Guam’s Forest Resources, 2002

Joseph A. Donnegan, Sarah L. Butler, Walter Grabowiecki, 

Bruce A. Hiserote, and David Limtiaco

United States

Department of

Agriculture

Forest 

Service

Pacific Northwest

Research Station

Resource Bulletin

PNW-RB-243

November 2004
The Forest Service of the U.S. Department of Agriculture 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.

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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 USDA, Director, Office of Civil Rights, Room 326-

W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or 

call (202) 720-5964 (voice and TDD). USDA is an equal opportunity provider and employer.

USDA is committed to making its information materials accessible to all USDA customers 

and employees.

Authors

Joseph A. Donnegan is an ecologist, Sarah L. Butler is an ecologist, Walter 

Grabowiecki is a quality assurance technician, and Bruce A. Hiserote is an 

information management specialist, Forestry Sciences Laboratory, 620 SW Main 

Street, Suite 400, Portland, OR 97205-3028; David Limtiaco is the chief of 

forestry for the government of Guam, 192 Dairy Road, Mangilao, Guam 96923.
Abstract

Donnegan, Joseph A.; Butler, Sarah L.; Grabowiecki, Walter; Hiserote, 

Bruce A.; Limtiaco, David. 2004. Guam’s forest resources, 2002. Resour. 

Bull. PNW-RB-243. Portland, OR: U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Research Station. 32 p.

The Forest Inventory and Analysis Program collected, analyzed, and summarized field data on 46 forested plots on the island of Guam. Estimates of forest 

area, tree stem volume and biomass, the numbers of trees, tree damages, and the 

distribution of tree sizes were summarized for this statistical sample. Detailed 

tables and graphical highlights provide a summary of Guam’s forest resources 

and a baseline from which to detect future change following remeasurement of 

the permanent field plots.

Keywords: Guam, biomass, damage, FIA, forest inventory, volume.
Summary

The Forest Inventory and Analysis (FIA) Program established a systematic sample 

of forest field plots on Guam to estimate forest area, tree stem volume and biomass, 

tree damages, and associated understory vegetation. Thirty-two permanent field 

plots were established in limestone forest and 14 in volcanic forest. Land cover 

was mapped from high-resolution satellite data and merged with the soil layer in 

a geographic information system to stratify the field sample. The forest area on 

Guam occupies approximately 63,830 acres, with limestone forest accounting for 

about 70 percent of that total. About 18 percent of Guam was classified as urban 

land and includes roads, towns, airstrips, and military facilities. We estimated gross 

tree stem volume to be about 91 million cubic feet, inclusive of all tree size classes. 

Aboveground stem weight for all trees greater than or equal to 5 inches in diameter 

at breast height was estimated to be about 1 million tons. About 21 percent of the 

trees sampled exhibited some form of physical damage. Of those damaged trees, 

evidence of decay was found in nearly one-third of the individuals. Physical breakage owing to weather and damage by falling trees were cited as the primary damaging agents in approximately 64 percent of the damage cases. The average number of 

tree species per sixth-acre plot was about four in both limestone and volcanic forest 

types. Forty-eight tree species and about 140 understory species were measured on 

46 sixth-acre plots.
1

Guam’s Forest Resources, 2002

Introduction

This summary of forest resources on the island of Guam (fig. 1) was based on a 

forest inventory conducted in 2002 by the USDA Forest Service, Pacific Northwest 

Forest Inventory and Analysis Program (FIA) in cooperation with Pacific Island 

foresters. The key inventory objectives were to help answer local and national 

questions about the status and trends in tropical forested ecosystems and to share 

forestry skills among cultures and agencies. The fieldwork for this inventory was 

conducted by a multinational crew including students and foresters from Guam, 

foresters from American Samoa, and mainland U.S. foresters and ecologists.

This systematic, sample-based field inventory on Guam was the first of its kind 

to be conducted across the island to establish estimates for the area of forest types, 

tree size distribution, volume, biomass, and damages for living and dead trees. It 

was designed to provide resource managers with information about the current 

situation so they can better manage their forested and nonforested lands and, after 

remeasurement, better manage or mitigate any changes in the resource. Empirically 

based knowledge of the status and trends in forest vegetation can help managers 

plan sustainable supplies of wood, control invasive species, control erosion, and 

manage disturbances such as fire and animal damage.

Objectives

1. Estimate the area of forest land by forest type group and stand size class.

2. Estimate the volume, biomass, and carbon storage for tree species by 

diameter class.

3. Estimate the numbers of trees affected by damaging agents, such as insects 

and diseases, and estimate the number of dead trees.

4. Share measurement and analysis techniques among groups involved in the 

inventory.

Methods

Site Description

The island of Guam is the largest and farthest south in the north-south sweeping 

Marianas archipelago (fig. 1). The highest point on Guam is Mount Lamlam with 

an elevation of 1,332 feet. Guam is an unincorporated territory of the United States 

with significant income generated from tourism. The climate is tropical marine with 

little annual variation in temperature, but a pronounced dry season from January to 

June (fig. 2). 
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RESOURCE BULLETIN PNW-RB-243

Figure 1—The unincorporated Territory of Guam is in the North Pacific Ocean approximately 1,600 miles east of Manila and 

3,500 miles north of Melbourne, Australia.
3

Guam’s Forest Resources, 2002

Average Maximum and Minimum Temperature

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

74

76

78

80

82

84

86

Monthly Precipitation Totals

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Precipitation (inches)

2

4

6

8

10

12

14

Anderson Air Force Base, Guam, 1953–2001

Temperature (°F)

Figure 2—Average maximum and minimum temperature, and total monthly precipitation for Anderson Air Force Base, Guam. Note the 

pronounced seasonality in precipitation (Western Regional Climate Center, http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?piande).
4

RESOURCE BULLETIN PNW-RB-243

A significant portion of land on Guam is occupied by the U.S. military. On a 

portion of these lands, a system of refuges and wildlife reserves are overlain on 

the military lands. For example, Ritidian Point, approximately 22,500 acres at the 

northern tip of Guam, is administered by the U.S. Fish and Wildlife Service as a 

national wildlife refuge.

Significant disturbance agents have shaped vegetation in Guam, including 

frequent tropical storms and typhoons, human-caused grassland and forest fires, 

animal rooting, browsing and trampling, mass soil movements and erosion, nonnative insects and pathogens, invasive weeds, historical military actions, and 

historical timber harvest.

The northern half of the island is generally flat limestone plateau with abrupt 

dropoffs and cliffs toward the ocean. Locally the limestone can be strongly weathered into rugged karstic terrain. Soils are related to vegetation communities in 

Guam. The limestone soils in the north are covered with forest in areas not cultivated or urbanized. The southern part of the island features rolling to mountainous 

terrain in the deeply weathered volcanic soils. The volcanic soils on the southern 

half of Guam are covered primarily by grassland, with some ravine forest occurring in sheltered and leeward sites.

Vegetation Types

The vegetation of Guam was categorized (Mueller-Dombois and Fosberg 1998) 

according to the major underlying soil types: (1) northern limestone vegetation, 

and (2) southern volcanic vegetation. The limestone vegetation was further broken 

down into five classes by Fosberg (1960): Artocarpus-Ficus forest, Mammea forest, Cordia forest, Merrilliodendron-Ficus forest, and Pandanus forest. A brief 

description of each forest community follows. Pure examples of these forest types 

are now rare on Guam; instead these forests tend to be mixtures with secondary 

species predominating. The FIA inventory does not currently provide sufficient 

plot density to reliably classify vegetation types in these highly diverse tropical 

forests but provides plot-level data to help refine remotely sensed estimates of 

forest type acreage and developmental stage. Some of the forest types discussed 

below may not be represented in the plot-level information provided from this 

inventory owing to the systematic sampling design having missed the forest type. 

Ongoing mapping efforts will provide greater spatial detail and better refinement 

of forest types.

