Caribbean Journal of Science, Vol. 42, No. 1, 53-66, 2006 Copyright 2006 College of Arts and Sciences University of , Mayagu¨ez

Phytosociology of Vascular on an International Biosphere Reserve: Virgin Islands National Park, St. John, US Virgin Islands

Sonja N. Oswalt1,*, Thomas J. Brandeis1, and Britta P. Dimick2

1U.S. Department of Agriculture Forest Service, Southern Research Station, Forest Inventory and Analysis, 4700 Old Kingston Pike, Knoxville, TN 37919, U.S.A 2University of Tennessee Department of Forestry, Wildlife & Fisheries, 274 Ellington Sciences, Knoxville, TN 37916, U.S.A. *Corresponding author: [email protected]

ABSTRACT.—We investigated the relationships of vegetation communities to environmental variables and compared the relative contribution of native and introduced in extant forest communities on St. John, US Virgin Islands, using an island-wide forest vegetation inventory and monitoring network of permanent plots. We detected 2,415 individuals of 203 species, 5 percent of which were introduced. Cluster analysis, Indicator Species Analysis, and Nonmetric Multidimensional Scaling (NMS) ordination detected four broad species communities divided primarily by moisture and disturbance gradients. Group 1 was characterized by rocky, low-to-mid elevation dry scrub forest on soils with fairly low soil nutrient content; Group 2 was distinguished by low soil nutrient content, high-elevation moist basin forests on steep slopes; Group 3 was indicative of disturbed communities on a wide range of elevations with gentle or no slope across a range of soil types; and Group 4 represented mid-elevation moist forests across a range of steep slopes on nutrient- rich soils. Though introduced species are present and widespread on the island, they do not appear to be dominating most plant communities. Exceptions may be those communities with long-standing histories of human disturbance. Achieving an adequate sample of forest types of limited extent or linear spatial patterns such as mangroves and gallery moist forests is difficult with a systematic design. Future sampling should consider some form of stratification targeting these under-sampled forest types.

KEYWORDS.— forest vegetation, disturbance, introduced species, inventory and monitoring, phy- tosociology, Virgin Islands National Park

INTRODUCTION lent itself to sugarcane, cotton, tobacco, and coffee cultivation (Woodbury and Weaver Extant Caribbean forests have been 1997). As a result, the vegetation has been shaped by natural disturbances, specifi- dramatically altered through time, and cally hurricanes, and a more recent history many of the current island landscapes are of human exploitation (Lugo et al. 2000). dominated by secondary forest (Weaver Land use history on St. John is similar to and Chinea-Rivera 1987; Ray and Brown that of other islands in the Lesser Antilles. 1995; Rogers and Reilly 1998). Some An- Forests on St. John were first utilized by tillean forest types, such as the once- pre-Columbian native Taino Indians for common subtropical dry forest, have been food, shelter, and raw materials for boat impacted to the point that they are now construction. Danish settlers arrived in the considered endangered ecosystems (Mur- early 1700’s and cleared large areas of for- est for agriculture, farming, and building phy and Lugo 1986; Ray et al. 1998; Linares- construction (Ray and Brown 1995; - Palomino and Ponce Alvarez 2005). On bury and Weaver 1997). A favorable cli- many of the islands, forest cover consists of mate contributing to long growing seasons fragments scattered across the landscape and much of what remains are secondary forests containing various introduced spe- ms. received November 14, 2005; accepted Decem- cies whose long-term impacts on species ber 22, 2005 composition and ecosystem function are 53 54 SONJA N. OSWALT ET AL. yet to be understood (Chinea and Helmer ent Caribbean forest landscape, surpris- 2003; Grau et al. 2003; Lugo and Helmer ingly few studies have documented pat- 2004). terns and composition of island vegetation, Though St. John shares a history of dis- particularly in the U.S. Virgin Islands. turbance-dominated landscapes common Studies that have explored species- in the Lesser Antilles, it has avoided the environment relationships and forest pro- subsequent urban development that has oc- ductivity in the Caribbean have focused curred across the Caribbean basin. In 1956 primarily on woody species (Weaver and approximately 65 percent of St. John was Chinea-Rivera 1987; Ray et al. 1995; Ray et donated to the U.S. National Park Service al. 1998; Rivera et al. 1998; Lugo et al. 2004; for the creation of the Virgin Islands Na- Linares-Palomino et al. 2005). Aside from a tional Park (VINP) (National Park Service limited number of field guides, taxonomic 2005). In recognition of its uniqueness, in floral descriptions, and broad scale com- 1976 the park was designated an interna- munity maps, the result has been a general tional biosphere reserve by the United Na- paucity of information regarding non- tions Educational, Scientific and Cultural woody species, their rela- (UNESCO) Man and Biosphere (MAB) pro- tionships to woody species, and plant com- gram. With the addition of offshore areas in munity-environment relationships in this 1962 and the establishment of the Virgin region. Current VINP management objec- Islands Coral Reef National Monument in tives that could benefit from a comprehen- 2001, the VINP mission “to protect, man- sive phytosociological study and vegeta- age, interpret and preserve the park’s tion monitoring include: reduction of soil unique natural and scenic resources and erosion, landslides, and freshwater runoff values unimpaired for the education, en- that damage near-shore coral reefs and re- joyment and inspiration of present and fu- duce their value as an ecotourism resource ture generations” extends from the island’s (National Park Service 2005); elimination or highest point, Bordeaux Mountain at 389 reduction of feral white-tail deer (Odocoi- m, into the surrounding Caribbean tropical leus virginianus), donkey (Equus asinus), do- marine ecosystems (National Park Service mestic goat (Capra hircus), wild hog (Sus 2005). The result has been the protection scrofa), and domestic sheep (Ovis aries) from development of one of the larger con- populations that threaten forest regenera- tiguous forest areas in the Lesser Antilles, tion and soils (National Park Service 2004); and a unique opportunity for the study of an Avian Conservation Implementation island flora and fauna. Plan, directed toward maintaining and in- The species composition and vegetation creasing the amount of over wintering structure in St. John’s forests, considered a habitat for Neotropical migrants (Watson global biodiversity “hotspot” (Myers et al. 2003); and monitoring of land use change 2000, Helmer et al. 2002), are of substantial from construction and over development importance to understand current biodiver- (U.S. Department of Transportation 2004). sity in the Caribbean basin, and to evaluate The U.S. Department of Agriculture availability of habitat for multitudes of (USDA) Forest Service, Forest Inventory wildlife species. In addition to contributing and Analysis (FIA) program conducts sys- to global wildlife and vegetation biodiver- tematic surveys of forestland across the en- sity, the Greater and Lesser Antilles are an tire and its territories. In 2004, important archipelago for wintering neo- FIA initiated a special, intensified study on tropical migratory songbirds (Askins et al. St. John to investigate the structure and di- 1992; Wunderle and Waide 1993; Rappole versity of forest vegetation, and additional 1995). Studies have suggested that defores- forest attributes. This intensified inventory tation has led to losses of wintering habitat was not limited to within the VINP bound- for many warbler species in the Caribbean, aries, but encompassed the entire island be- and to population declines in these species cause conditions and management activi- (Rappole 1995). ties in the non-park portions of the island Despite the need to understand the pres- directly and indirectly impact the unique FOREST VEGETATION OF ST. JOHN 55

