555

BIOTROPICA 33(4): 555–565 2001

Landscape Patterns of Tropical Forest Recovery in theRepublic of Palau1

Bryan A. Endress2 and J. Danilo Chinea

Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana–Champaign,1201 S. Dorner Drive, Urbana, Illinois 61801, U.S.A.

ABSTRACTA GIS (geographic information systems) database was constructed from aerial photographs, a vegetation map, andtopographic map data of the Ngeremeduu Bay Drainage Area (NBDA), Palau, to examine relationships betweenupland land cover dynamics, environmental variables, and past land use. In 1992, 82.9 percent of the NBDA wasforest, 16.6 percent was grassland, and 0.5 percent consisted of village areas. Between 1947 and 1992, there was a11.2 percent reduction of grassland area primarily due to a 10.9 percent increase in forest cover. These land coverchanges led to larger, more continuous stretches of forest and numerous, highly fragmented grassland patches. Sig-nificant relationships (P , 0.001) were found between the spatial distribution of forest and grassland cover and slope,elevation, soil pH, and percent soil organic matter. These patterns, however, may have resulted from past farm siteselection rather than from ecological relationships. Our results indicate that areas of forest expansion were significantly(P , 0.001) associated with the location of abandoned agricultural communities. In addition, over 92 percent of areasof forest expansion occurred within 100 m of established forest. These results suggest that the proximity of establishedforest facilitate forest recovery following human disturbance.

Key words: land cover change; landscape dynamics; Micronesia; Palau; tropical forest regeneration.

LANDSCAPE PATTERN DEVELOPMENT IN BOTH SPACE

AND TIME results from complex interactions ofphysical, biological, and social forces. These inter-actions result in landscape mosaics of natural andmanaged patches that differ in size, shape, connec-tivity, arrangement, and history. Numerous studieshave shown the importance of landscape pattern tomany ecological processes and phenomena, whichinclude plant and animal dynamics (Chapin 1996,Jules 1998), community structure and composition(Laurance et al. 1998), biodiversity (Didham et al.1998), nutrient and water cycling (Erickson & Kel-ler 1997), and soil erosion (Sarmiento 1997). Be-cause of the importance of landscape pattern onthese processes, it is important to document spatio-temporal changes in plant community distributionsas well as determine the factors that influence thesechanges. Despite the recent focus on land coverpatterns and dynamics, little is known about therelative importance of environmental and historicalfactors in influencing landscape pattern and land-scape-level responses of plant communities to hu-man disturbance in tropical regions.

In temperate regions, numerous studies have

1 Received 6 September 2000; revision accepted 23 Feb-ruary 2001.2 Current address: Department of Botany, Miami Uni-versity, Oxford, Ohio 45056, U.S.A.; E-mail: endresba@muohio.edu

examined landscape-level responses of forest com-munities to land clearing (Debussche & Lepart1992, Foster 1993); however, forest recovery fol-lowing human disturbances (logging, grazing, andagriculture) is not as well understood in tropicalregions because the majority of landscape levelstudies in the tropics have focused on forest loss,fragmentation, or land degradation (Stone et al.1991, Ranta et al 1998).

Studies on tropical forest recovery have gener-ally focused on either (1) the mechanisms and fac-tors that affect forest regeneration (e.g., soil fertility,propagule availability, seed and/or seedling preda-tion, root competition, and fire; Uhl et al. 1988,Nepstad et al. 1990, Aide & Cavelier 1994) or (2)the structure and composition of secondary forests(Brown & Lugo 1990, Aide et al. 1996, Turner etal. 1997). While these studies have provided valu-able information concerning tropical forest regen-eration, landscape-level patterns of tropical forestrecovery are still largely undocumented (Thomlin-son et al. 1996). In addition, many of the studiesconducted on forest recovery have been done atrelatively small spatial scales (one to several ha) andoften over short time periods (,10 y).

