San Juan Bay Estuary Watershed 

Urban Forest Inventory

Thomas J. Brandeis, Francisco J. Escobedo, Christina L. Staudhammer, 

David J. Nowak, and Wayne C. Zipperer

United States Department of Agriculture

Forest Service Southern Research Station General Technical Report SRS–190
About the Authors

Thomas J. Brandeis is a Research Forester with the Forest Inventory and Analysis Research 

Work Unit, Southern Research Station, U.S. Forest Service, Knoxville, TN 37919.

Francisco J. Escobedo is an Associate Professor and State Extension Specialist at the School of 

Forest Resources and Conservation, University of Florida, Gainesville, FL 32611-0410.

Christina L. Staudhammer is an Associate Professor in the Department of Biological Sciences, 

University of Alabama, Tuscaloosa, AL 35487.

David J. Nowak is a Research Forester with the Effects of Urban Forests and their Management 

on Human Health and Environmental Quality Research Work Unit, Northern Research Station, 

U.S. Forest Service, Syracuse, NY 13210.

Wayne C. Zipperer is a Research Forester with the Integrating Human and Natural Systems in 

Urban and Urbanizing Environments Research Work Unit, Southern Research Station, U.S. Forest 

Service, Gainesville, FL 32611-0806.

All photos by Tom Brandeis, Southern Research Station, unless otherwise 

noted.

Cover photo: The southern portion of the San Juan Bay Estuary watershed, 

a hilly area with soils derived from volcanic materials, is less denselydeveloped and has higher tree cover. Note the limestone “mogote” hills 

rising from the coastal plain in the distance to the north.

March 2014

Southern Research Station

200 W.T. Weaver Blvd.

Asheville, NC 28804

www.srs.fs.usda.gov
San Juan Bay Estuary Watershed

Urban Forest Inventory

Thomas J. Brandeis, Francisco J. Escobedo, Christina L. Staudhammer, 

David J. Nowak, and Wayne C. Zipperer

Structurally diverse, species rich patches of secondary subtropical forest are found throughout the San Juan metropolitan area.
ii

Foreword

This report presents information on the forests, urban tree 

cover, and land use within the watershed of Puerto Rico’s 

San Juan Bay Estuary (SJBE) as described by two urban 

forest inventories undertaken from 2001 to 2002 and 2010 

to 2011. These urban forest inventories were integrated with 

the island-wide forest inventories of Puerto Rico that were 

carried out concurrently.

Forest inventories of all States, Commonwealths, Territories, 

and possessions of the United States are mandated by 

the Agricultural and Research Extension and Education 

Reform Act of 1998 (Farm Bill). These surveys are part 

of a continuing nationwide undertaking by the U.S. Forest 

Service (USFS) through its regional research stations. 

Southern Research Station (SRS) Forest Inventory and 

Analysis (FIA)—operating from its headquarters in 

Knoxville, TN, and offices in Asheville, NC, and Starkville, 

MS—is responsible for surveying the 13 Southern States 

(Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, 

Mississippi, North Carolina, Oklahoma, South Carolina, 

Tennessee, Texas, and Virginia), the Commonwealth of 

Puerto Rico, and the Territory of the U.S. Virgin Islands. 

The USFS FIA program for Puerto Rico and the U.S. Virgin 

Islands is jointly funded and conducted by USFS, SRS, FIA, 

and the USFS International Institute of Tropical Forestry 

(IITF). The primary goal of these surveys is to develop 

and maintain the resource information needed to formulate 

sound forest policies and programs. Additional information 

about annual surveys is available at http://fia.fs.fed.us/.

The research project was a collaborative effort between the 

USFS, University of Florida School of Forest Resources 

and Conservation, and University of Alabama School of 

Biological Sciences. In addition to the SRS FIA program’s 

involvement, two other USFS research units participated 

in this work: the Northern Research Station’s Effects of 

Urban Forests and their Management on Human Health 

and Environmental Quality Research Unit, and the SRS’ 

Integrating Human and Natural Systems Research Unit. 

The mission of the first-mentioned research unit is to 

quantify the effects of urban forests and their management 

on human health and environmental quality. Scientists 

and technical staff collect and analyze various field 

measurements and develop computer programs to better 

understand the structure, functions, and benefits of urban 

forests across the world. Numerous user-friendly tools are 

also being developed to aid local constituents in analyzing 

the structure, functions, health, and value of their own 

urban forest resources. The Integrating Human and Natural 

Systems Research Unit seeks to improve the understanding 

of how people living in urban and urbanizing landscapes 

both influence and are influenced by natural environments. 

The unit’s technology transfer centers aim to develop and 

communicate guidelines, models, and tools for natural 

resource professionals, policymakers, planners, and citizens.

This urban forest inventory report characterizes forest cover 

and structure in the SJBE watershed and quantifies some of 

the ecosystem services this forest delivers. We also describe 

ground cover within the watershed, list tree and shrub 

species found there, and examine forest health in terms of 

pests, diseases, and tree crown conditions for different land 

uses. Finally, we discuss the forest’s current conditions, 

compare them to similar forests outside the watershed, and 

stress the importance of continued monitoring to provide 

decisionmakers and land managers with timely, useful 

information.

Additional information about any aspect of this survey may 

be obtained from:

Forest Inventory and Analysis Research Work Unit

U.S. Department of Agriculture Forest Service

Southern Research Station

4700 Old Kingston Pike

Knoxville, TN 37919

Telephone: 865-862-2000
iii

Acknowledgments

We thank Robin Morgan and Terry Hoffman of the U.S. 

Department of Agriculture Forest Service’s State and Private 

Forestry program for providing funding for the 2002 urban 

forest inventory. We also acknowledge funding of the 2010 

data collection under the grant “Spatial distribution of 

invasive horticultural woody plants in urban landscapes” 

Tropical and Subtropical Agricultural Research (TSTAR-C 

FY2008), Humfredo Marcano and Luis Ortíz of SRS FIA 

for their assistance with data collection, Angie Rowe of 

SRS FIA for her work with training, Olga Ramos and 

Eileen Helmer of the IITF for compiling of geographic 

information system datasets, Iván Vícens of the IITF for 

assistance with data collection, Jeffrey Glogiewicz of 

Consultores Ambiental and the Fundación Puertorriqueña 

de Conservación for collecting field data in 2001–02, and 

Jeffrey Glogiewicz and Edgardo González with Centro para 

la Conservación del Paisaje for data collection in 2010–11. 

We would also like to thank the reviewers for comments 

and suggestions on the draft publication. We appreciate the 

cooperation of other public agencies and private landowners 

in providing access to measurement plots.

Iván Vícens, U.S. Forest Service International Institute of Tropical Forestry; Luis Ortíz, U.S. Forest Service Southern Research Station; 

Edgardo González, Centro para la Conservación del Paisaje; Humfredo Marcano, U.S. Forest Service Southern Research Station; 

Jeffrey Glogiewicz, Centro para la Conservación del Paisaje; and Francisco Escobedo, University of Florida.
iv

Contents

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Study Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Urban Forest Inventory Sampling Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Field Plot Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Land Use, Cover, and Building Information . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Tree and Shrub Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Statistical Procedures and the Urban Forest Effects Models . . . . . . . . . . . . . . . . . . . 9 

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Inventory Plot Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Land Use and Forest Cover Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Tree and Shrub Canopy Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Ground Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Urban Forest Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Urban Forest Tree and Shrub Species Composition . . . . . . . . . . . . . . . . . . . . . . . 20

Species Diversity Indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Urban Tree Damage and Crown Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Urban Forest Effects Model Results for Ecosystem Services . . . . . . . . . . . . . . . . . . 31 

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Urban Forest Land and Tree Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Urban Forest Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Species Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Tree Damage and Forest Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Ecosystem Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Next Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Abstract

We present information on the urban forests and land uses within the 

watershed of Puerto Rico’s 21 658-ha San Juan Bay Estuary (SJBE) based 

on urban forest inventories undertaken in 2001 and 2011. We found 2548 ha 

of mangrove and subtropical moist secondary forests covering 11.8 percent 

of the total watershed area in 2011. Average tree cover in the study area 

was 24.1 percent overall, ranging from 12.2 percent cover on commercial/

industrial/transportation land uses to 69.0 percent on the watershed’s 

mangrove forests. This forest cover was created by approximately 

10.1 million trees, which stored 319 737 metric tons of carbon (C) in 2011 

and sequester C at a rate of 28 384 metric tons/year. The estimated value 

of the C storage by trees in the SJBE watershed was $8.1 million with an 

annual C sequestration value of $718,113 in 2011, up from the 2001 values 

of $4.0 million in stored C and an annual rate of $349,261.

In 2011 approximately 19 000 megawatts of energy required for cooling 

buildings were avoided due to tree shading and climate effects in 

residential and commercial areas and equated to 1986 metric tons of 

avoided C emissions due to building energy effects. 

The inventories identified 75 tree and shrub species in 2001 and 86 species 

in 2011. Red, black, and white mangroves (Rhizophora mangle, Avicennia 

germinans, and Laguncularia racemosa) were the most common species 

due to the watershed’s extensive mangrove forests, while tulipán africano 

(Spathodea campanulata) and María (Calophyllum antillanum) were 

predominant species in the moist forest patches and developed land uses. 

The occurrence of tree pests, diseases, and natural or anthropogenic damage 

was relatively low (12.7 percent) and generally of minor severity. Tree 

crowns did not show appreciable amounts of dieback or defoliation. 

Urban forest benefits can be increased by tree-establishment and protection 

programs as an estimated 16.8 percent of the estuary is potentially 

plantable. Benefits also can be lost by deforestation of the existing forest 

canopy caused by urban development and other activities. Proper planning 

and management can sustain or enhance the existing urban forest to 

increase the environmental and societal benefits from trees in the SJBE 

watershed.

Keywords: Caribbean, ecosystem services, FIA, forest inventory, Puerto 

Rico, subtropical forest, urban forest.

San Juan Bay Estuary Watershed

Urban Forest Inventory

Thomas J. Brandeis, Francisco J. Escobedo, Christina L. Staudhammer, 

David J. Nowak, and Wayne C. Zipperer

Introduction

The 21 658-ha San Juan Bay Estuary (SJBE) watershed lies 

along the northeast coast of the island of Puerto Rico. It is at 

the heart of the dynamic, expanding San Juan metropolitan 

area, which has a total population of 2,478,905 people 

according to the 2010 U.S. Census; the population density 

averages 3,215 persons/km2

 but in some areas exceeds 8,300 

people/km2 (Villanueva and others 2000). This ecologically 

important area encompasses San Juan Bay, several large 

lagoons and channels, and extensive wetlands and forests, 

all in close proximity to this densely populated area. 

Historically, forest covered much of the estuary’s watershed. 

Mangrove forest (composed of Rhizophora mangle, 

Avicennia germinans, and Laguncularia racemosa) fringed 

the bays and lagoons, where it was protected from the surf 

and wind. A diverse mix of species (Casearia guianensis, 

Calophyllum antillanum, Coccoloba uvifera, Manilkara 

bidentata, Sideroxylon foetidissium, and Tabebuia 

heterophylla, to name just a few of the principal species) 

that grew in the moist coastal plain forests and scattered 

karst hills (known locally as mogotes) was found farther 

inland (Departamento de Recursos Naturales y Ambientales 

2007, Little and Wadsworth 1989, Wadsworth 1950).

A subtropical moist forest remnant in the 

San Juan Bay Estuary watershed.
2

Today, however, forest cover in the estuary has been 

greatly reduced by human activities. The native Taíno 

peoples had little impact on the forests of the estuary 

(Domínguez Cristóbal 1989), but since the founding of 

San Juan by Juan Ponce de León in 1521, the estuary has 

been subjected to a long and continued decrease in forest 

cover, filling in of mangrove swamps, wetlands draining, 

stream channelization, and other hydrological modifications 

(Seguinot Barbosa 1996). Historical records and previous 

island-wide forest inventories have shown a pattern of 

deforestation for agricultural production, then abandonment 

of the agricultural land as manufacturing began to dominate 

the Puerto Rican economy, and finally reversion of the land 

to secondary forest (Birdsey and Weaver 1982, Franco and 

others 1997). This pattern of land use change also occurred 

in the SJBE watershed. Currently, near San Juan and other 

urban areas of Puerto Rico, former agricultural land and the 

remaining forest are now cleared for urban development 

(Grau and others 2008, López and others 2001, Martinuzzi 

and others 2007, Parés-Ramos and others 2008, Ramos 

González 2001, Thompson and others 2007). 

The estuary’s remaining forest cover is now greatly 

reduced, highly fragmented, and threatened by further 

urban development. Mangroves have been removed from 

the margins of many waterways, with losses ranging from 

28 to 67 percent from 1936 to 1995 (U.S. Environmental 

Protection Agency 2007). But mangrove areas are generally 

not suitable for most types of urban development, so a large, 

relatively contiguous mangrove forest still exists within the 

estuary. This mangrove forest, the largest in Puerto Rico, is 

to some extent protected within the Piñones Commonwealth 

Forest (Villanueva and others 2000). Conversely, virtually 

none of the original moist coastal forest remains. Most of 

the estuary’s land that formerly held moist coastal forest has 

been cleared due to its suitability for urban development. 

Small stands of secondary forest consisting of a mix of 

native and introduced species remain scattered in a matrix of 

urban development, particularly on the mogotes across the 

metropolitan area. 

The reduction of forest cover in the SJBE watershed has 

wide-ranging consequences for both terrestrial and marine 

wildlife and human residents. Ten of the 160 species of 

birds, 4 of the 19 species of amphibians and reptiles, and 

the manatee (Trichechus manatus) that reside in the SJBE 

watershed are listed as threatened or endangered at a Federal 

or Commonwealth level (Villanueva and others 2000). The 

estuary and its wildlife provide recreational opportunities 

for San Juan residents. Activities in the estuary’s lagoons 

and canals include picnicking, swimming, boating, fishing 

for tarpon (Megalops atlanticus) and snook (Centropomus

undecimalis), crabbing, and shellfish collection. 

Recognizing the estuary’s importance and the seriousness of 

the threats that this ecosystem faces, the U.S. Environmental 

Protection Agency added the SJBE to its National Estuary 

Program in 1992. The Río Piedras subwatershed within the 

larger SJBE was designated an Urban Long Term Research 

Area (ULTRA) by the National Science Foundation and 

Forest Service, U.S. Department of Agriculture (USFS) in 

2010.

The presence of trees and green areas improves the quality 

of life for all San Juan residents (Seguinot Barbosa 1996). 

Street trees in a 

residential neighborhood 

in San Juan, Puerto Rico.
3

San Juan’s urban trees also have important aesthetic, 

psychological, and environmental value in this heavily 

urbanized environment. Street trees, gardens, green areas, 

and urban parks complement the architecture of a city and 

increase the economic value of properties. People form 

strong emotional and spiritual associations with trees, 

and their presence helps reduce stress for urban dwellers 

(Nowak and others 1997a). Additional evidence indicates 

that exposure to trees contributes positively to human health 

(Donovan and others 2013). 

While some aspects of the importance of urban tree cover 

are difficult to express in concrete terms, trees also provide 

ecosystem services whose benefits can be valued monetarily 

to more clearly quantify their importance to planners and 

policymakers (see Jones and others 2013 for a review of the 

economic benefits of urban trees and Escobedo and others 

2011 for a review of urban forest ecosystem services and 

disservices related to pollution mitigation). Dollar values 

can be placed on the benefits provided by urban trees in 

sequestering carbon (C), moderating temperature, improving 

air quality, and mitigating hydrological problems brought 

on by development. Trees in the urban forest can affect 

building energy use and can sequester C as they grow, 

contributing to an area’s larger C balance (McPherson 1998, 

Nowak 1993, Nowak and Crane 2002, Rowntree and Nowak 

1991). The blocking of solar radiation by tree canopies is 

particularly important in the tropical climate of San Juan. 

A single shade tree can block 70 to 90 percent of the solar 

radiation that would otherwise shine on a single-story home 

(Heisler 1986a, 1986b). Shade trees have the potential to 

reduce interior temperatures of adjacent, non-air-conditioned 

buildings, and provide significant energy savings to homes 

with air conditioning (Donovan and Butry 2009, Simpson 

and McPherson 1998). 

On a larger scale, the cumulative effect of solar energy 

interception by tree canopies (shading buildings, cement, 

and asphalt surfaces) and their subsequent transpiration 

has the potential to counteract the urban “heat island” 

effect (Heisler and others 1995, McPherson 1994). Trees 

also facilitate the removal and mitigation of many air 

pollutants commonly emitted in urban areas. Some airborne 

pollutants are directly absorbed by tree leaf tissues, while 

other pollutants, especially particulates, are also physically 

filtered from the air as it passes through tree crowns 

(Abdollahi and Ning 1996; Nowak 1994, 2002; Nowak 

and others 1997b, 2006). Pollutants are eventually washed 

or blown off the tree, absorbed in the leaves, or deposited 

onto the forest floor, where chemical processes or the 

biologically active soil community can potentially break 

them down. Urban forests also can mitigate the effects of 

heavy precipitation. The prevalence of impervious surfaces 

in urban areas exacerbates problems with stormwater as 

tropical depressions and hurricanes can yield high levels of 

precipitation in a short time. Interception and evaporation 

of rainwater by trees, and infiltration through the permeable 

forest floor, can substantially reduce runoff and flooding 

(Armson and others 2013, McPherson and Rowntree 1991, 

Nowak and others 1997a). 

Management to maximize the delivery of beneficial services, 

particularly in an area as biologically and ecologically 

diverse as the SJBE watershed, requires accurate 

assessments of current ecosystem conditions. The study’s 

overall goal is to provide some of the necessary information 

to make such assessments. Specifically, we estimate how 

much of the watershed is covered with forest and trees; how 

many trees are in the urban forest; how much C is stored 

in those trees; what the average ground, tree, and shrub 

covers are for each developed land use; what the species 

composition of both forested land and planted trees is in 

urbanized areas; and how healthy those trees are in terms of 

pests, diseases, and tree crown conditions. Additionally, we 

examine how some of these factors have changed between 

2001 and 2011.
4

Methods

Study Site

The SJBE watershed, bound north to south by 

18°28’ N and 18°19’ N latitude and west to 

east by 66°90’ W and 65°53’ W longitude, falls 

within the subtropical moist forest life zone 

(sensu Holdridge 1967) (fig. 1). Mean annual 

rainfall is between 1500 and 1700 mm, with an 

average annual temperature measured at the Old 

San Juan Climate Station of 27.2 °C to 23.9 °C, 

with an annual average of 25.9 °C (Lugo and 

others 2011). The upper reaches of the SJBE 

watershed lie on the northern foothills of the 

Puerto Rican Cordillera Central. These volcanic 

rock-derived hills slope down to meet the coastal 

plains upon which lies the San Juan metropolitan 

area. The coastal plains are made of surficial 

deposits of alluvial materials, sands, and other components 

over sedimentary bedrock. The plains are punctuated in 

areas by steep-sided limestone mogote outcroppings. 