Significant disturbance 

agents have shaped 

the vegetation on 

Guam. Humans 

and tropical storms 

continue changing 

forest structure and 

composition.
5

Guam’s Forest Resources, 2002

Limestone forest types

Artocarpus-Ficus—

A widespread forest type on limestone in Guam with canopy dominants including 

Artocarpus mariannensis (breadfruit) and Ficus prolixa (banyan).

Mammea forest—

Mueller-Dombois and Fosberg (1998) characterize this forest type as dominated by 

Mammea odorata and by its occurrence on the eastern escarpment of the northern 

limestone plateau.

Cordia forest—

This is an open scrub-forest type on steep slopes and cliffs dominated by Cordia 

subcordata Lam.

Merrilliodendron-Ficus forest—

Merrilliodendron megacarpum (Hemsley) Sleumer and Ficus prolixa dominate this 

tall forest type.

Pandanus forest—

This moderate-height forest is dominated by Pandanus tectorius. Other common 

genera include Flagellaria, Cestrum, Triphasia, and Nephrolepis (Fosberg 1960).

Volcanic Types

The southern, volcanic half of the island is vegetated with a mix of grassland and 

patchy forest. The forest tends to follow topographic features, such as river drainages, sheltered depressions, and ravines. Grassland savannas are dominated by the 

dense, sharp-leaved Miscanthus floridulus (Labill.) Warb. ex K. Schum. & Laut. 

(swordgrass), and smaller areas of Pennisetum polystachyon (L.) J.A. Schultes 

(mission grass) and Dimeria chloridiformis (Gand.) K. Schum. & Laut. Common 

ravine forest trees include Areca catechu, (see table 1 for common names of trees) 

Ficus prolixa, Glochidion mariana Mueller-Arg., L., Hibiscus tiliaceus, Pandanus 

tectorius, P. dubius Sprengel, and Premna serratifolia L. (Fosberg 1960).

Low-lying, halophytic (sea-salt adapted) vegetation is found along beaches 

in the north and south, and may be composed solely of Casuarina equisetifolia

or a mixture of species including Casuarina, Cocos nucifera, Guettarda speciosa, 

Hernandia sonora, Pandanus tectorius, Scaevola taccada (Gaertn.) Roxb., Thespesia populnea (L.) Soland. ex Correa, and Tournefortia argentea L. f. (Fosberg 

1960). Areas of swamp, mangrove, and marsh are also found on Guam.
6

RESOURCE BULLETIN PNW-RB-243

Table 1—Scientific and common names and estimated specific gravitiesa of species measured as trees 

on Guam

Scientific name Common name

Specific gravity 

(for biomass)

Number 

measured

Adenanthera pavonina L. kulalis 0.50 7

Aglaia mariannensis Merrill mapuñao .50 54

Annona reticulata L. annonas .50 8

Areca catechu L. puguá .50 9

Artocarpus altilis (Park.) Fosb. lemai, breadfruit .50 8

Artocarpus mariannensis Trec. dugdug, Marianas breadfruit .50 1

Averrhoa bilimbi L. bilimbi, pikue .50 9

Barringtonia asiatica (L.) Kurz puting .50 2

Barringtonia racemosa (L.) Spreng. langaasag .50 2

Bauhinia monandra Kurz Saint Thomas tree, mariposa .50 3

Calophyllum inophyllum L. daok, Alexandrian laurel .57 5

Cananga odorata (Lam.) Hook. f. & Thoms. ilang-ilang .29 7

Carica papaya L. papaya .50 23

Casuarina equisetifolia L. gagu,australian pine .84 9

Ceiba pentandra (L.) Gaertn. algodon de manila .23 1

Cerbera dilatata Markgraf chuite .50 19

Citrus aurantifolia (Christm.) Swingle lime .50 1

Cocos nucifera L. niyok, coconut palm .50 81

Cyathea lunulata (Forst. f.) Copel. chacha, tree fern .50 2

Cycas micronesica K.D. Hill fandan (C. circinalis L.) .50 112

Cynometra ramiflora L. gulos .70 5

Eugenia reinwardtiana (Benth.) a’abang .50 2

Eugenia thompsonii Merrill atoto .50 2

Ficus prolixa Forst. f. nunu .50 11

Ficus tinctoria Forst. f. hoda, tagete .50 2

Guamia mariannae Merrill paipai .50 47

Guettarda speciosa L. pano .50 1

Hernandia ovigera L. .50 9

Hernandia sonora L. nonak .29 2

Heterospathe elata Scheffer palma brava .50 80

Hibiscus tiliaceus L. sea-hibiscus, pago .57 104

Inocarpus fagifer (Park.) Fosb. budo buoy, Tahitian chestnut .50 1

Intsia bijuga (Colebr.) O. Ktze. ifit, ifil .50 5

Kleinhovia hospita L. .36 5

Leucaena leucocephala (Lam.) de Wit tangantangan .64 147

Macaranga thompsonii Merrill .50 13

Mangifera indica L. mango .52 18

Morinda citrifolia L. lada, Indian mulberry .50 41

Neisosperma oppositifolia (Lam.) Fosb. & Sachet faag .50 3

Pandanus tectorius Park. aggag .50 61

Pipturus argenteus (Forst.) Wedd. amahazan .50 2

Polyscias grandifolia Volk. .50 1

Premna obtusifolia R. Br. ahgao .50 50

Psidium guajava L. guava, abas .50 3

Spathodea campanulata Beauv. African tulip tree .25 4

Triphasia trifolia (Burm. f.) P. Wils. limon de China .50 38

Vitex parviflora Juss. .70 92

Xylosma nelsonii Merrill .50 1

aSpecific gravity was used to calculate biomass and carbon mass.
7

Guam’s Forest Resources, 2002

Regions of barren, eroding soil are prevalent in the southern part of the island. 

The erosion problem is being addressed individually by multiple agencies and 

coordinated jointly through a multiagency consortium working on the vegetation 

strategy for southern Guam (Bell et al. 2002).

Field Methods

This inventory was based on the national FIA inventory design that was implemented across the mainland United States. Adaptations were made to the national 

design to use topography to define site productivity and to include additional 

branching and rooting forms, additional tree crown measurements, and specialinterest species ranging from invasive plants to pathogens to culturally or economically important species of various life forms. In the mainland FIA Program, 

plots are spaced within forest land on a 3.3-mile grid. With the assistance of the 

Government of Guam, Division of Forestry, plots were spaced across all vegetation 

types at 1.9-mile intervals, yielding a triple intensification of the mainland inventory 

plot grid.

The FIA plot-cluster is composed of four 24-foot-radius subplots (fig. 3). Three 

of those subplots are equally spaced, as if on spokes of a wheel, around the central 

subplot. The distance from the middle of the central subplot to the middle of each 

subplot on the three spokes was 120 feet.

A variety of information was collected at the plot, subplot, and tree levels 

(USDA Forest Service 2002). Differences in forest-type conditions are also mapped. 

For example, roads that intersect subplots are mapped, as are clear boundaries in 

forest tree size classes. The primary variables collected include plot location, slope, 

aspect, elevation, subplot slope position and shape, tree species, diameters, heights, 

damages, branching and rooting forms, decay, epiphytic loadings, crown characteristics, tree locations, and regeneration information. The fieldwork for this inventory 

was performed February to April 2002, before the two major typhoons, Chata’an 

and Pongsona, struck the island in July and December of 2002, respectively.

Analysis Methods

The FIA estimates of forest land are based on a system that uses aerial photography or satellite imagery to define different types of land (strata) across the landscape. The simplest stratification is separating land into forest and nonforest strata. 

However, stratifications can be assisted or refined by using ancillary data such as 

topography, soil information, life zone or climatically based information, and prior 

inventories of vegetation groups.