Thomlinson et al. (1996) examined landscape-level patterns of forest recovery in Puerto Rico,where forest cover has increased from ca 5 to over30 percent in the past 50 years (Birdsey & Weaver1987). Results showed that the regeneration of sub-

556 Endress and Chinea

tropical forests in Puerto Rico was characterizedprimarily by an expansion of remnant forests, sug-gesting that remnant forests play a critical role inforest reestablishment and regeneration.

Beyond the work of Thomlinson et al. (1996),landscape patterns of tropical forest recovery havenot been described, and no study has examined thelandscape patterns of forest recovery in tropical rainforests. This is not surprising considering that (1)large-scale forest recovery is not a common trendin the tropics and (2) long-term data (e.g., aerialphotos or satellite imagery) do not exist for muchof the tropics; however, for those cases in whichthese conditions are met, there is an opportunityto examine the forest recovery process at spatial andtemporal scales not often studied in tropical re-gions. Additional studies beyond the work inPuerto Rico are necessary because many site-spe-cific factors such as disturbance regimes, land usehistory, and primary methods of seed dispersal mayinfluence land change trajectories, thereby leadingto different patterns of forest recovery. Site-specificfactors have been shown to influence forest regen-eration at small spatial scales (references citedabove), although it is unclear if they influencelarge-scale, landscape-level patterns of forest regen-eration.

Further understanding of the forest recoveryprocess is also valuable because the importance oftropical secondary forests to biodiversity conserva-tion and ecosystem processes (including carbonbudgets) have recently been recognized (Brown etal. 1994, Franklin et al. 1999, Helmer 2000). Theecological importance of secondary forests will like-ly increase in the future as current deforestationtrends continue throughout the tropics. Additionalinformation on how tropical forests respond to pastdisturbances may provide insight on ways to facil-itate forest recovery.

The island of Babeldaob in the Republic of Pa-lau represents one case for which data exists (in theform of aerial photographs) that allows us to ex-amine landscape-level patterns of forest dynamicsover a time period longer than that of many otherstudies (45 yr). In addition, Palau has experiencedperiods of intense human use followed by aban-donment. Prior to European contact (1596), Pa-lauans cleared substantial areas of forest and createdextensive terraces situated on mountain slopes andcoastal hillsides (still evident today), most of whichsupport grassland plant communities. Babeldaob’spopulation in the late 19th century was estimatedat ca 40,000 compared to ca 5,000 inhabitants to-day. Prior to colonization, Palauans engaged in sev-

eral subsistence activities, including fishing and thecultivation of taro (Colocasia esculenta) and numer-ous fruit trees (Parmentier 1987). That changed in1914, when Japan occupied Palau and initiated ex-port-oriented commercial agriculture on the island(Parmentier 1987, Cassell et al. 1992). Areas ofnative forest were cleared, and sugarcane, copra,and pineapple plantations were established (U.S.Department of Defense 1956). Changes in socio-economic conditions following the takeover of Pa-lau by the United States in 1944 led to the aban-donment of these plantations and virtually allforms of commercial export (Otobed & Maiava1994). Agriculture thus returned to subsistence-lev-el production of taro, cassava (Manihot esculenta),and various fruit trees in small (,0.5 ha) plots nearvillages.

Previous research on the terrestrial plant com-munities of Babeldaob has focused either on flo-ristics (Otobed 1971, Fosberg et al. 1980) or onplant community descriptions (Cole et al. 1987,Canfield et al. 1992). Although these studies haveprovided insight into the composition and struc-ture of Palau’s plant communities, no research hasinvestigated the response of forest communities topast human disturbances or examined the role ofenvironmental and historical factors in influencingforest distribution or dynamics.

This study addressed two questions concerningthe relative contribution of past land use and en-vironmental factors in influencing forest distribu-tion and landscape pattern in the Ngeremeduu BayDrainage Area (NBDA), Republic of Palau: (1)what are the spatial and temporal forest patterns/trends within the area, and (2) how are these trendsrelated to environmental variables and past landuse? Results from this study will not only enableecologists to better understand landscape-level re-sponses of tropical rain forests to human distur-bance, they will also provide information on re-gional plant community dynamics in Palau. Thisis particularly important in light of the rapid de-velopment plans for the entire island of Babeldaob,the NBDA in particular. Without a proper under-standing of these issues, unplanned development ofthe region could have deleterious ecological andeconomic consequences.