Detailed description of the substrates underlying the SJBE 

watershed can be found in Lugo and others (2011).

Figure 1—San Juan Bay Estuary’s watershed in Puerto Rico.

048 2 Kilometers

San Juan Bay Estuary watershed

Public forests and nature reserves

6

Atlantic Ocean

San Juan Ecological Corridor

San Patricio

Forest

Ciénaga las Cucharillas

San Juan

Bay

Laguna Los Corozos

Canal San Antonio

Laguna de Condado Laguna La Torrecilla

Piñones Forest

Laguna

de Piñones

Laguna San Jose

Caño Martin Peña

Río Grande de Loíza

67°20'0"W 66°15'0"W

18°20'0"N

67°20'0"W 66°15'0"W

18°20'0"N

The southern portion of the San Juan Bay Estuary watershed, a 

hilly area with soils derived from volcanic materials is less densely 

developed and has higher tree cover. Note the limestone “mogote” 

hills rising from the Coastal Plain in the distance to the north.
5

The geomorphology and hydrology of the area are 

naturally complex and have been modified by human 

activities. The Río Bayamon lies along the western 

edge of the watershed and the Río Loíza along 

the west. The Río Piedras, Río Puerto Nuevo, and 

numerous smaller streams and channels also bring 

freshwater from the mountains into the watershed, 

where it mixes with saltwater from the Atlantic 

Ocean in the many estuarine areas such as San Juan 

Bay, Condado, San José Lagoon, La Torrecilla and 

Piñones Lagoons, and the channels that connect them. 

Dredging, landfills, and other human activities have 

changed the hydrological characteristics of the estuary 

considerably (Lugo and others 2011, Villanueva and 

others 2000).

Although most of the land in the SJBE watershed is 

privately owned, there are public forests and nature 

reserves, too. The 630-ha Piñones Commonwealth 

Forest falls entirely within the SJBE watershed to 

the northeast. The San Juan Ecological Corridor, a 

series of connected green spaces that includes the 

University of Puerto Rico’s Experimental Station and 

Botanical Garden, and the Bosque Nuevo Milenio Urban 

Commonwealth Forest, has a total area of approximately 

200 ha. Additionally, there is the small Bosque San Patricio 

Urban Commonwealth Forest (approximately 27 ha) 

on mogote hills within the watershed. The Ciénega las 

Cucharillas is an extensive saltwater and freshwater 

wetlands area in the northeastern portion of the watershed.

Mangrove forest in the Piñones Commonwealth Forest, Puerto Rico.

Laguna San Jose in the San Juan Bay Estuary watershed, 2001. 

(photo by Jeffery Glogiewicz, Consultores Ambiental and the 

Fundación Puertorriqueña de Conservación)
6

Urban Forest Inventory Sampling Design

Because we lacked a current land use map of the watershed 

to use for stratification, we chose to systematically sample 

the watershed. This approach will allow us to better follow 

long-term changes in land use in this highly dynamic 

landscape and to fully incorporate the urban forest inventory 

data into the concurrent island-wide forest inventory carried 

out by the USFS’ Forest Inventory and Analysis (FIA) 

program (see Brandeis 2003, Brandeis and Turner 2013, 

Brandeis and others 2007 for details on the island-wide 

forest inventories). 

The island-wide forest inventory provided a framework for 

our systematic sampling grid. The FIA program overlays 

a hexagonal sampling grid over the area to be inventoried 

(McCollum 2001, Reams and others 2005). Each hexagon 

in the standard FIA grid used on the continental United 

States has an area of approximately 2400 ha with a sampling 

plot in the center of each hexagon, or located a random 

distance and azimuth from that center (Reams and others 

2005). Studies in North American cities indicated that two 

hundred 0.004-ha (1

/10-acre) plots (for a total of 8.08 ha or 

20 acres sampled) produced standard errors of 10 percent 

for estimates of the mean number of trees/ha over the 

entire urban area.1 Broken down by land uses in the urban 

area, data from these studies yielded standard errors of 

5.9 percent for mean percent tree cover in urban parks, 

2.4 percent for mean percent tree cover in residential areas, 

and 1.0 percent for mean percent tree cover in commercial/

industrial areas. Based on these results, for our study we 

chose to decrease within-plot variation by increasing the 

size of sampling plots rather than decrease between-plot 

variation by increasing the number of sampling plots. 

The SJBE watershed is covered with 11 standard-sized 

FIA hexagons. Therefore, we intensified the base grid 

(decomposed it into smaller hexagons) by a factor of 12. We 

had 108 sampling points within the watershed boundaries 

once we removed points that fell onto census water (streams, 

sloughs, estuaries, canals, and other moving bodies of 

water at least 200 m wide, and lakes, reservoirs, ponds, and 

other permanent bodies of water at least 1.8 ha in area). 

Preliminary classification of the points using a 1991 land 

cover map from Helmer and others (2002) produced from 

LandSat Thematic Mapper imagery indicated that there are 

potentially 21 points with forest, 60 urban/barren points, 

and 28 points on agricultural land (primarily pasture). This 

point distribution within the strata exceeded the minimum 

of 10 points per stratum for urban areas recommended by 

Nowak and others (2001). 

Field Plot Designs

Field crews first visited the plots from July 2001 to February 

2002 (hereafter referred to as the 2001 data), and then again 

from May 2010 to March 2011 (hereafter referred to as 

the 2011 data). Two plot designs were used, each with the 

same total sampled area 0.067 ha (1

/6 acre). This plot size 

and number of plots gave a potential total sampled area 

of 7.35 ha, although the final sampled area depended on 

whether we had access to the sampling point. 

A standard FIA subplot cluster was installed in areas 

that met the Caribbean FIA criteria for forested land: a 

contiguous area >0.4 ha, or >30 m wide for forested strips, 

with >10 percent canopy coverage in trees (Bechtold and 

Scott 2005, U.S. Department of Agriculture Forest Service 

2007). In 2011, additional urban forest inventory data were 

collected on the FIA-style plots following FIA urban forest 

inventory protocols (U.S. Department of Agriculture Forest 

Service 2006). Single 14.6-m-radius circular plots used to 

collect data are part of the Urban Forest Effects (UFORE) 

model and thus urban forest inventories were installed in 

urban and agricultural lands that did not meet minimum 

requirements for forest under FIA’s definitions (Nowak 

and others 2005). Small patches (>0.4 ha) of tree-covered 

land that did not meet the minimum area requirements were 

considered urbanized and usually categorized as vacant. 

All plots are considered permanent and fully monumented 

to allow future remeasurement and assessment of forest and 

land use changes. Plot center is located on the ground by 

using Global Positioning System (GPS) units, road maps, 

and aerial photographs, and relocated by using previous 

measurements to buildings, trees, and other nearby features. 

When a plot center fell on a building or other surface where 

the center point could not be accessed, a GPS unit was used 

to calculate the offset location. Photographs were taken of 

most plot centers at both measurement periods.

1

Nowak, D.J.; Crane, D.E.; Steven, J.C. 2002. [Untitled]. [Unpaged]. 

Unpublished data. On file with: Northern Research Station, Forest Inventory 

and Analysis, Syracuse, NY 13210.
7

A modified UFORE plot design was used in some areas with 

extremely high stem densities that still did not meet the FIA 

minimum size requirements for forest. These dense stands 

of small (diameter at breast height [d.b.h.] <12.5 cm) trees 

were a problem because the UFORE plot design does not 

subsample saplings, unlike the FIA plot design. Numbers of 

small trees to be measured could reach into the hundreds in 

these situations. In those cases tree, shrub, and ground cover 

estimates (described below) were made for the entire plot 

area, but tree measurements were taken on only one-fourth 

(northeast quadrant between 0 and 90 degrees) or one-half 

(between 0 and 180 degrees) of the plot. There was one plot 

in 2001 where a 1⁄4-UFORE plot was used. In 2011 there 

were three plots where a 1⁄4-UFORE plot was installed and 

one instance of installing a 1⁄2-UFORE plot.

Land Use, Cover, and Building Information

At each plot crews noted the land use or forest type. 

Developed land uses were commercial/industrial, 

institutional, park, residential, transportation, and vacant. 

The commercial/industrial land use was typified by the 

presence of buildings dedicated to business activities, 

manufacturing, etc., and included outdoor storage/staging 

areas as well as parking lots in downtown areas that are not 

connected with any institutional or residential use. Examples 

of a typical institutional land use would be schools, grounds 

of a large government agency, hospitals/medical complexes, 

colleges, religious buildings, and government buildings. 

Residential areas had buildings that were predominantly 

single-family or multifamily structures and their related 

Urban forest inventory field crews described land use and measured vegetation at systematically selected points 

across the San Juan Bay Estuary watershed. (photo by Jeffery Glogiewicz, Consultores Ambiental and the Fundación 

Puertorriqueña de Conservación)
8

green spaces. Roads; median strips; limited access 

roadways; railroad stations, tracks, and yards; shipyards; 

airports; etc.; and their related green spaces were considered 

transportation land uses. Vacant areas had no apparent 

use, few or boarded-up buildings, and vacant structures 

in the immediate vicinity; were not being actively used 

or developed; and were not large enough or did not have 

enough tree cover to be considered forest. Undeveloped, 

nonforest land uses were agriculture (cropland, pasture, 

orchards, vineyards, nurseries, farmsteads, and related 

buildings, feed lots, and rangeland) and noncensus water 

bodies and wetlands (wide streams, rivers, lakes, and 

other water bodies). The two broadly defined forest types 

encountered in the SJBE watershed were mangrove forest 

and moist secondary forest. 

At each plot the percentages of tree and shrub canopies 

covering the plot were estimated by visualizing the cover in 

a cylinder bounded by the plot boundaries and projecting it 

onto the ground. Trees (woody vegetation that was at least 

2.5 cm d.b.h.) and shrubs (<2.5 cm d.b.h.) had to be at least 

30 cm tall to be included in the estimate. 

Crews estimated ground cover as a percentage of the plot 

area covered by buildings, impervious surfaces (for example, 

concrete or asphalt), permeable surfaces (such as gravel, 

bare soil, sand, mulch, or leaf litter), herbaceous vegetation 

(agricultural crops, grass, low shrubs <30 cm tall), and 

water. Additionally, an estimate was made of the proportion 

of the plot area that was available for planting trees. Cover 

amounts were estimated to the nearest 5 percent.

Tree and Shrub Information

The term “tree” here applies to individual woody plants that 

are capable of growing to a minimum d.b.h. of 12.5 cm and 

5-m height. This category included palms but not nonwoody 

vegetation such as bananas or bamboo. Distinguishing 

between a tree and a shrub was often difficult and subjective. 

For each tree within the plot (regardless of the plot layout), 

distance and azimuth from plot center to the tree were noted, 

as were tree status (live, dead, or removed) and whether the 

tree falls on public lands (for example, Federal, State, or 

municipality). Crews also noted whether the tree is a street 

tree, typically meaning it was planted in the space between 

the edge of the road and the sidewalk; however, other 

examples may include areas where no sidewalks are present 

or median strips. A reconcile code (such as ingrowth, 

through growth, missed live, or missed dead) was added for 

plot remeasurement and every effort was made to account 

for all trees previously measured.

Urban forest inventory field crew assessing the condition of street 

trees planted along the Avenida Isla Verde, San Juan.

Tree species was noted and d.b.h. was measured at 1.4 m. 

For forked trees with multiple possible d.b.h. measurements, 

we followed the FIA guidelines described in the FIA field 

manuals (U.S. Department of Agriculture Forest Service 

2011). Multistemmed trees such as Dypsis species palms, 

Ficus species, and other hedges >1.8 m tall with multiple 

stems >2.5 cm were treated as multiple individuals as per 

FIA guidelines. Height to top of tree and height to base of 

live crown were measured. Type, severity, and location of 

damage, if present, were recorded (as per the guidelines in 

U.S. Department of Agriculture Forest Service 2006). 

Tree crown condition was evaluated by using the following 

parameters (see Schomaker and others 2007 for details on 

crown data collection protocols). Uncompacted live crown 
9

ratio is a percentage determined by dividing the live crown 

length by the actual tree height. Crown light exposure 

describes how many sides of the tree crown receive direct 

sunlight. Crown density is the amount of crown branches, 

foliage, and reproductive structures that blocks light 

visibility through the crown and serves as an indicator of 

expected growth in the near future. Crown dieback is a 

percentage of the live crown area, including the dieback 

area. Foliage transparency is the amount of skylight visible 

through the live, normally foliated portion (where foliage, 

normal or damaged, or remnants of its recent presence can 

be seen) of the crown. Crown width is described with two 

measurements: one at the crown’s widest diameter and the 

second at 90 degrees to the widest diameter.

For trees >6 m tall and within 18 m of buildings, distance 

and azimuth to the buildings were recorded to estimate the 

tree’s impacts on building energy. Buildings were defined 

as space-conditioned residential structures (heated and 

cooled) that are ≤3 stories (2 stories + attic) in height above 

ground level. The UFORE model utilizes an algorithm for 

single standing structures ≤370 m2

 in total inhabitable space, 

although larger single-family homes or duplexes should be 

included regardless of size. Unheated garages, sheds, and 

similar outbuildings are not included.

Statistical Procedures and the Urban Forest 

Effects Models

Individual plot and tree data were compiled and processed 

by using the Urban Forest Effects (currently known as 

i-Tree Eco) suite of models (UFORE ACE2

 ver. 6.5). The 

UFORE models use data collected in the field along with 

hourly air pollution and meteorological data from other 

sources to quantify urban forest structure and function 

(Nowak and Crane 2000). The total number of trees 

and C stored in the forests and urban areas in the SJBE 

watershed were estimated by multiplying the estimated 

area in each stratum by the per-hectare estimate of each 

parameter calculated from the plot data. For instance, 

where a 1⁄4- or 1⁄2-UFORE plot was installed, the individual 

tree’s expansion factors were adjusted accordingly. Totals, 

averages, and standard errors were calculated for species, 

land uses, and watershed totals.

Individual tree aboveground biomass and C were estimated 

by using allometric equations published in scientific 

literature and described in Nowak (1994) and Nowak and 

others (2002). When multiple allometric equations existed 

for a single species, those equations were combined, or 

“splined” into a single equation (Nowak and Crane 2002). 

The distance and direction of trees to adjacent 

buildings were measured to model the effects of tree 

shading on energy use.

2

ACE = Anatomy, Carbon, Energy; formerly the old nomenclature for the 

different submodels in UFORE, now referred to as Eco.
10

When a species-specific equation was not available, the 

results from using other equations for species of the same 

genus were averaged, and if there were no equations for 

the genus, then the average result from all broadleaf or 

conifer species equations was used (Nowak and Crane 

2002). Belowground biomass and C were estimated by 

using an aboveground-to-belowground biomass ratio of 

0.26 derived from Cairns and others (1997). Biomass 

values were converted to C by multiplying by 0.5 as per 

Nabuurs and others (2003). Leaf area and leaf biomass 

were estimated by using regression equations for temperate 

deciduous urban species described in Nowak (1996). Gross 

and net C sequestration were calculated according to the 

methods described in Escobedo and others (2010) and 

Nowak and others (2002, 2008).

Species richness (the number of species sampled within 

the watershed or each land use type) and diversity indices 

(Shannon-Wiener, Menhinick’s, Simpson’s, Shannon-Wiener 

evenness, and Sander’s Rarefaction) for the watershed and 

land uses were calculated.

The reduction of energy use and the associated C emissions 

from power plants were estimated by using the methods 

described in McPherson and Simpson (1999). These savings 

are presented as megawatt-hours (MWh) saved that would 

be otherwise used for cooling residential structures, and the 

mega British Thermal Units and MWh that would have been 

used for heating.

Native and introduced trees growing on the grounds of a site that was formerly used for a governmental institution and 

is now classified as vacant land.
11

Results

Inventory Plot Distribution

Urban forest inventory plots were installed on 99 of 

the 108 potential sampling points during the 2001–02 

measurement period, and 94 points were relocated and 

measured during 2010–11 (table 1). Some potential 

sampling points were inaccessible, so no plots were 

established at either time, for example, in flooded 

wetland areas that were far from any roads. We 

categorized these areas by using aerial photographs. 

Other points were not sampled after landowners denied 

access during one or both time periods. In 2011, six 

sampling points were inaccessible because they were 

in flooded wetlands or in a mangrove forest far from 

any roads. Five sampling points fell in water, and 

access was denied by the landowner at three sampling 

points. Agricultural land in the Loíza area of the 

watershed is underrepresented by the field sampling 

because of a combination of impassable seasonally 

flooded pasture and our inability to contact landowners 

to gain access.

Table 1—Number of plots and trees sampled by land use, area, plots sampled, number of trees, and 

sampled area, San Juan Bay Estuary watershed, 2001 and 2011

Land use Area

Total

area

Plots

sampled Trees 

Sampled

area

2001 2011 2001 2011 2001 2011

- ha - - percent - - - - - - - - - number - - - - - - - - - - - - ha - - - -

Commercial/industrial/transportation 4 459 20.6 21 23 32 84 1.42 1.55

Institution/park 2 336 10.8 11 9 81 22 0.74 0.61

Mangrove forest 1 486 6.9 4 3 547 730 0.27 0.20

Residential 7 856 36.3 34 34 240 190 2.29 2.29

Moist forest 1 062 4.9 7 6 1,044 766 0.47 0.40

Vacant 3 185 14.7 15 11 86 419 1.01 0.74

Wetland/water/agriculture 1 274 5.9 7 8 NA NA 0.47 0.54

Total 21 658 100.0 99 94 2,030 2,211 6.68 6.34

NA = not applicable.