Caribbean ecosystems the park is meant to sign (USDA Forest Service 2003). In an ef- preserve. Specifically, our objectives were fort to minimize impacts to forest vegeta- to: 1) install a permanent forest vegetation tion, Forest Service FIA protocol does not inventory and monitoring network across allow for repeated visits to sample sites the island of St. John, 2) document the com- within a given year. To minimize any po- position, relative abundance, and vertical tential effects of seasonality we sampled at position of all vascular plants on forested the mid-point of the rainy season—the plots, 3) quantify and compare populations point at which any seasonal growth flushes of native and introduced vegetation occur- would have already occurred but drought ring on forestland, and 4) identify environ- would not have resulted in senescence. mental variables influencing the distribu- Twenty-six plots were arranged on an un- tion of vascular plants across the island. biased, systematic sample grid across St John. The grid on St. John was composed of hexagons covering approximately 200 ha MATERIALS AND METHODS each, with one sample plot located within each hexagon. Plots were located and Study site mapped using differentially corrected glo- bal positioning system (GPS) equipment to The island of St. John, U.S. Virgin Is- ensure a high degree of accuracy. Plots lands, is located in the Caribbean Sea were installed and data collection took (13°22’N, 64°40’W) approximately 30 km place where there was at least 10 percent east of Puerto Rico. St. John is approxi- canopy coverage and a minimum for- mately 5180 ha in area and covers rugged ested area of 0.4 ha around each plot center, terrain ranging in elevation from sea level as designated by USDA Forest Service FIA to 387 m above sea level, and slopes in ex- sample design guidelines. Plots consisted cess of 30 percent across the bulk of the of a cluster of four subplots each with a 7.3 island (Rankin 2002). The Virgin Islands m radius (Fig. 1). There were 167 m2 per National Park occupies over half of the vol- subplot, for a total sampled area of 670 m2 canically formed island. Primary forest (0.017 ha) in a fully forested plot. types include mangrove and salt flats; moist upland, gallery, and basin forests; dry evergreen woodlands, thickets, thorn and scrubs, coastal hedges; and pas- ture (Woodbury et al. 1997). The climate on St. John is primarily subtropical, and the island is subject to tropical trade winds, hurricanes, and variable rainfall. Precipita- tion on St. John ranges from 890 to 1400 mm y−1, and averaged 1141 mm y−1 for the years 1971 to 2000 at the Cruz Bay weather station at the west end of the island (Weaver and Chinea-Rivera 1987; South- east Regional Climate Center 2005). The majority of rainfall on St. John occurs dur- ing late spring, summer, and early fall, with a dry season December through April (Weaver and Chinea-Rivera 1987).

