The Sand Dune Ecology of the Palisadoes, Kingston Harbour, Jamaica

BULLETIN OF MARINE SCIENCE, 73(2): 507–520, 2003

THE SAND DUNE ECOLOGY OF THE PALISADOES, KINGSTON HARBOUR, JAMAICA

Heather P. Thompson and Dale F. Webber

ABSTRACT The Palisadoes is a 16 km tombolo forming the south border of the Kingston Harbour, characterized by dune vegetation on the southern windward side and mangrove thickets on the northern leeward side. The coastal vegetation there was investigated and characterized using four transects. Research was limited to the windward side where the little studied sand dune community exists. Vegetation attributes investigated along the transects were species composition, height, percentage vegetation cover, vegetation life form and leaf size class. Environmental parameters and variables assessed were relative humidity, air temperature, wind velocity, light intensity and salt spray. Edaphic parameters assessed were soil temperature, soil moisture, soil salinity, soil nutrient content and organic content. The sand dune community of the Palisadoes exhibits zonation indicated by change in percentage cover, height and species composition. The three zones identified (strand beach, strand dune and strand thorn-scrub) are characterized by different dominant species assemblages. Strand beach zone was characterized by a Sporobolus sp., Gomphrena sp. and Sesuvium sp. assemblage, the narrow strand dune zone by replacement with a Capparis and Calliandra assemblage and the strand thorn-scrub by further addition of a dominant Acacia-cacti assemblage. The presence of particular zones in each transect was influenced by different climatic and edaphic conditions with salt spray, soil moisture, organic content, and soil water content being important factors. Two endemic species were identified, Calliandra pilosa and Opunitia jamaicensis, compared with five in a 1953 report. Recent road realignment and widening/resurfacing development as well as environmental theft have increased the pressure on the rare and endemic species found in this sand dune community. Continued vegetation loss could impact on dune stability and integrity of Palisadoes and associated communities within Kingston Harbour.

Most coastal plants endure harsh living conditions with the combined effects of high salinity, temperatures and wind speed as well as substrate mobility. Curiosity as to how these plants adapt to such conditions has been the basis of investigations worldwide. However, except for significant studies carried out on mangrove communities, information on other coastal vegetation is lacking in the tropics. Waisel (1972) cited that studies on coastal halophytes are more descriptive for inventory purposes. This has been the case in Jamaica in the works of Asprey and Robbins (1953), Loveless and Asprey (1957). Investigations in recent decades have carried studies beyond the descriptive level in re- sponse to the need for more in-depth ecophysiological studies (Thompson, 1997). This study was aimed at describing the strand dune vegetation of the Palisadoes and associated environmental parameters. The supra-littoral (marine influenced land) communities on exposed shorelines across Jamaica may be broadly categorized into sandy beaches or limestone shelves. The Palisadoes spit is a 16 km strip (Fig.1) with sand and gravel as its substratum and illus- trates aggregations of distinct plant communities. The sand dunes are low at the mouth of the Hope River and gradually rise to the western end towards Port Royal. Physical stability of the spit depends on replenishment by coarse sediments eroded from the Hope, Cane

507 508 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 2, 2003

Figure 1. Map of Kingston Harbour showing Palisadoes tombolo, sample transect locations Pal 0– 3, Port Royal and the International Airport situated on the tombolo. and Yallahs Rivers, which enter the sea to the east of the spit. The presence of sand dune flora initiates and consolidates sand deposition, dune formation and stability. Dune flora typifies the vegetation found on the Palisadoes (Chapman, 1939). Pioneer species are followed by Acacia sp. scrub, which gives way on the harbor side to the mangrove thicket. While the major development on the Palisadoes has been the town of Port Royal and the Norman Manley International Airport, other significant changes have occurred to alter the original topography and vegetation. Landscaping connected to the airport’s ongoing beautification, has resulted in the removal of vegetation and the replanting of other species along the strip. A mining quarry is in operation at the eastern end of the spit on the harbor side and the access road to Port Royal has been raised and widened recently. In this particular operation, vegetation from both sides of the road was cleared significantly, removing mangrove and other natural strand vegetation species. The objectives of this study were to describe the sand dune community of the Palisadoes and identify the factors influencing the zonation, hence the stability of the tombolo.