The Guam stratification was conducted via a classification of 2002 IKONOS 

satellite data, masking out and replacing clouds by using aerial photography taken 

The FIA inventory 

design was adapted 

for the conditions and 

vegetation in Guam.
8

RESOURCE BULLETIN PNW-RB-243

in 1994. The initial land cover classification divided the landscape into forest, 

urban, nonforest vegetation, barren, and water land types. At least 10 percent cover 

of trees was used as the basis for the forest land classification, and includes both 

agroforests and mangrove forests. Nonforest vegetation includes other vegetation 

types with less than 10 percent cover of trees. A geographic information system 

(GIS) was used to sum acreage for each type. Forest was further classified into 

volcanic and limestone forest types by overlaying a soils layer (USDA NRCS 1988) 

on the land-cover-classification layer in a GIS. We used the GIS to individually 

Forest Inventory and Analysis

Pacific Islands Plot Design

N

Overall plot footprint ~1.5 acres

Distance between subplot centers: 120 feet

Distance from subplot center to microplot center: 12 feet

1

2

4 3

Plot

Subplot Microplot

6.8-foot-radius microplot: 

 seedlings + saplings

Key

24-foot-radius subplot: 

 all trees (�5 inches in 

 diameter at breast height)

Figure 3—Forest Inventory and Analysis Pacific Islands plot design.
9

Guam’s Forest Resources, 2002

sum the acreage of limestone and volcanic forest types. From this limestone/

volcanic stratification, field plot condition types were expanded to the landscape 

level. The acreage each field plot represents was derived by dividing the total 

acreage of forest in each of the limestone and volcanic soil types by the number of 

field plots that fell within the geographic bounds of that specific type. Average stand 

size was expanded from the plots to the landscape level by using the same expansion factors. The forested areas on our initial land cover classification will be 

further refined into more detailed forest types in an ongoing project by the USDA 

Forest Service, Pacific Southwest Region, Remote Sensing Laboratory.

Wood volume was estimated for individual trees by using tree height and two 

stem diameter measurements. These measurements are expanded to tree-level 

volume estimates by using equations for sections of a cone. Both gross stem volume 

and net stem volume estimates were calculated. Net stem volume subtracts damage and rotten defects from gross stem volume. Biomass for individual tree stems 

was estimated by using the specific gravity for known species (31 out of 50 species 

measured on Guam have known specific gravities). For species where specific gravity was not known, an average specific gravity, according to forest type, was used. 

These estimates of aboveground tree biomass are derived from bole volume and 

include only biomass for the main stem, excluding branches, roots, and foliage.

An additive combination of relative density and relative basal area (importance 

value IV) was used to classify forest types and assess the species dominance in a 

stand. Traditional site productivity estimates require forest stand age, derived from 

the annual rings of forest trees. Because tropical trees do not produce consistent 

annual rings, a modified topographic relative moisture index (TRMI) (Parker 1982) 

was used as a proxy for site productivity. The TRMI used a weighted, additive combination of slope steepness, slope shape, and slope position to assess the potential 

moisture retention in a forested stand. The TRMI serves as a proxy for productivity until we can obtain growth rate data from remeasurement of these plots. We 

recognize moisture is not likely to be the only factor limiting tree growth and that 

an excess of moisture can be detrimental to tree growth.

Reliability of FIA Data

The area of land cover types mapped from the IKONOS classification was assumed 

to be accurate and was used as the basis for the expansion of the numbers of trees, 

tree volume, and tree biomass from the plot scale to the forest-type scale. Possible 

sources of error not accounted for in our estimates include errors in the land cover 

map owing to incorrect interpretation of the image, errors from rounding when 

working with pixel-based imagery, and measurement errors on field plots. Standard 

Because tropical 

trees do not produce 

consistent annual 

rings, an index 

based on slope 

characteristics 

affecting moisture 

retention was used 

as a proxy for site 

productivity.
10

RESOURCE BULLETIN PNW-RB-243

errors for the expansion of our estimates from field plots to the forested landscape 

were calculated according to the proportion of area occupied by either volcanic or 

limestone forest types. Volcanic and limestone forest areas were treated as known 

rather than estimated, and variance was calculated by using methods in Cochran 

(1977). Using one standard error as our basis for evaluation gives a 68 percent 

chance that the true total gross tree stem volume on Guam lies between 83,229,532 

and 99,520,878 cubic feet. There is a 68-percent chance that the true number of 

trees (>1 inch diameter) on Guam lies within the range of 68,809,072 to 84,733,366.

Resource Highlights

Land Cover

Based on the IKONOS satellite imagery, we estimate that Guam is approximately 

48 percent forested, with an additional 33 percent covered by grass and shrublands. 

Barren lands account for slightly over 1,500 acres (1 percent), and urban lands total 

approximately 24,000 acres (18 percent) (figs. 1 and 4, table 2).

Productivity

Forest land productivity was approximated as the potential of a site to retain moisture (TRMI) (Parker 1982) relative to that particular broad soil-based forest type 

(limestone vs. volcanic; table 3). Topography at each plot was used to rank sites 

within a forest type in terms of whether water would be expected to accumulate 

on or flow from the site. The limestone forests tended to occur on lands that would 

be classified predominantly (80 percent) as moderately high productivity (fig. 5). 

However, when basing potential productivity on topography, one must consider 

that limestone soil and parent material quickly drain water owing to high porosity. 

This may lead to an overestimate of productivity depending on the moisture-holding capacity of the soil. Sixty percent of the area of volcanic forest was classified 

as moderately high productivity or higher (fig. 6).

Forest Structure

Stand size class summarizes the predominant diameter of all live trees in a forested 

condition. In Guam, trees tend to be somewhat small in diameter, with the majority 

of forested area having trees in the 5- to 10.9-inch category (fig. 7). No stands were 

sampled in this inventory for the 20+ inch category. Such large-diameter stands are 

expected to be rare on Guam owing to frequent disturbance by typhoons.

The diameter distribution for trees on Guam follows a typical “reverse-J” 

pattern (fig. 8). More individuals are found in the smallest tree size classes (tables 

4 and 5), which through time either grow into larger trees, die in the understory 

Based on satellite 

imagery, we estimate 

that Guam is 48 

percent forested.

In Guam, trees tend 

to be relatively small 

owing to disturbance 

by typhoons and 

historical land use 

practices.
11

Guam’s Forest Resources, 2002

Guam's Land Cover, 2002

1%

14%

34%

33%

18%

Barren

Volcanic forest

Limestone forest

Nonforest vegetation

Urban

Fresh water

<1%

Figure 4—Land cover was mapped for Guam by using high-resolution satellite imagery. Guam is 

currently about 48 percent forested and 18 percent urban.

Table 2—Estimated land area by land status

Land status Area

Acres

Accessible forest land:

 Unreserved forest land— 

 Limestone forest 44,704

 Volcanic forest 19,129

 Total unreserved 63,833

 Reserved forest land 0

 All accessible forest land 63,833

Nonforest and other area:

 Nonforest urban 23,956

 Nonforest savanna or fernland 44,455

 Barren lands 1,539

 Area not classified 1,622

 Water 255

 All nonforest and other 71,827

Total forest and nonforest area 135,660

Area not included in field sample:

 Access denied 2,096

 Hazardous conditions 1,716
12

RESOURCE BULLETIN PNW-RB-243

Table 3—Estimated area of accessible forest land by forest-type group and productivity class

Site productivity classa

Forest-type group 0–4.9 5–9.9 10–14.9 15–19.9 20–24.9 25–29.9 30–40 All classes

Acres

Limestone forest — — 6,020 3,010 35,675 — — 44,704

Volcanic/ravine forest — 2,455 4,911 254 6,953 3,328 1,228 19,129

 Total — 2,455 10,930 3,264 42,628 3,328 1,228 63,833

a

Classes are based on topographic relative moisture index. Higher numbers correspond to greater potential moisture retention on a site.

Limestone Forest Potential Productivity

(Percentage of forest-type acres)

80%

7%

13%

Medium low

Medium

Medium high

Figure 5—The majority of forest on limestone soils was estimated to be moderately high 

productivity. However, owing to the rapid runoff expected in karstic topography, productivity will depend greatly on the depth of soil present locally.

as other individuals outcompete them for space, or are killed by typhoon or other 

damaging agents. Likewise, cubic-foot volume (fig. 9, tables 6, 7, and 8), biomass 

(fig. 10, table 9), and carbon mass (table 10) follow similar trends, with the majority of each distributed among the smaller tree size classes. For trees greater than or 

equal to 5 inches in diameter, the mean tree height is approximately 25 feet with a 

standard deviation of 11.3 feet. Approximately 63 percent of the trees fall within the 

height range of 15 to 30 feet, and about 30 percent of the trees are greater than 30 

feet tall.
13

Guam’s Forest Resources, 2002

Figure 6—Much of the forest on volcanic soil occurs on steeper slopes. Productivity on 

these steeper slopes is expected to be lower than in areas where the moisture collects.