Sample design and data collection

Vegetation and environmental data were FIG. 1. Forest Inventory and Analysis plot design. collected June through July of 2004 using Figure recreated with permission of USDA Forest Ser- the USDA Forest Service FIA sample de- vice, Forest Inventory and Analysis program. 56 SONJA N. OSWALT ET AL.

Field crews collected forest inventory, species-area relationships, we eliminated understory structure and composition, and all subplots that contained less than 100 physiographic data on each subplot. Diam- percent forest area, and analyzed data us- eter at breast height (DBH) (taken at 1.37 ing subplots as the primary sample unit m), total height, crown ratio, crown width, (Gray and Azuma 2005; Oswalt in press; and other parameters were measured on all Rudis et al. in press). Some degree of spa- with DBH Ն 12.5 cm within the sub- tial autocorrelation was expected among plots; and DBH and total height were mea- subplots comprising a “plot” unit; how- sured on saplings with DBH Ն 2.5 cm ever, the nonparametric statistical methods within a 2.1 m radius microplot nested in used for this analysis helped to avoid this each subplot (Bechtold and Patterson 2005). problem, and we were interested in allow- Details on the plot and subplot-level site ing for the potential separation of subplots characteristic categories used in FIA are occurring in different environmental condi- found in the USDA Forest Service, FIA- tions (e.g., aspect or percent slope) (King Southern Research Station Field Guide, and Buckney 2002). The remaining sample Supplement C to Southern Research Station size consisted of 75 100-percent forested Regional Manual 1.62, Field Procedures for subplots in 20 plots. Puerto Rico and the Virgin Islands (http:// www.srs.fs.usda.gov/fia/manual/). Additional Species composition and structure environmental information was derived from mapped Soil Survey Geographic Da- Species richness values, including jack- tabase (SSURGO) soil data (USDA Natural knife estimates and rarefaction curves, Resources Conservation Service 2004). were estimated using PC-Ord and Esti- All vascular plant species were recorded mateS 7.5 (McCune and Mefford 1999; Col- within each subplot within a 45 minute well 2005). Nomenclature followed the time limit. The USDA Forest Service FIA USDA PLANTS database species list for the program limits the subplot search for vas- U.S. Virgin Islands and Puerto Rico. Spe- cular plant species to 45 minutes in order to cies were assigned to life form as given in maintain the efficiency of the data collec- USDA PLANTS database (http://plants tion process, and to standardize the process .usda.gov/) and Acevedo-Rodríguez nationwide (Gray and Azuma 2005). Per- (1996). Rushes and sedges were lumped cent foliar cover was then estimated to the into the “herb” category, and subshrubs nearest percent for each species on the sub- were lumped in the “” category for plot in each of five categories: total cover, simplification. Cacti were split into “shrub” cover in the 0 to 61 cm height layer, cover in or “tree” as given in PLANTS. Species were the 61 to 183 cm height layer, cover in the assigned to three groups: native, intro- 183 to 488 cm height layer, and cover above duced, or unknown using the PLANTS da- 488 cm. Voucher specimens for unknown tabase. Definitions of broad forest types species and species with which the botanist (e.g., Secondary Forest) follow Woodbury was less familiar were documented in the and Weaver (1997) and Acevedo-Rodrí- field, collected, and submitted to a botanist guez (1996). from the University of the Virgin Islands for identification and comparison against Community determination herbarium records. Field methods as designated by FIA al- Cluster analysis was used to group sub- low for multiple conditions on each sub- plots by species composition (McCune and plot, resulting in subplots with various per- Grace 2002). Sample by species matrices centages of forest acreage. The nonforest were populated using total percent cover portions are not sampled, resulting in “par- values on a subplot basis, arcsine-square tial” subplots and “partial” plots. Because root transformed to improve normality partially sampled subplots and plots vio- (McCune and Grace 2002). Rare species late statistical assumptions, particularly (species occurring in two or fewer plots) FOREST VEGETATION OF ST. JOHN 57 were removed from each matrix prior to (NMS) to identify environmental character- analysis to reduce noise in the dataset istics that play a role in determining com- (Weaver and Chinea-Rivera 1987; McCune munity composition. One subplot con- et al. 2000; McCune and Grace 2002; Pick- tained no environmental data and was ing and Veneman 2004). One subplot was removed from the ordination analysis, re- an outlier falling in a dry salt lagoon and sulting in a sample size of 73. The ordina- containing only one species, Laguncularia tion was performed on the “autopilot slow- racemosa (L.) Gaertn. f., and was removed and-thorough” mode in PC-Ord using from analysis. Sorenson distance measures, 40 runs with Clustering was performed using Soren- real data and 50 runs of randomized data son distance measures with flexible beta set (McCune and Grace 2002). Average stress at -0.25, the preferred distance measure for and instability were used to determine the community datasets (McCune et al. 2000; best solution, and Pearson and Kendall cor- McCune and Grace 2002). We then used relations of environmental variables with nonmetric multiresponse permutation pro- axes were used to interpret the ordination cedures (MRPP) to test within-group ho- (McCune and Grace 2002). Species matrices mogeneity and to validate the separation were initially analyzed at the whole com- between clusters (McCune et al. 2000). munity level. Subsequent analysis sepa- Nonmetric MRPP utilizes rank transformed rated the species matrix into two life-form distance matrices rather than raw data (as categories, trees (overstory) and herb- in MRPP) to calculate test statistics, and has shrub-epiphyte (understory). been successfully used for evaluating dif- Categorical class variables in the envi- ferences in groups of community data (Mc- ronmental matrix were used like continu- Cune et al. 2000). The A statistic in MRPP ous variables by assigning values based on describes “chance-corrected within-group the midpoint of the class in which they fell. agreement,” or the difference in within- Environmental variables used in the com- group homogeneity compared to random munity NMS ordination are given in Table chance (McCune and Grace 2002). The A 1. Ordination on the understory layer alone statistic is significant and A > 0 when the also included structural components of the within-group homogeneity and between- overstory in the environmental matrix, as group heterogeneity is greater than that ex- we suspected that the herb layer was influ- pected by chance (McCune et al. 2000; Mc- enced both by the environment and by the Cune and Grace 2002). When A > 0.3, structure of the overstory component differences between groups are considered (Table 1). Ordinations are shown with clus- to be fairly large, though McCune et al. ter groups and species as the identifying (2000) point out that A values in commu- characteristics. nity data often fall below A = 0.1 even if differences between groups exist. Indicator species analysis was used to flag species RESULTS indicative of each cluster, as determined by Species composition and structure importance values and significance levels. We determined where to break the dendro- We recorded 2,415 individuals of 203 gram by plotting the average P-values from species (63 families) on St. John forestland. the indicator species analysis by the num- This richness represents 21 percent of all ber of groups to find the level of maximum vascular species described on the island significance as suggested by (Dufrene and (Acevedo-Rodríguez 1996). First-order Legendre 1997). jackknife estimates indicated the true spe- cies richness was 255.3 (± 12.15 standard Ordination deviations (SD)). The species area curve for samples recorded on St. John began to pla- Arcsine square root-transformed matri- teau at 70 subplots (Fig. 2). ces from the cluster analysis were subjected Subplots contained 32.20 (± 8.29 SD) spe- to nonmetric multidimensional scaling cies, on average, and beta diversity (gamma 58 SONJA N. OSWALT ET AL.