METHODS

STUDY AREA.—The NBDA is an 84 km2 area lo-cated along the west-central coast of the volcanicisland of Babeldaob, Republic of Palau (ca 78309N,1348309E; Fig. 1). The drainage area and adjacent

Forest Recovery in Palau 557

FIGURE 1. Ngeremeduu Bay Drainage Area, Island ofBabeldaob, Republic of Palau.

marine communities not only have the highest spe-cies richness found in Micronesia but also containthe most diverse set of habitats of all areas studiedin Micronesia (Maragos 1992).

Studies by Canfield et al. (1992) and Cole etal. (1987) have shown that upland areas of theNBDA (excluding the lowland, coastal communi-ties of mangrove forest, swamp forest, and fresh-water marsh) consist of two major land cover cat-egories: forest and grassland. Canfield et al. (1992)identified 56 tree species within the NBDA in arapid ecological assessment of the region. Commoncanopy species include Maranthes corymbosa,Campnosperma brevipetiolata, Pinanga insignis,Gmelina palawense, and Manilkara udoido.

The climate is tropical, with a mean annual

temperature of 278C and mean annual rainfall of3730 mm (van der Brug 1984). The NBDA rangesin elevation from sea level to 250 m. Upland soilsare a complex of Inceptisols and Oxisols, whileHistosols and Entisols dominate soils at lower ele-vations (USDA 1983).

The land use history of the NBDA is similarto that of all of Babeldaob. Between 1914 and1944, several Japanese agricultural communitieswere established within the NBDA and areas ofnative forest were converted to sugarcane and pine-apple plantations (U.S. Department of Defense1956). Agricultural activity was halted when theUnited States took possession of Palau duringWorld War II. Since then, human activity withinthe NBDA has been minimal and restricted to twosmall communities, Nekking and Ibobong. Thepopulation within the NBDA today is estimated at492 people (Otobed & Maiava 1994).

GIS DATABASE CONSTRUCTION AND ANALYSIS.—Landcover maps for three years were produced from twosets of aerial photographs (1947: black and white,1:40,000; 1992: color, 1:20,000), and a vegetationmap from 1976 (based on black-and-white 1:10,000 photographs; Cole et al. 1987). The 1992aerial photographs were scanned at a pixel resolu-tion of 2 m2 and then orthorectified and registeredto the UTM georeferencing system. At least fourground control points, also visible in a 1987, 1:25,000 USGS topography map, were collected fororthorectification and registration (maximum RMSerror for any of the orthophotos was 1.84 pixels;mean RMSE x 5 0.95, y 5 0.94). The orthorec-tified photos were then assembled into a photo mo-saic for on-screen digitizing of land cover categories(see below). The 1947 images were also scannedand registered at a pixel size of 2 m2. Due to thelack of complete camera calibration information, itwas not possible to orthorectify the 1947 aerialphotos. These photos were registered and rectifiedusing a second-order polynomial transformationwith at least seven ground control points per photoand then placed into a mosaic for on-screen digi-tizing (maximum RMSE 5 1.58 pixels; meanRMSE x 5 0.72, y 5 1.1). The 1976 vegetationmaps were also scanned, registered (7 ground con-trol points per map), and assembled into a mosaicfor on-screen digitizing at a resolution of 2 m2

(maximum RMSE 5 0.96; mean RMSE x 5 0.51y 5 0.49).

Four land cover categories were delineatedwithin the NBDA: upland forest, grassland, villag-es, and wetland. Several factors were taken into

558 Endress and Chinea

TABLE 1. Definitions and reclassification scheme of upland land cover categories found within NBDA.