Figure 7—The majority of Guam’s forests are characterized by small-diameter stands of trees.

26%

1% 37%

17%

6% 13%

Volcanic Forest Potential Productivity

(Percentage of forest-type acres)

Low

Medium low

Medium

Medium high

High

Highest

Stand size class

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

Less than

5 inches

5–10.9

inches

11–19.9

inches

20+ inches All sizes

Area (acres)

Limestone forest

Volcanic/ravine forest

Forest total
14

RESOURCE BULLETIN PNW-RB-243

Figure 8—The distribution of number of trees by size class indicates Guam’s forests tend to be dominated by smaller individuals.

Figure 9—The woody stem volume for trees in Guam is concentrated in the smaller size classes.

Number of Trees by Size Class

Total trees in all classes: 76,771,219

0

5

10

15

20

25

30

35

40

45

1.0–2.9

3.0–4.9

5.0–6.9

7.0–8.9

9.0–10.9

11.0–12.9

13.0–14.9

15.0–16.9

17.0–18.9

19.0–20.9

21.0–22.9

23.0–24.9

25.0–26.9

27.0–28.9

29.0+

Diameter (inches)

Estimated number of trees (millions)

50

0

2

4

6

8

10

12

14

16

5.0–6.9

7.0–8.9

9.0–10.9

11.0–12.9

13.0–14.9

15.0–16.9

17.0–18.9

19.0–20.9

21.0–22.9

23.0–24.9

25.0–26.9

27.0–28.9

29.0+

Diameter (inches)

Gross volume (million cubic feet)

18

Volume by Size Class

Total volume, all classes: 59,937,557
15

Table 4—Estimated number of live trees on forest land by species group and diameter class

Diameter class (inches)

Species group 1–2.9 3–4.9 5–6.9 7–8.9 9–10.9 11–12.9 13–14.9 15–16.9 17–18.9 19.0+ 

Number of trees

Limestone 

 forest type 42,779,542 14,079,343 3,622,623 1,275,163 652,072 231,848 130,414 57,962 28,981 72,45Volcanic/ravine 

 forest type 5,338,023 4,601,744 2,039,186 620,622 502,408 413,748 192,097 73,884 44,330 14,77 Total 48,117,565 18,681,087 5,661,809 1,895,785 1,154,480 645,596 322,512 131,846 73,311 87,22
16Table 5—Estimated number of live trees on forest land by species and diameter class

Diameter class (inches)

Species 5.0–6.9 7.0–8.9 9.0–10.9 11.0–12.9 13.0–14.9 15.0–16.9 17.0–18.9 19.0–20.9 21.0–22.9 23.0–24.9 25.0–26

Number of trees

Adenanthera pavonina — 14,490 14,490 14,490 14,490 — — — — — — Aglaia mariannensis 260,829 28,981 — — — — — — — — — 

Annona reticulata 86,943 — — — — — — — — — — 

Areca catechu 29,553 — 14,777 — — — — — — — — Artocarpus altilis 58,534 14,777 14,777 — — — — — — — — Artocarpus mariannensis 14,490 — — — — — — — — — — 

Averrhoa bilimbi 57,962 — — — — — — — — — — 

Barringtonia asiatica — 14,490 — — — — — — — 14,490 — Barringtonia racemosa 14,777 — — — 14,777 — — — — — — Calophyllum inophyllum 44,330 — — — — — — — — — — Cananga odorata — — — — — — 14,490 — — — — 

Carica papaya 43,471 43,471 14,490 — — — — — — — — Casuarina equisetifolia 43,471 43,471 14,490 14,490 — — — — — — — 

Ceiba pentandra — 14,490 — — — — — — — — — 

Cerbera dilatata 101,433 28,981 28,981 — — — — — — — — Cocos nucifera 43,758 117,927 498,974 339,292 132,990 14,777 14,777 — — — — 

Cyathea lunulata 14,777 14,777 — — — — — — — — — Cycas micronesica 900,700 510,316 117,069 28,981 14,490 — — — — — — 

Eugenia thompsonii 29,553 — — — — — — — — — — 

Ficus prolixa 43,471 — 14,490 — — 44,044 14,777 — — — — 

Ficus tinctoria 14,490 14,490 — — — — — — — — — 

Guamia mariannae 86,943 43,471 — — — — — — — — — Hernandia ovigera 72,452 — 43,471 14,490 — — — — — — — Hernandia sonora 14,490 — 14,490 — — — — — — — — Heterospathe elata 1,075,552 — — — — — — — — — — Hibiscus tiliaceus 437,291 115,924 14,490 — — — — — — — — 

Intsia bijuga 14,490 14,490 — — 43,471 — — — — — — Kleinhovia hospita 43,471 14,490 — — — — — — — — — 

Leucaena leucocephala 729,390 146,050 14,777 — — — — — — — — Macaranga thompsonii 43,471 57,962 28,981 43,471 — 14,490 — — — — — 

Mangifera indica — 29,553 14,777 88,660 44,330 29,553 14,777 14,777 — — — 

Morinda citrifolia 58,821 — — — — — — — — — — Neisosperma oppositifolia — 14,490 — — — — — — — — — 

Pandanus tectorius 527,668 249,487 14,490 — — — — — — — — 

Premna obtusifolia 405,734 159,395 43,471 — — — — — 14,490 14,490 — 

Spathodea campanulata 28,981 — 14,490 — — 14,490 — — — — — 

Vitex parviflora 306,018 189,808 218,502 101,720 57,962 14,490 14,490 — — — — Xylosma nelsonii 14,490 — — — — — — — — — — 

 Total 5,661,809 1,895,785 1,154,480 645,596 322,512 131,846 73,311 29,267 14,490 28,981 — 
17

Guam’s Forest Resources, 2002

Table 6—Estimated gross volume of all live trees on forest land by forest-type 

group and diameter class

Diameter class (inches)

Forest-type group Less than 5 5–10.9 11–19.9 20+ All sizes

Cubic feet

Limestone forest type 25,000,018 22,194,860 8,278,410 3,937,467 59,410,755

Volcanic/ravine forest type 6,437,631 14,069,581 11,457,238 31,964,450

 Total 31,437,648 36,264,441 19,735,649 3,937,467 91,375,205

Table 7—Estimated net volume of all live trees >5 inches in diameter on forest 

land by forest-type group and diameter class

Diameter class (inches)

Forest-type group 5–10.9 11–19.9 20+ All sizes

Cubic feet

Limestone forest type 21,467,757 8,248,484 3,756,956 33,473,197

Volcanic/ravine forest type 14,048,885 11,318,163 25,367,048

 Total 35,516,642 19,566,647 3,756,956 58,840,245

300

250

200

150

100

50

0

5.0-6.9

7.0-8.9

9.0-10.9

11.0-12.9

13.0-14.9

15.0-16.9

17.0-18.9

19.0-20.9

21.0-22.9

23.0-24.9

25.0-26.9

27.0-28.9

29.0+

Diameter (inches)

Stem biomass (thousand tons)

Biomass by Size Class

Biomass total, all classes: 1,000,200 tons

Figure 10—Small-diameter trees account for the greatest relative share of biomass in Guam’s forests.
18Table 8—Estimated gross volume of all live trees on forest land by species and diameter class

Diameter class (inches)

Species 5.0–6.9 7.0–8.9 9.0–10.9 11.0–12.9 13.0–14.9 15.0–16.9 17.0–18.9 19.0–20.9 21.0–22.9 23.0–24.9 25.0–26