TABLE 1. Quantitative environmental variables evaluated in the community and understory NMS matrices and correlations to community-level NMS axes.

Community axis correlations Community Understory Variable name matrix matrix 123 Slope Yes Yes 0.36 0.12 −0.03 Aspect Yes Yes −0.21 0.3 −0.06 Elevation Yes Yes 0.41 −0.1 −0.49 Distance to water Yes Yes −0.06 −0.15 −0.11 Distance to improved roads Yes Yes 0.07 −0.05 0.55 Distance to urban areas Yes Yes 0.07 −0.17 0.17 Distance to agriculture Yes Yes −0.27 0.03 −0.29 Acres of contiguous forest Yes Yes −0.05 0.07 −0.02 Total clay (percent)a Yes Yes −0.11 −0.47 −0.44 Bulk densitya Yes Yes 0.13 0.42 0.46 Available water capacitya Yes Yes −0.04 −0.48 −0.41 Linear extensibility (percent)a Yes Yes −0.13 −0.38 −0.46 Organic matter (percent)a Yes Yes −0.14 −0.17 −0.39 Cation exchange capacitya Yes Yes −0.1 −0.47 −0.44 pH (water extraction method)a Yes Yes 0.03 −0.41 −0.33 Calcium carbonatea Yes Yes 0.14 0.2 0.4 Gypsuma Yes Yes −0.13 −0.4 −0.47 Sodium adsorption ratioa Yes Yes −0.13 −0.4 −0.47 Overstory trees per hectare No Yes N/A Overstory basal area per hectare No Yes N/A Aboveground biomass (mg/ha) No Yes N/A Belowground biomass (mg/ha) No Yes N/A Total biomass (mg/ha) No Yes N/A Crown area No Yes N/A Crown volume No Yes N/A Overstory trees per hectare No Yes N/A aDenotes variables derived from SSURGO mapped soil data, Natural Resources Conservation Service, 2004. diversity divided by alpha diversity) was percent of any single subplot, and averaged estimated at 7.93. Twenty-six percent of 8 percent total foliar cover across subplots. species occurred in only one subplot and 10 Randia aculeata L. was the most common percent of recorded species occurred in shrub sampled, occurring in 75 percent of only two subplots. Twenty-two species subplots sampled; Acacia retusa (Jacq.) were unknown, but distinct, and were as- Howard was the most common liana, oc- signed generic names as given in the PLANTS database. Eighty-four percent of species detected were native to the Virgin Islands, 5 percent were introduced, and na- tivity could not be determined for 11 per- cent of species. Fifty-nine subplots (79%) contained at least one introduced species; 29 subplots (39%) contained two or more. We detected 9 out of 25 known species that are endemic to the combined Virgin Islands and Puerto Rico (Acevedo-Rodríguez 1996). Guapira fragrans (Dum.-Cours.) Little was the most common vascular plant (and tree) FIG. 2. Species area curve with Sorenson distance sampled, occurring on 91 percent of sub- measures for samples on St. John, U.S. Virgin Islands, plots, though it never covered more than 25 2004. FOREST VEGETATION OF ST. JOHN 59 curring in 61 percent of subplots; Anthu- percent. On subplots containing at least one rium cordatum (L.) Scott was the most com- introduced species, the average cover of in- mon herb, appearing in 37 percent of troduced species was 1.86 (± 0.22 SD) per- subplots; Lasiacas divaricata (L.) A.S. Hitchc. cent. Average foliar cover on subplots was was the most common grass, sampled in 55 highest in the 61 to 183 cm vegetation layer, percent of subplots; and Tillandsia utricul- and was lowest in the 0 to 61 cm and >488 lata L. was the most common epiphyte, oc- cm layers. curring in 52 percent of subplots. The ma- jority of species detected were , Community determination followed by trees and vines. Species averaged 4.19 (±0.29 SD) percent Clustering and indicator species analysis total foliar cover on subplots, with a range yielded maximum results at four clusters. of averages from 1 percent (trace) to 22.14 The resulting dendrogram was pruned percent. The maximum cover exhibited by with <25 percent of information remaining any one species on a given subplot was 90 (Fig. 3). The four highest scoring indicator

