Station is situated at 

265-m elevation within a small community at the outskirts of the 

LEF. The station is managed by the Forest Service, International 

Institute of Tropical Forestry (IITF), and houses laboratory 

facilities and living quarters for visiting students and senior 

researchers. High-speed Internet is also available within facilities. 

The site has year-round road access and is just a short drive 

from the three Bisley Experimental Watersheds where several 

researchers conduct their studies. The IITF has conducted 

detailed watershed studies in these adjacent watersheds since 

1987, and the site has been part of the LUQ LTER Program 

since its creation in 1988. The Bisley Experimental Watersheds 

are located within forest dominated by tabonuco trees and 

consist of three drainages dedicated to the study of watershed 

processes within steep forest areas. The research area contains 

two canopy towers with climate stations and three gauged 

watersheds nested within the gauged Río Mameyes watershed. 

Experimental research areas also include a network of permanent 

vegetation and soil plots and areas dedicated to the study 

20. Facilities

E. Meléndez Ackerman
128 Luquillo Experimental Forest: Research History and Opportunities

of biomass removal, coarse woody debris, and both aquatic 

and riparian ecology, all of which have generated numerous 

publications. Long-term environmental records include climate, 

throughfall, litterfall, and plant succession. Scientists from the 

U.S. Geological Survey and Forest Service IITF at Bisley and the 

Sabana Field Research Station collect climate data at hourly and 

daily time intervals from recording climate stations. In 2008, 

IITF integrated ozone monitoring at Bisley (Grizelle González, 

pers. comm.).

Luquillo Parrot Aviary—The U.S. Fish and Wildlife Service 

administers the Luquillo Aviary, one of two aviaries on the island 

of Puerto Rico that house the endangered Puerto Rican Parrot 

(Amazona vittata), which currently has very few individuals 

in the wild. The Luquillo Parrot Aviary was established in 

1972 as a management strategy for the conservation of this 

endangered species. The Luquillo Aviary was originally located 

at a high elevation site (~800 m) and was used for breeding and 

reintroduction purposes. It has been recently relocated to a 

lower elevation site situated at 550-m elevation in the Luquillo 

Mountains with state-of-the-art facilities. It is currently used for 

breeding purposes only and is a source of Puerto Rican parrots to 

the Jose L. Vivaldi Memorial Aviary (administered by the Puerto 

Rico Department of Natural and Environmental Resources) 

located in the Río Abajo Commonwealth Forest. Parrots are 

currently trained at the Río Abajo facilities for reintroduction 

into the Río Abajo and in the future will be trained for 

subsequent releases in other parts of Puerto Rico. The Luquillo 

Aviary facilities sit on a 1.2-acre lot in the LEF. Facilities include 

a nursery, a hospital, a hurricane shelter room, a kitchen, and 

outdoor cages.

The library at IITF—The library was founded in 1939. The 

library contains major publications on forestry, ecology, 

management, and utilization of tropical forests, and other 

documents and materials related to the tropical forests from 

around the world, with emphasis on New World Tropics. The 

collection of bound monographs, journals, geographically 

referenced documents, research studies, reports, proceedings, 

historical books, photographs, aerial photographs, and maps 

is used by a heterogeneous group of local, national, and 

international customers. The library also has a small videotape 

and slide collection available to staff and the general public. The 

library provides support for research programs by acquiring new 

books and subscriptions to journals and other services; acting as 

a liaison to other libraries for lending and information exchanges; 

functioning as a repository of relevant documents in tropical 

forestry and natural resources management in Puerto Rico 

and the tropics; disseminating research results through broad 

publication distribution, and by serving in an outreach capacity 

to the general public seeking information on tropical forestry.
Luquillo Experimental Forest: Research History and Opportunities 129

Web Resources

The technological advances related to Internet-based data 

archival during the past 15 years have greatly facilitated data 

transfer and dissemination, particularly for long-term projects at 

the Luquillo Experimental Forest (LEF). These advances have 

generated a proliferation of Web resources that enhance research 

and education activities. Some of the most important Web 

resources are listed below and are arranged by topic of interest.