Cubic feet

Adenanthera pavonina — 101,166 215,151 237,852 295,831 — — — — — — 

Aglaia mariannensis 745,310 147,832 — — — — — — — — — 

Annona reticulata 243,248 — — — — — — — — — — Areca catechu 66,619 — 62,911 — — — — — — — — 

Artocarpus altilis 166,087 62,781 87,091 — — — — — — — — Artocarpus mariannensis 69,114 — — — — — — — — — — 

Averrhoa bilimbi 170,238 — — — — — — — — — — 

Barringtonia asiatica — 94,617 — — — — — — — 678,079 — 

Barringtonia racemosa 36,026 — — — 185,433 — — — — — — Calophyllum inophyllum 162,457 — — — — — — — — — — 

Cananga odorata — — — — — — 481,325 — — — — 

Carica papaya 97,698 153,709 147,836 — — — — — — — — 

Casuarina equisetifolia 158,514 212,708 156,081 171,682 — — — — — — — 

Ceiba pentandra — 68,581 — — — — — — — — — 

Cerbera dilatata 324,750 168,655 212,986 — — — — — — — — 

Cocos nucifera 157,742 598,026 5,659,657 4,275,006 2,066,260 284,146 578,822 — — — — 

Cyathea lunulata 27,992 56,791 — — — — — — — — — 

Cycas micronesica 1,735,701 1,810,718 724,419 265,754 165,066 — — — — — — 

Eugenia thompsonii 78,386 — — — — — — — — — — Ficus prolixa 258,381 — 251,358 — — 685,362 210,436 936,398 — — — Ficus tinctoria 42,561 71,029 — — — — — — — — — 

Guamia mariannae 197,414 259,918 — — — — — — — — — 

Hernandia ovigera 165,222 — 361,133 178,057 — — — — — — — 

Hernandia sonora 65,398 — 115,645 — — — — — — — — 

Heterospathe elata 3,562,810 — — — — — — — — — — 

Hibiscus tiliaceus 1,233,493 610,807 81,339 — — — — — — — — 

Intsia bijuga 63,488 59,862 — — 916,945 — — — — — — 

Kleinhovia hospita 126,462 83,586 — — — — — — — — — 

Leucaena leucocephala 2,071,080 654,665 84,420 — — — — — — — — Macaranga thompsonii 128,274 343,322 274,918 567,175 — 256,535 — — — — — Mangifera indica — 109,580 192,508 1,284,968 945,215 746,063 473,038 520,927 — — — 

Morinda citrifolia 167,806 — — — — — — — — — — 

Neisosperma oppositifolia — 53,968 — — — — — — — — — 

Pandanus tectorius 1,531,375 1,140,927 140,841 — — — — — — — — Premna obtusifolia 1,185,221 797,452 393,112 — — — — — 483,181 601,705 — Spathodea campanulata 98,688 — 114,276 — — 334,289 — — — — — 

Vitex parviflora 1,040,617 938,346 2,161,915 1,444,560 1,181,825 457,650 525,425 — — — — Xylosma nelsonii 49,624 — — — — — — — — — —  Total 16,227,799 8,599,046 11,437,596 8,425,054 5,756,576 2,764,045 2,269,047 1,457,325 483,181 1,279,783 — 
19

Table 9—Estimated aboveground dry stem weight of all live trees on forest land by species and diameter class

Diameter class (inches)

Species 5.0–6.9 7.0–8.9 9.0–10.9 11.0–12.9 13.0–14.9 15.0–16.9 17.0–18.9 19.0–20.9 21.0–22.9 23.0–24.9 25.0–26

Tons

Adenanthera pavonina — 1,578 3,356 3,710 4,615 — — — — — — 

Aglaia mariannensis 11,627 2,306 — — — — — — — — — 

Annona reticulata 3,795 — — — — — — — — — — Areca catechu 1,039 — 981 — — — — — — — — Artocarpus altilis 2,591 979 1,359 — — — — — — — — 

Artocarpus mariannensis 1,078 — — — — — — — — — — 

Averrhoa bilimbi 2,656 — — — — — — — — — — 

Barringtonia asiatica — 1,476 — — — — — — — 10,578 — 

Barringtonia racemosa 562 — — — 2,893 — — — — — — Calophyllum inophyllum 2,889 — — — — — — — — — — 

Cananga odorata — — — — — — 4,355 — — — — 

Carica papaya 1,524 2,398 2,306 — — — — — — — — 

Casuarina equisetifolia 4,154 5,575 4,091 4,499 — — — — — — — Ceiba pentandra — 492 — — — — — — — — — 

Cerbera dilatata 5,066 2,631 3,323 — — — — — — — — 

Cocos nucifera 2,461 9,329 88,291 66,690 32,234 4,433 9,030 — — — — 

Cyathea lunulata 437 886 — — — — — — — — — 

Cycas micronesica 27,077 28,247 11,301 4,146 2,575 — — — — — — Eugenia thompsonii 1,223 — — — — — — — — — — Ficus prolixa 4,031 — 3,921 — — 10,692 3,283 14,608 — — — 

Ficus tinctoria 664 1,108 — — — — — — — — — 

Guamia mariannae 3,080 4,055 — — — — — — — — — 

Hernandia ovigera 2,577 — 5,634 2,778 — — — — — — — 

Hernandia sonora 592 — 1,046 — — — — — — — — 

Heterospathe elata 55,580 — — — — — — — — — — 

Hibiscus tiliaceus 21,936 10,863 1,447 — — — — — — — — Intsia bijuga 990 934 — — 14,304 — — — — — — Kleinhovia hospita 1,420 939 — — — — — — — — — Leucaena leucocephala 41,355 13,072 1,686 — — — — — — — — 

Macaranga thompsonii 2,001 5,356 4,289 8,848 — 4,002 — — — — — 

Mangifera indica — 1,778 3,123 20,847 15,335 12,104 7,675 8,452 — — — 

Morinda citrifolia 2,618 — — — — — — — — — — 

Neisosperma oppositifolia — 842 — — — — — — — — — 

Pandanus tectorius 23,889 17,798 2,197 — — — — — — — — 

Premna obtusifolia 18,489 12,440 6,133 — — — — — 7,538 9,387 — 

Spathodea campanulata 770 — 891 — — 2,607 — — — — — 

Vitex parviflora 22,727 20,493 47,216 31,549 25,811 9,995 11,475 — — — — 

Xylosma nelsonii 774 — — — — — — — — — — 

 Total 271,673 145,576 192,590 143,068 97,767 43,833 35,817 23,059 7,538 19,965 — 
20Table 10—Estimated aboveground weight of carbon for stems of all live trees on forest land by species and diameter class

Diameter class (inches)

Species 5.0–6.9 7.0–8.9 9.0–10.9 11.0–12.9 13.0–14.9 15.0–16.9 17.0–18.9 19.0–20.9 21.0–22.9 23.0–24.9 25.0–26

Tons

Adenanthera pavonina — 773 1,645 1,818 2,261 — — — — — — 

Aglaia mariannensis 5,697 1,130 — — — — — — — — — 

Annona reticulata 1,859 — — — — — — — — — — Areca catechu 509 — 481 — — — — — — — — 

Artocarpus altilis 1,270 — 666 — — — — — — — — 

Artocarpus mariannensis 528 — — — — — — — — — — 

Averrhoa bilimbi 1,301 — — — — — — — — — — Barringtonia asiatica — 723 — — — — — — — 5,183 — Barringtonia racemosa 275 — — — 1,417 — — — — — — Calophyllum inophyllum 1,416 — — — — — — — — — — 

Cananga odorata — — — — — — 2,134 — — — — 

Carica papaya 747 1,175 1,130 — — — — — — — — 

Casuarina equisetifolia 2,036 2,732 2,004 2,205 — — — — — — — 

Ceiba pentandra — 241 — — — — — — — — — Cerbera dilatata 2,482 1,289 1,628 — — — — — — — — Cocos nucifera 1,206 4,571 43,262 32,678 15,794 2,172 4,425 — — — — Cyathea lunulata 214 434 — — — — — — — — — 