FIG. 3. Results of cluster analysis on community-level data, St. John, U.S. Virgin Islands. The dendrogram was broken into 4 groups with less than 25 percent of information remaining. 60 SONJA N. OSWALT ET AL. species, importance values, and P-values var. rigidus Muell.-Arg., Eugenia cordata for each group are given in Table 2. Major (Sw.) DC., and Plumeria alba L.; species in- group separations occurred between Group dicative of dry scrub forest (Acevedo- 3 and Groups 1, 2, and 4. Results of the Rodríguez 1996). Conversely, indicators of nonmetric MRPP showed a chance- both Groups 2 and 4 were indicative of corrected within-group agreement of A = moist forests, although species of Group 2 0.50 (P < 0.0001), and a T-test statistic of were more typical of basin moist forests, −31.22, thus validating the separation be- while species of Group 4 were more typical tween selected groups. of secondary moist forests (Acevedo- Group 1 split apart from Groups 2 and 4 Rodríguez 1996). Primary indicators of along what appeared to be a moisture gra- Group 2 included Anthurium cordatum (L.) dient. Primary indicators of Group 1 were Schott, baducca L., Chionanthus Acacia muricata (L.) Willd., Croton flavens L. compactus Sw., alliodora (Ruiz &

TABLE 2. Indicator species detected on the island of St. John in June and July, 2005 by cluster analysis group. Life form, indicator value for indicator species analysis, and nativity are denoted.

Primary Indicator Group Scientific name Plant family life form valuea Nativityb 1 Plumeria alba L. Apocynaceae tree 75.7 N Acacia muricata (L.) Willd. Fabaceae tree 63.9 N Eugenia cordata (Sw.) DC. Myrtaceae tree 63.8 N Croto flavens L. var. rigidus Muell.-Arg. Euphorbiaceae shrub 52.6 N Reynosia guama Urban Rhamnaceae shrub 48.3 N* Brufelsia americana L. Solanaceae shrub 45.1 N spinosa (Jacq.) Raf. shrub 44.4 N Ipomoea eggersii (House) D. Austin Convolulaceae vine 43.5 N Amyris elemifera L. Rutaceae shrub 41.5 N Agave missionum Trel. Agavaceae herb 41 N* 2 Morisonia americana L. tree 61.3 N Cordia alliodora (Ruiz & Pavo´n) Oken tree 46.4 N Chionanthus compactus Sw. Oleaceae shrub 45.9 N Anthurium cordatum (L.) Schott Araceae herb 40.6 N Capparis baducca L. Capparaceae shrub 40.5 N 3 repens Bello Cactaceae shrub 69.7 N* Leucaena leucocephala (Lam.) de Wit Fabaceae tree 65 N frutescens Jacq. shrub 60.7 N Acacia retusa (Jacq.) Howard Fabaceae liana 58.1 N Erythroxylum brevipes DC. Erythroxylaceae tree 51.1 N Capparis flexuosa (L.) L. Capparaceae shrub 50.1 N Serjania polyphylla (L.) Radlk. Sapindaceae liana 48.7 N Commelina erecta L. Commelinaceae herb 46.8 N Capparis indica (L.) Druce Capparaceae shrub 44.2 N Tragia volubilis L. Euphorbiaceae vine 43.5 N 4 Savinea (Vahl) DC. Fabaceae shrub 61.2 N Ardisia obovata Desv. ex Hamilton Myrsinaceae tree 60.7 N Guettarda scabra (L.) Vent. Rubiaceae shrub 60.6 N Eugenia monticola (Sw.) DC. Myrtaceae tree 57.5 N Psychotria nervosa Sw. Rubiaceae shrub 51.9 N Nectandra coriacea (Sw.) Griseb. Lauraceae shrub 51.4 N Cestrum laurifolium L’He´r. Solanaceae shrub 48.2 N Chiococca alba (L.) A.S. Hitchc. Rubiaceae liana 47.8 N Lasiacis divaricata (L.) A.S. Hitchc. Poaceae grass 47.7 N aIndicator values represent a combination of relative abundance and relative frequency. Values are given where p Ն 0.001. bNative species with an asterisk indicate species endemic to the combined U.S. Virgin Islands and Puerto Rico. FOREST VEGETATION OF ST. JOHN 61