Numbers in rows and columns may not sum to totals due to rounding.

Pasture that is periodically inundated is a widespread land use 

in the eastern portion of the San Juan Bay Estuary watershed in 

the Loíza municipality. (photo by Jeffery Glogiewicz, Consultores 

Ambiental and the Fundación Puertorriqueña de Conservación)
12

Table 1 presents plot distribution according to primary 

land use categories and accessibility. Eight plots fell on 

wooded areas with either mangrove or moist secondary 

forest that was large enough to meet the FIA definition of 

forest, so an FIA-style subplot cluster with nested microplot 

was installed instead of a single, circular plot. We decided 

that there were too few plots in the transportation land use 

to analyze separately. Therefore, we grouped those plots 

with the commercial/industrial land. We also grouped the 

institutional and park categories into one category for the 

same reason.

From 2001 to 2011, only two sampling points changed land 

use category and both of these points had plots installed 

on them. One plot that fell on institutional land changed to 

vacant land after that institution’s buildings were demolished 

and the area had yet to be redeveloped. Another area was 

originally categorized as vacant land recently cleared for 

development and had new houses on it when revisited.

Land use change from vacant, recently cleared land to a residential 

area (Urbanización El Coquí II) in the 10 years between urban forest 

inventories in the San Juan Bay Estuary watershed. (left photo 

by Jeffery Glogiewicz, Consultores Ambiental and the Fundación 

Puertorriqueña de Conservación; right photo by Edgardo González, 

Centro para la Conservación del Paisaje)

Trees naturally regenerate on unused lands, in this case at the site of 

a closed public institution, from 2001 (left) to 2010 (right). (left photo 

by Jeffery Glogiewicz, Consultores Ambiental and the Fundación 

Puertorriqueña de Conservación; right photo by Angie Rowe, 

U.S. Forest Service)
13

Land Use and Forest Cover Estimates

We estimated that mangrove and moist forests covered 

2548 ha in the SJBE watershed, 11.8 percent of the area 

(table 1) in 2011. It should be remembered that for an 

area to be considered forest, it had to meet a minimum 

area requirement of 0.4 ha. Scattered patches of trees 

around the SJBE watershed were not included in the 

forest categories; rather they were put in the urbanized 

categories such as vacant/barren land use, which occupied 

14.7 percent of the watershed. Residential areas were the 

largest single use of land in the watershed (36.3 percent), 

followed by urbanized areas for commercial, industrial, 

and transportation uses (20.6 percent) (table 1). In 2011 

residential land had 24.1 percent tree cover, vacant land 

22.7 percent tree cover, and land used for commercial/

industrial/transportation purposes 12.2 percent tree 

cover (table 2). Overall, there seemed to be a pattern of 

increasing tree cover from 2001 to 2011 in the developed 

land uses, particularly in vacant land (table 2).

Tree and Shrub Canopy Cover

Field crews estimated tree and shrub canopy cover for all 

land uses, along with an estimate of the percentage of the 

plot area available for planting with additional trees or 

shrubs (excluding forested areas). These estimates were 

summarized by land use during each sampling period 

(table 2). There were definitional changes between the 

two time periods on how to estimate tree cover, shrub 

Table 2—Mean percent tree cover, shrub cover, and plantable space with standard errors of the mean by land 

use, San Juan Bay Estuary watershed, 2001 and 2011

Land use

Tree cover Shrub cover Plantable space

2001 2011 2001 2011 2001 2011

% SE % SE % SE % SE % SE % SE

Commercial/industrial/transportation 4.9 2.5 12.2 4.7 0.8 0.5 2.5 1.3 13.0 5.2 11.1 4.3

Institution/park 11.5 2.4 14.4 4.1 1.6 1.0 3.6 1.5 40.3 11.3 18.9 9.0

Mangrove forest NA NA 69.0 6.7 NA NA 53.0 18.5 NA NA 0.0 0.0

Residential 19.3 2.8 24.1 3.4 7.6 1.3 9.5 1.5 19.0 3.4 12.1 1.8

Moist forest NA NA 65.5 9.6 NA NA 60.9 7.8 NA NA 0.0 0.0

Vacant 10.0 5.0 22.7 8.3 6.1 3.2 25.9 9.8 76.9 8.6 50.0 13.4

Wetland/water/agriculture NA NA 0.0 0.0 NA NA 1.3 1.3 NA NA 12.5 12.5

Total NA NA 24.1 2.1 NA NA 14.9 2.1 NA NA 16.8 2.6

SE = standard error; NA = not applicable.

cover, and plantable space in the forested and wetlands 

areas, so 2001 estimates are not comparable to the 2011 

estimates. The later estimates were made with a superior 

procedure. Therefore, we did not include those estimates 

for 2001 in table 2. Estimates for the developed land uses, 

however, were made by using consistent procedures and are 

comparable. 

Natural tree regeneration on vacant land in the San Juan Bay 

Estuary watershed. (photo by Jeffery Glogiewicz, Consultores 

Ambiental and the Fundación Puertorriqueña de Conservación)
14

In 2011 the highest percentages of tree cover were found, 

of course, in the forested areas, and ranged from 65.5 to 

69.0 percent (table 2, fig. 2). Residential and vacant areas 

had the next highest tree covers, 24.1 and 22.7 percent, 

respectively. Vacant areas had the highest percentage 

of shrub cover (25.9 percent) for developed land uses. 

Developed, nonvacant land uses had between 11.1 and 

18.9 percent of their area available for planting additional 

trees or shrubs. From 2001 to 2011 there were increases 

in the average tree and shrub cover percentages in the 

developed land uses, and some indication of minor decreases 

in plantable space (table 2).

Figure 2—Percent shrub, tree and building cover by land use, 

San Juan Bay Estuary watershed, 2011.

Land use

Mangrove

Moist forest

Commercial/industrial/

transportation

Institution/park

Residential

Vacant/barren

Average

Percent cover

0

10

20

30

40

50

60

70

80

Shrub 

Tree 

Building

Ground Cover

In table 3 we present the mean ground cover percentages 

by land use for 2011. The ground cover percentages for 

2001 were not appreciably different from those recorded 

in 2011. At a typical point within the SJBE watershed, 

excluding larger bodies of water considered census waters, 

10.6 percent of the ground was covered in duff/mulch, 

20.7 percent in herbaceous plants (excluding grass), 

15.9 percent in grass (both maintained and unmaintained), 

8.2 percent in water, 17.0 percent with buildings, and 

27.6 percent with impervious (for example, cement or tar) 

and other surfaces (table 3, fig. 3). For both developed land 

uses and undeveloped forests, on average, 55.4 percent 

of the ground in the SJBE watershed was covered with 

permeable surfaces (such as grass, vegetation, or water) and 

44.6 percent was covered in impervious surfaces (such as 

cement, tar, or buildings).

As would be expected, forested and vacant land uses 

were covered almost entirely in permeable surfaces while 

impervious surfaces and buildings predominated in the 

developed land uses. Average ground cover of the two 

predominant urbanized land uses in the SJBE watershed, 

commercial and residential, is presented in figures 4 and 5. 

Urban Forest Structure

Two thousand and thirty trees and shrubs from 75 species 

were tallied in 2001 and 2,211 trees and shrubs from 

86 species were tallied in 2011. The largest trees found 

on the inventory plots were a mango (Mangifera indica) 

with a d.b.h. of 105 cm and a tulipán africano (Spathodea 

campanulata) with a height of 26.0 m.

As residential areas are developed and mature, the tree and shrub cover on them changes, here from 2001 (left) to 2011 (right). (photos by 

Jeffery Glogiewicz, Consultores Ambiental and the Fundación Puertorriqueña de Conservación)
15

Table 3—Mean percent ground cover with standard errors of the mean by land use, San Juan Bay Estuary 

watershed, 2011

Land use

Ground cover

Duff/

mulch Herbaceous Grass Water Building

Impervious 

and other

% SE % SE % SE % SE % SE % SE

Commercial/industrial/transportation 7.0 3.0 4.3 2.4 17.5 5.6 0.0 0.0 16.4 5.3 54.8 6.9

Institution/park 3.6 2.2 3.3 1.4 43.3 13.4 1.7 1.7 15.3 9.1 32.8 9.9

Mangrove forest 28.0 17.1 17.0 8.5 0.0 0.0 55.0 24.7 0.0 0.0 0.0 0.0

Residential 6.1 1.4 9.5 2.1 16.4 3.0 0.9 0.9 32.5 3.8 34.6 3.6

Moist forest 39.4 9.3 60.6 9.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Vacant 11.0 9.0 65.8 13.4 11.4 9.2 9.1 9.1 0.9 0.9 1.8 1.4

Wetland/water/agriculture 18.8 9.2 37.5 15.7 0.0 0.0 43.8 17.5 0.0 0.0 0.0 0.0

Total 10.6 2.1 20.7 2.5 15.9 2.5 8.2 2.4 17.0 2.0 27.6 2.2

SE = standard error.

Figure 3—Average ground cover for all developed land uses 

combined, San Juan Bay Estuary watershed, 2011.

Water

8.2%

Buildings

17.0%

Impervious/other

27.6%

Duff/mulch

10.6%

Herbaceous

20.7%

Grass

15.9%

Revegetation of land cleared adjacent to residential 

developments at an urban forest inventory plot 

center in 2001 (top) and 2011 (bottom) in the 

San Juan Bay Estuary watershed. (top photo by 

Jeffery Glogiewicz, Consultores Ambiental and 

the Fundación Puertorriqueña de Conservación; 

bottom photo by Edgardo González. Centro para la 

Conservación del Paisaje)
16

These sampled trees represent a total population in the 

watershed of 6.8 million trees (±958,000 trees) in 2001 and 

10.1 million trees (±1.8 million trees) in 2011 (table 4). This 

tree population stored 175 255 metric tons of C in 2001 

and net-sequestered 15 318 metric tons per year (table 4). 

The amount of C stored by the watershed’s trees increased 

to 319 737 metric tons in 2011 and the net C sequestration 

rate had increased to 28 384 metric tons per year (table 4). 

Side-by-side comparisons of trees/ha with d.b.h. ≥2.5 cm by 

inventory year and land use show that mangrove and moist 

forest areas have much higher stem densities, C storage, and 

net C sequestration rates than the developed land uses, as 

would be expected (figs. 6, 7, and 8).

Figure 4—Average ground cover for the combined commercial/

institutional/transportation land use, San Juan Bay Estuary 

watershed, 2011.

Water

0.0%

Buildings

16.4%

Impervious/other

54.8%

Duff/mulch

7.0%

Herbaceous

4.3%

Grass

17.5%

Figure 5—Average ground cover for residential land use, San 

Juan Bay Estuary watershed, 2011.

Water

0.9%

Buildings

32.5%

Impervious/other

34.6% Duff/mulch

6.1%

Herbaceous

9.5%

Grass

16.4%

Patches of young, dense secondary subtropical moist forest are 

scattered across the San Juan Bay Estuary watershed.

Forests have many more trees/ha but occupy less of the 

landscape than developed urban areas and undeveloped 

forests; therefore, these three land uses within the watershed 

store comparable amounts of total C. The 2011 estimates 

show that developed areas have far fewer trees (54.1 trees/ha 

for commercial to 564.7 trees/ha for vacant areas) and less 

C stored/ha (6.0 metric tons C/ha for commercial land uses 

to 20.3 metric tons C/ha for vacant areas) than does forest 

(3,607 trees/ha with 52.9 metric tons C/ha for mangroves, 

and 1,893 trees/ha and 43.2 metric tons C/ha for moist 

forest) on average (figs. 6 and 7). But there are fewer 

forested hectares in the watershed (2548 ha of forest, which 

is 11.8 percent of the total watershed land area); thus even 

though 72.6 percent of the trees are in mangrove and moist 

forests, only 38.9 percent of stored C was found there. Trees 

growing in urbanized areas held 61.1 percent of the stored C.

A high percentage of the trees in the SJBE watershed are 

of a relatively small diameter (fig. 9). The “reverse-J” 

distribution seen in figure 9 is commonly observed in natural 

forests but also typical of most urbanized landscapes. In 

mangrove and moist forests, 36.0 percent and 45.6 percent 

of tree stems were between 2.5 and 5.1 cm d.b.h. and 

73.5 percent and 78.3 percent were <10.3 cm, respectively 

(table 5). The developed land uses had fewer saplings, 

ranging from a high of 63.9 percent of trees with 

d.b.h. <10.3 cm in vacant areas to a low of 37.4 percent in 

residential areas (table 5). 
17

Table 4—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch stored carbon, net carbon 

sequestration by land use for the urban forest inventory, San Juan Bay Estuary watershed, 2001 and 2011

Land use

Number of trees Stored carbon

Net carbon 

sequestration

2001 2011 2001 2011 2001 2011

- - - n - - - - - SE - - - - - n - - - - - SE - - metric 

tons

- - SE - - metric 

tons

- - SE - - metric 

tons/

year

- SE - metric 

tons/

year

- SE -

Commercial/

industrial/

transportation 100,328 92,881 241,438 123,663 2 137.2 2069.9 26 585.2 14 113.5 268.9 267.9 1546.2 744.6

Institution/

park 254,390 237,875 84,656 42,453 6 433.6 5807.8 16 922.5 8 332.8 670.9 614.7 993.5 440.1

Mangrove forest 3,012,865 1,735,841 5,362,580 1,497,690 50 590.3 35 076.4 78 596.3 5 313.7 5 736.0 890.1 11 262.2 469.3

Residential 821,738 584,172 650,906 125,081 72 081.9 55 832.9 86 928.5 25 622.1 4 582.4 3 287.4 4 361.4 907.3

Moist forest 2,347,976 2,191,968 2,009,218 474,573 31 581.1 29 389.5 45 871.7 14 353.2 3 267.1 2 977.9 4 424.2 1 149.5

Vacant 271,044 232,855 1,798,653 918,495 12 429.5 11 396.1 64 833.1 34 855.6 830.3 730.7 5 796.4 2 941.4

Total 6,809,586 958,293 10,147,451 1,828,843 175 255.4 34 515.2 319 737.2 48 726.6 15 318.5 1 870.0 28 383.9 3 430.0

D.b.h. = diameter at breast height; n = number of trees; SE = standard error.

Figure 6—Number of trees/ha (with d.b.h. ≥2.5 cm) by land use, 

San Juan Bay Estuary watershed, 2001 and 2011.

Land use

Number of trees (thousand) 1

2

3

4

5

2001

2011

Mangrove

Moist forest

Commercial/industrial/

transportation

Institution/park

Residential

Vacant/barren

Average

0

Figure 7—Carbon stored in trees/ha (with d.b.h. ≥2.5 cm) by land 

use, San Juan Bay Estuary watershed, 2001 and 2011.

Land use

Carbon (tons/ha)

0

10

20

30

40

50

60

2001

2011

Mangrove

Moist forest

Commercial/industrial/

transportation

Institution/park

Residential

Vacant/barren

Average
18

Figure 8—Net carbon annually sequestered by trees/ha (with d.b.h. 

≥2.5 cm) by land use, San Juan Bay Estuary watershed, 2001 and 

2011.

Land use

Net carbon sequestration

(tons/year/ha)

0

1

2

3

4

5

6

7

8

2001

2011

Mangrove

Moist forest

Commercial/industrial/

transportation

Institution/park

Residential

Vacant/barren

Average

The author in mangrove forest in the Piñones Commonwealth Forest.

Figure 9—Number of trees (with d.b.h. ≥2.5 cm) by diameter at breast height, San Juan Bay Estuary 

watershed, 2011.

Diameter at breast height (cm)

5 8

10

131518202325283033363841

43464851

5356

5861

6971768184

8694

102

Number of trees (million)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0
19

Table 5—Percentage of trees in each d.b.h. class by land use, San Juan Bay Estuary 

watershed, 2011

D.b.h. class 

Land use

Total

Commercial/ 

industrial/ 

transportation

Institution/

park

Mangrove 

forest Residential

Moist

forest Vacant

- - cm - - - - - - - - - - - - - - - - - - - - - - - - - - percent - - - - - - - - - - - - - - - - - - - - - - - -

2.5–5.1 16.7 9.1 36.0 15.8 45.6 27.9 34.5

5.2–7.6 9.5 27.3 22.2 13.2 26.2 24.3 22.5

7.7–10.2 14.3 18.2 15.3 8.4 6.5 11.7 12.5

10.3–12.7 8.3 4.5 15.6 10.0 8.4 7.2 12.1

12.8–15.2 11.9 0.0 3.7 14.7 3.4 9.3 5.5

15.3–17.8 7.1 0.0 3.2 7.9 1.3 3.8 3.3

17.9–20.3 7.1 4.5 2.6 4.2 2.5 5.0 3.2

20.4–22.9 6.0 4.5 0.7 3.2 1.7 3.3 1.7

23.0–25.4 4.8 0.0 0.1 3.7 1.3 1.2 0.9

25.5–27.9 4.8 0.0 0.3 2.1 1.0 1.4 0.9

28.0–30.5 1.2 4.5 0.3 1.1 0.4 2.1 0.7

30.6–33.0 1.2 0.0 0.0 4.7 0.3 1.2 0.6

33.1–35.6 2.4 0.0 0.0 1.1 0.0 1.0 0.3

35.7–38.1 1.2 4.5 0.0 3.2 0.5 0.2 0.4

38.2–40.6 0.0 0.0 0.0 0.0 0.1 0.2 0.1

40.7–43.2 0.0 4.5 0.0 0.5 0.3 0.0 0.1

43.3–45.7 0.0 4.5 0.0 0.5 0.3 0.0 0.1

45.8–48.3 0.0 4.5 0.0 1.1 0.0 0.0 0.1

48.4–50.8 1.2 0.0 0.0 0.0 0.0 0.0 0.0

50.9–53.3 0.0 0.0 0.0 0.5 0.1 0.0 0.1

53.4–55.9 0.0 0.0 0.0 0.0 0.1 0.0 0.0

56.0–58.4 0.0 4.5 0.0 0.0 0.0 0.0 0.0

58.5–61.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0

61.1–63.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0

63.6–66.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

66.1–68.6 0.0 4.5 0.0 1.1 0.0 0.0 0.1

68.7–71.1 1.2 0.0 0.0 0.0 0.0 0.0 0.0

71.2–73.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0

73.8–76.2 0.0 0.0 0.0 0.5 0.0 0.0 0.0

76.3–78.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0

78.8–81.3 0.0 0.0 0.0 0.5 0.0 0.0 0.0

81.4–83.8 0.0 0.0 0.0 0.5 0.0 0.0 0.0

83.9–86.4 0.0 0.0 0.0 0.5 0.0 0.0 0.0

86.5–88.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0

89.0–91.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0

91.5–94.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0

94.1–96.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0

96.6–99.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0

99.2–101.6 1.2 0.0 0.0 0.0 0.0 0.0 0.0

D.b.h. = diameter at breast height.