MATERIALS AND METHODS

COASTAL VEGETATION SURVEY — PROFILES.—The study of the structure of vegetation and vegetation systematics defines vegetation ecology (MuellerDombois and Ellenberg, 1974) and concerns itself with the interactions between the environment and vegetation. Field studies were undertaken during August to December 1995, after a number of preliminary assessments were conducted. Gradient analysis has been employed in previous coastal studies (Randall, 1970a,b); whereby, gradients of vegetation and environment are described and examined to discover relationships be- THOMPSON AND WEBBER: SAND DUNE ECOLOGY OF THE PALISADOES 509 tween environmental variations and changes in the coastal vegetation. Gradient analysis of vegetation, particularly coastal vegetation, requires belt transects to be laid orthogonal to the shoreline. One 20 ¥ 5 m and three 50 ¥ 10 m transects were established at the Palisadoes, Kingston and named Pal 0, 1, 2, and 3 respectively (Fig. 1). Each belt was divided into 5 ¥ 5 m squares. Assess- ments were made in contiguous quadrats along the length of the belt, noting species presence, height and their relative abundance in terms of percentage cover. The decision in selecting individual sites was done subjectively to accommodate any obvious variability in the vegetation inland from the shoreline. Delimitation of sample sites was done with lengths of rope and a compass to ensure consistency in angle measurements. Transects began at the highest water mark, with assessments made every 5 m in a contiguous fashion. The percentage cover, debris and bare ground were estimated for each quadrat. Species were identified as far as possible in situ. Those species that could not be identified in the field were transported to the U.W.I. Herbarium for confirmation. The recorded flora were categorized according to life form (after Raunkier, 1934). The definition of the leaf characteristics of a community is valuable in distinguishing it from other communities. Two features of the foliage were considered: leaf size and leaf shape. For compound leaves, the size of the leaflet was assessed instead. This was done using the spectrum of Raunkier (1934). Estimates of height and percentage cover were made for each species represented in each quadrat. The percentage cover is the percentage of ground within the quadrat covered by a vertical projection of the aerial parts of each species. By combining percentage cover of each species with height and distance at which it is found in each transect, zonation patterns are distinguishable. The information derived above was used to construct vegetation profiles representing each transect. The horizontal axis represents the ground distance from the start (0 m) to the transect end (50 m) while the vertical axis represents plant height. Based on numbers and size, the more dominant species can be represented. ENVIRONMENTAL AND EDAPHIC DATA.—A number of environmental parameters were measured in the center of each quadrat along each transect. These were air temperature relative humidity and light flux density at ground level. At each quadrat, three consecutive readings were taken so that an average value could be obtained. Wind measurements were obtained with a Sper Scientific an- emometer 84003 (± 0.5 m s-1), relative humidity and air temperature with a Bacharach Inc. (12- 9015) whirling psychrometer (± 1 % R.H. and ± 0.5ºC). Light intensity (in this case, interception of light at the ground) was measured with the Hobolite™ light meter (± 10 mE m-2 s-1). Salt spray was determined by erecting wooden stakes at 5 m intervals for 50 m. Hangers stretched into a diamond shape, were covered with cheesecloth and secured to the tops of the stakes. After a period of 48 hrs the cloths were removed and analyzed for salt content with a YSI salinity probe in mmho/20 ml (± 0.5 %). Slope of land was determined using an inclinometer at 10 m intervals to identify the dune area. Fresh soil samples collected from all sites were transported to the laboratory and stored in cool, dry cupboards until analysis. Samples for water content were collected using a 20 mm diameter soil core while soil samples for other analyses were collected using a trowel from the upper 80 mm of the soil. Water content and organic content were determined gravimetrically on portions of fresh samples using a Mettler balance ± 0.001 mg. Water content was determined by repeated weighing of 10 g portions of soil after oven drying for 24 hrs at 60ºC. Organic content was determined by repeated weighing of 10 g portions of soil after exposure to 450ºC for 3 hrs. Values were expressed as percentages. Twenty grams of fresh soil from each quadrat was placed in separate conical flasks containing 20 ml of distilled water. The flasks were placed in a shaker for 24 hrs. After allowing time for settling, the supernatant was decanted and salinity measured with a YSI salinity probe in mmho/20 ml (± 0.05 %). Nitrite + nitrate-N concentrations were determined using the Cadmium Reduction method. This was measured using automated versions of the sulphanilamide-diazo colorimetric method at a wavelength of 550 nm by a Technicon™ autoanalyser (Technicon Instruments Corporation, 1972a). OrthoPhosphate concentrations were also determined using automated versions of the phosphomolybdenum colorimetric method at a wavelength of 880 nm in a Technicon™ autoanalyser (Technicon Instruments Corporation, 1972b). 510 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 2, 2003

