project pmis # 80373 viis strategic plan goal # 1a2b...

FINAL REPORT

Zanthoxylum thomasianum Survey, Mapping and Population Status Update

for the Virgin Islands National Park, St. John, US Virgin Islands

Project PMIS # 80373

VIIS Strategic Plan Goal # 1a2B

Cooperative Agreement # H5000-01-0557

Gary Ray, Ph.D.

& Alice Stanford, Ph.D.

December 31, 2005

Final Report to NPS, Population Status of Zanthoxylum thomasianum

Table of Contents

Introduction Background 3 Objectives 4 Legal issues 4

Methods Systematic surveys 5 Data Management 6 Results Overview 7 Population habitat descriptions 12 Community floristics and similarity 12 Community Structure and Z. thomasianum populations 14 Point Rendezvous - Gifft Hill 14 Bordeaux Heights 18 Cob Hill Southeast 20 Cob Gut 22 Johnson Bay Ridge 24 Management Recommendations 27 References 28


G. Ray and A. Stanford 3

Introduction

This study updates the status of five populations from St. John of a federally listed endangered plant species, Zanthoxylum thomasianum (Krug & Urban) ex P. Wilson, known locally as St. Thomas prickly ash. Species descriptions may be found most recently in the Flora of St. John (Acevedo-Rodriguez 1996), and also in The Trees of Puerto Rico and the Virgin Islands (Little, et al. 1974). St. Thomas prickly ash, (Zanthoxylum thomasianum), a thorny shrub of the citrus (Rutaceae) family, was the first plant species in the U.S. Virgin Islands to be listed “Endangered” by the U.S. Fish and Wildlife Service. This survey focused particularly on occurrences within the Virgin Islands National Park, but included large populations external to its authorized boundaries. After a discussion with Mr. Rafe Boulon, Chief of Resources Management of the Virgin Islands National Park, it was decided prior to commencement of our study to forego surveys outside of St. John, and to focus in particular on past sightings within VINP boundaries. The most recent status report on this species was released in 1981 (Vivaldi and Woodbury 1981). In the intervening decades development in the Virgin Islands has continued almost unabated. Historically, it is likely that forest clearing for charcoaling, pasturing and other agricultural activities has been significant to its decline. In Puerto Rico at Cuomo, clandestine agricultural activities have added significantly to the threat to very small populations. More recently, forest alteration or removal associated with residential development has become a more important threat to its survival. Grazing activity, particularly on St. John where free-roaming goats and feral donkeys, along with growing population of introduced Key deer, is becoming a secondary threat. Prior reporting has suggested it mode of reproduction, dioecy (male and female flowers on separate individuals), given its rarity, as contributing to decline.

Background

The type specimen was collected by Eggers (No. 293) from Flag Hill, St. Thomas in 1880. Early assessments of population numbers were reported by Vivaldi and Woodbury (1981). Larger numbers of plants have been discovered in the 1990’s in various locations on publicly and privately owned real estate. The discovery in the early 1990’s of high densities yet fairly restricted spatial distributions on Cobb Gut by E. Gibney, Cobb Hill by G. Ray, and on Minna Hill by P. Acevedo – all within the Virgin Islands National Park, has spurred interest in more extensive surveys park wide.



Objectives

We include four separate actions to advance the knowledge of the ecology, genetics, and geographical distribution of Zanthoxylum thomasianum, and to further document key threats to its existence.

1. to survey known populations on St. John, and to search for new populations, primarily within the boundaries of the Virgin Islands National Park,

2. to characterize the demographic structure of the known populations, and to present an assessment of the composition of communities in which this species occurs,

3. to sample each population and conduct genetic fingerprinting analysis to determine levels of genetic variation overall, and to examine inter-population differences, and

4. to tag specimens permanently and record their positions to aid future surveys and conservation research.

Legal issues

Zanthoxylum thomasianum was Federally listed ”Endangered” in 1985 under the U.S. Endangered Species Act. Under the Virgin Islands Indigenous and Endangered Species Act (V.I. Code, Title 12, Chapter 2) Zanthoxylum thomasianum also is listed as “Endangered.” This research was conducting with the required Scientific Research and Collecting Permit from the National Park Service and notification and permitting under the V.I. Fish and Game under V.I. Code, Title 12, Ch. 2. This project contains inventory monitoring elements in its basic design that do not require significant land alteration. Therefore, we anticipate that this project will be in compliance with provisions of the National Environmental Policy Act of 1969, and the Federal Land Policy and Management Act of 1976. Future studies should include new surveys on Virgin Gorda and elsewhere in the British Virgin Islands, in addition to St. Thomas, and locations in Puerto Rico adjacent islands under its jurisdiction, including Vieques and Culebra.