Pavón) Oken, and Morisonia americana L.. subplots were relegated across low-to-mid Primary indicators of Group 3 included elevations over a range of slopes, and were Acacia retusa (Jacq.) Howard, Capparis flex- distinguished by higher bulk densities, uosa (L.) L., Capparis indica (L.) Druce, Com- lower clay content, low AWC, low CEC, melina erecta L., Erythroxylum brevipes DC., and slightly acidic soils. Group 2 subplots Leucaena leucocephala (Lam.) de Wit, Opuntia were generally arrayed towards higher el- repens Bello, Jacq., Seja- evations with steep slopes, and were char- nia polyphylla (L.) Radlk., and Tragia volubi- acterized by high bulk densities, lower clay lis L.. These species are typical of secondary content, low AWC and CEC, and higher dry forest and disturbed areas (Acevedo- pH values. The subplots in Group 2 were Rodríguez 1996). Significant indicators of scattered across southeast to northwest as- Group 4 were Ardisia obovata Desv. ex. pects. Group 3 subplots occurred primarily Hamilton, Cestrum laurifolium L’Hér, Chio- at the gradient between lower elevation dry cocca alba (L.) A.S. Hitchc., Eugenia monticola forests and higher elevation moist forests (Sw.) DC., Lasiacis divaricata (L.) A.S. on areas with gentle to no slopes, higher Hitchc., Nectandra coriaceae (Sw.) Griseb., organic content, and spanning a range of and Sabinea florida (Vahl) DC. nutrient contents. The subplots in Group 3 fell on sites with moist, northwesterly as- Ordination pects. Group 4 subplots occurred primarily at higher elevations with less-steep slopes, Ordination was performed on two sepa- fell on south—or east-facing sites, and were rate species matrices. Community is used on soils distinguished by low bulk densi- here to describe the plant composition of ties, high clay content, high organic matter combined understory and overstory spe- content, high AWC, LEP, and CEC, and cies, including all woody and nonwoody neutral pH values (Fig. 4). species. Understory is used here to describe Nonmetric multidimensional scaling of all non-tree woody and nonwoody species. subplots in understory species space with Nonmetric multidimensional scaling of tree species removed and environmental subplots in community species space re- variables related to forest structure added sulted in a three-dimensional solution. The to the environmental matrix resulted in a final stress for the three-dimensional solu- three-dimensional solution, as well (Fig. 5). tion after 108 iterations was 13.52, and the Final stress for the three-dimensional final instability was 0.00001. Overall, the model was 14.50, and final instability was three-dimensional solution explained 78 0.00001. Combined axes explained 76 per- percent of the variation in the dataset. Axis cent of the variation in the dataset. Axis one one explained 27 percent of the variation, explained 33 percent, and was negatively and was positively correlated with slope correlated with overstory basal area, over- and elevation (Table 1). Axis two explained story biomass, overstory crown area and 18 percent of the variation, and was nega- volume, and elevation. Axis two explained tively correlated with percent total clay, 22 percent, and was positively correlated available water capacity (AWC), linear ex- with overstory basal area, overstory bio- tensibility or shrink-swell (LEP), cation ex- mass, total percent clay, AWC, LEP, CEC, change capacity (CEC), pH, gypsum, and pH, gypsum, and SAR, and was negatively sodium adsorption ratio (SAR), and posi- correlated with bulk density. Axis three ex- tively correlated with bulk density, and plained 20 percent, and was negatively cor- weakly positively correlated with aspect related with distance to agricultural land, (Table 1). Axis three explained 32 percent of total clay, AWC, LEP, organic matter, CEC, the variation, and was positively correlated gypsum, and SAR, and was positively cor- with distance to improved roads, bulk den- related with bulk density and calcium car- sity, and calcium carbonate, and negatively bonate. Slope and aspect, which were im- correlated with percent total clay, organic portant in the model when overstory matter content, elevation, AWC, LEP, CEC, species were included, were no longer pH, gypsum, and SAR (Table 1). Group 1 strongly correlated to ordination axes. 62 SONJA N. OSWALT ET AL.