Cycas micronesica 13,268 13,841 5,537 2,031 1,262 — — — — — — Eugenia thompsonii 599 — — — — — — — — — — 

Ficus prolixa 1,975 — 1,921 — — 5,239 1,609 7,158 — — — Ficus tinctoria 325 543 — — — — — — — — — 

Guamia mariannae 1,509 1,987 — — — — — — — — — Hernandia ovigera 1,263 — 2,761 1,361 — — — — — — — Hernandia sonora 290 — 513 — — — — — — — — Heterospathe elata 27,234 — — — — — — — — — — 

Hibiscus tiliaceus 10,749 5,323 709 — — — — — — — — 

Intsia bijuga 485 458 — — 7,009 — — — — — — 

Kleinhovia hospita 696 460 — — — — — — — — — Leucaena leucocephala 20,264 6,405 826 — — — — — — — — 

Macaranga thompsonii 981 2,624 2,101 4,335 — 1,961 — — — — — 

Mangifera indica — 871 1,530 10,215 7,514 5,931 — 4,141 — — — 

Morinda citrifolia 1,283 — — — — — — — — — — Neisosperma oppositifolia — 413 — — — — — — — — — Pandanus tectorius 11,706 8,721 1,077 — — — — — — — — 

Premna obtusifolia 9,060 6,096 3,005 — — — — — 3,693 4,599 — Spathodea campanulata 377 — 437 — — 1,278 — — — — — 

Vitex parviflora 11,136 10,042 23,136 15,459 12,647 4,898 5,623 — — — — Xylosma nelsonii 379 — — — — — — — — — — 

 Total 133,120 71,332 94,369 70,103 47,906 21,478 17,550 11,299 3,693 9,783 — 
21

Guam’s Forest Resources, 2002

Number of Canopy and Understory Species

The field sample of this inventory considers both overstory and understory vegetation. Understory vegetation cover was surveyed on field plots for individual species 

that occupy at least 3 percent cover on an entire subplot (tables 11 and 12). Relatively rare species are not documented on plots unless they have been identified 

as a special-interest species by island foresters. More species were found in the 

limestone forest sample than in the volcanic forest sample (fig. 11). However, the 

limestone sample area was over twice the area of the volcanic forest sample, and 

the number of species found tends to increase with the area sampled. These species 

figures are only a rough estimate of cover and species counts for the most common 

plants. Stone (1971) details 931 species in the flora of Guam.

On a per-plot basis, using the sum of FIA subplot area of approximately onesixth of an acre (summing four subplots at 24-foot radius = 7,238 square feet), the 

distribution of tree species numbers reveals a pattern of a relatively high number 

of species per plot (fig. 12). Plots where only one or two species are found are often 

plots dominated by the widespread Hibiscus tiliaceus, Casuarina equisetifolia, or 

the recently broadcast (aerial seeded after World War II) Leucaena leucocephala.

Tree Damage and Mortality

We estimate that about 20 percent of the individual trees on Guam have been 

damaged (table 13). These damages include mechanical damage from storms and 

people, diseases, insects, decay, and damage by other plant species. Approximately 

one-third of the damaged trees show signs of decay with visible growth of woodrotting fungus (conks) (fig. 13).

Tree damages are categorized according to causal agents. Because a tree can 

exhibit secondary damages caused by another agent, we have classified damages 

according to frequency for the primary and secondary damage agents. Thirty-six 

percent of primary tree damages in Guam forests are attributed to weather (fig. 

14). Damage caused by other vegetation, insects, and diseases follow in declining 

frequency for primary damage types. Insects frequently play a secondary role following damage or weakening by another damaging agent (fig. 15).

Tree damages can contribute to mortality at different rates for different species. 

Some species are more sensitive than others to insects, some to fungal, viral, or 

bacterial pathogens that gain entry into plant tissues through mechanical wounding. 

Overall, about 2 percent of the trees in our sample were dead. However, by species, 

the percentage dead ranges as high as 33 percent for breadfruit trees (Artocarpus 

altilis) in the inventory (fig. 16).

About 20 percent of 

the trees on Guam 

have been damaged—

most from weather, 

other vegetation, and 

insects.
22

RESOURCE BULLETIN PNW-RB-243

Table 11—Average nontree understory covera on FIA field subplots by species 

and forest type

Forest type

Species Limestone Volcanic

Percent

Unknown 12 —

Abutilon indicum (L.) Sweet — 3

Aeschynomene indica L. — 10

Allophylus timoriensis (DC) Bl. 3 —

Alocasia macrorrhiza (L.) Schott — 5

Antrophyum plantagineum (Cav.) Kaulf. — 5

Antigonon leptopus Hook. & Arn. 20 17

Arundina graminifolia (D. Don) Hochr. — 3

Asplenium laserpitifolium Lam. — 7

Axonopus compressus (Sw.) Beauv. 9 15

Bambusa vulgaris Schrad. ex J.C. Wendl. 10 —

Belvisia spicata Safford 3 4

Bidens alba (L.) DC. 27 8

Blechum pyramidatum (Lam.) Urban 33 —

Bothriochloa pertusa (L.) A. Camus — 32

Buddleja asiatica Lour. 5 —

Calopogonium mucunoides Desv. 11 5

Calyptocarpus vialis Less. 10 —

Cassytha filiformis L. 3 —

Centotheca lappacea (L.) Desv. 5 11

Centrosema pubescens Benth. 9 —

Chamaecrista nictitans (L.) Moench — 3

Chamaesyce sp. S.F. Gray 3 —

Chamaesyce hirta (L.) Millsp. — 3

Chamaesyce hypericifolia (L.) Millsp. 3 —

Chloris barbata Sw. — 8

Chloris radiata (L.) Sw. 30 —

Chromolaena odorata (L.) King & H.E. Robins. 13 12

Clerodendrum inerme (L.) Gaertn. 5 —

Colubrina asiatica (L.) Brongn. 28 —

Conyza canadensis (L.) Cronq. 4 —

Cynodon dactylon (L.) Pers. 10 3

Cyperus ligularis L. 3 —

Davallia solida (Forst.) Sw. 3 —

Decaspermum fruticosum Forst. — 3

Dendrobium guamense Ames — 9

Desmanthus virgatus (L.) Willd. 8 —

Desmodium umbellatum (L.) DC. 10 —

Dichanthium bladhii (Retz.) Clayton — 75

Dicranopteris linearis (Burm.) Underwood — 19

Digitaria bicornis (Lam.) Roemer & J.A. Schultes ex Loud. 10 28

Digitaria ciliaris (Retz.) Koel. 30 —

Dimeria chloridiformis (Gaud.) K. Schum. & Lauterb. — 36

Discocalyx megacarpa Merrill — 5

Elephantopus mollis Kunth — 6

Eleusine indica (L.) Gaertn. 36 5

Entada phaseoloides (L.) Merr. 5 22
23

Guam’s Forest Resources, 2002

Table 11—Average nontree understory covera on FIA field subplots by species 

and forest type (continued)