Methods

Systematic surveys of VIIS and adjacent areas We visited five sites island-wide, four of which had either been documented sites prior to our study, or had been the product of sightings by botanists in the past. The fifth site (JBR), located on a ridgeline northwest of Johnson Bay, was established from prior work on Solanum conocarpum (Ray and Stanford 2005). At each site, upon encountering a Zanthoxylum thomasianum shrub, we fixed its position using a Garmin GPS III Plus hand-held device. All geographic coordinates were recorded as degrees, decimal minutes. We tagged each shrub sequencing identification numbers by site with a site code preceding it, e.g. PRGH-001 indicating the first shrub from Point Rendezvous – Gifft Hill. We measured the height of the plant to the nearest 0.1 m. Due to the multiple-stemmed habit of this shrub, we measured the diameter of the shoot base at the shoot-root interface. If the base was not a single piece (often the case), we measured the length and width to nearest cm of the entire collection of stems at their bases, and calculated its average during analysis. All shrubs were flagged in the field to facilitate further searching at each site, or to give warning to surveyors at sites undergoing active development activities. Seedlings (individuals having primary stems < 1.0 cm) were flagged in the field, but not tagged. If a reproductively mature female was proximal, its tag number was associated with the seedlings. Subsequent surveys will track surviving seedlings. Floristic composition and habitat characterizations were also performed at each site. We recorded a list of all woody and important herbaceous species in communities harboring Z. thomasianum. We noted species origin, (native or non-native), and noted the more dominant species in each site. We recorded canopy height, slope aspect, slope inclination, range in elevation, and estimated surface rockiness. Genetic analyses were conducted at the University of the Virgin Islands on leaf samples from each of the five populations. At least ten individuals were sampled from all populations except the tiny Johnson Bay Ridgeline population, from which we collected 3 of 6 individuals. Our geneticist and her teams developed protocols for DNA extraction (CTAB). Fragments were separated using agarose gel electrophoresis, followed by PCR amplification on the fragments. RAPDs (randomly amplified polymorphic DNA) were also performed. The lab calculated heterozygosity (H), percent polymorphic loci (P), and genetic distance (FST -according to Fei 1982) between populations. Heterozygosity (H) calculates the average amount heterozygosity (having two different alleles for a locus, as opposed to two of the same) found in a population. Polymorphism (P) estimates the proportion of polymorphic (having more than one allele) loci found in a population. Both of these are used to assess genetic variability, ranging from 0 –



1 (or 0 – 100%), of a population. Higher values are associated with greater hardiness, i.e. faster growth, higher disease resistance, and resilience in the face of sudden environmental change. Using ArcGPS, we produced digital maps of each of the five populations. We also produced a map of the entire island of St. John showing the locations of all five populations. Report manuscripts with graphics files and quantitative data files generated from this study will be archived on CD, with copies made available by the principal investigator to the project manager, (Chief of Resource Management, VIIS), the NRPC project coordinator, and the co-principal investigator. To assess community structure we recorded a list of vascular plants and categorized them by relative abundance. The coding system, summarized in the table below, includes “dominant”, “co-dominant”, “common”, “frequent”, “infrequent” and “rare”. Table 1. Summary of coding used to estimate relative abundance and spatial distribution of community associates in surveys for Z. thomasianum. Code Description DOM Dominant, significantly more abundant than nearest

associate; likely to occur in every placement of a quadrat of appropriate size

CoD Highly abundant; likely to occur in all but a fraction of quadrats, but less so than a dominant

COM Likely to occur in the majority of quadrats FREQ Likely to occur in numerous, up to 50%, of the quadrats INF Likely to occur in only a few quadrats Rare Unlikely to occur in sample quadrats by chance Data Management We will archive all data files on compact disks and submit them to the Division of Resource Management at the Virgin Islands National Park. These will include GPS coordinates, information regarding tag ID numbers, basal diameter and height measurements of every shrub tallied in all populations, and species lists at each site. These data should be updated as research progresses, in cooperation among the National Park Service, the University of the Virgin Islands, the U.S. Fish and Wildlife Service (lead agency mandated by US Endangered Species Act) and the authors.



RESULTS Overview Z. thomasianum populations on St. John were found to occur in dry scrub thickets and woodlands at elevations ranging from 29 (95 ft) to 310 meters (1015 ft), on slopes facing predominantly south to east or along ravines. Scrub (shrubland) communities in which this species is found exhibit canopies of approximately 2 to 3 m. Also, this shrub occurs as an understory element in dry semi-evergreen woodlands containing canopies of 4-7 m. While at least 10 populations were known prior to this study, we have added a new location on St. John - a small scattering of six individuals above Flannagan’s Passage (our moniker is Johnson’s Bay Ridge, JBR), which overlooks western Coral Bay. Mean abundance across five populations was 38 individuals, but population size varied sharply; ranging from 6 individuals at Johnson Bay Ridge to 112 at Point Rendezvous on Gifft Hill. Mean density for all sites was 35.3 individuals per hectare. Population areas averaged one hectare. However, the Point Rendezvous population at Gifft Hill is likely to be larger, as privately-owned land east of the surveyed area has not been searched systematically. Table 1. Population abundance and density across five populations of Z. thomasianum on St. John, U.S. Virgin Islands.

Site Population

size Population area (ha)

Density (indiv./ha)

PRGH 112 2.61 42.98 BDHT 22 0.43 50.97 COBSE 36 1.08 33.18 COBGUT 13 1.05 12.35 JBR 6 0.16 37.04 Mean 37.8 1.07 35.30

A size class distribution (Fig. 1) reveals the range in size and age of the 5 subpopulations. These demographic data show a bias in moderate to older age classes. There are few seedlings (0-1.9 cm) represented in these populations. This indicates a trend toward an aging population lacking recruitment of young individuals on an island-wide basis. The mean basal diameter across all populations was 7.5 cm, and shrubs attained a mean height of 2.4 m. Growth rate tends to be inversely proportional to stem wood density. Dense-wooded, slow-growing species such as Z. thomasianum in Virgin Islands dry forests have shown stem growth increments of approximately 1 mm or slightly less per year. Based on the mean stem base diameter, the mean age of the live shrubs is likely to be between 75 and100 years, with some bases suggesting 3-4 centuries of



growth. Despite this, most individual stems of this multi-stemmed shrub die prior to reaching 5 cm near their base. Although main stems on some shrubs reach 6 cm, this species often exhibits dieback of stems as they reach 3 – 4 cm basal diameter. Thus, the root-shoot interface yields the truest estimate of the age of the individual. Root sprouting also occurs on occasion, allowing new clonal shoots to emerge where canopy gaps exist. Moreover, the long, narrow “canes” often grow diagonally toward these canopy openings, reaching 3-4 meters in length. Fig. 1. Size class distribution (correlated with age class) in 2005 for 188 individual Zanthoxylum thomasianum shrubs across 5 populations on St. John, Virgin Islands.