1976 land cover(Cole et al. 1987)

Land coverreclassification Definition of land cover category

Upland forest Upland forest Areas dominated by trees with a canopy cover $ 30%.Secondary vegetation Upland forest Areas consisting of numerous small trees, shrubs and vines.Grassland Grassland Dominated by herbaceous, fern or low shrub cover. Tall trees and

shrubs, if present, are widely scattered (canopy cover , 30%).Agroforest Village Mixture of food producing trees, forest trees, and other culturally

important plants (e.g., Areca catachu).Coconut plantation Village Areas planted exclusively with Cocus nucifera.Cropland Village Cultivated land without tree cover.Urban Village Areas developed for non-forest, non-agricultural use.

consideration when selecting these categories. First,much of the NBDA is inaccessible by road, and itwas therefore important to define land cover cate-gories that were interpreted easily without extensiveground checking. Moreover, the large scale of the1947 photos (1:40,000) made it difficult to accu-rately define more specific land cover categoriesbased on community structure or composition.Cole et al. (1987) identified several subcategoriesof plant communities based on differences in treesize and crown density, while Canfield et al. (1992)classified forest land cover into three plant com-munities based on species composition; however,because of the factors mentioned above, we chosesimple, easily interpretable categories. In addition,the categories used in this study needed to be con-sistent with the categories used in the 1976 vege-tation map (Table 1). Areas identified by Cole etal. (1987) as mangrove forest, swamp forest, orfreshwater marsh were grouped together, classifiedat wetland, and were omitted from the study be-cause the focus of the research was on upland landcover dynamics. Thus, land cover analyses were re-stricted to upland forest, grassland, and village landcover categories.

Photos were viewed stereoscopically and landcover types were differentiated based on differencesin tone, texture, pattern, and in the case of the1992 photos, color. Land cover was then delineateddirectly onto the digital photo mosaics. Because ofthe general land cover categories used in this pro-ject, these categories were readily identified on thephotos, and a subsequent visit to the NBDA (con-ducted in 1995) showed that the land cover delin-eations based on the aerial photographs were ap-propriate.

Soil survey maps of the NBDA (1:10,000;USDA 1983) were also scanned and registered asa mosaic, and maps of soil pH, organic matter, andslope were generated (pixel resolution 2 m2; max-

imum RMSE 5 0.99; mean RMSE x 5 0.49, y 50.41). In addition, the location of houses from Jap-anese agricultural communities were digitized fromthe 1947, 1:25,000 U.S. Defense Mapping Agencytopographic maps.

Once the land cover, slope, and various soilsmaps were assembled and georeferenced, the pixelresolution of the entire GIS database was resampledto 10 m2. Approximately 85 ha within the NBDAwere omitted from the study due to lack of photocoverage in at least one of the three sample datesresulting from cloud coverage or areas missed bynon-overlapping photos.

Spatial and temporal changes in land cover dis-tribution were determined by performing overlayanalysis of the land cover maps using IDRISI GIS.To characterize the spatial pattern of the land covercategories, we used the program FRAGSTATS(McGarigal & Marks 1995) to determine the fol-lowing parameters for grassland and forest landcover categories: their absolute and relative area,and the number and size of land cover patches. Inaddition, we distinguished between edge and coreareas of land cover patches. Areas farther than 100m from a patch edge were considered core area.Distinguishing between edge and core areas of landcover is important because several studies haveshown the negative effect of edges on the structureand composition of tropical rain forests (Lovejoy etal. 1986, Bierregaard et al. 1992). We used 100 mas the distance to differentiate edge from core areasbecause Laurance et al. (1997) found evidence offorest degradation up to 100 m from patch edgesin Amazonia. A patch of land cover was defined asa group of pixels of the same land cover categoryadjacent to one another (including diagonals) withan area not smaller than 1 ha.

Relationships between vegetation and environ-mental variables were examined by cross tabulationof land cover maps with maps of elevation and

Forest Recovery in Palau 559

slope, soil pH, and organic matter (%). Chi-squareanalysis was used to test for independence of landcover and the environmental variables. Cramer’scontingency coefficient V was subsequently calcu-lated to assess the strength of significant relation-ships. Cramer’s V ranges between zero (indicatingno relationship) and one (indicating 100% corre-spondence between variables).