Forest type

Species Limestone Volcanic

Percent

Entada pursaetha DC. 12 —

Epipremnum pinnatum (L.) Engl. 27 —

Eragrostis tenella (L.) Beauv. ex Roemer & J.A. Schultes 60 —

Euphorbia chamaesyce L. 10 —

Eustachys petraea (Sw.) Desv. — 8

Fimbristylis dichotoma (L.) Vahl 8 —

Fimbristylis sp. Vahl — 10

Flagellaria indica L. 28 14

Flemingia strobilifera (L.) Ait. & Ait. f. 36 —

Freycinetia reineckei Warb. — 15

Gleichenia linearis (Burm.) Clarke — 16

Glochidion mariana Mueller-Arg., L. — 3

Gossypium hirsutum L. 5 —

Grasses — 10

Heterogonium pinnatum (Copel.) Holtt. — 3

Humata heterophylla (Sm.) Desv. — 5

Hyptis capitata Jacq. — 9

Hyptis pectinata (L.) Poit. — 15

Hyptis rhomboidea Mart. & Galeotti — 5

Ipomoea indica (Burm. f.) Merr. 13 5

Ipomoea littoralis Blume 5 3

Ipomoea obscura (L.) Ker-Gawl. 3 —

Ipomoea triloba L. 7 3

Jasminum marianum DC. 10 25

Lepturus repens (G. Forst.) R. Br. 75 —

Lycopodium cernuum L. — 7

Lygodium microphyllum (Cav.) R. Br. 10 11

Macroptilium lathyroides (L.) Urban 3 —

Mariscus javanicus (Houtt.) Merr. & Metcalfe — 70

Maytenus thompsonii Fosb. 5 —

Medinilla sp. Gaud — 8

Melastoma malabathricum L. — 4

Merremia peltata (L.) Merr. 7 —

Microlepia speluncae (L.) T. Moore — 8

Mikania micrantha Kunth 7 18

Mikania scandens (L.) Willd. 9 —

Mimosa pudica L. 9 3

Miscanthus floridulus (Labill.) Warb. ex K. Schum. & Laut. 22 47

Momordica charantia L. 11 70

Myrtella bennigseniana (Volk.) Diels — 3

Nephrolepis biserrata (Sw.) Schott 10 12

Nephrolepis hirsutula (J.R. Forst.) K. Presl 7 8

Ochrosia mariannensis A. DC. 7 —

Oplismenus compositus (L.) Beauv. 14 20

Oplismenus sp. Beauv. — 23

Panicum maximum Jacq. 34 —

Paspalum conjugatum Berg. 10 —
24

RESOURCE BULLETIN PNW-RB-243

Table 11—Average nontree understory covera on FIA field subplots by species 

and forest type (continued)

Forest type

Species Limestone Volcanic

Percent

Paspalum longifolium Roxb. 3 7

Passiflora foetida L. 11 6

Passiflora suberosa L. 8 —

Pennisetum polystachyon (L.) J.A. Schultes 17 27

Pennisetum purpureum Schumacher 39 10

Phragmites karka (Retz.) Trin ex Steud. — 44

Phyllanthus saffordii Merrill — 3

Phymatosorus scolopendria (Burm. f.) Pic. Serm. 7 —

Pilea microphylla (L.) Liebm. 19 —

Piper guahamense DC. 10 3

Pluchea carolinensis (Jacq.) G. Don — 3

Polygonum minus Huds. — 25

Polypodium punctatum (L.) Sw. 5 5

Polypodium scolopendria Burm. f. 19 13

Premna serratifolia L. — 3

Pteris ensiformis Burm. f. — 50

Pteris quadriaurita Retz. — 8

Pteris tripartita Sw. 3 —

Pycreus polystachyos (Rottb.) Beauv. — 15

Pyrrosia lanceolata (L.) Farw. 9 6

Pyrrosia sp. Mirbel — 7

Rhynchospora rubra Domin — 13

Saccharum spontaneum L. — 28

Scaevola frutescens (Miller) Krause — 3

Scleria polycarpa Boeck. 15 11

Senna alat (L.) Roxb. 6 —

Unknown shrub 10 —

Solanum americanum P. Mill. — 3

Spathoglottis plicata Blume — 3

Stachytarpheta jamaicensis (L.) Vahl 16 4

Stachytarpheta urticifolia Sims 19 6

Stacytharpheta jamaicensis (L.) Vahl 3 4

Stictocardia tiliaefolia (Desr.) H. Hallier — 8

Stylosanthes guianensis (Aubl.) Sw. — 7

Syngonium augustatum Schott 8 —

Thelypteris gretheri (Wagner) Stone 13 —

Thelypteris maemonensis (Wagner & Grether) Stone 3 —

Thelypteris opulenta (Kalfuss) Fosberg 5 10

Thelypteris parasitica (L.) Fosberg — 9

Thelypteris subpubescens Blume — 13

Thelypteris unita (L.) C.V. Morton — 6

Trichomanes brevipes (C. Presl.) Baker — 5

Waltheria indica L. 17 8

Wikstroemia elliptica Merr. 6 3

Zoysia matrella (L.) Merr. 7 —

a

 The cover estimates are averaged among subplots where each species was found.
25

Guam’s Forest Resources, 2002

Table 12—Average understory tree cover on FIA 

field subplots by species and forest type

Forest type

Species Limestone Volcanic

Percent

Adenanthera pavonina 3 —

Aglaia mariannensis 4 —

Annona muricata L. 7 3

Annona reticulata 8 —

Areca catechu — 8

Artocarpus altilis 3 —

Averrhoa bilimbi 5 —

Barringtonia racemosa — 3

Bauhinia monandra — 10

Calophyllum inophyllum — 4

Cananga odorata 3 —

Carica papaya 3 —

Casuarina equisetifolia — 3

Cerbera dilatata 4 —

Cocos nucifera 7 14

Cycas micronesica 8 6

Diospyros kaki L. f. 3 —

Eugenia reinwardtiana 3 —

Ficus prolixa 7 —

Guamia mariannae 11 3

Heterospathe elata 20 10

Hibiscus tiliaceus 7 5

Kleinhovia hospita 8 —

Leucaena leucocephala 11 4

Mangifera indica — 3

Morinda citrifolia 7 6

Neisosperma oppositifolia 3 —

Pandanus tectorius 5 8

Pipturus argenteus 3 —

Polyscias grandifolia — 7

Premna obtusifolia 6 —

Psidium guajava 3 4

Triphasia trifolia 11 13

Vitex parviflora 5 10
26

RESOURCE BULLETIN PNW-RB-243

90

80

70

60

50

40

30

20

10

0

Tree species Understory

species

Number of plots

Number of species

Limestone

Volcanic

Figure 11—Forty-eight tree species and about 140 understory species were measured on 46 plots in 

Guam.

1–2 3–4 5–6 7–8 9–10 11–12

12

10

8

6

4

2

0

Number of plots

Number of tree species on FIA plots

Limestone

Volcanic

Figure 12—The average number of tree species per sixth-acre plot on both limestone and volcanic 

soils was four.
27

Guam’s Forest Resources, 2002

Table 13—Number of trees by primary damage type and species for all trees (>5 inches diameter at breast 

height; includes dead trees)

Species

No 

damage

Broken 

bole

Broken 

branches

Broken 

roots Conks

Damaged 

shoots

Lost apical 

dominance

Open 

wound

Vines in 

crown

All 

damages

Number of trees

Adenanthera pavonina 14,490 — 28,981 — 14,490 — — — — 43,471

Aglaia mariannensis 275,319 14,490 — — 14,490 — — 14,490 — 43,471

Annona reticulata 72,452 — — — 14,490 — — — — 14,490

Areca catechu 44,330 — — — — — — — — —

Artocarpus altilis 73,311 — — — — — 43,758 — — 43,758

Artocarpus mariannensis 14,490 — — — — — — — — —

Averrhoa bilimbi 57,962 — — — — — — — — —

Barringtonia asiatica 14,490 — — — — — — 14,490 — 14,490

Barringtonia racemosa — — — — 29,553 — — — — 29,553

Calophyllum inophyllum 59,107 — — — — — — — — —

Cananga odorata 14,490 — — — — — — — — —

Carica papaya 57,962 — 28,981 — — — 14,490 — — 43,471

Casuarina equisetifolia 115,924 — — — — — — — — —

Ceiba pentandra 14,490 — — — — — — — — —

Cerbera dilatata 144,905 — — — 14,490 — 28,981 — — 43,471

Cocos nucifera 1,074,979 — — — 29,553 14,490 — 58,248 — 102,292

Cyathea lunulata 29,553 — — — — — — — — —

Cycas micronesica 1,280,888 14,490 43,758 — 115,924 — — 72,452 87,515 334,140

Eugenia thompsonii 29,553 — — — — — — — — —

Ficus prolixa 131,273 — — — — — 14,490 — — 14,490

Ficus tinctoria 14,490 — — — — — 14,490 — — 14,490

Guamia mariannae 86,943 — — — — 14,490 14,490 14,490 — 43,471

Hernandia ovigera 115,924 — 14,490 — — — — — — 14,490

Hernandia sonora — — — — 28,981 — — — — 28,981

Heterospathe elata 1,045,998 — — — — — — 29,553 — 29,553

Hibiscus tiliaceus 219,933 — 101,433 — 159,682 101,433 14,777 14,490 — 391,816

Intsia bijuga 57,962 — — — 14,490 — — — — 14,490

Kleinhovia hospita 57,962 — — — — — — — — —

Leucaena leucocephala 772,576 — — — 58,534 — 14,777 44,330 — 117,641

Macaranga thompsonii 144,905 — — — 14,490 — 14,490 14,490 — 43,471

Mangifera indica 177,321 14,777 — — 14,777 — 29,553 29,553 — 88,660

Morinda citrifolia 29,267 — 29,553 — — — — — — 29,553

Neisosperma oppositifolia 14,490 — — — — — — — — —

Pandanus tectorius 688,781 58,248 29,267 — — — — — 44,330 131,846

Premna obtusifolia 376,753 14,490 — 43,471 130,414 — 14,490 14,490 57,962 275,319