FIG. 4. Vegetation community (combined overstory FIG. 5. Understory vegetation joint biplot ordina- and understory) joint biplot ordination along NMS tion along NMS axes, coded by cluster analysis axes, coded by cluster analysis groups. Vector length groups. Vector length and direction indicates the and direction indicates the strength and direction of strength and direction of relationships with environ- relationships with environmental axes. Plot (a) shows mental axes. Plot (a) shows ordinations for axes 1 and ordinations for axes 1 and 2, and plot (b) shows ordi- 2, and plot (b) shows ordinations for axes 2 and 3. nations for axes 2 and 3. eas overall, and a wide range of crown vol- Subplots grouped by community from umes and crown areas. Group relationships the cluster analysis, and understory species to other environmental variables did not were plotted simultaneously on the ordina- change between the community ordination tion axes (Fig. 5). Subplots in Group 1 ap- and the understory ordination. peared to have lower basal areas, crown volumes, and crown areas, and—as in the DISCUSSION whole community ordination—fell along lower elevations. Subplots in Group 2 cov- The island of St. John contains diverse ered a range of basal areas, but tended to vascular plant assemblages covering a have large crown volumes and crown ar- range of forest structures, all of which are eas. Subplots in Group 3 exhibited fairly important to the maintenance of species low basal areas and crown volumes and biodiversity in the Caribbean. The flora on areas in the middle of the range of values. St. John is important because of the loss of Finally, Group 4 exhibited higher basal ar- tropical and subtropical dry forests and the FOREST VEGETATION OF ST. JOHN 63 level of preservation afforded those forests from many studies conducted on St. John. on St. John as a result of federal protection Rather than focusing on a single landform through the VINP. or landholding, our study crossed physi- Cluster analysis, indicator species analy- ographic and political boundaries. More- sis, and NMS ordination detected four over, our study included all vascular plant broad species communities on St. John. species. Group 1 was characterized by rocky, low- The major division in our dendrogram, to-mid elevation dry-scrub forest on soils rather than occurring along a moisture gra- with fairly low nutrient content; Group 2 dient, occurred between “disturbed” for- was distinguished by nutrient poor, high- ests (Group 3) and “undisturbed” forests in elevation moist basin forests on steep both dry and moist forests (Fig. 3). From slopes; Group 3 was indicative of disturbed there, the larger group of “undisturbed” communities on a wide range of elevations forests was divided along a moisture gra- with gentle or no slope across a range of dient, as others have found in studies of soil types; and Group 4 represented mid- subtropical and tropical vegetation (Wood- elevation moist forests across a range of bury and Weaver 1997; Sánchez-Sánchez steep slopes on nutrient-rich soils. Due to and Islebe 2002). Group 3 is of interest to us the systematic nature of the FIA sample de- because disturbed forest community types sign, we probably undersampled rare com- appear to have been overlooked in most munities or communities that form linear previous studies. Species composition in features on the landscape. For instance, we Group 3 consisted primarily of weedy spe- probably undersampled mangrove forests, cies typical of roadsides and other open, which tend to form linear bands along the disturbed areas. While introduced species coast, as well as gallery moist forests, were so widespread they were not strong which form linear bands in riparian areas indicators of any group (>50 percent Indi- (Woodbury and Weaver 1997). We may cator Value), Group 3 was the only group also have under-represented partially for- that contained any introduced species indi- ested areas due to FIA rules for installing cators at significant levels (P < 0.5) (Table plots. However, the systematic design was 2). Our species associations in cluster useful in that we sampled disturbed com- groups were fairly closely aligned descrip- munities and transitions that other studies tively with the Woodbury and Weaver spe- may have overlooked by sampling discrete cies lists for species associations (1997). stands based on aerial photography, land- However, preliminary mapping indicates owner constraints, or study objectives. For that the spatial distribution of our vegeta- example, though Ray et al. (1998) con- tion communities does not necessarily ducted thorough woody-species botanical agree with vegetation maps produced in surveys of dry forest communities on St. 1997 by Woodbury and Weaver. Future re- John, the sample design was necessarily search will attempt to further compare and constrained by the study objectives to mini- contrast forest community typing based on mize effects of human alterations, thereby extensive, systematically-collected forest missing portions of the vegetation commu- inventory data and with previous mapping nity. Similarly, Woodbury and Weaver efforts that used more intensive, selective (1997) produced a thorough report with sampling. maps describing vegetation communities All subplots classified in Group 3 (dis- on St. John. Their study focused on verify- turbed subplots) fell on the north side of ing, describing, and delineating communi- the island, though they were scattered from ties derived from aerial photographs, how- the far west end of the island, near Cruz ever, and again, their approach minimized Bay, to the northeast side of the island near the potential for detecting transitions and Brown Bay, and not all disturbed subplots disturbed vegetation, and did not focus on were facing the same aspect. This surprised the environmental factors that structure the us, given that hurricanes are an important, communities, their structure, development, naturally-occurring periodic disturbance in and occurrence. Our study differs in scope Caribbean forests, and major hurricanes, 64 SONJA N. OSWALT ET AL. like Hurricane Hugo in the late 1980s, tend phy on St. John and are most likely related to approach the island from the