Relative forest age was estimated by overlayanalysis of the 1947, 1976, and 1992 land covermaps, producing a map with three forest age cat-egories: (I), forest since 1947 (II), forest since 1976and (III) forest since 1992. The number and sizeof new forest patches were also calculated for forestage categories (II and III). Forest patches smallerthan 1 ha were omitted to reduce potential errorsresulting from map registration. The spatial rela-tionship between established forest (forest categoryI) and new forest areas (II and III) was also ex-amined by proximity analysis and cross tabulation.Chi-square analysis was then performed to deter-mine if new forest establishment was related to theproximity of established forest.

Agricultural activity within the NBDA ceasedin 1944, three years prior to the first year of avail-able land cover data. Thus, Japanese agriculturalfields were not visible in the 1947 aerial photosand were indistinguishable from grassland; there-fore, the locations of abandoned buildings from theagricultural settlements on the 1947 topographicmaps were used to infer areas of prior agriculturalactivity. Analysis of spatial relationships betweenabandoned buildings and land cover changes wasdone by performing proximity analysis and crosstabulation. Chi-square analysis was then performedto determine if new forest expansion was relatedspatially to the location of abandoned buildings.

RESULTS

Upland areas accounted for 88.3 percent (7420 ha)of the NBDA; the remaining areas consisted ofwetland plant communities. Throughout the studyperiod, forest was the dominant land cover cate-gory (Fig. 2). Village areas more than doubled insize since 1947, from 16 to 37 ha, but still ac-counted for only 0.5 percent of upland areas in1992. Analysis of the NBDA showed a 11.2 per-cent reduction of grassland area, due primarily toa 10.9 percent increase in forest (Fig. 3). The ma-jority of this transition occurred between 1947 and1976 when 41.6 percent of grassland cover wasconverted to forest. This conversion slowed sub-

stantially after 1976, as only 3.6 percent of grass-land areas were further converted to forest by 1992.

As land cover changed through time, so didland cover pattern. While forest area increased be-tween 1947 and 1992, the number of forest patch-es decreased (Table 2). The reduction in forestpatches resulted in an increase of mean patch sizeas well as core area; during the study, the amountof interior forest (core area) increased 13.2 percent.Forest distribution in 1947 was characterized byone large, continuous forest patch, accounting for86.7 percent (4632 ha) of all forested areas, andnumerous smaller forest patches. By 1992, the larg-est forest patch increased to contain 89 percent(5473 ha) of all forested area.

The total area of grassland decreased as forestcover increased (Table 2). With this change, thenumber and size of grassland patches also de-creased. Thus, by 1992, the landscape consisted ofa largely forested matrix embedded with numerous,small grassland patches.

The distribution of forest and grassland landcover was significantly (P , 0.001) related to slope,elevation, soil pH, and percent soil organic matterin each of the three study years. Areas with slopesless than 2 percent or greater than 13 percent con-tained a high percentage of forest cover (Table 3).Conversely, areas with slope between 3 and 12 per-cent tended to have a higher proportion of grass-land than forest. Forest expansion after 1947 wasnot restricted to a particular slope category, al-though the largest change occurred on slopes of lessthan 2 percent; however, these areas only accountfor 6 percent of upland areas in the NBDA.

Forest cover also dominated mid-elevations(51–150 m), while disproportionate amounts ofgrassland occupied the lowest elevation class (1–50m). Although these areas had greater than 50 per-cent forest cover, they contained nearly 73 percentof all grassland areas in 1947. Areas containing soilswith high pH and percent organic matter alsotended to have a larger percentage of forest coverthan grassland. Percent increases in forest coversince 1947 were higher in areas with high soil pHvalues or with high organic matter content. Despitethe significance of the chi-square analyses, Cramer’sV values were low, indicating weak relationshipsbetween the environmental variables and land cover(Table 4).