Percentages may not sum to 100 due to rounding.
20

Urban Forest Tree and Shrub Species Composition

Tables 6 through 12 present species composition and 

population estimates as well as their associated standard 

errors for trees with d.b.h. ≥2.5 cm, metric tons of stored C, 

and net annual sequestration rates for the study area and 

by land use. The species in these tables are ranked by total 

number of individuals. While this is a useful estimate, it 

should not be taken as the only way to assess the species’ 

“relative importance” in the watershed. Stored C takes into 

account tree size, so if we were to rank species by stored 

carbon, a species that is less common but generally larger 

would rank higher than a more common species that tends 

to be smaller. Also, it is important to be mindful of the 

standard errors associated with these population estimates. 

In some cases our sample size was far less than ideal and 

this shortcoming is reflected in high sampling errors. 

Our results show that red mangrove (Rhizophora mangle) 

was the most commonly encountered species overall 

(table 6) and in the mangrove forest (table 7). But other 

studies (Brandeis and others 2007, Jimenez and others 1985) 

have shown that the mangrove forests in the SJBE watershed 

have higher densities of white mangrove (Laguncularia 

racemosa) rather than red mangrove. This discrepancy 

can be explained by sampling methodology and sample 

size. One microplot (the smaller, nested plot for sampling 

regeneration) in our small sample of mangrove forest (three 

plots total in 2011) held an unusually high concentration of 

small red mangrove saplings. These saplings are sampled 

in a relatively small area so each sapling represents a 

large number of trees in the total population. While these 

mangrove forests tend toward relative overall homogeneity, 

it is possible that our admittedly small sample does not 

accurately represent the SJBE watershed’s mangrove forest. 

We can see that the watershed’s white mangrove trees store 

considerably more C than the red mangrove, indicating 

the predominance of smaller stems in the red mangrove 

population estimate.

The mangrove forests of the San 

Juan Bay Estuary watershed 

provide valuable ecosystem 

services to San Juan’s population.
21

Moist forest showed a similar, seemingly incongruous result 

(table 8). With only six plots sampled in 2011, the presence 

of a single plot with a dense concentration of saplings can 

skew population estimates, particularly the estimates for 

total numbers of individual trees. Siamese cassia (Senna 

siamea) has the most stems in the watershed, but the tulipán 

africano holds the greatest amounts of stored C by far 

(table 8). To more accurately assess the relative importance 

of these species in the watershed, one should look at the 

total number of individuals and also their stored C, an 

estimate that incorporates the overall size of the trees. Note 

that the tulipán africano, while again not having the most 

individuals, also stores considerable amounts of C in the 

institutional/park and vacant land uses (tables 10 and 12).

Introduced species are a very important part of the SJBE 

watershed’s forests. Tulipán africano, Siamese cassia, 

albizia (Albizia procera), flamboyán (Delonix regia), and 

guamá americano (Pithecellobium dulce) are all introduced 

species that were once purposely planted but now regenerate 

naturally. Flamboyán is still commonly planted for its showy 

flowers. 

Landscapers and homeowners in the SJBE watershed 

preferred introduced species for planting in commercial and 

residential areas. Among these nonnative plants are small, 

ornamental palms, such as palma areca (Dypsis lutescens), 

and fruit trees such as mango, panapén (Artocarpus altilis), 

aguacate (Persea americana), and Citrus species. Many 

native moist forest species, like María, are also important 

parts of this urban forest community.

The showy red flowers of Spathodea campanulata, 

make it easy to see how common this introduced tree 

has become in the forest patches of the San Juan Bay 

Estuary watershed.
22

Table 6—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch by common name, scientific name, number of trees measured, stored carbon, and net carbon sequestration for mangrove forest, San Juan Bay Estuary watershed, 2011Common name Scientific name Number of trees Stored carbon

Net carbon 

sequestration

% - - - n - - - - - SE - - % metric 

tons

- SE - % metric 

tons/

year

- SE -

Red mangrove Rhizophora mangle L. 27.5 2,791,480 1,523,408 5.8 18 582 10 144 13.9 4 020 2 015White mangrove Laguncularia racemosa (L.) C.F. Gaertn. 12.7 1,285,550 800,457 9.2 29 349 14 799 12.8 3 693 1 874African tulip tree Spathodea campanulata P. Beauv. 11.9 1,203,904 631,001 13.3 42 631 19 384 13.4 3 862 1 854Black mangrove Avicennia germinans (L.) L. 8.5 859,482 837,484 5.9 18 772 17 810 8.0 2 315 2 209Button mangrove Conocarpus erectus L. 3.5 351,967 324,051 3.6 11 653 10 090 4.7 1 358 1 207Antilles calophyllum Calophyllum antillanum Britton 3.4 345,098 255,170 2.2 6 887 4 304 2.9 823 512Siamese cassia Senna siamea (Lam.) Irwin & Barneby 3.0 304,268 301,074 0.9 2 976 2 708 1.3 376 354Other species — 2.5 259,796 115,281 2.2 7 293 2 942 3.1 837 294Granadillo bobo Miconia prasina (Sw.) DC. 2.3 230,824 192,785 0.2 513 351 0.7 193 144Malabar plum Syzygium jambos (L.) Alston 1.8 186,233 113,787 0.6 1 864 1 024 1.1 332 188Citrus Citrus spp. 1.6 158,771 109,014 0.6 2 074 1 297 1.3 370 227American muskwood Guarea guidonia (L.) Sleumer 1.4 142,613 137,451 0.9 2 979 2 589 1.3 371 338Australian pine Casuarina equisetifolia L. 1.4 137,367 137,351 0.8 2 653 2 652 0.8 244 244Tall albizia Albizia procera (Roxb.) Benth. 1.3 127,948 70,938 10.5 33 580 18 910 5.0 1 377 857Gumbo limbo Bursera simaruba (L.) Sarg. 1.1 107,069 82,325 1.2 3 807 2 597 1.3 372 268Matchwood Schefflera morototonii (Aubl.) Maguire, Steyerm. & Frodin 1.0 105,648 103,047 4.0 12 707 12 311 2.8 817 791Guyanese wild coffee Casearia guianensis (Aubl.) Urb. 1.0 97,051 66,809 0.1 383 256 0.4 112 71Royal poinciana Delonix regia (Bojer ex Hook.) Raf. 0.9 88,753 47,977 1.7 5 542 4 167 1.9 523 309White cedar Tabebuia heterophylla (DC.) Britton 0.8 82,819 40,437 0.9 2 931 2 016 1.2 335 185Guamuchil Pithecellobium dulce (Roxb.) Benth. 0.8 77,568 69,502 2.4 7 568 6 016 1.6 439 352Ratwood Erythroxylum rotundifolium Lunan 0.6 65,575 65,562 0.2 505 505 0.4 114 114Punchberry Myrcia splendens (Sw.) DC. 0.6 65,575 65,562 0.3 842 842 0.5 149 149Crabwood Carapa guianensis Aubl. 0.6 62,952 62,940 0.0 108 108 0.2 48 48Spanish lime Melicoccus bijugatus Jacq. 0.5 54,930 28,254 3.0 9 533 7 728 1.5 403 211Sour orange Citrus ×aurantium L. ssp. aurantium 0.4 41,110 23,521 0.0 150 86 0.3 85 48Avocado Persea americana Mill. var. americana 0.4 37,132 19,589 0.7 2 290 1 566 0.8 231 136Cabbagebark tree Andira inermis (W. Wright) Kunth ex DC. 0.3 34,815 22,301 0.8 2 471 1 668 0.6 151 93Sapodilla Manilkara zapota (L.) P. Royen 0.3 34,342 34,338 0.0 61 61 0.1 23 23Wild honeytree Casearia decandra Jacq. 0.3 34,099 34,092 0.0 85 85 0.1 33 33Blackrodwood Eugenia biflora (L.) DC. 0.3 34,099 34,092 0.0 30 30 0.1 18 18Red stopper Eugenia rhombea (Berg) Krug & Urb. 0.3 34,099 34,092 0.0 34 34 0.1 20 20Black mampoo Guapira fragrans (Dum. Cours.) Little 0.3 34,099 34,092 0.3 886 886 0.4 123 123

Geno geno Lonchocarpus domingensis (Turp. ex Pers.) DC. 0.3 31,476 31,470 0.2 484 484 0.3 84 84Bitterbush Picramnia pentandra Sw. 0.3 31,476 31,470 0.0 24 24 0.1 16 16

continued
23

Table 6—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch by common name, scientific name, number of trees measured, stored carbon, and net carbon sequestration for mangrove forest, San Juan Bay Estuary watershed, 2011 (continued)Common name Scientific name Number of trees Stored carbon

Net carbon 

sequesetration

% - - - n - - - - - SE - - % metric 

tons

- SE - % metric 

tons/

year

- SE -

Bullytree Pouteria multiflora (A. DC.) Eyma 0.3 31,476 31,470 0.0 45 45 0.1 22 22Coconut palm Cocos nucifera L. 0.3 30,244 16,488 0.2 796 457 0.0 10 6Casearia Casearia spp. 0.3 29,182 18,728 0.0 107 83 0.1 37 25Almendra Terminalia catappa L. 0.3 29,043 23,391 1.3 4 139 2 886 0.9 249 180Rose Rosa spp. 0.3 25,378 12,418 2.7 8 756 4 292 1.4 388 190Mahogany Swietenia macrophylla King 0.2 23,981 12,803 5.8 18 541 10 509 2.2 559 300Portia tree Thespesia populnea (L.) Sol. ex Corrêa 0.2 23,088 23,085 0.0 152 152 0.2 53 53Sweet orange Citrus ×sinensis (L.) Osbeck (pro sp.) [maxima × reticulata] 0.2 20,555 14,311 0.1 283 275 0.2 65 56Carolina indigo Indigofera caroliniana Mill. 0.2 19,240 19,237 0.0 72 72 0.1 34 34Malaysian apple Syzygium malaccense (L.) Merr. & L.M. Perry 0.2 17,171 17,169 0.2 510 510 0.2 64 64Common guava Psidium guajava L. 0.2 17,129 9,995 0.1 352 241 0.2 58 36Black olive Bucida buceras L. 0.1 14,548 10,303 1.3 4 224 3 157 0.9 264 199Bejamin fig Ficus benjamina L. 0.1 13,703 8,176 0.1 292 213 0.2 51 33Guacima Guazuma ulmifolia Lam. 0.1 13,703 13,701 0.0 78 78 0.1 34 33Wild banyantree Ficus citrifolia Mill. 0.1 13,395 8,519 0.2 684 456 0.3 73 46Mango Mangifera indica L. 0.1 13,152 6,428 2.5 7 889 5 153 1.0 265 148Queens crapemyrtle Lagerstroemia speciosa (L.) Pers. 0.1 11,122 8,423 4.8 15 306 11 284 1.8 469 334Guanabana Annona muricata L. 0.1 10,277 7,566 0.3 1 022 976 0.2 67 51Breadfruit Artocarpus altilis (Parkinson) Fosberg J.R. Forst. & G. Forst. 0.1 10,277 7,566 2.3 7 215 7 108 0.8 198 175

Cypress Cupressus spp. 0.1 10,277 5,750 0.0 44 25 0.1 17 9Yellow butterfly palm Dypsis lutescens (H. Wendl.) Beentje & Dransf. 0.1 10,277 5,750 0.0 98 74 0.0 1 1Broadleaf lancepod Lonchocarpus heptaphyllus (Poir.) DC. 0.1 10,277 10,276 1.3 4 174 4 174 0.7 173 173Barbados cherry Mancoa pubens (A. Gray) Rollins 0.1 10,277 5,750 0.0 105 97 0.1 27 18Queen palm Syagrus romanzoffiana (Cham.) 0.1 10,277 10,276 0.1 226 226 0.0 3 3Lemon bottlebrush Callistemon pallidus (Bonpl.) DC. 0.1 8,623 8,621 0.1 422 422 0.2 51 51

River koko Inga vera Willd. 0.1 7,869 7,867 0.2 787 787 0.2 59 59Grapefruit Citrus ×paradisi Macfad. (pro sp.) [maxima × sinensis] 0.1 6,852 4,770 0.1 410 288 0.2 44 31Sea grape Coccoloba uvifera (L.) L. 0.1 6,852 6,851 0.3 1 007 1 007 0.3 69 69Higuillo de hija menuda Crescentia portoricensis Britton 0.1 6,852 6,851 0.0 9 9 0.0 10 10Barbados nut Jatropha curcas L. 0.1 6,852 6,851 0.1 163 163 0.1 31 31Mulberry Morus spp. 0.1 6,852 6,851 0.1 214 214 0.1 34 34

Apamate Tabebuia rosea (Bertol.) DC. 0.1 6,722 4,802 0.7 2 351 2 245 0.5 134 120Clitoria Barbieria pinnata (Pers.) Baill. 0.1 5,749 5,748 0.2 692 692 0.2 57 57Bastard stopper Petitia domingensis Jacq. 0.1 5,246 5,245 0.2 792 792 0.1 30 30White pricklyash Zanthoxylum martinicense (Lam.) DC. 0.1 5,246 5,245 0.4 1 381 1 381 0.2 67 67Peepul tree Ficus religiosa L. <0.1 4,293 4,292 <0.1 32 32 <0.1 8 8

continued
24

Table 6—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch by common name, scientific name, number of trees measured, stored carbon, and net carbon sequestration for mangrove forest, San Juan Bay Estuary watershed, 2011 (continued)Common name Scientific name Number of trees Stored carbon

Net carbon 

sequesetration

% - - - n - - - - - SE - - % metric 

tons

- SE - % metric 

tons/

year

- SE -

Frangipani Plumeria rubra L. <0.1 4,293 4,292 0.0 148 148 0.1 18 18

Areca palm Areca spp. <0.1 3,426 3,425 <0.1 10 10 <0.1 0 0

Ilan-ilan Cananga odorata (Lam.) Hook. f. & Thomson <0.1 3,426 3,425 0.2 564 564 0.1 40 40

Star apple Chamaecrista calycioides (DC. ex Collad.) Greene <0.1 3,426 3,425 0.3 1 109 1 109 0.2 52 52Guara Cupania americana L. <0.1 3,426 3,425 <0.1 16 16 <0.1 9 9

Chinese banyan Ficus microcarpa L. f. <0.1 3,426 3,425 <0.1 33 33 <0.1 9 9

Tahitian gooseberry tree Phyllanthus acidus (L.) Skeels <0.1 3,426 3,425 <0.1 6 6 <0.1 6 6

Pumpwood Cecropia schreberiana Miq. <0.1 2,623 2,622 0.1 474 474 0.1 29 29

Laurel avispillo Cinnamomum elongatum (Vahl ex Nees) Kosterm. <0.1 2,623 2,622 0.1 443 443 0.1 28 28Ear tree Enterolobium cyclocarpum (Jacq.) Griseb. <0.1 2,623 2,622 <0.1 159 159 0.1 15 15Leadwood Krugiodendron ferreum (Vahl) Urb. <0.1 2,623 2,622 <0.1 121 121 <0.1 13 13

Sweetpea Lathyrus odoratus L. <0.1 2,623 2,622 0.1 314 314 0.1 23 23

Bastard hogberry Margaritaria nobilis L. f. <0.1 2,623 2,622 <0.1 108 108 <0.1 12 12

Loblolly sweetwood Ocotea leucoxylon (Sw.) De Laness. <0.1 2,623 2,622 0.1 375 375 0.1 26 26

Maricao Scolosanthus portoricensis Borhidi <0.1 2,623 2,622 0.1 162 162 0.1 15 15

Total 99.6 10,147,453 NA 99.2 319 737 NA 100.0 28 384 NA

D.b.h. = diameter at breast height; n = number of trees measured; SE = standard error; — = no scientific name; NA - not applicable.Percentages may not sum to 100 due to rounding.
25

Table 7—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch by 

common name, scientific name, number of trees, stored carbon, and net carbon sequestraton, for mangrove forest 

in the urban forest inventory, San Juan Bay Estuary watershed, 2011

Common name Scientific name Number of trees Stored carbon

Net carbon 

sequestration

- - - n - - - - - SE - - metric 

tons

- SE - metric 

tons/

year

- SE -

Red mangrove Rhizophora mangle L. 2,791,480 1,523,408 18 582 10 144 4 020 2 015

White mangrove Laguncularia racemosa (L.) C.F. Gaertn. 1,285,550 800,457 29 349 14 799 3 693 1 874

Black mangrove Avicennia germinans (L.) L. 859,482 837,484 18 772 17 810 2 315 2 209

Button mangrove Conocarpus erectus L. 323,224 323,202 9 947 9 946 1 198 1 198

Other species — 95,498 95,491 1 558 1 558 -4 4

Wild banyantree Ficus citrifolia Mill. 7,346 7,345 388 388 40 40

Total 5,362,580 1,497,690 78 596 5314 11 262 469

D.b.h. = diameter at breast height; n = number of trees measured; SE = standard error; — = no scientific name.