0

5

10

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20

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0-1.92-3.9

4-5.96-7.9

8-9.910-11.9

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14.15.9

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18-19.9

20-21.9

22-23.9

24.25.9

26-27.9

28-29.9

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Basal Stem Size Class (cm)

Num

ber o

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uals

Genetic analyses of 11 loci revealed very low heterozygosity (HTMean = 13.8%) and polymorphisms (PMean = 43.6%) in samples taken from all five populations (Table 2). Both of these measures are key indicators of genetic diversity. Therefore, genetic variability in these populations is low, even when compared to other local rare species, including Solanum conocarpum, a St. John endemic. Polymorphism (P) values manifested by the two smallest of the five populations, those of Johnson Bay Ridge (0.182) and Cob Gut (0.273), manifest the lowest polymorphism values. Values of (P) from the three larger populations show a range from 0.455 – 0.727. Based on FST genetic distance analysis (sensu Nei 1982), the largest population at Point Rendezvous – Gifft Hill is most distinct from the four other populations, all at least 8 km to its east (Table 3). Point Rendezvous was most distinct from the Bordeaux Heights population, yet comparison of Bordeaux Heights with Cob Gut, which lies in a separate drainage to the southwest (Fig. 2), reveals no distinctions among the limited sampling of 11 loci. Cob Gut loci exhibited slightly



more affinity to the Bordeaux Heights population, than to a proximal population on the southeast slope of Cob Hill (COBSE), only a kilometer distant. Table. 2. Comparisons among five Z. thomasianum populations on St. John of heterozygosity (HT) and polymorphisms (P) derived from 11 loci.

H P

PRGH 0.196 0.545

JBR 0.136 0.182

COB GUT 0.111 0.273

COB SE 0.157 0.455

BDHT 0.088 0.727

(Mean) 0.1376 0.4364 The Johnson Bay Ridge (JBR) population appeared to have more affinity with the Cob Hill Southeast than with Cob Gut or Bordeaux Heights. These results suggest that topography has no controlling influence upon the distribution of Table 3. Pairwise genetic distance (FST) among five populations based upon 11 loci isolated from 45 samples.

PRGH JBR COB GUT

COB SE BDHT

PRGH 0 JBR 0.351 0 COB GUT 0.401 0.355 0 COB SE 0.229 0.194 0.127 0 BDHT 0.405 0.363 0 0.109 0

shrubs from population to population. By contrast, intra-population distribution, keenly manifested within the Cob Gut population that flourishes along the banks of a cascading ravine, may influence spatial distribution. Scarce observational information, and no published data, has identified the dispersal agent for the small (3-4 mm diameter) seeds of Z. thomasianum, which are incased singly within dry capsules. The author has spotted the Lesser Antillean bullfinch landing on one of the larger shrubs in Cob Gut, but no ripe fruits were visible on the plant at the time. Fresh water (and gravity) could play some role in dispersal during heavy rainfall events.


Fig. 2. Distribution of five Zanthoxylum thomasianum populations assessed in 2004-2005.


Some analysis of gender ratios of this dioecious (male and female flowers on separate individuals) species suggest an approximate 50:50 balance. Visitation by Apis mellifera, the introduced honey bee, is very high, and seed crops are copious in all five populations. Three of the five populations grow on land in private ownership, including the largest at Point Rendezvous on Gifft Hill. A total of 138 (74%) of individuals tagged in this survey derive from private property. Recruitment of new seedlings was very low in four of five populations, despite relatively large seed crops noted at all sites. Four of five populations representing 84% of 188 individual plants tagged, had produced a total of 8 (4.8%) seedlings. A fifth population at Bordeaux Heights produced 18 seedlings out of 28 individuals tagged or flagged. However, these seedlings are likely the offspring of only 4 parents. Survival to maturity of most seedlings is not assured. This shrub reaches reproductive maturity, i.e. generates flowers and fruits, at a stem size of 2 cm, which is approximately 20 years of age. According to these data, despite a paucity of new seedlings, the bulk of the population (60%) island-wide was “youthful” and productive (between 2 and 10 decades old). All five subpopulations of Z. thomasianum on St. John reside in plant communities containing very high richness and distributional abundance of indigenous species, and very low richness and abundance of exotic species. These diverse woody plant communities consist of many other species associated with relatively low anthropogenic disturbance. A notable fact about the compositional structure of Z. thomasianum communities is that Acacia muricata, a leguminous tree, is a dominant to very abundant community associate in all of them. Moreover, all habitats share at least one aspect of the land use history – none of the sites has ever been cultivated. Some of these sites have been coppiced (cut at or near the base) during the 20th century. Coppicing sharply changes many abiotic factors, including light regime, ambient and soil moisture levels and so forth. However, in contrast to clearing and cultivation, coppicing for charcoal is an activity typified by relatively low modification of the top strata of the soil profile across a hillside. Most coppiced trees of Caribbean dry forests survive the impact. Woody species here are adapted for regenerating new shoots in response to damage to their stems. Their intact root systems facilitate rapid shoot sprouting and maturation in place. Despite wholesale cutting, if the root zone is not altered, myriad existing mycorrhizal and bacterial mutualisms remain intact, and key biotic components of the community apparently facilitate rather rapid recovery. The role of Acacia muricata in this context in not well understood, but the fidelity of the rare Zanthoxylum thomasianum shrub to these isolated patches of Acacia woodland and scrub thicket is quite remarkable.