Areas of forest regeneration were strongly as-sociated with both the location of abandonedbuildings and areas of previously established forest.Nearly 80 percent of all new forest areas occurredwithin 500 m of abandoned buildings. Chi-square

560 Endress and Chinea

FIGURE 2. Ngeremeduu Bay Drainage Area Land Cover 1992. Black points in the map represent the location ofabandoned buildings found on 1947 typography maps.

Forest Recovery in Palau 561

FIGURE 3. Major transitions in land cover between1947 and 1992. Values inside boxes indicate land coverarea (ha) and the percentage (%) of upland area in a givenyear. Arrows indicate changes (ha) in land cover betweenyears. Transitions # 5 ha are not illustrated, nor are tran-sitions between villages and other land cover types.

TABLE 2. Land cover characteristics of forest and grassland areas, 1947–1992, NBDA, Palau.

Percentupland

landscapeClass area

(ha)Number

of patchesMean patch

size (ha)Patch sizeSD (ha)

Number ofcore areas

Mean corearea size

(ha)Percent

core area

Upland forest194719761992

72.082.682.9

534061286152

604242

89146147

592829833

835855

44.591.092.6

50.062.463.2

Grassland194719761992

27.817.116.6

206412661231

156142134

13.28.99.2

362118

1115756

1.20.70.7

8.87.47.2

analysis showed that forest establishment was notrandomly distributed in relation to the proximityof abandoned buildings (P , 0.001). Farther awayfrom abandoned buildings, forest establishmentwas infrequent. In addition, areas of forest estab-lishment occurred adjacent to areas of establishedforest (forest category I); 92.3 percent of new forestareas (categories II and III) occurred within 100 mof established forest (chi-square; P , 0.001). Thus,forest regeneration was more common near areasof past disturbance (as inferred by the location ofabandoned buildings) and areas of established for-est. Further analysis showed that the presence offorest was the primary factor initiating grasslandconversion to forest (Table 5). Areas disturbed inthe past, as indicated by the presence of an aban-doned building that were also adjacent to estab-lished forest (within 100 m), transitioned to forestnearly 50 percent of the time, whereas disturbedareas located farther than 100 m from establishedforest rarely transitioned to forest.

The pattern of forest establishment within theNBDA varied dramatically as time progressed. Be-tween 1947 and 1976, there were 151 separate ar-

eas of forest regeneration (i.e., new forest patches$1 ha) averaging 4.8 ha. Of these patches, 19(12.6%) were large enough to contain core area.As forest recovery slowed between 1976 and 1992,the number of new forest patches decreased to 13(mean patch size of 1.4 ha), none of which con-tained core area.

DISCUSSION

This study begins to provide an understanding ofupland land cover dynamics within the NBDA. Re-sults show that the abandonment of agriculturalcommunities was a primary factor in initiating landcover change, which resulted in a modest increasein forest cover. Changes in landscape pattern ac-companied land cover changes leading to contin-uous stretches of forest and numerous, fragmentedgrassland areas.

Landscape-level patterns of tropical forest re-covery are not well understood as few studies haveexamined this phenomenon. Results from thisstudy support the findings of Thomlinson et al.’s(1996) study of forest recovery in the Luquillo areaof Puerto Rico. Data from this study, as well asThomlinson et al. (1996), indicate that establishedforest is crucial for the facilitation of forest recoveryacross the landscape; forest recovery in both studieswas characterized primarily by the expansion ofpreviously established forest patches. These resultssuggest that established forest act as nuclei for theforest recovery process. Within the NBDA, sec-ondary forest establishment was composed of nu-merous, small forest patches near areas of previ-ously established forest. The underlying mecha-nisms that facilitate forest recovery in the NBDAare still unknown, although it appears likely thatincreased seed dispersal is an important factor.

In contrast to the area studied by Thomlinsonet al. (1996), the rate of forest cover increase in the

562 Endress and Chinea

TABLE 3. Changes in forest and grassland cover in relation to environmental factors in NBDA between 1947 and 1992.