Figure 2. Vegetation profile at Pal 1 for the 50 m transect. Species identification by numbers is given in Table 5.

RESULTS

VEGETATION PROFILES.—Nanophanerophytes were the most dominant life form category at the Palisadoes, consistent with observations by Waisel (1972). While climbers were not represented along the Palisadoes, succulent herbs and grasses were always present, but not always dominant. Variations in height, canopy coverage, density and species composition contribute to the unique characteristics of various transects. Thus, mesophytes, geophytes, therophytes, and epiphytes were not represented. The transects represented a typical coastal dune profile according to Davis (1942) with the following zones represented; strand beach, strand dune and strand scrub mixed with a thorn thicket (after Asprey and Robbins, 1953). At Pal 0 the only species present were a few pioneer species including Sesuvium sp. and Sporobolus sp., occupying a very narrow zone. Thereafter, the majority of the area consisted of bare ground. The width of the tombolo at Pal 0 was quite narrow (< 50 m) with the roadway occupying most of the space, thus zonation of species was not observed. However, zonation was apparent in Pal 1–3 (Figs. 2,3,4), with a pioneer zone followed by a narrow strand dune zone, then an extensive thorn-scrub association. While the herb layer (Sesuvium sp., other prostrate runners, grasses and short herbaceous plants) was well established throughout these transects, the Acacia sp. thorn scrub vegetation type was the most dominant association. The strand beach or pioneer zone extended to 0–15 m inland and was dominated by succulent creepers and other pioneer species such as Sporobolus sp. and Gomphrena sp. This zone had less leaf litter and more bare ground than the strand dune and strand thorn- scrub. The narrow strand dune zone between 15 and 20 m was evident by change in percentage cover and height at sites Pal 1 and Pal 2. However, zonation was more evident at Pal 3 by an introduction of additional species to form a Conocarpus sp. and Calliandra sp. assemblage mixed with Acacia sp. and the strand thorn-scrub zone further evidenced THOMPSON AND WEBBER: SAND DUNE ECOLOGY OF THE PALISADOES 511

Figure 3. Vegetation profile at Pal 2 for the 50 m transect. Species identification by numbers is given in Table 5.

by a change in dominance to the Acacia-cacti assemblage. This Acacia-cacti assemblage, while being dominated by the legumes Acacia sp. and Calliandra sp., distribution of other species such as the columnar cacti (Opuntia spinossisima) was fairly regular. Cepalocereus swartzii and Stenocereus hystrix also combined with Acacia sp. to form the distinctive vegetation type at Pal 1and Pal 2. In general, plant height increased with distance over Pal 1 and Pal 2. However, vegetation at Pal 3 showed constant height that offered more coverage because of the single layer canopy provided by the Acacia sp.

Figure 4. Vegetation profile at Pal 3 for the 50 m transect. Species identification by numbers is given in Table 5. 512 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 2, 2003

Figure 5. Variation in total vegetation percentage cover, percentage soil water content, soil salt content in mmho/20 mL and salt spray in mmho/20 mL with distance along 50 m transect at Pal 1.