Healthy, well-maintained trees in residential areas beautify and add value to properties.
26

Table 8—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch stored carbon, net carbon 

sequestration by common name, scientific name, number of trees, stored carbon, and net carbon sequestration for moist forest in the 

urban forest inventory, San Juan Bay Estuary watershed, 2011

Common name Scientific name Number of trees Stored carbon

Net carbon 

sequestration

- - - n - - - - SE - metrictons- SE - metric 

tons/

year

- SE -

Siamese cassia Senna siamea (Lam.) Irwin & Barneby 304,268 301,074 2 976 2 708 376 354

African tulip tree Spathodea campanulata P. Beauv. 254,431 210,183 13 334 10 915 893 755

Granadillo bobo Miconia prasina (Sw.) DC. 230,824 192,785 513 351 193 144

Malabar plum Syzygium jambos (L.) Alston 186,233 113,787 1 864 1 024 332 188

Citrus Citrus spp. 141,642 108,332 1 892 1 286 319 223

Guyanese wild coffee Casearia guianensis (Aubl.) Urb. 97,051 66,809 383 256 112 71

Antilles calophyllum Calophyllum antillanum Britton 94,428 85,318 4 190 3 628 361 310

Gumbo limbo Bursera simaruba (L.) Sarg. 81,313 78,193 2 424 2 198 267 247

Ratwood Erythroxylum rotundifolium Lunan 65,575 65,562 505 505 114 114

Punchberry Myrcia splendens (Sw.) DC. 65,575 65,562 842 842 149 149

Lancewood Nectandra coriacea (Sw.) Griseb. 65,575 65,562 80 80 43 43

Crabwood Carapa guianensis Aubl. 62,952 62,940 108 108 48 48

Wild honeytree Casearia decandra Jacq. 34,099 34,092 85 85 33 33

Blackrodwood Eugenia biflora (L.) DC. 34,099 34,092 30 30 18 18

Red stopper Eugenia rhombea (Berg) Krug & Urb. 34,099 34,092 34 34 20 20

Black mampoo Guapira fragrans (Dum. Cours.) Little 34,099 34,092 886 886 123 123

Geno geno Lonchocarpus domingensis (Turp. ex Pers.) DC. 31,476 31,470 484 484 84 84

Bitterbush Picramnia pentandra Sw. 31,476 31,470 24 24 16 16

Bullytree Pouteria multiflora (A. DC.) Eyma 31,476 31,470 45 45 22 22

Cabbagebark tree Andira inermis (W. Wright) Kunth ex DC. 26,230 20,583 2 426 1 667 138 92

White cedar Tabebuia heterophylla (DC.) Britton 26,230 23,221 2 048 1 959 173 161

Coconut palm Cocos nucifera L. 13,115 13,112 170 170 3 3

Rose Rosa spp. 10,492 7,780 3 646 2 330 152 100

River koko Inga vera Willd. 7,869 7,867 787 787 59 59

American muskwood Guarea guidonia (L.) Sleumer 5,246 5,245 425 425 34 34

Pelargonium Pelargonium spp. 5,246 5,245 792 792 30 30

White pricklyash Zanthoxylum martinicense (Lam.) DC. 5,246 5,245 1 381 1 381 67 67

Maricao Scolosanthus portoricensis Borhidi 2,623 2,622 162 162 15 15

Pumpwood Cecropia schreberiana Miq. 2,623 2,622 474 474 29 29

Laurel avispillo Cinnamomum elongatum (Vahl ex Nees) Koterm. 2,623 2,622 443 443 28 28

Ear tree Enterolobium cyclocarpum (Jacq.) Griseb. 2,623 2,622 159 159 15 15

Wild banyantree Ficus citrifolia Mill. 2,623 2,622 229 229 19 19

Other species — 2,623 2,622 314 314 23 23

Leadwood Krugiodendron ferreum (Vahl) Urb. 2,623 2,622 121 121 13 13

Bastard hogberry Margaritaria nobilis L. f. 2,623 2,622 108 108 12 12

Loblolly sweetwood Ocotea leucoxylon (Sw.) De Laness. 2,623 2,622 375 375 26 26

Matchwood Schefflera morototonii (Aubl.) Maguire, Steyerm. & Frodin 2,623 2,622 400 400 26 26

Almendra Terminalia catappa L. 2,623 2,622 711 711 37 37

Total 2,009,218 474,573 45 872 14 353 4 424 1 149

D.b.h. = diameter at breast height; n = number of trees measured; SE = standard error; — = no scientific name.
27

Table 10—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch stored 

carbon, net carbon sequestration, and number of street trees by common name and scientific name for the 

institutional/park land use in the urban forest inventory, San Juan Bay Estuary watershed, 2011

Common name Scientific name

Number of 

trees Stored carbon

Net carbon

sequestration

Street 

trees

- - n - - - SE - metric 

tons

- SE - metric 

tons/

year

SE - - n - -

Portia tree Thespesia populnea (L.) Sol. ex Corrêa 23,088 23,085 152 152 53 53 0

Carolina indigo Indigofera caroliniana Mill. 19,240 19,237 72 72 34 34 0

African tulip tree Spathodea campanulata P. Beauv. 15,392 15,390 6 127 6 126 316 316 0

Black olive Bucida buceras L. 7,696 7,695 2 835 2 835 181 181 7,696

Queens crapemyrtle Lagerstroemia speciosa (L.) Pers. 7,696 7,695 5 390 5 389 262 262 7,696

Other species — 7,696 7,695 105 105 28 28 0

Pink trumpet-tree Tabebuia rosea (Bertol.) DC. 3,848 3,847 2 242 2 242 119 119 3,848

Total 84,656 42,453 16 923 8 333 993 440 19,240

D.b.h. = diameter at breast height; n = number of trees measured; SE = standard error; — = no scientific name.

Table 9—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch stored 

carbon, net carbon sequestration, and number of street trees by common name and scientific name for the 

commerical/industrial/transportation land use in the urban forest inventory, San Juan Bay Estuary watershed, 2011

Common name Scientific name Number of trees Stored carbon

Net carbon 

sequestration

Street 

trees

- - n - - - SE - metrictons- SE - metric 

tons/

year

- SE - - n -

Guamuchil Pithecellobium dulce (Roxb.) Benth. 68,982 68,970 5 729 5 728 337 337 0

Royal poinciana Delonix regia (Bojer ex Hook.) Raf. 37,365 31,872 905 751 162 135 0

Button mangrove Conocarpus erectus L. 28,743 23,443 1 706 1 695 160 149 28,743

Other species — 22,994 14,158 1 049 865 124 83 0

Almendra Terminalia catappa L. 22,994 22,990 2 702 2 701 171 171 0

Tall albizia Albizia procera (Roxb.) Benth. 17,246 10,361 10 985 9 599 286 204 0

Spanish lime Melicoccus bijugatus Jacq. 17,246 17,243 722 722 83 83 0

Lemon bottlebrush Callistemon pallidus (Bonpl.) DC. 8,623 8,621 422 422 51 51 0

Clitoria Barbieria pinnata (Pers.) Baill. 5,749 5,748 692 692 57 57 0

Mango Mangifera indica L. 2,874 2,874 248 248 24 24 0

Avocado Persea americana Mill. var. americana 2,874 2,874 539 539 37 37 0

Rose Rosa spp. 2,874 2,874 778 778 39 39 0

Pink trumpet-tree Tabebuia rosea (Bertol.) DC. 2,874 2,874 109 109 15 15 0

Total 241,438 123,663 26 585 14 114 1 546 745 28,743

D.b.h. = diameter at breast height; n = number of trees measured; SE = standard error; — = no scientific name.
28

Table 11—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch stored carbon, net 

carbon sequestration, and number of street trees by species for the residential land use in the urban forest inventory, San Juan Bay 

Estuary watershed, 2011

Common name Scientific name Number of trees Stored carbon

Net carbon 

sequestration

Street 

trees

- - n - - - - SE - - metric 

tons

- SE - metric 

tons/

year

SE - - n - -

Other species — 133,607 48,097 4 581 2 076 689 272 37,684

Royal poinciana Delonix regia (Bojer ex Hook.) Raf. 51,387 35,861 4 637 4 099 361 278 0

Sour orange Citrus ×aurantium L. ssp. aurantium 41,110 23,521 150 86 85 48 0

Spanish lime Melicoccus bijugatus Jacq. 37,684 22,382 8 811 7 694 319 194 0

Avocado Persea americana Mill. var. americana 34,258 19,377 1 752 1 470 194 131 0

White cedar Tabebuia heterophylla (DC.) Britton 30,832 27,513 609 439 113 85 27,407

Mahogany Swietenia macrophylla King 23,981 12,803 18 541 10 509 559 300 3,426

Tall albizia Albizia procera (Roxb.) Benth. 20,555 14,311 8 441 8 221 268 237 0

Sweet orange Citrus ×sinensis (L.) Osbeck (pro sp.) [maxima ×

reticulata] 20,555 14,311 283 275 65 56 0

Citrus Citrus spp. 17,129 12,176 183 169 51 41 0

Coconut palm Cocos nucifera L. 17,129 9,995 625 424 7 5 6,852

Common guava Psidium guajava L. 17,129 9,995 352 241 58 36 0

Benjamin fig Ficus benjamina L. 13,703 8,176 292 213 51 33 0

Guacima Guazuma ulmifolia Lam. 13,703 13,701 78 78 34 33 0

Guanabana Annona muricata L. 10,277 7,566 1 022 976 67 51 0

Breadfruit Artocarpus altilis (Parkinson) Fosberg J.R. Forst.

& G. Forst. 10,277 7,566 7 215 7 108 198 175 0

Antilles calophyllum Calophyllum antillanum Britton 10,277 7,566 416 396 57 47 3,426

Cypress Cupressus spp. 10,277 5,750 44 25 17 9 0

Yellow butterfly palm Dypsis lutescens (H. Wendl.) Beentje & Dransf. 10,277 5,750 98 74 1 1 3,426

Broadleaf lancepod Lonchocarpus heptaphyllus (Poir.) DC. 10,277 10,276 4 174 4 174 173 173 0

Barbados cherry Mancoa pubens (A. Gray) Rollins 10,277 5,750 105 97 27 18 0

Mango Mangifera indica L. 10,277 5,750 7 641 5 147 242 146 0

Munz's sage Salvia munzii Epling 10,277 10,276 226 226 3 3 6,852

Black olive Bucida buceras L. 6,852 6,851 1 389 1 389 82 82 6,852

Grapefruit Citrus ×paradisi Macfad. (pro sp.) [maxima × sinensis] 6,852 4,770 410 288 44 31 0

Sea grape Coccoloba uvifera (L.) L. 6,852 6,851 1 007 1 007 69 69 0

Barbados nut Jatropha curcas L. 6,852 6,851 163 163 31 31 0

Mulberry Morus spp. 6,852 6,851 214 214 34 34 0

Higuillo de hija menuda Crescentia portoricensis Britton 6,852 6,851 9 9 10 10 0

African tulip tree Spathodea campanulata P. Beauv. 6,852 4,770 65 61 18 14 0

Areca palm Areca spp. 3,426 3,425 10 10 0 0 0

Ilan-ilan Cananga odorata (Lam.) Hook. f. & Thomson 3,426 3,425 564 564 40 40 0

Casearia Casearia spp. 3,426 3,425 9 9 7 7 0

Star apple Chamaecrista calycioides (DC. ex Collad.)

Greene 3,426 3,425 1 109 1 109 52 52 0

Guara Cupania americana L. 3,426 3,425 16 16 9 9 0

Wild banyantree Ficus citrifolia Mill. 3,426 3,425 66 66 14 14 0

Chinese banyan Ficus microcarpa L. f. 3,426 3,425 33 33 9 9 0

Queens crapemyrtle Lagerstroemia speciosa (L.) Pers. 3,426 3,425 9 916 9 915 207 207 0

Tahitian gooseberry tree Phyllanthus acidus (L.) Skeels 3,426 3,425 6 6 6 6 0

Rose Rosa spp. 3,426 3,425 941 941 47 47 3,426

Almendra Terminalia catappa L. 3,426 3,425 726 726 41 41 0

Total 650,906 125,081 86 929 25 622 4 361 907 99,349

D.b.h. = diameter at breast height; n = number of trees measured; SE = standard error; — = no scientific name.
29

Table 12—Population estimates and standard error of the estimates for number of trees with d.b.h. ≥1.0 inch by common name, 

scientific name, number of trees, stored carbon, net carbon sequestration, and number of street trees by species for the vacant land 

use in the urban forest inventory, San Juan Bay Estuary watershed, 2011

Common name Scientific name Number of trees Stored carbon

Net carbon 

sequestration

Street 

trees

- - - n - - - - - SE - - metric 

tons

- SE - metric 

tons/

year

- SE - - - n - -

African tulip tree Spathodea campanulata P. Beauv. 927,229 594,748 23 104 14 800 2 636 1 664 0

Antilles calophyllum Calophyllum antillanum Britton 240,393 240,365 2 281 2 281 404 404 0

Australian pine Casuarina equisetifolia L. 137,367 137,351 2 653 2 652 244 244 0

American muskwood Guarea guidonia (L.) Sleumer 137,367 137,351 2 554 2 554 336 336 0

Matchwood Schefflera morototonii (Aubl.) Maguire,

Steyerm. & Frodin 103,025 103,013 12 306 12 305 790 790 0

Tall albizia Albizia procera (Roxb.) Benth. 90,147 68,702 14 154 14 066 823 798 0

Sapodilla Manilkara zapota (L.) P. Royen 34,342 34,338 61 61 23 23 0

Gumbo limbo Bursera simaruba (L.) Sarg. 25,756 25,753 1 383 1 383 105 105 12,878

Casearia Casearia spp. 25,756 18,412 98 83 29 23 0

White cedar Tabebuia heterophylla (DC.) Britton 25,756 18,412 274 186 50 34 4,293

Malaysian apple Syzygium malaccense (L.) Merr. & L.M. Perry 17,171 17,169 510 510 64 64 0

Cabbagebark tree Andira inermis (W. Wright) Kunth ex DC. 8,585 8,584 45 45 13 13 0

Guamuchil Pithecellobium dulce (Roxb.) Benth. 8,585 8,584 1 839 1 839 102 102 0

Rose Rosa spp. 8,585 8,584 3 391 3 391 149 149 0

Peepul tree Ficus religiosa L. 4,293 4,292 32 32 8 8 0

Frangipani Plumeria rubra L. 4,293 4,292 148 148 18 18 0

Total 1,798,653 918,495 64 833 34 856 5 796 2 941 17,171

D.b.h. = diameter at breast height; n = number of trees measured; SE = standard error.

Species Diversity Indices

We sampled a slightly smaller area in 2011 (6.34 ha) 

compared to 2001 (6.68 ha) (table 1) but found 

more species overall in 2011 (86 vs. 75 species) 

(table 13). Residential areas were the single most 

species-rich land use. A diverse assemblage of 

native and introduced species was found there, with 

notable numbers of fruit trees (table 11). An increase 

in vacant land species richness and loss of species 

from the institutional/park land use in 2011 reflect 

the change in status of one sampling point from 

institutional/park to vacant land. We speculate that the 

increased species richness in vacant land and moist 

forest is due to the continued reversion of these lands 

to denser, more diverse forest vegetation.

Table 13 presents different species diversity indices 

for each land use with trees in the SJBE watershed. 

The Shannon-Wiener diversity index, which assumes 

that all species within land use type or city have been 

sampled, is an indicator of species richness and has a 

moderate sensitivity to sample size; therefore, values 

may not be comparable across land uses or cities. 

The residential areas in the San Juan Bay Estuary watershed hold a 

wide variety of trees and shrubs, such as this patio in Barrio Obrero, 

San Juan. (photo by Jeffery Glogiewicz, Consultores Ambiental and 

the Fundación Puertorriqueña de Conservación)
30

Table 13—Tree species richness, number of species per hectare, Shannon-Wiener diversity index, Menhinick's diversity index, Simpson's 

diversity index, Shannon-Wiener's evenness index, and Sanders' Rarefaction technique value, San Juan Bay Estuary watershed, 2001 and 

2011

Land use

Species 

richness

Number of

species/ha

ShannonWiener Menhinick Simpson Evenness Rarefaction

2001 2011 2001 2011 2001 2011 2001 2011 2001 2011 2001 2011 2001 2011

Commercial/industrial/

transportation 9 13 6.35 8.38 1.8807 2.1439 1.5910 1.4184 5.9048 7.1288 0.8560 0.8359 7.87 7.55

Institution/park 16 7 21.56 11.53 2.2816 1.7955 1.7778 1.4924 6.5988 6.7941 0.8229 0.9227 9.79 6.53

Mangrove forest 5 6 18.53 29.65 1.2316 1.2257 0.2138 0.2221 3.0656 2.7996 0.7653 0.6841 3.82 3.84

Residential 41 41 17.88 18.31 3.1934 3.2242 2.6465 3.0470 18.2095 16.3079 0.8599 0.8626 13.64 11.45

Moist forest 32 38 67.78 93.90 2.4428 2.8763 0.9904 1.3730 6.6416 12.9478 0.7048 0.7907 9.71 10.01

Vacant 11 16 10.87 21.56 1.9318 1.7250 1.1862 0.7817 5.8762 3.3272 0.8056 0.6222 7.18 5.89

Total 75 86 11.23 13.72 3.1007 3.2766 1.6646 1.8502 11.9471 14.0792 0.7182 0.7337 NA NA

NA = not applicable.

Note that the total watershed value for species richness will not necessarily equal the sum of the land use species richness values.

Menhinick’s and Simpson’s diversity indices, which are 

indicators of species richness and dominance, respectively, 

have a low sensitivity to sample size and therefore may 

be more appropriate for comparison between land use 

types. Shannon-Wiener’s evenness index, which again 

assumes that all species within land use type or city have 

been sampled, is an indicator of species evenness and has 

a moderate sensitivity to sample size; thus, values may not 

be comparable across land uses or cities. Finally, Sander’s 

Rarefaction technique value is the number of species one 

would expect to find if 21 trees were sampled in the land use 

type. For the watershed, this value is the number of species 

one would expect to find if 250 trees were sampled within 

the city. 

Urban Tree Damage and Crown Health

Despite differences in damage assessment protocols from 

2001 to 2011, there did not appear to be appreciable 

changes in the types or frequencies of pests, pathogens, or 

human damage agents. Individual tree damage assessments 

indicated that 12.7 percent of all trees in the SJBE watershed 

with d.b.h. ≥12.5 cm were damaged or suffered from 

pathogens. Damage frequency was lower in the mangrove 

and moist secondary forest than in developed land uses. 