1947

Percentforest

Percentgrassland

1992

Percentforest

Percentgrassland

Percentforest change

Slope (%)0–23–67–12

52.038.343.9

48.057.255.7

80.345.149.5

19.550.648.1

128.316.815.6

13–3031–5051–75

82.965.680.4

17.234.419.6

90.074.290.9

9.825.49.1

17.118.6

110.5Elevation (m)

1–5051–100

101–150151–213

58.485.685.559.6

41.214.514.543.2

74.390.287.961.0

24.69.8

12.139.0

115.914.612.411.4

Soil pH4.0–4.54.6–5.05.11

73.555.271.1

26.344.828.9

82.071.586.6

17.428.413.3

18.5116.3115.5

Organic matter (%)1–45–9

101

33.679.953.4

65.120.146.6

38.990.178.8

58.29.8

20.9

15.3110.2125.6

TABLE 4. Cramer’s V contingency coefficients indicatingthe strength of significant relationships (P ,0.001) between land cover and environmentalvariables.

1947 landcover

1976 landcover

1992 landcover

ElevationSlopeSoil organic matterSoil pH

0.300.300.380.12

0.200.350.470.10

0.200.350.470.09

NBDA was surprisingly small. This difference canbe attributed in part to the large proportion of for-est area at the beginning of the study period andto the apparent stability of the grassland areas inthe NBDA. While dense forest in Luquillo (in1936) covered 15 percent of the area (Thomlinsonet al. 1996), in the NBDA (in 1947), forest ac-counted for almost five times that proportion (Ta-ble 2). Therefore, the area with the potential to bereforested in the NBDA was much smaller than inLuquillo. Furthermore, only a subset of the NBDAnon-forested area recovered its forest. Forest recov-ery occurred on areas of prime agricultural utility:predominantly on nearly level slopes (0–2%), atelevations below 100 m, and on soils with high pHor organic matter content. As suggested by theanalysis of distance to agricultural structures, thesewere the areas most likely cultivated by the Japa-

nese. The remaining non-forested areas were most-ly anthropogenic grasslands from precolonial times.

Previous investigators (Cole et al. 1987, Mar-agos 1992) have suggested that grassland areas onBabeldaob are the result of human disturbancessuch as land clearing and agriculture prior to Eu-ropean contact. The presence of large terraces onmany grassland areas within the NBDA is furtherevidence for this argument. The purpose of theseterraces remains unclear, although it has been sug-gested they were created for agricultural use to sus-tain a population of ca 40,000 (Osborne 1966,Parmentier 1987). A prolonged period of humandisturbance and periodic burning, still done by lo-cals (Otobed & Maiava 1994), appears to have in-hibited forest establishment into these areas.

Land clearing followed by intense land usetypes, such as agriculture or grazing, have led tothe formation of grassland plant communities inseveral tropical areas, particularly when grasslandsare periodically burned (Aide & Cavelier 1994,Cohen et al. 1995). Several barriers to forest re-covery have been identified once grasslands are es-tablished: soil infertility, low propagule availability,high levels of seed and seedling predation, rootcompetition, low light levels underneath a thicklayer of herbaceous material, and annual fires (Aide& Cavelier 1994). Nepstad et al. (1990) found thatforest colonization of anthropogenic grasslands inAmazonia was not restricted by low soil nutrient

Forest Recovery in Palau 563

TABLE 5. Proportion of area transitioning from grassland to forest in relation to the proximity of established forest (I)and abandoned structures. Numbers in parentheses indicate the amount of area (ha) transitioning from grass-land to forest in each class. Data for areas further than 400 m of established forest and 500 m from abandonedstructures are not shown (transitions presented account for 66.7 percent of all area converted from grassland toforest).