Damage caused by a shoot borer, was noticed on 2 plants at Point Rendezvous – Gifft Hill. This was not noticed at other sites. The stem borer warrants close monitoring, yet the major threat to these populations is residential development, which destroys trees directly or fragments the forest community, thereby severely diminishing habitat quality. Population Habitat Descriptions We summarize some environmental characteristics of the five sites harboring populations of Z. thomasianum in Table 3. Populations occurring on slopes represented greater than 93% of all individuals surveyed. All of these slopes were east-facing (Table 4). Slope inclination ranged from 12 to 34 degrees, except for the ravine population. All sites exhibited exposed rocks and boulders at values up to 70%. Z. thomasianum can thrive in near-pristine scrub thickets having canopies at 1.5-2.5 m in height. Although the species was not found in littoral stands near sea-level, on St. John it favors slopes above the sea spray zone from 30 to 300 meters elevation. Table 4. Selected physical characteristics of Z. thomasianum habitats at Point Rendezvous, Bordeaux Heights, Southeast Cob Hill, Cob Gut, and Johnson Bay Ridge, St. John, U.S. Virgin Islands.

Site Aspect Slope Inclination Surface

Rockiness Canopy Ht (m)

Range of Elev. (ft)

PRGH 1000 (E) 28 degrees 50-60% 1.5 - 6 351-700 BDHT 700 (ENE) 34 degrees 25-40% 2.0 - 2.5 940-1015 COBSE 1300 (SE) 28 degrees 50-60% 2.0 - 3.5 207-500 COBGUT Ravine banks 0 -2 degrees 60-70% 4.0 - 5.0 95-411 JBR 700 (ENE) 12 degrees 20-24% 2.5 - 4.5 409-635

Community Floristics and Similarity Plant communities harboring Z. thomasianum populations tend to be species-rich, and nearly absent of non-indigenous plants. (We give complete listings of woody species from each of the five sites in electronic appendices). Plant communities at all five sites exhibited a single leguminous tree, Acacia muricata, as the dominant canopy species (Table 5). Across five sites we tallied 118 species, only two of which were non-native. Seven other shrub and tree species were quite prevalent across the five sites. Three of the shrub species are classified in the same genus, Eugenia, of the Myrtaceae (Myrtle Family). Eugenia cordata was common or co-dominant at all five sites. E. ligustrina and E. pseudopsidium occurred at high abundance three of the five sites. Acacia, and members of the myrtle family, have been the topic of much research on fungal (mycorrhizal) relationships in soils of moderate to low nutrient levels. Fidelity of St. Thomas prickly ash to communities dominated by these taxa is



probably not coincidental. Four other species: a creeping, liana-like shrub of the Acanthaceae (Oplonia spinosa), an epiphytic bromeliad (Tillandsia utriculata), a tall shrub of the coffee family locally called “inkberry” (Randia aculeata), and black mampoo (Guapira fragrans) an evergreen canopy tree, are likely on our list due to their prevalence in most dry forest communities of the Virgin Islands. While numerous species are present across all five sites, the species we emphasize are common to quite abundant across these communities. Table 5. Species occurring in high abundance in at least two of the five communities we studied that harbor Z. thomasianum. Dominants are indicated with a score of 6; co- dominants = 5; common = 4. No. Species PRGH BDHT COBSE COBGUT JBR Totals

1 Acacia muricata 6 6 6 6 6 30 2 Oplonia spinosa 4 5 4 4 4 21 3 Eugenia cordata 4 4 4 4 4 20 4 Tillandsia utriculata 4 4 4 4 16 5 Eugenia ligustrina 4 4 4 12 6 Eugenia pseudopsidium 4 4 4 12

7 Guapira fragrans 4 4 4 12 8 Randia aculeata 4 4 4 12 9 Agave missionum 4 5 9

10 Maytenus laevigata 5 4 9 11 Brunfelsia americanum 4 4 8 12 Capparis flexuosa 4 4 8 13 Comocladia dodonaea 4 4 8 14 Reynosia guama 4 4 8 15 Serjania polyphylla 4 4 8

Similarity in floristic composition can be directly measure between any two communities on the basis of species comparisons or in terms of the relative abundance of those species. We estimated abundance, rather than produce direct measurements, so we present here (Table 6) a matrix of similarity consisting of pair-wise comparisons of the percent overlap in species presence across five sites. Using 35 or more of the most abundant species (all sites consisted of this minimum number of species estimated at “frequent” to “dominant) from each of the five sites, we found an overall mean similarity in floristic composition of 34.7% (Table 6). We find the greatest floristic affinities among the five sites between the community at Cob Gut and those of Point Rendezvous, Bordeaux Heights, and Cob Hill, ranging between 41 and 47% species overlap. Point Rendezvous and Cob Gut exhibited the most similar species lists among all other comparisons. Floristically, the Johnson Bay Ridge community was more distinct from all other sites.



Table 6. Pairwise comparisons of community similarity in floristic composition among five St. John populations of Z. thomasianum. Infrequent and rare species are omitted.