south to the vegetation only as a byproduct of (Reilly 1991). Reilly (1991) found that eleva- other topographical features. For instance, tion, slope, and aspect played a more im- the primary road on St. John follows a portant role in the relative damage caused ridgetop across the length of the island. by Hurricane Hugo, though, than did lati- Subplots falling close to the top of the ridge tude or longitude on the island. In the would be positively autocorrelated with Reilly (1991) study, lower elevation wind- both elevation and close proximity to im- ward slopes received more damage than proved roads, even if the proximity of the did higher elevation leeward slopes. How- road has no ecological impact on the plant ever, the north side of St. John is flatter and community structure. Therefore, topogra- more suitable for agricultural and residen- phy has a confounding effect on the rela- tial development, and has more attractive tionship between plant communities, land- white sand beaches. By contrast, the south use features, and disturbance on St. John. side is composed primarily of rocky Group 3 subplots exhibited a strong affinity beaches and cliffs. As a result, more devel- to axis two, and to axis three, correspond- opment has occurred along the north side ing to mid-elevation areas of low slope of the island, and human impacts are prob- with a northern aspect and a wide range of ably more heavily concentrated there, even soil parameters. Aspect may be a lesser in- within National Park boundaries. Four of fluence as degree of slope declines. It is the 16 Group 3 subplots fell on Mary Point, logical that mid-elevation areas with lower a site with a history of over 250 years of percent slope on a rugged island would be grazing and cultivation (Ray and Brown subject to higher rates of disturbance, as 1995). Vegetation profiles in 1995 indicated those areas would be most attractive for a young forest characterized by a sparse cultivation, development, and recreational canopy and dominated by nonnative spe- activities, particularly if soil productivity is cies and thorny vines (Ray and Brown relatively high compared to other island 1995). Authors noted that grazing by feral soils. The same sites might also be more donkeys still occurred frequently at Mary attractive to grazing mammals like feral Point (Ray and Brown 1995). At least one donkeys, goats, and cows, particularly if Group 3 subplot occurred near Cruz Bay in the sites are already characterized by an area affected by a blowdown, presum- sparse canopies and lush understory ably associated with a tropical storm or growth (Rudman 1998). Highly disturbed hurricane. Four subplots were located near community types like Group 3 could be historic ruins, and signs of feral donkeys priority areas for VINP rehabilitation ac- (scat, tracks) suggested grazing may have tivities. Long term re-measurement and affected the vegetation. monitoring directed at understanding the Beyond major divisions that appear to be disturbed communities detected in Group 3 based on moisture and land-use or land- will be valuable, particularly with regards history patterns, species groups were struc- to the impacts of feral grazing mammals on tured primarily on the basis of elevation, island flora. slope, soils, and aspect, which strongly in- When canopy tree species were removed fluence vegetation development in the sub- and overstory characteristics were added to tropics (Lugo et al. 2000). Influential soil the environmental matrix, understory com- parameters were primarily indicative of the munities were primarily influenced by el- texture and productivity of sites on which evation, soil characteristics, tree species each community fell, which is related, in basal area, and canopy characteristics. part, to moisture and topography. Al- Therefore, it is reasonable to expect that un- though distance to agriculture and distance derstory species that occur at similar eleva- to improved roads were important in the tions at similar moisture gradients vary in community ordination and appeared as relation to the characteristics of the forest vectors on ordination graphs, these fea- overstory, which are partially dictated by tures are heavily constrained by topogra- aspect, slope, stand age, disturbance, and FOREST VEGETATION OF ST. JOHN 65 overall tree morphology. Fine-scale soil pa- samples of all vegetation types found on rameters like macronutrient availability, the island. which we were unable to include in analy- sis, might also, play an important role in Acknowledgments.—Thanks to the South- structuring the understory community. ern Research Station and the International Chemical extraction of macronutrients on Institute of Tropical Forestry for project the subplots measured in this study is cur- funding. We gratefully acknowledge Hum- rently underway for future analyses. Fac- fredo Marcano, Luis Ortiz, and Hana Blu- tors which we were not able to measure, menfeld for their expertise in field data col- like precipitation at various locations lection on St. John, Gary Ray for his help in across the island, also play a role, but are identifying voucher specimens, and the closely correlated with the characteristics National Park Service, particularly Rafe we did measure. Boulon, for use of facilities over the course This study provides a more comprehen- of data collection. The authors also thank sive look at St. John’s forests, as well as a David Buckley, Eileen Helmer, and anony- baseline for future vegetation monitoring. mous reviewers for their valuable assis- The island-wide network of permanent tance. plots captures not only the relatively undis- turbed forests but also the more heavily LITERATURE CITED disturbed, secondary forests that do not necessarily fit into traditional Caribbean Acevedo-Rodríguez, P. 1996. Flora of St. John, U.S. Vir- vegetation classes, but reflect each island’s gin Islands. Bronx: The New York Botanical Gar- unique land use history. We hope that the den. Askins, R. A., D. N. Ewert, and R. L. Norton. 1992. knowledge gained by this inventory and Wintering migrants in fragmented and continuous monitoring network will contribute to the forests, Virgin Islands. 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