Climatic and Hydrological Data—El Yunque National Forest 

houses three meteorological stations and eight hydrological 

stations managed by the U.S. Geological Survey (USGS), the 

International Institue of Tropical Forestry (IITF), the Luquillo 

Long Term Ecological Research (LUQ LTER) Program and the 

Institute for Tropical Ecosystems Studies (ITES) of University of 

Puerto Rico in Río Piedras. Hydrological data date from 1945 

and meteorological data date from 1975. Data can be accessed 

at http://pr.water.usgs.gov/public/webb/.

Long-term records under the LUQ LTER Program—The LUQ 

LTER Program Web site currently consists of 45 data sets in the 

following topics: meteorology, hydrology, disturbance, nutrient 

and carbon dynamics, vegetation dynamics, animal population 

dynamics, biological diversity, maps and photos, primary 

productivity, population dynamics, organic matter accumulation, 

inorganic inputs and nutrient movements, patterns and 

frequency of disturbance, environmental monitoring, and 

simulation models. Data can be accessed at http://luq.lternet.

edu/.

Publications—Active research at LEF has resulted in a large 

number of publications, particularly through research sponsored 

by IITF, USGS, the LUQ LTER Program, and institutions 

affiliated with these research groups. The Luquillo Water, 

Energy, and Biogeochemical Budgets (WEBB) research project 

is a cooperative effort led by USGS Caribbean Water Science 

Center and USGS National Research Program researchers 

and in collaboration with LTER researchers. It is part of a 

larger network of WEBB sites across the United States and its 

territories aimed at improving the ability to predict water, energy, 

and biogeochemical budgets over a range of spatial and temporal 

scales. Many of the bibliographies related to this and other 

research programs at the LEF can be searched at the following 

Web sites:

n	 LUQ LTER Program—http://luq.lternet.edu/publications/.

n	 IITF-FS—http://www.fs.fed.us/global/iitf/pubdir1.html. 

n	 Luquillo WEBB Project (USGS)—http://pr.water.usgs.gov/

public/webb/webb_biblio.html.

Research Field Stations—Web addresses for the three main 

research facilities at LEF are as follows:

n	 El Verde Field Station (Managed by ITES, University of 

Puerto Rico at Río Piedras)—http://ites.upr.edu/EVFS/.

n	 Sabana Field Station (Managed by the IITF–Forest 

Service)—http://www.tropicalforestry.net/facilities/sabana. 

n	 Luquillo Aviary (Managed by the U.S. Fish and Wildlife 

Service)—http://www.fws.gov/caribbean-ecoteam/PRP_

aviaries.htm.

Educational Resources and Public Outreach—El Yunque National 

Forest has the largest collection of educational and public 

outreach Web resources in the Caribbean. Educational sites 

target different audiences from Kindergarten to Grade 12 

students and teachers to higher education. Public outreach 

and educational resources offer information that promotes the 

integrated management of El Yunque’s Ecosystem.

n	 Journey to El Yunque—http://elyunque.net/journey.html.

n	 Ecoplexity–Luquillo-LTER—http://ecoplexity.org/tropical_

forest (in English and Spanish).

n	 LUQ LTER Schoolyard Program—http://luq.lternet.edu/

outreach/schoolyard/index.html. 

n	 Hydrology for the Environment, Life, and People (HELP)—

http://luquillohelp.upr.edu/index.html (Spanish only).

n	 Research Experience for Undergraduates in Ecology and 

Evolution—http://ites.upr.edu/REU/.

n	 El Portal—http://www.fs.fed.us/r8/el_yunque/ (Spanish and 

English).

n	 ITES—http://www.ites.upr.edu/.

n	 IITF—http://www.fs.fed.us/global/iitf/.

Additional Resources

Current sources for nomenclature and taxonomic identification 

guides are available at no cost from the Antilles Flora project, 

directed by Pedro Acevedo Rodríguez, Department of Botany, 

National Museum of Natural History, and Smithsonian 

Institution6.

Guide to Vines: http://botany.si.edu/Antilles/PRFlora/vines.

html.

Guide to Monocotyledons: http://botany.si.edu/Antilles/

PRFlora/monocots.html.

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