PRGH BDHT COBSE COBGUT JBR PRGH 0 BDHT 0.375 0 COBSE 0.381 0.353 0 COBGUT 0.467 0.408 0.429 0 JBR 0.342 0.313 0.339 0.254 0

Mean 0.391 0.358 0.384 0.254 Community Structure of Five Sites and their Z. thomasianum populations We describe the community structure of the five sites below, focusing upon attributes such as nativity, species richness, and dominance. Gifft Hill – Point Rendezvous The east-southeast (aspect = 1000) facing slope (mean inclination = 280) of Gifft Hill harbors the largest population of Z. thomasianum yet documented on St. John, containing 112 individuals in an area of approximately 10 acres. Regrettably, the site is presently under development as a residential subdivision (Fig. 3). Road construction and land survey cutting have already caused significant damage to the population. Thirteen shrubs (12% of the total) we documented had been cut by surveyors, and preliminary assessments of the effect of road building reveal that at least 15 shrubs were subsequently eliminated by bulldozing (Fig. 6). Fig. 3. Fresh slabs of concrete displace several individuals of Z. thomasianum at Upper Point Rendezvous residential development.



The forest ranks with the most diverse and pristine of any St. John dry forest tract on the Caribbean Sea side of St. John. Exotic plant species are few, and those species that do occur, “Tan tan” (Leucaena leucocephala) and “Guinea grass” (Urochloa maxima), and Scleria lithosperma, a small sedge are uncommon. The terrain is rocky. The surface rockiness greatly exceeds 50%, and very large rock outcroppings, some more than a half hectare in size and up to 10 m (33 ft) tall, are scattered throughout area. These outcrops and large boulders are strewn with bromeliads (Fig. 4a&b) Tillandsia fasciculata, T. utriculata, and T. lineatispica, and orchids (Epidendrum ciliare, Psychillis macconnellae and Tolumnia prionochila). The orchids and the sterile hybrid, T. lineatispica, are locally protected. The communities clinging to large boulders had mostly survived a thousand or more years of Amerindian occupation, the colonial era, and much of the contemporary period without human alteration. Fig. 4a. Epilithic herbs Tillandsia fasciculata (golden spikes) and T. lineatispica (red spikes) cling to boulder top sheared by excavators.

Fig. 4b. Epiphytic herbs Epidendrum ciliare (native orchid) and Tillandsia utriculata share an outcropping partially destroyed by road-building activities.

The canopy ranges from 5-6 meters at mid-slope (300 feet elevation) to less than 2 meters at shoulder-slope approaching the upper reaches (700 ft). Stem density is high and bole size of most tree species is rather small (5-8 cm), except the soft wooded, rapidly growing trees, such as Turpentine (Bursera simaruba). The community of woody and common herbaceous plants harboring this Z. thomasianum population is species rich. Discounting several herbs and vines most of which were indigenous, the authors counted 83 species on the site. Only two of them were non-native. Already some non-native grasses and herbs are colonizing the road shoulders, so Z. thomasianum habitat in any area left “green” is likely to decline in short order. The Z. thomasianum population here numbers 112, five (4%) of which are older seedlings. This is the largest and genetically the best endowed population we



surveyed. The mean basal diameter was 6.5 cm, and the mean height was 2.8 m, making it the tallest of the populations, likely owing to the 5 – 6-m canopy height in the lowest zone of elevation at the site. Additional prickly ash habitat warrants exploration on privately-owned land to the east of this population. Some natural threats were noted. Dieback on one or more stems was evident on a significant portion of the population. Two individuals looked dead one year following tagging and measuring. Shoot borers were noted on two trees near the shoulder of new subdivision road. However, despite clear signs of low seedling recruitment, seed crops were copious. Human-induced threats are acute at Point Rendezvous as road construction already has permanently converted forest habitat. While goat grazing is a secondary, low-level threat, clearing for residential development is presently in its early stages. Excavation for home sites and driveways is likely to triple the area of lost habitat, further reducing population size (Fig. 6). Ultimately, as much as half of the forested area will be removed, and the remainder fragmented into pieces of less than a quarter acre. Edge effect, the increase in light penetration from perimeter areas of small wood lots, will encourage weed infestation, further reducing habitat quality. Fig. 5. Careless construction practices magnify problems of soil erosion, pollution with concrete waste, and weedy infestations of forests not yet altered by home building at Point Rendezvous.



Fig. 6. Distribution of Z. thomasianum at the upper Point Rendezvous subdivision, on the east flank of Gifft Hill, St. John. Residential development is ongoing. Road construction subsequent to our rare plant survey has obliterated 15–20 individuals, as depicted by yellow dots (proportional in size to the number of shrubs at a given coordinate) within the estate road clearing, and a new home site (bottom right).



Bordeaux Heights The Bordeaux Heights population lies between 940 and 1015 ft. (300m), and is divided between a small population on the ridge crest to the west (Fig. 7), and a tightly packed subpopulation in a near-pristine scrub thicket. The population consists of 22 tagged individuals, 12 of which are large, tagged seedlings. Six additional seedlings were flagged, but not tagged, near mature shrubs. Fig. 7. Distribution of Z. thomasianum shrub locations at Bordeaux Heights subdivision, St. John, VI. The easternmost subpopulation is densely packed in a scrub thicket; boxes indicate multiple shrubs per GPS coordinate (expressed in thousandths of minute, longitude and latitude).