Only 5.6 percent of mangroves showed any damage, that 

is, the presence of fungal fruiting bodies or advanced 

decay. Only 2.9 percent of moist secondary forest trees 

were damaged; loss of apical dominance caused by a dead 

terminal leader was the most common damage type. Overall, 

20.3 percent of trees in developed land uses showed at 

least one urban damage agent, 9.1 percent had two damage 

agents, and 3.0 percent had three. The presence of wounds 

or cracks in the bole was most common (10.3 percent), 

followed by cankers or decay (9.5 percent), and having the 

tree crown in conflict with overhead wires (8.5 percent) 

(table 14).

No appreciable amounts of crown dieback or defoliation 

were observed on the trees in the watershed during either 

time period, nor did there appear to be changes in tree crown 

conditions over time. Average crown density and foliage 

transparency for the most commonly encountered broadleaf 

trees in the watershed (with a minimum of 10 individuals) 

are presented in table 15 to provide a baseline for future 

forest health monitoring. Future decreases in crown density 

and foliage transparency would indicate a loss of tree vigor.

The roots of urban trees can conflict with sidewalks if they are not 

given sufficient growing space.
31

Table 14—Percentage of trees in developed 

land uses affected by urban damaging agents, 

San Juan Bay Estuary watershed, 2011

Damage agent Percent

Wound or crack 10.3

Canker or decay 9.5

Conflict with tree crown 8.5

Severe topping or poor pruning 7.9

Vines in crown 6.2

Conflict with roots 4.6

Improper planting 0.6

Chlorotic, necrotic 0.6

Borers/bark beetles 0.6

Bark inclusion 0.6

Stem girdling 0.4

Defoliation 0.2

Excessive mulch 0.0

Dead top 0.0

Dead or dying crown 0.0

Urban Forest Effects Model Results for Ecosystem 

Services

The number, size, and distribution of trees in the SJBE 

watershed affect the functions or benefits that accrue to the 

local environment. An important benefit is the sequestration 

of C. As trees grow, they sequester or store C from the 

atmosphere within their biomass, thereby reducing the 

amount of carbon dioxide (CO2) in the atmosphere. As CO2

is thought by many to contribute to global climate change, 

reductions in this greenhouse gas through tree growth can 

help mitigate climate changes. 

As previously mentioned, the trees in the SJBE in 2011 

stored approximately 319 737 metric tons of C and 

sequester 28 384 metric tons of C per year (table 6). Given 

an estimated marginal social cost of CO2 emissions of 

Table 15—Mean crown attributes and standard errors for the estimates for the most commonly encountered 

broadleaf trees by common name, scientific name, number, crown ratio, crown density, and foliage 

transparency, San Juan Bay Estuary watershed, 2011

Common name Scientific name Number

Crown 

ratio

Crown 

density

Foliage 

transparency 

n mean SE mean SE mean SE

African tulip tree Spathodea campanulata P. Beauv. 99 32.0 1.6 38.4 1.4 43.6 0.6

Black mangrove Laguncularia racemosa (L.) C.F. Gaertn. 49 30.9 2.5 44.2 2.8 52.1 1.6

White mangrove Avicennia germinans (L.) L. 37 42.0 2.7 61.1 1.0 30.8 2.7

Red mangrove Rhizophora mangle L. 35 50.4 2.9 64.0 3.7 35.0 1.6

Coconut palm Cocos nucifera L. 33 53.1 4.3 59.5 1.4 42.9 0.9

Australian pine Casuarina equisetifolia L. 32 88.6 1.7 28.3 0.9 59.8 0.2

Antilles calophyllum Calophyllum antillanum Britton 31 39.3 3.8 39.1 3.3 41.9 1.6

Flametree Delonix regia (Bojer ex Hook.) Raf. 28 55.0 3.6 29.6 2.3 46.6 2.7

Guamuchil Pithecellobium dulce (Roxb.) Benth. 26 52.1 4.2 25.6 2.2 47.3 1.2

White cedar Tabebuia heterophylla (DC.) Britton 25 57.8 4.1 50.0 3.0 39.0 1.4

Tall albizia Albizia procera (Roxb.) Benth. 19 58.8 4.6 35.0 3.3 48.2 2.4

Button mangrove Conocarpus erectus L. 19 60.0 5.5 37.5 4.6 41.0 5.4

Spanish lime Melicoccus bijugatus Jacq. 17 72.1 3.8 38.8 2.2 40.6 1.7

Gumbo limbo Bursera simaruba (L.) Sarg. 14 38.6 4.4 37.5 5.0 41.2 2.4

Fiddlewood Citharexylum spinosum L. 13 36.9 5.0 38.3 6.0 45.0 1.4

Malaysian apple Syzygium jambos (L.) Alston 13 40.4 4.5 48.6 5.7 47.9 4.3

Sour orange Citrus ×aurantium L. ssp. aurantium 12 50.0 6.2 27.1 2.3 47.9 1.1

Siamese cassia Senna siamea (Lam.) Irwin & Barneby 12 22.7 2.5 65.0 15.0 37.5 2.5

Avocado Persea americana Mill. 11 72.3 2.9 39.1 3.1 46.4 2.0

American muskwood Guarea guidonia (L.) Sleumer 10 66.0 5.3 40.0 6.8 46.5 1.7

Almendra Terminalia catappa L. 10 68.5 7.2 40.0 3.9 41.5 1.7

N = number of trees measured; SE = standard error.
32

$10.26/metric ton of C (based on the price of a ton of CO2

trading on global markets on the http://www.pointcarbon.

com/ Web site, accessed in March 2013), the estimated value 

of the C storage by trees in the SJBE watershed was $3.3 

million with an annual C sequestration value of $291,219 in 

2011, up from the 2001 values of $1.8 million in stored C 

and an annual rate of $157,167.

In 2011, approximately 19,034 MWh of energy required 

for cooling buildings were avoided due to appropriately 

located tree shading and climate effects in residential and 

commercial areas (table 16). These energy savings translate 

into 1 986 metric tons of avoided C emissions per year or 

$20,376 in economic benefits (table 17). Assuming that the 

price per kilowatt-hour of electricity is $0.26, an average 

price for the San Juan area in 2012, the energy savings from 

shade and climate amelioration alone equate to $4,948,840 

in benefits to residents in 2011. 

Table 16—Avoided energy use mega British thermal units and megawatt-hours due to the heating and cooling effects of residential houses by urban trees, with standard errors for each estimate, San Juan Bay Estuary watershed, 2011Land use

Heating (MBtus avoided) Heating (MWhs avoided) Cooling (MWhs avoided)

Shade Windbreak Climate Shade Windbreak Climate Shade Climate

MBtu SE MBtu SE MBtu SE MWh SE MWh SE MWh SE MWh SE MWh SE

Commercial/industrial/transportation -24,467 23,410 12,305 11,312 2,268 1,885 -1,051 1,005 530 487 95 79 3,160 3,029 1,914 1,786

Institution/park -335 334 1,625 1,624 2,684 1,988 -14 14 70 70 112 83 14 10 403 327

Residential -30,728 10,787 15,828 6,953 23,439 12,540 -1,320 463 682 300 979 524 8,695 2,937 4,849 1,747

Total -55,530 25,777 29,758 13,377 28,390 12,836 -2,385 1,107 1,282 576 1,186 537 11,869 4,219 7,165 2,520

MBtu = mega British thermal units; MWh = megawatt-hours; SE = standard error.

Table 17—Avoided tons of carbon (C) emissions due to the heating and cooling effects of 

residential houses by urban trees with standard errors for each estimate, San Juan Bay 

Estuary watershed, 2011

Land use

Heating (metric tons C avoided) Cooling (metric tons C avoided)

Shade Windbreak Climate Shade Climate

metric

tons

SE metric

tons

SE metric

tons

SE metric

tons

SE metric

tons

SE

Commercial/

industrial/

transportation -538 515 271 249 49 41 321 308 194 182

Institution/park -7 7 36 36 58 43 1 1 41 33

Residential -676 237 348 153 511 273 884 298 493 178

Total -1,221 567 655 295 618 280 1,206 429 728 256

C = carbon; SE = standard error.
33

Discussion

Urban Forest Land and Tree Cover

Ramos González and others (2005) (as cited in Lugo and 

others 2011) analyzed 1999 IKONOS satellite images 

and estimated that forest covered 26 percent of the greater 

San Juan metropolitan area. It is difficult to compare this 

estimate to the 11.8 percent forest reported in this study 

because in addition to the SJBE watershed, the Ramos 

González and others (2005) study area included more of 

the surrounding San Juan metropolitan area, particularly 

to the south with the more forested Central Cordillera 

foothills, and excluded the Piñones Commonwealth Forest. 

The spatial distribution of forest in the San Juan area is 

important to note, even if not explicitly addressed in this 

study. Tree cover is not spread equally over the watershed; 

rather most is concentrated in mangrove forests of the Loíza 

municipality, in and around the Piñones Commonwealth 

Forest. Outside of the concentration of mangrove located 

in Piñones and fringing the estuary’s waterways, forest 

is found in scattered subtropical moist secondary forest 

remnants. These remnant patches are more concentrated in 

the upper reaches of the watershed or scattered across the 

area on mogote outcroppings that are too large and steep to 

be economically feasible for development. While the SJBE 

watershed’s vacant areas held substantial numbers of trees, 

it is best to consider this forest cover as relatively ephemeral 

and likely to be cleared in the future. 

Average U.S. urban area tree cover is 27.1 percent (Nowak 

and others 2001), comparable to the tree cover percentage 

observed in the residential and vacant land uses in the 

SJBE watershed, but somewhat higher than observed in the 

other developed land uses there. Perhaps more comparable 

are the results from urban areas in Florida. Using UFORE 

methodology very similar to that used in San Juan, Zhao 

and others (2010) found the city of Gainesville, FL to have 

51 percent tree canopy cover, and the Miami-Dade, FL 

metropolitan area to have 14 percent tree canopy cover. 

Both of these urban areas had forested patches within the 

urbanized matrix: remnant oak-pine forest in Gainesville 

and mangrove forest in Miami-Dade. 

Urban Forest Structure

To put the urban forests of the SJBE watershed into the 

larger context of the urban forest ecosystem, we can 

compare their structural characteristics to those of other 

urban forests. The average number of trees/ha compiled 

in a study of several U.S. cities ranged from 62 to 276 

(Nowak and others 2001). For the SJBE watershed, 

the overall average tree density was 314 trees/ha, with 

a high of 2,211 in moist forest and a low of 22 in the 

commercial/institutional/transportation land use. Perhaps 

more comparably, Gainesville, FL, had 374 trees/ha and 

Miami-Dade, FL, had an average of 227 trees/ha (Escobedo 

and others 2010). These urban forest trees stored an average 

of 9.3 metric tons of C/ha in Gainesville and 38.4 metric 

tons of C/ha in Miami-Dade (Escobedo and others 2010). 

Similar to the situation in San Juan, remnant forests within 

the urbanized matrix and vacant land uses (2.1 metric tons/

ha/year) showed the highest per-area sequestration rates in 

Gainesville, while Miami-Dade’s mangrove forests were 

found to have the highest C storage density (74.7 metric 

tons/ha) and to be sequestering the most C per area 

(2.2 metric tons/ha/year) (Escobedo and others 2010). 

This comprehensive forest inventory of the SJBE watershed 

also provides a new perspective from which to view other, 

more detailed, smaller-scale studies of forests in the San 

Juan urban landscape, and shows another dimension of the 

highly diverse, and heavily disturbed, subtropical moist 

forest of Puerto Rico. Lugo and others (2001) found an 

average tree density of 300 stems/ha (ranging from 22 to 

879 stems/ha) for trees with a d.b.h. ≥2.5 cm in the riparian 

forest along the Río Piedras in the SJBE watershed. These 

values fall within this study’s averages for forested land 

and urbanized areas. The riparian zones studied in Lugo 

and others (2001) represent heavily disturbed, less dense 

secondary forest found along accessible waterways in the 

heart of the watershed. Despiau-Batista (1997) inventoried 

a moist forest woodlot adjacent to the University of 

Puerto Rico’s Botanical Garden, also in Río Piedras, and 

found an average of 1,701 stems/ha ≥4.0 cm d.b.h. for 

the 15 dominant tree species. These stem densities are 

comparable to those observed in this study over the entire 
34

watershed. Plots installed along an elevational gradient on 

the forested mogotes that form the Bosque San Patricio 

Urban Commonwealth Forest found stem densities that 

ranged from 510 to 3,568 trees/ha and basal areas from 

0.6 to 3.0 m2/ha (Suárez and others 2005).

Other studies also indicate that the urban forest of the 

SJBE watershed falls within the wide range of structural 

characteristics currently found in Puerto Rican subtropical 

moist forests. A stem density of 4,500 stems/ha was 

observed in abandoned pastures and coffee shade in the karst 

region (Rivera and Aide 1998). Also working in the karst 

region, Alvarez-Ruiz (1997) found 520 to 7,970 stems with 

d.b.h. ≥2.5 cm in subtropical moist forest stands at various 

successional stages and with varied land use histories. 

Chinea’s (2002) study of abandoned agricultural land that 

had reverted to moist forest in the Humacao area found tree 

densities that ranged from a per-hectare low of 500 stems 

with d.b.h. ≥2.5 cm on very young sites to a high of 2,600 

stems/ha on sites that were 43 to 45 years old (Chinea 2002). 

Scattered secondary subtropical moist forest patches store and annually sequester considerable amounts of carbon every 

year in the San Juan Bay Estuary watershed.
35

Species Composition

Species composition, richness, and diversity of the two 

forest types found in the SJBE watershed were typical of 

those found in similar areas island-wide (Brandeis and 

others 2007). But there were also some notable differences. 

A striking change in species frequency occurred in the 

SJBE watershed’s mangrove forest. The numbers of red 

mangrove seem to have increased considerably, but this 

high concentration was driven by saplings and seedlings 

that appeared on a single plot and has a large amount 

of uncertainty ascribed to it. This plot falls on a small, 

low-elevation island in one of the watershed’s bays where 

debris has been cleared from the bay’s side channels to 

increase water flow. This cleanup is part of larger efforts 

to rehabilitate the area since the first plot measurement. 

These activities might be causing changes in seawater flow 

around this island such that the more saltwater-tolerant 

red mangrove is now favored over the white mangrove, 

which makes up most of that small island’s forest overstory. 

However, the standard errors associated with both of these 

species show these estimates overlap considerably, with 

standard errors >50 percent of their density estimates. 

Therefore, we do not see this shift in species composition 

as a widespread phenomenon in the watershed, but rather 

as a small-scale event. A similar circumstance arose with 

the moist forest data. Siamese cassia, while not rare, is not 

a ubiquitous tree in the island’s subtropical moist forest. 

This species was found in a high concentration on one 

of the SJBE watershed’s forested plots, inflating its total 

numbers, but again, the estimate is highly uncertain with 

a standard error >90 percent of its density estimate. These 

two examples highlight the sensitivity of these data to small 

sample sizes. 

Introduced species have not become established in 

mangrove forests of the SJBE watershed; all species found 

in this forest type were native to Puerto Rico. However, 

outside the mangrove forests, the introduced tulipán africano 

is the most frequently encountered species and the species 

with the highest total C storage. The prevalence of tulipán 

africano in the SJBE watershed’s forest is notable, and 

consistent with findings from Despiau-Batista’s (1997) 

study of a Río Piedras woodlot where tulipán africano was 

the most important (as defined by a calculated importance 

value) species found there. Tulipán africano and albizia were 

also predominant in the Bosque San Patricio when surveyed 

by Suárez and others (2005). Island-wide forest inventories 

also found that tulipán africano was the most frequently 

encountered species and had the highest total basal area 

in the island-wide forest inventories of 1990 (Franco and 

others 1997) and 2003 (Brandeis and others 2007). This 

species has steadily increased due to colonization of new 

areas and continued growth of established trees since the 

first forest inventory in 1980 (Birdsey and Weaver 1982, 

Brandeis and others 2007, Franco and others 1997). Some 

evidence suggests that tulipán africano becomes established 

and grows particularly well on more fertile soils on less 

steep slopes (Brandeis and others 2009). 

Other introduced species, such as Siamese cassia and 

albizia, may be indicators of more intensely disturbed 

land within the SJBE watershed. Albizia occurred in areas 

which generally showed a high degree of disturbance while 

still allowing for some natural tree regeneration. Chinea 

(2002) found that albizia was more common on abandoned 

agricultural land that had been bulldozed, and hypothesized 

that the species had a competitive advantage on these 

heavily disturbed sites. Lugo and others (2001) reported 

that albizia had the highest importance value in Rio Piedras 

riparian zones, which are also heavily affected by human 

activity. 

Species compositions in the developed land uses differed 

greatly and consisted of a mix of purposely planted trees 

and shrubs and natural regeneration by both native and 

introduced species in the unmaintained areas. The high 

level of species richness observed in residential areas was 

not surprising, even when taking into account the larger 

number of plots sampled in that land use. Species diversity 

was highest in residential areas in the Miami-Dade urban 

forest inventory as well (Zhao and others 2010). During data 

collection we observed a wide variety of both native and 

introduced species being planted and naturally regenerating 

in residential areas. Tropical plant species from around the 

world are being used as ornamentals in San Juan and more 
36

are being introduced, often without consideration of their 

potential environmental impact (for example, potential as 

invasive species or vectors for pests and diseases). Vacant 

areas were particularly diverse and dynamic. These small 

treed areas are undergoing primary or secondary succession 

depending on the time since they were last cleared or 

disturbed. The changing nature of the urban tree species 

composition in vacant areas illustrates how difficult it is 

to hypothesize about changes in species richness, species 

diversity, and persistence of introduced species and to then 

assess the risk from potentially invasive ones. Puerto Rico’s 

forests are a highly diverse mix of native and introduced 

species that are novel assemblages, and these assemblages 

are still evolving (Brandeis and others 2009, Lugo and 

Brandeis 2005).

Tree Damage and Forest Health

These first two inventories of the watershed provided 

the baseline data needed to assess any future changes in 

tree and forest health. Although an appreciable number 

(12.7 percent) of the watershed’s trees showed signs of 

natural or anthropogenic damage, there were no indications 

of widespread vigor declines, pests, or pathogens. 