Distance(m) from

abandonedstructure

Distance (m) from established forest

50 100 150 200 250 300 350 400

50100150200250

0.47 (35.8)0.54 (83.9)0.53 (77.7)0.50 (63.5)0.46 (52.3)

0.42 (11.6)0.49 (25.9)0.47 (24.7)0.43 (20.3)0.41 (17.6)

0.14 (0.9)0.25 (2.8)0.29 (4.1)0.30 (5.6)0.30 (3.8)

0.01 (0.1)0.06 (0.3)0.17 (1.2)0.25 (1.6)0.21 (1.1)

—0.13 (0.4)0.10 (0.5)0.05 (0.2)0.04 (0.1)

—0.01 (0.1)0.04 (0.1)

——

—————

—————

300350400450500

0.44 (43.8)0.39 (30.3)0.35 (22.5)0.30 (17.5)0.28 (14.3)

0.33 (10.4)0.28 (7.4)0.25 (4.6)0.28 (4.6)0.28 (3.8)

0.10 (0.9)0.10 (1.0)0.16 (1.0)0.20 (0.6)0.05 (0.1)

0.01 (0.1)0.01 (0.1)

—0.11 (0.1)

—

——

0.03 (0.1)——

——

0.47 (0.3)0.58 (0.2)0.38 (0.1)

——

0.12 (0.1)0.94 (0.2)0.93 (0.3)

————

0.20 (0.1)

availability, but rather by the other barriers men-tioned above. The NBDA may represent a similarsituation; data from the soil survey of Palau (USDA1983) suggest that the soils that underlie grasslandareas are not too degraded to support forest vege-tation. This indicates that other factors may be lim-iting further forest expansion.

Dispersed seeds may be the main source of treepropagules into the NBDA grasslands, since regen-eration via seedlings, tree sprouts, and the seedbank is often eliminated after several years of cul-tivation. This conclusion is supported by the factthat the majority of forest area expansion occurredwithin 100 m of previously established forest.Thomlinson et al. (1996) found a similar relation-ship in abandoned pastures in Puerto Rico whereforest regeneration decreased with increasing dis-tance from established forest.

Even when seeds are dispersed into the grass-land, other factors may further limit tree establish-ment. A thick herbaceous mat of Ischaemum spp.,Miscanthus floridulus, and Gleichenia linearis com-prise much of the NBDA grassland plant com-munities; this may inhibit seeds from reaching thesoil and prevent seedling establishment by causinglow light availability and root competition. Onceestablished, small seedlings and saplings must stillcompete for soil nutrients and growing space withthe established herbaceous layer and also surviveannual or semiannual burning.

Despite past disturbances from human landuse, the NBDA contains a large portion of mature

forest. Seventy-two percent of the upland areas ofthe NBDA have been forested since 1947. Whileit is unclear how disturbed these forests are, Frank-lin et al. (1999) found that secondary forests 30–50 years old in the Vava’u island group of Tongawere of great conservation value. The forests of theNBDA, and Palau in general, are unique becauseendemic species play a major role in forest com-position, structure, and succession (Cole et al.1987, Canfield et al. 1992). In addition, the up-land forests of the NBDA reduce soil erosion,flooding, and siltation of the rivers draining intoNgeremeduu Bay, an important ecological and eco-nomic area. Monitoring future changes in land-scape pattern is necessary, as plans for developmentwithin the NBDA are likely to occur rapidly fol-lowing the completion of a paved highway throughthe region that is currently under construction.

ACKNOWLEDGMENTSWe would like to thank the Palau Conservation Society,especially Tom Graham and Noah Idechong, for theirassistance and support of this project. In addition, wewould like to thank Demei Otobed of the Palau Divisionof Conservation and Marcelo Brel of the Palau Divisionof Forestry for sharing their vast knowledge of the ecologyof Palau’s terrestrial plant communities. We are gratefulfor the assistance and support of Ann and Clarence Ki-talong, Maren Peterson, and Ron Gonzales while con-ducting fieldwork. The Department of Natural Resourcesand Environmental Sciences at the University of Illinoisat Urbana–Champaign provided funds for this project.We also thank two anonymous reviewers for their helpfulcomments on a previous draft of this manuscript.

564 Endress and Chinea

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