Thus, 18 of 28 (64%) of the individuals we encountered are seedlings. Apparently, this population is rapidly expanding. Density of the spatial distribution of prickly ash shrubs is the highest among the five populations. The scrub thicket contains a Z. thomasianum population of 16 tagged individuals (of the total of 22 tagged individuals) on the Parcels 6 and 5. The physiognomy is low, below two meters, and is perhaps the most densely-packed vegetation on the island. Non-native species were absent from this evergreen thicket, characterized chiefly by Acacia muricata, three shrubs of the Myrtle Family, including Eugenia pseudopsidium, E. ligustrina and E. cordata, combined with a tight mat of a native bromeliad, Aechmea lingulata. Maytenus laevigata, and guama (Reynosia guama), both dense-wooded tree species, were also common. A scrambling, spiny liana (Oplonia spinosa) was also highly abundant. A second Z. thomasianum subpopulation is contained in low-statured, dry semi-evergreen woodland. It is located west of the scrub thicket (parcel numbers unknown), situated on a shoulder slope facing south (Fig. 7). This community has a taller canopy (4 m), and is less densely wooded. Recent surveying activity at Bordeaux Heights has damaged this population, as three individuals were chopped clean at 30 cm from the base. Five seedlings were flagged near the adults. The population is evenly divided between very young shrubs and quite old shrubs, but most of the mature individuals seem to be productive female plants. Heterozygosity of 0.088 is extraordinarily low. Of 10 plants sampled for genetic analysis, 7 came from the scrub thicket and 3 from the low woodland. All except one sample came from older, reproductively mature shrubs. Transfer of alleles from other populations is apparently needed. Fig. 8. Margins of Bordeaux Heights habitat, view to south.

Absence of weeds in the scrub thicket, and a modest encroachment of non-natives in the woodland segment of these Z. thomasianum communities has likely kept community integrity high. However, all individuals are under imminent threat of destruction by excavators as land speculation slows, and residential development resumes. The past decade has seen no homes built. Further excavation for subdivision roads, driveways and home sites looms in the future.



Horses owned by a neighboring landowner have browsed and grazed the woodland. Increasing densities of introduced key deer have also been observed on this site. Access by free-roaming goats and feral donkeys is also wide open. Cob Hill (Southeast) Fig. 9. Distribution of Z. thomasianum population at Cob Hill, southeast-facing slope, Estate Concordia, St. John, within the boundaries of the Virgin Islands National Park. The northwest face is in semi-shadow, at left. Boxes indicate multiple occurrences of Individual shrubs.



Cob Hill is the location of a one-hectare permanent forest monitoring plot established one decade ago by the Smithsonian Institution’s Biodiversity Program in collaboration with this report’s first author and in cooperation with the National Park Service (Fig. 9). More than 8,000 stems of 2.0 cm and greater were measured and tagged within the plot. No woody exotics were encountered. This NPS-owned property may contain the most pristine scrub thicket ever documented in the region. We measured and tagged 36 Z. thomasianum shrubs on this site. This is the largest NPS-protected population. Hundreds of hectares of potential habitat may exist between Cob Hill and Minna Hill. We focused on areas proximal to shrubs encountered in the past two decades. Also, further searches to the northeast may turn up more individuals. Most of the prickly ash shrubs we encountered on the southeast slope of Cob Hill were outside the plot boundaries. The range in elevation of the population was 207 to 500 ft, and the mean height of the shrubs was 1.6 meters. The mean basal diameter of the stems was 13.2 cm, shifting the demographic profile to the older age classes. Moreover, the proportion of the individuals with multiple stems was the highest among all populations. It is possible that prickly ash was among those species coppiced for charcoal early in the 20th century. A charcoal pit lies in the center of the slope, and Acacia muricata, Coccoloba microstachya and several other harder wooded species are multi-stemmed, showing direct evidence of being cut near the base. Few seedlings were found. Only two of 36 (5.6%) tagged individuals were seedlings. Most shrubs in fruit during our survey or at the time of subsequent visits late in the fall rainy season of 2005 exhibited large seed crops. We tallied 59 plant species in the community harboring this population. The most abundant species were the canopy dominant, Acacia muricata (densest stand of the five sites), and Croton fishlocki, a rare subshrub, which dominated the ground layer. Croton fishlocki is known from only two sites in the U.S. Virgin Islands, but is also present in the British Virgin Islands. There are likely more than one thousand of these shrubs on the hillside. The community is a mostly a scrub thicket, but some patches of low woodland appear at mid-slope. A few large rock outcrops add to the diversity of microenvironments on the otherwise homogenous shrubland. A tree cactus, Pilosocereus royenii, a few species of Capparis, a seagrape relative (Coccoloba microstachya), Christmas bush (Comocladia dodonaea), the ever-present Eugenia cordata are quite common. The herbaceous perennial monocot -- century plant (Agave missionum) – has been all but destroyed by an introduced Mexican agave weevil (Scyphophorus acupunctatus). This community has seen recent intrusions by goats and donkeys, and should be fenced off for protection.



Cob Gut Fig.10. Distribution of Z. thomasianum population at Cob Gut is scattered over the lower reach of an intermittent stream flowing into Mandahl Pond, lower left. The summit of Cob Hill is in the lower center. A proximal population lies on its southeast flank.