Surprisingly little crown damage or dieback had occurred, 

for perhaps two reasons. First, past hurricanes and posthurricane maintenance activities likely resulted in the 

removal of dead limbs and trees in poor condition. Sufficient 

time has passed since Hurricane Georges in 1998; most trees 

have regrown broken branches and crowns to such an extent 

that the damage is no longer very noticeable. The second 

possible reason for not recording more tree damage is the 

damage guidelines themselves. Our procedures may be 

oriented more toward recent, obvious tree damage and were 

not sensitive or detailed enough to record past damages. 

Future tree health monitoring, and unfortunately the passage 

of damaging hurricanes through the watershed, will provide 

the information needed for the assessment of our protocols. 

Ecosystem Services

These initial attempts to quantify the value of the ecosystem 

services provided by San Juan’s urban forest cover make it 

possible to analyze the costs and benefits related to resource 

management decisions. Land managers and policymakers 

can see that an average hectare of land (excluding the 

water/agriculture land use) in the SJBE watershed held 

15.7 metric tons of C. For developed land uses (excluding 

water/agriculture, mangrove, and moist forest) the average 

was 10.9 metric tons of C/ha. The watershed’s moist forest 

held an average of 43.2 metric tons/ha. This estimated 

C storage density is comparable to that observed in the 

non-urban subtropical moist forests of mainland Puerto Rico 

and Vieques. Using the FIA estimates for aboveground dry 

biomass and adding 26 percent for the belowground portion 

of the tree (as per UFORE calculations described in the 

Methods section), the forest inventory estimated C density 

to be 47.1 metric tons/ha in 2004 and 54.2 metric tons/ha in 

2009 (Miles 2013). 

Estimates for mangrove forests, however, were much more 

variable. We observed an average of 52.9 metric tons/ha 

of C. The forest inventories (using the same adjustment 

described above) found values of 45.5 metric tons/ha in 

2004 and 25.1 metric tons/ha in 2009, but with sampling 

errors of 79.1 and 94.6 percent, respectively. These sampling 

errors produce 67-percent confidence intervals that range 

from a high estimate of 82.0 metric tons/ha in 2004 to a low 

Note the removal of street trees that were present at the first plot 

measurement in 2001 (top) and their replacement with small palms 

when revisited in 2010 (bottom). (top photo by Jeffery Glogiewicz, 

Consultores Ambiental and the Fundación Puertorriqueña de 

Conservación; bottom photo by Edgardo González, Centro para la 

Conservación del Paisaje)
37

estimate of 1.4 metric tons/ha in 2009 (Miles 2013). The 

sampling intensity of mangrove forest, an important C sink 

in the watershed and the island as a whole, needs to be 

increased if we are to estimate its size accurately.

Looking farther afield, the U.S. national average urban forest 

C storage density is 25.1 metric tons/ha, and 53.5 metric 

tons/ha for all forest land (Nowak and Crane 2002). We 

hypothesize that the SJBE urban forest’s C density values 

are lower than these national averages due to a combination 

of lower tree density in San Juan’s developed land uses and 

smaller average tree size in the moist and mangrove forested 

areas.

Carbon was being sequestered at an average rate of 

1.4 metric tons/ha/year across all land uses by the trees in 

the SJBE watershed. The highest rates of C sequestration/ha 

were found in mangrove forest, where C was sequestered 

at a rate of 7.6 metric tons/ha/year and moist forest at 

4.2 metric tons/ha/year. The average rate for developed land 

uses only was 0.7 metric tons/ha/year. In comparison, we 

can cite rates of 0.8 metric tons/ha/year for developed land 

uses in the continental United States (Nowak and Crane 

2002) and 0.98 metric tons/ha/year in a study of urban 

forests in northern New England (Zheng and others 2013). 

The C sequestration rates observed in the SJBE watershed 

were near the highest values (1.7 metric tons/ha/year for 

urban areas in Georgia) estimated by Nowak and Crane 

(2002), but it should be remembered that the overall SJBE 

watershed rate includes a considerable area of forest, which 

is shown to sequester C at a higher rate than developed 

land uses. So it might be more accurate to compare the 

SJBE watershed developed land use sequestration rate of 

0.7 metric tons/ha/year to U.S. urban areas.

Planted tree growth at an urban forest inventory plot center in a median strip of Paseo de los Gigantes, San Juan, from 2001 (left) to 2010 

(right). (photos by Jeffery Glogiewicz, Consultores Ambiental and the Fundación Puertorriqueña de Conservación)

An illustration of rapid palm tree growth over the course of the 10-year (2001 to 2011) urban forest inventory remeasurement period at an 

urban forest inventory plot center in the San Juan Bay Estuary watershed. (left photo by Jeffery Glogiewicz, Consultores Ambiental and the 

Fundación Puertorriqueña de Conservación; right photo by Edgardo González, Centro para la Conservación del Paisaje)

37
38

In addition to the ecosystem services directly quantified in 

this study, trees in the SJBE watershed provided benefits that 

positively affected both environmental quality and human 

health. Closely related to the energy savings generated 

by urban tree shade on residential structures is the overall 

reduction of ambient temperature brought about by tree 

cover in urban areas. The formation of an urban heat island 

has been observed in San Juan and temperatures are thought 

to be increasing by 0.06 °C annually (González and others 

2005, Lugo and others 2011, Velazquez-Lozada and others 

2006). The SJBE watershed’s urban trees will be valuable 

in counteracting this phenomenon, as has been shown in 

a meta-analysis of data collected in multiple cities, where 

urban green areas averaged 0.94 °C cooler than non-green 

sites (Bowler and others 2010).

This study did not attempt to quantify some of the most 

important ecosystem services trees provide in the SJBE 

watershed. Urban tree cover can greatly influence the flow 

of water through the watershed. Flooding and occasional 

landslides following torrential rainfall events associated with 

tropical depressions and hurricanes affect many San Juan 

residents. The prevalence of impervious surfaces in urban 

areas means coping with stormwater can be a problem. Tree 

cover reduces the impact of precipitation, stabilizes the soil, 

reduces rainwater runoff, and increases infiltration. The 

more forested Río Piedras watershed, a subwatershed within 

the larger SJBE watershed, has been shown to export less 

nitrogen than less forested urban and agricultural watersheds 

(Ortiz-Zayas and others 2006). In addition to reducing 

the amounts of nitrogen pollutants in the estuary’s waters, 

increased forest cover could potentially help reduce the fecal 

coliform pollution known to be present there (Hunter and 

Arbona 1995). 

Protection of the remaining forest cover would also help 

ameliorate erosion and landslides produced by poorly 

planned development, which are all too common throughout 

the upper reaches of the watershed. Natural vegetation, 

particularly the mangrove forest, stabilizes shorelines 

throughout the estuary. The economically vital San Juan 

Bay, through which most of the island’s shipping and 

all of its cruise ship traffic passes, is being harmed by 

deteriorating water quality due to sedimentation and nutrient 

loads carried downstream from the surrounding urban 

areas (Hunter and Arbona 1995, Lugo and others 2011, 

Ortiz-Zayas and others 2006, U.S. Environmental Protection 

Agency 2007, Villanueva and others 2000). Increased tree 

cover in the surrounding urban area could mitigate some 

of these negative human-caused environmental impacts. 

Selective plantings in residential and commercial areas 

could increase property values and provide for other 

recreation opportunities. Studies have shown that trees along 

streets and in urban lots can increase the value of properties 

and shorten the time it takes to sell them (Donovan and 

Butry 2009, Pandit and others 2013, Sander and others 2010, 

Saphores and Li 2012).

The delivery of all these benefits can be increased by treeestablishment programs to increase forest cover on the 

estimated 16.8 percent of the estuary that is potentially 

plantable. More efforts are needed to protect and connect 

urban green areas, like the consolidation of previously 

isolated forest patches into a more connected San Juan 

Ecological Corridor, which provides long-term benefits 

for San Juan’s residents. But these benefits can also be 

lost by deforestation of the existing forest canopy due to 

poorly regulated urban development and other activities. In 

addition, there can be social (for example, allergies, damage 

to sidewalks), economic (maintenance, storm debris), and 

environmental (biogenic emissions, carbon emissions from 

decomposition) costs associated with trees (Escobedo and 

others 2011). Therefore, proper planning and management 

is needed to sustain or enhance the existing urban forest to 

increase the environmental and societal benefits from trees 

in the SJBE watershed.

Next Steps

Identifying the ecosystem services provided by subtropical 

urban forests that are of priority in Caribbean communities 

is a key point for further consideration. For example, air 

pollution mitigation has been identified as an important 

benefit of urban trees in the temperate-zone cities of the 

continental United States. But is it as important in a coastal 

city like San Juan, where regular trade winds keep air 

pollution concentrations low? Is the mitigation of windstorm 

damage by urban trees of greater value in the Caribbean? 

The long-term ecological implications of the many 

introduced species and how they will interact with the native 

species as these forests grow and mature, are still uncertain. 

Continued inventory and monitoring efforts will help 

answer this and many more questions about the health and 

functioning of this urban forest ecosystem.
39

Moreover, future ecosystem service delivery estimates for 

the SJBE watershed and other tropical urban areas will 

become more accurate as models and coefficients developed 

in those climates are incorporated into the UFORE/i-Tree 

program. For example, C sequestration was estimated in this 

study by using a standardized growth rate of 0.83 cm/year, 

which was then adjusted upward or downward according 

to whether the tree was in the forest or a developed land 

use, growing season length, crown light exposure, and tree 

condition (Nowak and Crane 2002). This standardized 

growth rate was derived from observations of trees growing 

in the temperate Northeast and North-Central United 

States. While this approach is clearly not ideal, this and 

other studies are now providing the information needed to 

adjust these growth rates for the subtropical tree species 

found in the SJBE watershed. Brandeis (2009) observed a 

mean annual growth rate of 0.37 cm/year for 2,315 trees 

in the subtropical moist forest life zone on mainland 

Puerto Rico. Average growth rates by species were as 

follows: S. campanulata, 0.64 cm/year; C. antillanum, 

0.41 cm/year; A. procera, 0.65 cm/year, with a maximum 

observed periodic annual increment of 2.22 cm/year; and 

Pithecellobium dulce, 1.64 cm/year (Brandeis 2009). 

Weaver (1979) found that trees in thinned secondary forest 

stands in what was then the St. Just Forest in the San Juan 

area grew at an average rate of 0.47 cm/year. For mangrove 

forest, Weaver (1979) observed that natural regeneration in 

cleared stands in the Piñones Commonwealth Forest grew 

at a rate of 0.46 cm/year from 1938 to 1975. The results 

from these studies and this current research will allow us to 

refine the models and coefficients used for future estimates 

of urban forest ecosystem services in San Juan and other 

subtropical and tropical urban areas.

Municipal park improvements and tree 

planting at an urban forest inventory plot 

center from 2002 (top) to 2010 (bottom) 

in the San Juan Bay Estuary watershed. 

(top photo by Jeffery Glogiewicz, 

Consultores Ambiental and the Fundación 

Puertorriqueña de Conservación; bottom 

photo by Tom Brandeis, U.S. Forest Service)
40

Conclusions

The SJBE watershed holds species-rich, diverse forests with 

trees and shrubs brought there from the World’s tropical 

regions. Despite being heavily urbanized and densely 

populated, the SJBE watershed maintains a relatively 

large, contiguous mangrove forest centered on the Piñones 

Commonwealth Forest that provides valuable ecosystem 

services to the adjoining city. In addition there are important 

biologically diverse secondary subtropical moist forest 

patches scattered across the developed urban matrix that 

are still in a developing stage, recovering and maturing 

after both recent and historical disturbances. These forests 

should be conserved for their ecosystem services, such 

as aesthetic value, recreational opportunities, and role as 

ecological refugia. As shown, subtropical San Juan receives 

considerable benefits from its diverse and vibrant urban 

forests.

Literature Cited

Abdollahi, K.K.; Ning, Z.H. 1996. Research update on environmental 

influences of trees on mitigating air pollution effects and improving 

urban air quality. In: Sustaining people, sustaining forests: proceedings 

of the 1995 Society of American Foresters national convention; 

October 28-November 1, 1995; Portland, ME. Bethesda, MD: Society of 

American Foresters: 177–185.

Alvarez-Ruiz, M.; Acevedo-Rodríguez, P.; Vázquez, M. 1997. Quantitative 

description of the structure and diversity of the vegetation in the 

limestone forest of Río Abajo, Arecibo-Utuado, Puerto Rico. Acta 

Científica. 11(1–3): 21–66.

Armson, D.; Stringer, P.; Ennos, A.R. 2013. The effect of street trees and 

amenity grass on urban surface water runoff in Manchester, U.K. Urban 

Forestry & Urban Greening. 12: 282–286.

Bechtold, W.A.; Scott, C.T. 2005. The forest inventory and analysis 

plot design. In: Bechtold, W.A.; Patterson, P.L., eds. The enhanced 

Forest Inventory and Analysis program—national sampling design 

and estimation procedures. Gen. Tech. Rep. SRS–80. Asheville, NC: 

U.S. Department of Agriculture Forest Service, Southern Research 

Station: 27–42. 

Birdsey, R.A.; Weaver, P.L. 1982. The forest resources of Puerto Rico. 

Resour. Bull. SO–85. New Orleans: U.S. Department of Agriculture 

Forest Service, Southern Forest Experiment Station. 56 p.

Bowler, D.E.; Buyung-Ali, L.; Knight, T.M.; Pullin, A.S. 2010. Urban 

greening to cool towns and cities: a systematic review of the empirical 

evidence. Landscape and Urban Planning. 97: 147–155.

Brandeis, T.J. 2003. Puerto Rico’s forest inventory: adapting the Forest 

Inventory and Analysis program to a Caribbean island. Journal of 

Forestry. 101(1): 8–13.

Brandeis, T.J. 2009. Diameter growth of subtropical trees in Puerto Rico. 

Res. Pap. SRS–47. Asheville, NC: U.S. Department of Agriculture Forest 

Service, Southern Research Station. 39 p.

Brandeis, T.J.; Helmer, E.H.; Marcano, H.; Lugo, A.E. 2009. Climate 

shapes the novel plant communities that form after deforestation 

in Puerto Rico and the U.S. Virgin Islands. Forest Ecology and 

Management. 258(7): 1704–1718.

Brandeis, T.J.; Helmer, E.H.; Oswalt, S.N. 2007. The status of Puerto Rico’s 

forests, 2003. Resour. Bull. SRS–119. Asheville, NC: U.S. Department 

of Agriculture Forest Service, Southern Research Station. 75 p.

Brandeis, T.J.; Turner, J.A. 2013. Puerto Rico’s Forests, 2009. Resour. Bull. 

SRS–191. Asheville, NC: U.S. Department of Agriculture Forest Service, 

Southern Research Station. 85 p.

Cairns, M.A.; Brown, S.; Helmer, E.H.; Baumgardner, G.A. 1997. Root 

biomass allocation in the world’s upland forests. Oecologia. 111: 1–11.

Chinea, J.D. 2002. Tropical forest succession on abandoned farms in the 

Humacao Municipality of Eastern Puerto Rico. Forest Ecology and 

Management. 167: 195–207.

Departamento de Recursos Naturales y Ambientales. 2007. Inventario 

de flora en el corredor ecológico de San Juan, San Juan, Puerto Rico. 

San Juan, PR: Puerto Rico Departamento de Recursos Naturales y 

Ambientales, Negociado de Servicio Forestal, División de Investigación 

Forestal. 30 p.

Despiau-Batista, A. 1997. Distribución de las especies arbóreas de acuerdo 

al gradiente en topographía en el bosque de Río Piedras, Puerto Rico, 

luego de 60 años de abandono agrícola. Acta Científica. 11(1–3): 3–20.

Domínguez Cristóbal, C.M. 1989. La situacíon forestal pre-hispanica de 

Puerto Rico. Acta Científica. 3(2–3): 63–66.

Donovan, G.H.; Butry, D.T. 2009. The value of shade: estimating the effect 

of urban trees on summertime electricity use. Energy and Buildings. 41: 

662–668.

Donovan, G.H.; Butry, D.T.; Michael, Y.L. [and others]. 2013. The 

relationship between trees and human health. American Journal of 

Preventive Medicine. 44(2): 139–145. 

Escobedo, F.J.; Kroeger, T.; Wagner, J.E. 2011. Urban forests and pollution 

mitigation: analyzing ecosystem services and disservices. Environmental 

Pollution. 159: 2078–2087.

Escobedo, F.J.; Varela, S.; Zhao, M. [and others]. 2010. Analyzing the 

efficacy of subtropical urban forests in offsetting carbon emissions from 

cities. Environmental Science and Policy. 13: 362–372.

Franco, P.A.; Weaver, P.L.; Eggen-McIntosh, S. 1997. Forest resources 

of Puerto Rico, 1990. Resour. Bull. SRS–22. Asheville, NC: U.S. 

Department of Agriculture Forest Service, Southern Research Station. 

45 p.
41

González, J.E.; Luvall, J.C.; Rickman, D. [and others]. 2005. Urban heat 

islands developing in coastal tropical cities. Eos, Transactions, American 

Geophysical Union. 86(42): 397–403..

Grau, R.H.; Hernández, M.E.; Gutierrez, J. [and others]. 2008. A peri-urban 

Neotropical forest transition and its consequences for environmental 

services. Ecology and Society. 13(1): 16. http://www.ecologyandsociety.

org/vol13/iss1/art35/.

Heisler, G.M. 1986a. Effects of individual trees on the solar radiation 

climate of small buildings. Urban Ecology. 9: 337–359.

Heisler, G.M. 1986b. Energy savings with trees. Journal of Arboriculture. 

12(5): 113–125.

Heisler, G.M.; Grant, R.H.; Grimmond, S.; Souch, C. 1995. Urban forests—

Cooling our communities? In: Kollin, C.; Barratt, M., eds. Inside 

urban ecosystems: proceedings, 7th national urban forest conference; 

September 12–16; New York, NY. Washington, DC: American Forests: 

31–34.

Helmer, E.H.; Ramos, O.; del Mar-López, T. [and others]. 2002. Mapping 

the forest type and land cover of Puerto Rico, a component of the 

Caribbean Biodiversity Hotspot. Caribbean Journal of Science. 38(3–4): 

165–183.