Spathodea campanulata 57,962 — — — — — — — — —

Vitex parviflora 743,022 29,267 58,248 — 28,981 — 57,962 14,490 — 188,949

Xylosma nelsonii 14,490 — — — — — — — — —

 Total 8,138,752 145,764 334,712 43,471 683,343 130,414 276,750 335,571 189,808 2,139,834
28

RESOURCE BULLETIN PNW-RB-243

Figure 13—Broken boles, branches, and roots accounted for 25 percent of the damages on Guam. An additional 

31 percent of the damages were due to rot evidenced by shelf-fungus conks.

31%

16%

16%

9%

6%

7% 2%

13%

Tree Damages

8.1 million no damage

2.1 million with damage

Conks

Open wound

Lost apical dominance

Broken branches

Vines in crown

Damaged shoots

Broken bole

Broken roots

Figure 14—Weather and mechanical impacts by other vegetation accounted for 64 percent of the primary 

damages.

36%

16%

10%

8%

1%

28%

1% <1%

Primary Damage Agent

Weather

Vegetation

Insect

Disease

Physical

Unknown

Human

Animal
29

Guam’s Forest Resources, 2002

Figure 15—Secondary damage was generally attributed to weather.

Figure 16—Soft-wooded species, such as Artocarpus altilis, tended to suffer high mortality.

82%

8%

6%

2%

2%

Secondary Damage Agent

Weather

Insect

Vegetation

Physical

Disease

Artocarpus altilis

Calophyllum inophyllum

Cerbera dilatata

Mangifera indica

Aglaia mariannensis

Hibiscus tiliaceus

Pandanus tectorius

Vitex parviflora

Cycas circinalis

Premna obtusifolia

Cocos nucifera

35

30

25

20

15

10

5

0

Dead trees (percent)

Percentage Dead by Species
30

RESOURCE BULLETIN PNW-RB-243

Epiphytes

Epiphytic plants growing on trees use space and moisture that might otherwise be 

available for the growth of the host tree. Physical breakage of tree limbs can occur 

depending on the amount and species of plants growing in the branching network 

of the host tree. In extremely wet areas, the weight loading of epiphytes can be 

tremendous, stunting the growth of individual trees. We rated each tree sampled in 

the inventory according to epiphyte loading. The majority of trees on Guam show 

little evidence of epiphyte loading (table 14). Frequent storm damage to trees may 

prevent the accumulation of epiphytes to the point of tree limb damage on Guam.

Acknowledgments

The FIA Pacific Island inventories are the result of cooperative effort between 

a number of groups and agencies. The USDA Forest Service, State and Private 

Forestry in the Pacific Southwest Region has been a strong supporter of natural 

resource assessments in the Pacific Islands. Many thanks to the Pacific Islands Imagery Consortium partners for sharing costs on imagery and interpretation. Thanks 

to the University of Guam (UOG) for logistical support, to Lynn Raulerson at UOG 

for species identification and training, to Lisa Fischer at the Remote Sensing Lab in 

the Pacific Southwest Region for imagery interpretation and mapping support, and 

to Walter Grabowiecki, Dale Baer, Falaniko Mika, Ritofu Lotovale, Tony Gaison, 

Patrick Quenga, Vince Tenorio, Albert Blas, De’rick Baza, D’wuane Baza, and 

Vince Perez for field assistance and logistical support. We also thank Dale Weyermann, John Chase, Chuck Veneklase, and Ron Wanek with FIA for GIS, programming, and technical support. Tom Brandeis, Dave Azuma, and Chuck Bolsinger 

offered very helpful comments on prior drafts.

Metric Equivalents

When you know: Multiply by: To find: 

Inches 2.54 Centimeters

Feet .3048 Meters

Miles 1.609 Kilometers

Acres .405 Hectares

Cubic feet .0283 Cubic meters

Tons 907 Kilograms

Tons per acre 2.24 Tonnes or megagrams per hectare

Cubic feet per acre .06997 Cubic meters per hectare

Trees per acre 2.471 Trees per hectare 
31

Guam’s Forest Resources, 2002

Table 14—Estimated number of trees on forest land by epiphyte loading 

(amounts of nontree vegetation in the canopy and branches) and species 

(includes dead trees)

Epiphyte loadings

Species None Low Moderate High All loadings

Number of trees

Adenanthera pavonina 28,981 — 28,981 — 57,962

Aglaia mariannensis 217,357 72,452 28,981 — 318,791

Annona reticulata 43,471 28,981 14,490 — 86,943

Areca catechu 44,330 — — — 44,330

Artocarpus altilis 58,248 — 44,044 14,777 117,069

Artocarpus mariannensis — 14,490 — — 14,490

Averrhoa bilimbi 14,490 43,471 — — 57,962

Barringtonia asiatica 14,490 14,490 — — 28,981

Barringtonia racemosa 14,777 14,777 — — 29,553

Calophyllum inophyllum 44,330 — 14,777 — 59,107

Cananga odorata 14,490 — — — 14,490

Carica papaya 101,433 — — — 101,433

Casuarina equisetifolia — 115,924 — — 115,924

Ceiba pentandra 14,490 — — — 14,490

Cerbera dilatata 101,433 72,452 — 14,490 188,376

Cocos nucifera 499,832 515,181 73,597 88,660 1,177,271

Cyathea lunulata — — — 29,553 29,553

Cycas micronesica 565,415 596,686 379,043 73,884 1,615,028

Eugenia thompsonii — 29,553 — — 29,553

Ficus prolixa 73,311 28,981 43,471 — 145,764

Ficus tinctoria 28,981 — — — 28,981

Guamia mariannae 86,943 43,471 — — 130,414

Hernandia ovigera — 86,943 43,471 — 130,414

Hernandia sonora 14,490 14,490 — — 28,981

Heterospathe elata 883,454 177,321 14,777 — 1,075,552

Hibiscus tiliaceus 450,923 160,827 — — 611,749

Intsia bijuga 14,490 57,962 — — 72,452

Kleinhovia hospita 57,962 — — — 57,962

Leucaena leucocephala 247,769 350,920 277,037 14,490 890,217

Macaranga thompsonii 144,905 14,490 28,981 — 188,376

Mangifera indica 73,884 162,544 14,777 14,777 265,981

Morinda citrifolia 58,821 — — — 58,821

Neisosperma oppositifolia — 14,490 — — 14,490

Pandanus tectorius 351,493 248,628 176,176 44,330 820,627

Premna obtusifolia 202,867 202,867 202,867 43,471 652,072

Spathodea campanulata 28,981 28,981 — — 57,962

Vitex parviflora 566,560 306,876 28,981 29,553 931,971

Xylosma nelsonii 14,490 — — — 14,490

 Total 5,077,896 3,418,252 1,414,451 367,987 10,278,586
32

RESOURCE BULLETIN PNW-RB-243

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instructions for the inventory of the Pacific Islands. Portland, OR: Pacific 

Northwest Research Station, Forestry Sciences Laboratory, Forest Inventory and 

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