The Cob Gut population occupies the primary flood terrace of an intermittent stream that drains the east flank of Minna Hill (Fig. 10). At Cob Flats the stream bed has a narrow flood terrace not more than 1 to 3 meters on either side at base of sloping terrain to the east and west of the gut. As it departs the flats flowing south it cascades over large boulders down more than 100 meters of topographic relief, leveling off about 500 meters north of Mandahl Pond, into which it empties. The entire Z. thomasianum population of Cob Gut lies inside Park boundaries. Most plants are on the stream banks at the edge of the stream. Two trees are on an island in the stream. A third individual, labeled CobGut #1, lies in the stream bed against the base of a boulder. The populations numbers 13 shrubs averaging 2.4 m in height, having a mean basal diameter of 6.5 cm. Only one of the 13 plants (7.7%) is of seedling size. All plants appear to be vigorous, despite their low levels of heterozygosity. Our genetic data suggest that there are some close affinities between Cob Gut and the Bordeaux Heights population situated above it and to the northwest. The community along Cob Gut numbers 60 trees, shrubs, lianas and more conspicuous herbs. It is dominated by Acacia muricata, but the other abundant species are more evenly distributed spatially. Three Eugenia shrubs, E. cordata, E. pseudopsidium, and E. ligustrina are very common. Four more myrtles, Eugenia biflora, E. sessiliflora, E. monticola, and guavaberry (Myrciaria floribunda) are frequent. Agave missionum is common in some areas, as are Brunfelsia americana, and water mampoo (Pisonia subcordata). Christmas bush (Comocladia dodonaea) is very common locally around large boulders or along the stream bed. Four orchid species were recorded: Vanilla barbellata, Epidendrum ciliare, Psychilis macconnelliae, and Tolumnia prionochila. It appears that more habitat may exist on the east flank of Minna Hill. Access is to this area is difficult, but a focused effort would likely turn up more prickly ash. Explorations further upstream along Cob Gut would likely produce more plants as well. This population’s tiny size may be a threat to its existence, yet it appears to be otherwise free of the human threats posed at other sites. Few weeds have invaded. Fig. 11. East flank, Minna hill.



Johnson Bay Ridge Fig. 12. Distribution of Z. thomasianum population (yellow dots) at the ridgeline overlooking Johnson Bay, St. John. A new subdivision under construction at Flannagan’s Passage infringes upon, and includes, the population (center-right). The boundary of the Virgin Islands National Park is west and south of the ridgeline (center).



The Z. thomasianum population at Johnson’s Bay Ridge is the smallest of the five, consisting of 6 individuals thus far (a seedling was found recently close to the cluster of four depicted in Figure 12). The population lies on an east facing slope gently inclined (12 degrees) in its lower reaches, but rising steeply toward a ridgeline. The Park boundary lies west of the population a few tens of meters from a ridge top shrub labeled JBR #3. Shrubs in this population are between 410 and 635 ft elevation. None are seedlings, although two of them are small (2.0 cm at the base). Fig. 13. Forest interior below Johnson’s Bay Ridge, north of Flannagan’s Passage subdivision. Litter layer is very thick, typical of sites dominated by Acacia muricata.

The community is species rich and very diverse, as the topography is varied. Sixty-eight species were tallied. Many of them were abundant and distributed broadly. Acacia muricata dominated, as it did in the remaining four sites. Agave was common, and often destroyed by weevil attack. The usual myrtles, and Brunfelsia, joined by three caper species were abundant. The canopy height varied from 6 meters to 3 near the ridgeline. This population has most of its members within surveyed parcels destined for home sites (Fig. 13). Developers have been notified, but ex situ propagation followed by some restoration higher on the slope or inside Park boundaries is probably warranted.



Fig. 14. View to west toward Park boundary along Ridgeline above Johnson’s Bay. Acacia muricata, a leguminous tree, dominates the community.

The substrate appears to be distinctive, with lighter soils, fewer large boulders, and deeper litter horizon than the other sites. Much of the forest to the north and east, most of which is privately owned proximal to the population we studied, requires further exploration for various rare plants. Fig. 15. Road construction on steep hillside fragments Zanthoxylum thomasianum habitat, and that of rare plant other populations in an area adjacent to Park boundaries.



Management and Recommendations This survey has broken new ground through the establishment of a baseline database on which more intensive and extensive surveys may be performed in the future. Moreover, genetic analysis has given us insights into a general lack of genetic variation across all populations. This work facilitates management steps, particularly how to approach ex situ and in situ conservation measures germane to the Virgin Islands National Park in the near future. We offer some general recommendations below.

A. As we gain experience with patterns in community structure across these populations, future surveys should be more efficient. The prevalence of dense stands of Acacia muricata, very visible at a distance for much of the year, will assist us as we search for new populations. East facing slopes from Saunders Gut to Gifft Hill along the south side of St. John are fair game for survey, as long as A. muricata is a major community component.

Further surveys internal and external to Park boundaries.

B.

Our original intention was to conduct a region-wide survey, but that notion was impractical due to difficulty in accessing scrub thickets on any large spatial scale. Nevertheless, few sites have been documented thus far in the British Virgin Islands other than Gorda Peak and its south and east flanks. Puerto sites at Piedras Chiquitas, Guajataca, and Coamo, should be revisited soon. Monte Pirata, on Vieques and hillsides on Culebra may well harbor intact populations.

Future surveys of BVI and Puerto Rico

C.

As residential development proceeds at Point Rendezvous, Bordeaux Heights, and Flannagan’s Passage below Johnson’s Bay Ridge, the urgency has increased for conservation actions such as seed collection and off site propagation. The Virgin Islands National Park will be well served by an effort to increase the genetic variation of park populations of prickly ash by collected seed from larger populations on private land outside the Park. Z. thomasianum is slow-growing. Hand-crossing from population to population in the wild is ideal for increasing heterozygosity in more genetically marginal populations. The advantage is that the parents (F1) are ready at stage one to make seeds for further propagation. The alternative, collect seed from one population, germinating them and propagating them to maturity is time-consuming. Off site propagation is essential either way.

Ex situ conservation

D.