Holdridge, L.R. 1967. Life zone ecology. Revised ed. San José, Costa Rica: 

Tropical Science Center. 206 p.

Hunter, J.M.; Arbona, S.I. 1995. Paradise lost: an introduction to the 

geography of water pollution in Puerto Rico. Social Science and 

Medicine. 40(10): 1331–1355.

Jimenez, J.A.; Martinez, R.; Encarnación, L. 1985. Massive tree mortality 

in a Puerto Rican mangrove forest. Caribbean Journal of Science. 

21(1–2): 75–78.

Jones, R.E.; Davis, K.L.; Bradford, J. 2013. The value of trees: factors 

influencing homeowner support for protecting local urban trees. 

Environment and Behavior. 45: 650–676.

Little, E.L.; Wadsworth, F.H. 1989. Common trees of Puerto Rico and the 

Virgin Islands. 2nd ed. 2 vols. Vol. 1. Washington, DC: U.S. Department 

of Agriculture Forest Service. 556 p.

López, T.; Aide, T.M.; Thomlinson, J.R. 2001. Urban expansion and the 

loss of prime agricultural lands in Puerto Rico. Ambio. 30: 49–54.

Lugo, A.E.; Brandeis, T.J. 2005. A new mix of alien and native species 

coexists in Puerto Rico’s landscapes. Chapter 20. In: Burslem, D.F.R.P.; 

Pinard, M.A.; Hartley, S.E., eds. Biotic interactions in the Tropics: their 

role in the maintenance of species diversity. Cambridge, UK: Cambridge 

University Press: 484–509.

Lugo, A.E.; Ramos Gonzáles, O.M.; Rodriguez Pedraza, C. 2011. The Río 

Piedras watershed and its surrounding environment. Forest Ecology and 

Management. Rio Piedras, Puerto Rico: U.S. Department of Agriculture 

Forest Service, International Institute of Tropical Forestry. 48 p.

Lugo, S.; Bryan, B.; Reyes, L.; Lugo, A. 2001. Riparian vegetation of a 

subtropical urban river. Acta Cientifica. 15: 59–72.

Martinuzzi, S.; Gould, W.A.; Ramos González, O.M. 2007. Land 

development, land use, and urban sprawl in Puerto Rico: integrating 

remote sensing and population census data. Landscape and Urban 

Planning. 79: 288–297.

McCollum, J. 2001. Honeycombing the icosahedron and icosahedroning 

the sphere. In: Reams, G.A.; McRoberts, R.E.; Van Deusen, P.C., 

eds. Proceedings of the second annual Forest Inventory and Analysis 

symposium. Gen. Tech. Rep. SRS–47. Salt Lake City, UT: U.S. 

Department of Agriculture Forest Service, Southern Research Station: 

25–31. 

McPherson, E.G. 1994. Cooling urban heat islands with sustainable 

landscapes. In: Platt, R.H.; Rowntree, R.A.; Muick, P.C., eds. The 

ecological city: preserving and restoring urban biodiversity. Amherst, 

MA: The University of Massachusetts Press: 150–171. 

McPherson, E.G. 1998. Atmospheric carbon dioxide reduction by 

Sacramento’s urban forest. Journal of Arboriculture. 24(4): 215–223.

McPherson, E.G.; Rowntree, R.A. 1991. The environmental benefits of 

urban forests. In: A national research agenda for urban forestry in the 

1990s; October; Urbana, IL. International Society of Arboriculture: 

45–49.

McPherson, E.G.; Simpson, J.R. 1999. Carbon dioxide reduction through 

urban forestry: guidelines for professional and volunteer tree planters. 

Gen. Tech. Rep. PSW–171. Albany, CA: U.S. Department of Agriculture 

Forest Service, Pacific Southwest Research Station. 237 p.

Miles, P.D. 2013. Forest Inventory EVALIDator Web-application version 

1.5.1.04. http://apps.fs.fed.us/Evalidator/tmattribute.jsp. [Date accessed: 

March 4].

Nabuurs, G.J.; Ravindranath, N.H.; Paustian, K. [and others]. 2003. LUCF 

sector good practice guidance. In: Penman, J.; Gytarsky, M.; Hiraishi, 

T.; Krug, T. [and others], eds. Good practice guidance for land use, landuse change, and forestry. Hayama, Kanagawa, Japan: IPCC National 

Greenhouse Gas Inventories Program, Technical Support Unit: 3.1–3.95. 

Chapter 3.

Nowak, D.J. 1993. Atmospheric carbon reduction by urban trees. Journal of 

Environmental Management. 37: 207–217.

Nowak, D.J. 1994. Atmospheric carbon dioxide reduction by Chicago’s 

urban forest. In: McPherson, E.G.; Nowak, D.J.; Rowntree, R.A., eds. 

Chicago’s urban forest ecosystem: results of the Chicago urban forest 

climate project. Gen. Tech. Rep. NE–186. Radnor, PA: U.S. Department 

of Agriculture Forest Service, Northeastern Research Station: 83-94. 

Nowak, D.J. 1996. Estimating leaf area and leaf biomass of open-grown 

deciduous urban trees. Forest Science. 42(4): 504–507.

Nowak, D.J. 2002. The effects of urban trees on air quality. http://www.

fs.fed.us/ne/syracuse/gif/trees.pdf. [Date accessed: June 2002].
42

Nowak, D.J.; Crane, D.E. 2000. The urban forest effects (UFORE) model: 

quantifying urban forest structure and functions. In: Hansen, M.; Burk, T. 

eds. Integrated tools for natural resources inventories in the 21st century. 

Proceedings of the IUFRO conference. Gen. Tech. Rep. NE–212. St. 

Paul, MN: U.S. Department of Agriculture Forest Service, North Central 

Research Station: 714–720.

Nowak, D.J.; Crane, D.E. 2002. Carbon storage and sequestration by urban 

trees in the USA. Environmental Pollution. 116: 381–389.

Nowak, D.J.; Crane, D.E.; Stevens, J.C.; Hoehn, R.E. 2001. The Urban 

Forest Effects (UFORE) model: field data collection procedures. 

Syracuse, NY: U.S. Department of Agriculture Forest Service, 

Northeastern Research Station. 51 p.

Nowak, D.J.; Crane, D.E.; Stevens, J.C. 2006. Air pollution removal by 

urban trees and shrubs in the United States. Urban Forestry and Urban 

Greening. 4: 115–123.

Nowak, D.J.; Crane, D.E.; Stevens, J.C.; Hoehn, R.E. 2005. The Urban 

Forest Effects (UFORE) model: field data collection manual. Syracuse, 

NY: U.S. Department of Agriculture Forest Service, Northern Research 

Station. 34 p. http://www.fs.fed.us/ne/syracuse/Tools/downloads/

UFORE_Manual.pdf. [Date accessed: January 2014].

Nowak, D.J.; Crane, D.E.; Stevens, J.C.; Ibarra, M. 2002. Brooklyn’s urban 

forest. Gen. Tech. Rep. NE–290. Newtown Square, PA: U.S. Department 

of Agriculture Forest Service, Northeastern Forest Experiment Station. 

107 p.

Nowak, D.J.; Dwyer, J.F.; Childs, G. 1997a. Los beneficios y costos del 

enverdecimiento urbano. In: Krishnamurty, L.; Rente-Nascimento, J., 

eds. Areas Verdes Urbanas en Latinoamérica y el Caribe. Mexico City, 

Mexico: Banco Interamericano de Desarrollo: 17–38. 

Nowak, D.J.; Hoehn, R.E., III; Crane, D.E. [and others]. 2008. A groundbased method of assessing urban forest structure and ecosystem services. 

Arboriculture & Urban Forestry. 34(6): 347–358.

Nowak, D.J.; McHale, P.J.; Ibarra, M. [and others]. 1997b. Modeling the 

effects of urban vegetation on air pollution. In: 22nd NATO/CCMS 

international technical meeting on air pollution modelling and its 

application. Clermont-Ferrand, France. Imprimerie des UFR Sciences: 

276–283.

Nowak, D.J.; Nobel, M.H.; Sisinni, S.S.; Dwyer, J.F. 2001. Assessing the 

U.S. urban forest resource. Journal of Forestry. 99(3): 37–42.

Ortiz-Zayas, J.R.; Cuevas, E.; Mayol-Bracero, O.L. [and others]. 2006. 

Urban influences on the nitrogen cycle in Puerto Rico. Biogeochemistry. 

79: 109–133.

Pandit, R.; Polyakov, M.; Tapsuwan, S.; Moran, T. 2013. The effect of street 

trees on property value in Perth, Western Australia. Landscape and Urban 

Planning. 110: 134–142.

Parés-Ramos, I.K.; Gould, W.A.; Aide, T.M. 2008. Agricultural 

abandonment, suburban growth, and forest expansion in Puerto Rico 

between 1991 and 2000. Ecology and Society. 13(2): 19. http://www.

ecologyandsociety.org/vol13/iss2/art1/. [Date accessed: January 2014].

Ramos González, O.M. 2001. Assessing vegetation and land cover changes 

in northeastern Puerto Rico: 1978–1995. Caribbean Journal of Science. 

37(1–2): 95–106.

Ramos González, O.M.; Rodríguez-Pedraza, C.D.; Lugo, A.E.; Edwards, B. 

2005. Distribution of forests and vegetation fragments in the San Juan 

metropolitan area [Abstract]. In: 9th annual urban and community 

forestry conference: managing the Caribbean urban and community 

forest, June 14–18, 2004; St. John, U.S. Virgin Islands. St. Thomas, 

U.S. Virgin Islands: University of the Virgin Islands, Cooperative 

Extension Service. 111 p.

Reams, G.A.; Smith, W.D.; Hansen, M.H. [and others]. 2005. The Forest 

Inventory and Analysis sampling frame. In: Bechtold, W.A.; Patterson, 

P.L., eds. The enhanced Forest Inventory and Analysis program—

national sampling design and estimation procedures. Gen. Tech. Rep. 

SRS–80. Asheville, NC: U.S. Department of Agriculture Forest Service, 

Southern Research Station: 11–26. 

Rivera, L.W.; Aide, T.M. 1998. Forest recovery in the karst region of Puerto 

Rico. Forest Ecology and Management. 108: 63–75.

Rowntree, R.A.; Nowak, D.J. 1991. Quantifying the role of urban forests in 

removing atmospheric carbon dioxide. Journal of Arboriculture. 17(10): 

269–275.

Sander, H.; Polasky, S.; Haight, R.G. 2010. The value of urban tree cover: 

a hedonic property price model in Ramsey and Dakota Counties, 

Minnesota. Ecological Economics. 69: 1646–1656.

Saphores, J.D.; Li, W. 2012. Estimating the value of urban green areas: 

a hedonic pricing analysis of the single family housing market in Los 

Angeles, CA. Landscape and Urban Planning. 104: 373–387.

Schomaker, M.E.; Zarnoch, S.J.; Bechtold, W.A. [and others]. 2007. 

Crown-condition classification: a guide to data collection and analysis. 

Gen. Tech. Rep. SRS–102. Asheville, NC: U.S. Department of 

Agriculture Forest Service, Southern Research Station. 78 p.

Seguinot Barbosa, J. 1996. La ecología urbana de San Juan (una 

interpretación geográfico social). Anales de Geografía de la Universidad 

Complutense. 16: 161–184.

Simpson, J.R.; McPherson, E.G. 1998. Simulation of tree shade impacts 

on residential energy use for space conditioning in Sacramento. 

Atmospheric Environment. 32(1): 69–74.

Suárez, A.; Vicéns, I.; Lugo, A.E. 2005. Composición de especies y 

estructura del bosque kárstico de San Patricio, Guaynabo, Puerto Rico. 

Acta Científica. 19(1–3): 7–22.

Thompson, J.; Lugo, A.E.; Thomlinson, J.R. 2007. Land use history, 

hurricane disturbance, and the fate of introduced species in a subtropical 

wet forest in Puerto Rico. Plant Ecology. 192: 289–301.

U.S. Department of Agriculture Forest Service. 2006. Forest Inventory 

and Analysis national field manual. Urban inventory pilot supplement. 

Section 15. Urban measurements and sampling. Knoxville, TN: U.S. 

Department of Agriculture Forest Service, Forest Inventory and Analysis 

Program. 68 p. http://www.fia.fs.fed.us/library/field-guides-methodsproc/docs/core_ver_4-0_10_2007_p2.pdf. [Date accessed: June 2007].
43

U.S. Department of Agriculture Forest Service. 2007. Forest Inventory 

and Analysis national core field guide. Field data collection procedures 

for phase 2 plots. Version 4.0. Washington, DC 224 p. Vol. 1. Internal 

report. On file with: U.S. Department of Agriculture Forest Service, 

Forest Inventory and Analysis, 201 14th Street, Washington, DC 20250. 

http://www.fia.fs.fed.us/library/field-guides-methods-proc/docs/core_

ver_4-0_10_2007_p2.pdf. [Date accessed: April 2007].

U.S. Department of Agriculture Forest Service. 2011. Forest Inventory 

and Analysis national core field guide. Field data collection procedures 

for phase 2 plots. Version 4.0. Washington, DC. Vol. I. Internal report. 

On file with: U.S. Department of Agriculture Forest Service, Forest 

Inventory and Analysis, 201 14th Street, Washington, DC 20250. 

http://www.fia.fs.fed.us/library/field-guides-methods-proc/docs/2006/

core_ver_3-0_10_2005.pdf. [Date accessed: August 2011].

U.S. Environmental Protection Agency. 2007. National estuary program 

coastal condition report. EPA-842/B-06/001. Puerto Rico: San 

Juan Bay Estuary Partnership Coastal Condition. Washington, DC: 

U.S. Environmental Protection Agency, Office of Water, Office of 

Research and Development: 385–400. Chapter 7. http://water.epa.gov/

type/oceb/nep/index.cfm#tabs-4. [Date accessed: January 2014].

Velazquez-Lozada, A.; Gonzalez, J.E.; Winter, A. 2006. Urban heat island 

effect analysis for San Juan, Puerto Rico. Atmospheric Environment. 

40(9): 1731–1741. 

Villanueva, E.; Rivera-Herrera, L.J.; Rivera-Colón, S. [and others]. 2000. 

Comprehensive conservation and management plan for the San Juan Bay 

Estuary. San Juan, Puerto Rico: U.S. Army Corps of Engineers. 420 p. 

Vol. 1.

Wadsworth, F.H. 1950. Notes on the climax forests of Puerto Rico and their 

destruction and conservation prior to 1900. Caribbean Forester. 11(1): 

38–47.

Weaver, P.L. 1979. Tree growth in several tropical forests of Puerto Rico. 

Gen. Tech. Rep. SO–152. New Orleans: U.S. Department of Agriculture 

Forest Service, Southern Forest Experiment Station. 15 p.

Zhao, M.; Escobedo, F.J.; Staudhammer, C. 2010. Spatial pattern of a 

subtropical, coastal urban forest: implications for land tenure, hurricanes, 

and invasives. Urban Forestry and Urban Greening. 9: 205–214.

Zheng, D.; Ducey, M.J.; Heath, L.S. 2013. Assessing net carbon 

sequestration on urban and community forests of northern New England, 

USA. Urban Forestry & Urban Greening. 12(1): 61–68.
44

San Juan Bay Estuary. (photo courtesy of U.S. Environmental Protection Agency)
Brandeis, Thomas J.; Escobedo, Francisco J.; Staudhammer, Christina L. [and others]. 2014. 

San Juan Bay Estuary watershed urban forest inventory. Gen. Tech. Rep. SRS–190. Asheville, 

NC. U.S. Department of Agriculture Forest Service, Southern Research Station. 44 p.

We present information on the urban forests and land uses within the watershed of Puerto Rico’s 

21 658-ha San Juan Bay Estuary based on urban forest inventories undertaken in 2001 and 2011. 

We found 2548 ha of mangrove and subtropical moist secondary forests covering 11.8 percent of 

the total watershed area in 2011. Red, black, and white mangroves (Rhizophora mangle, Avicennia 

germinans, and Laguncularia racemosa) were the most common species due to the watershed’s 

extensive mangrove forests, while tulipán africano (Spathodea campanulata) and María 

(Calophyllum antillanum) were predominant species in the moist forest patches and developed 

land uses. Approximately 10.1 million trees created an average tree cover of 24.1 percent, stored 

319 737 metric tons of carbon (C) and sequestered C at a rate of 28 384 metric tons/year. This 

stored C had an estimated value of $8.1 million with an annual C sequestration value of $718,113 

in 2011, up from the 2001 values. In 2011 approximately 19 000 megawatts of energy required 

for cooling buildings were avoided due to tree shading and climate effects in residential and 

commercial areas and equated to 1986 metric tons of avoided C emissions due to building energy 

effects.

Keywords: Caribbean, ecosystem services, FIA, forest inventory, Puerto Rico, subtropical forest, 

urban forest.

How do you rate this publication?

Scan this code to submit your feedback or go to 

www.srs.fs.usda.gov/pubeval 

You may request additional copies of this publication by email at pubrequest@fs.fed.us. 

Please limit requests to two copies per individual.
The Forest Service, United States Department of 

Agriculture (USDA), is dedicated to the principle of 

multiple use management of the Nation’s forest 

resources for sustained yields of wood, water, forage, 

wildlife, and recreation. Through forestry research, cooperation 

with the States and private forest owners, and management of the 

National Forests and National Grasslands, it strives—as directed 

by Congress—to provide increasingly greater service to a growing 

Nation.

The USDA prohibits discrimination in all its programs and activities 

on the basis of race, color, national origin, age, disability, and where 

applicable, sex, marital status, familial status, parental status, 

religion, sexual orientation, genetic information, political beliefs, 

reprisal, or because all or part of an individual’s income is derived 

from any public assistance program. (Not all prohibited bases apply 

to all programs.) Persons with disabilities who require alternative 

means for communication of program information (Braille, large 

print, audiotape, etc.) should contact USDA’s TARGET Center at 

(202) 720-2600 (voice and TDD).

To file a complaint of discrimination, write to USDA, Director, 

Office of Civil Rights, 1400 Independence Avenue, SW, Washington, 

D.C. 20250-9410, or call (800) 795-3272 (voice) or (202) 720-6382 

(TDD). USDA is an equal opportunity provider and employer.