There is no easy way to assure that free-roaming ungulates can be kept away from Park rare plant populations, other than fencing them off. Esthetically unappealing as it may be, if horse or goat owners refuse to cooperate, and as key deer populations continue to rise, the Park needs to consider some

Protecting the best populations in the VINP from feral grazers



contingencies in this regard. The Cob Hill population is the most important one in the Virgin Islands National Park. Herds of goats use the area frequently, and donkeys have unfettered access as well.

E. Mycorrhizal relationships are likely a key to the integrity of communities harboring Zanthoxylum thomasianum. The connection to the Myrtaceae is a mystery at this point, but the answer may be easy to find. Other edaphic factors, such as soil pH and macronutrient levels, are also likely to be important. These factors should be integrated into survey work. It will help us immensely on both ends of the conservation arena, on-site (habitat) conservation and off-site (propagation) efforts.

Examine edaphic factors

F.

Three of five Z. thomasianum populations we document on St. John occupy sites destined for residential development. By means of covenants, conservation easements or fee simple purchase, small fractions of the real estate from these new subdivisions currently occupied by our endangered shrub may be protected. Mitigation measures sufficient to conduct useful conservation in this instance require not only sampling seeds from intact plants prior to their destruction, but a segment of intact forest

Land protection initiatives

on-site

in which to plant prickly ash seedlings propagated off-site. This process could be promulgated by the National Park Service by means of accepting research proposals designed specifically to implement this specialized conservation work on St. John. The ultimate destination for many of these seedlings would be the small or weakened Z. thomasianum populations within the Virgin Islands National Park, as well as the privately owned “reserves within a subdivision” in locations such as Point Rendezvous, Bordeaux Heights, and Johnson Bay Ridge above Flannagan’s Passage. Of particular interest in this conservation scheme are: Parcels 5 & 6 and unknown parcels harboring the westernmost subpopulation at Estate Bordeaux Heights, the easternmost parcels of the Upper Point Rendezvous development on the Fish Bay flank of Gifft Hill, the westernmost parcels above Flannagan’s Passage, and portions of Cob Hill (within Park boundaries proximal to the public road, Rt. 107).

References Acevedo-Rodriguez, P. 1996. Flora of St. John, U.S. Virgin Islands. Memoirs of

The New York Botanical Garden, Vol. 78, Bronx, NY, USA. 581 pp. Ray, G.J. and A. Stanford. 2005. Population genetics, propagation and

reintroduction of Solanum conocarpum, a rare shrub of St. John, U.S. Virgin Islands. Final Report to National Park Service, PMIS No. 49192. Virgin Islands National Park, 18 pp.



Dallmeier, F., J.A. Comiskey and G. Ray. 1993. User’s Guide to the Virgin Islands Biosphere Reserve Biodiversity Plot 1, U.S. Virgin Islands. The Smithsonian Institution/Man and the Biosphere Biological Diversity Program, Washington, DC.

Dallmeier, F., J.A. Comiskey and G. Ray. 1995. User’s Guide to the Virgin

Islands Biosphere Reserve Biodiversity Plot 2, U.S. Virgin Islands. The Smithsonian Institution/Man and the Biosphere Biological Diversity Program, Washington, DC.

Densmore, D. 1986. Endangered Plants of our Caribbean Islands: A Unique

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Information Exchange. Virginia Tech. Available at http://fwie.fw.vt.edu/WWW/esis/ (Accessed 8/03/03).

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6-15. January. Little, E. L., Jr., R. O. Woodbury, and F. H. Wadsworth. 1974. Trees of Puerto

Rico and the Virgin Islands: Agriculture Handbook No. 449. U.S. Department of Agriculture, Forest Service. Washington, D.C.

Ray, G.J., F. Dallmeier and J.A. Comiskey. 1998. The structure of two

subtropical dry forest communities on the island of St. John, U.S. Virgin Islands. In Dallmeier, F. and J.A. Comiskey, (eds) Forest Biodiversity in North, Central, and South America, and the Caribbean: Research and Monitoring, Man and the Biosphere Series, Vol. 21. UNESCO and the Parthenon Publishing Group, Camforth, Lancashire, UK.

Ray, G.J. and B.J. Brown. 1995a. The structure of five successional stands in a

subtropical dry forest, St. John, U.S. Virgin Islands. Caribbean Journal of Science 31(3-4):212-222.

Ray, G.J. and B.J. Brown. 1995b. Restoring Caribbean dry forests: evaluation

of tree propagation techniques. Restoration Ecology 3(2):86-94. USFWS. 1985a. Proposed Endangered Status for Caribbean Plant. Endangered

Species Technical Bulletin 10 (3): 4. USFWS. 1985b. Proposed Endangered Status for Zanthoxylum thomasianum

(Prickly-ash). Federal Register 50 (28): 5647-5650. Monday, February 11.



USFWS. 1985c. Final Rule to Determine Zanthoxylum thomasianum (Prickly-ash) to be an Endangered Species. Federal Register 50 (245): 51867-51870. Friday, December 20, 1985.

USFWS. 1986. Caribbean Plants. Endangered Species Techn. Bulletin 11 (3): 4. USFWS. 1988. St. Thomas prickly ash recovery plan. U.S. Fish and Wildlife

Service. Atlanta, Georgia. USFWS. 1990. Endangered and Threatened Species Accounts. U.S. Fish and

Wildlife Service, Division of Endangered Species Vivaldi, J. L., and R. O. Woodbury. 1981. Zanthoxylum thomasianum (Krug and

Urban) P. Wilson. Status report submitted to the U.S. Fish and Wildlife Service. Mayaguez, Puerto Rico.

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