Throughout transects Acacia sp. heights were consistent while cacti heights were vari- able (Figs. 2,3,4). Percentage cover showed a general increase in the first few quadrats and maintained a maximum thereafter at Pal 1 and Pal 2 (Figs. 5,6). However, at Pal 3, percentage cover varied after 30 m, falling to below 70% at two quadrats along the transect (Fig. 7). Analy- sis of change in species composition and individual species percentage cover confirmed the presence of three zones at Pal 1, Pal 2 and Pal 3 (Tables 1,2,3). At Pal 1 the strand beach zone occurred between 0–10 m and was most influenced by the presence of salt spray and little soil water (Fig. 5). The Strand dune zone occurred between 10 and 25 m and was most influenced by a small increase in water content. The Strand thorn-scrub was the largest zone and extended from 25–50 m and was characterized by reduced water content and increased soil salinity (Fig. 5). At Pal 2 the Strand beach zone was only 5 m where salt spray was maximum. The Strand dune zone was extensive (5–20 m), but with few species present (Fig. 3). Increased height and species composition signaled the Strand thorn-scrub zone (20–50 m) with an increase in percentage soil organic content (Fig. 6). At Pal 3 percentage cover rather than height and species composition indicated the limits of the zones as Pal 3 was the most developed and stable of the transects along the Palisadoes.

Figure 6. Variation in total vegetation percentage cover, percentage soil water content, percentage soil organic content, soil salt content in mmho/20 mL and salt spray in mmho/20 mL with distance along 50 m transect at Pal 2. THOMPSON AND WEBBER: SAND DUNE ECOLOGY OF THE PALISADOES 513

Figure 7. Variation in total vegetation percentage cover, percentage soil water content, soil salt content in mmho/20 mL and salt spray in mmho/20 mL with distance along 50 m transect at Pal 3.

The Strand beach zone was comprised of tall shrub like vegetation with 50% cover at the waters edge increasing to 90% cover at 20 m where soil salinity and organic content increased. Strand dune community stabilized substrate and gave way at 35 m to a strand thorn-scrub community with further increases in organic content and small, but continu- ous increase in water content. The leaf size indicated that, generally, leptophylls were most represented. This is reflected in the vegetation that is observed at the Acacia sp. thorn scrub that characterizes the xerophytic vegetation at the Palisadoes (Tables 1,2,3). Pal 1, 2 and 3 together represent the vegetation of the Palisadoes tombolo with Pal 1 and Pal 3 being most similar, with a Jaccard similarity index of 52% (Table 4). Eight species were common to all three transects and represented 40, 53 and 73 % of total species number in each. No transect had more than 20 species.

Table 1. Species list and percentage cover for each species with increasing distance for the 50 m transect at Pal 1, on the Palisadoes tombolo.

D50istance in m 15102520353045405 Percentage cover per species B0are ground 709520302 D5151ebris Sporobolus virginicus 215 40 5 4 G10rass 1 155 Gomphrena decumbens 300 3 Canavalia maritima 55 Opuntia spinossima 15 Calliandra pilosa 500 3 Stenocereus hystrix 151013 Acacia tortuosa 300 4560703 Commicarpus scandens 5560 025106 Pithecellobium unguis-cati 10 Cassia emarginata 5 Melocactus communis 5 Capparis ferruginea 150 Stigmaphyllon emarginatum 200 102 Lippia alba 10 514 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 2, 2003

Table 2. Species list and percentage cover for each species with increasing distance for the 50 m transect at Pal 2, on the Palisadoes tombolo.

D50istance in m 15102520353045405 Percentage cover per species B0are ground 555125 D0ebris 25 5510 025 551 Ipomea pes-caprae 1 Stigmaphyllon emarginatum 55 Gomphrena decumbens 590 98959090207070709 Calliandra pilosa 155 10 52540 04071 Sporobolus virginicus 5 Capparis ferruginea 515525 0101015 Tephrosia purpurea 1 Acacia tortuosa 100 15 Stenocereus hystrix 501010351025 Guiacum officinale 55 Commicarpus scandens 5

Table 3. Species list and percentage cover for each species with increasing distance for the 50 m transect at Pal 3, on the Palisadoes tombolo.

D50istance in m 15102520353045405 Percentage cover per species B0are ground 504015 535 0 50 01 D105ebris 15155 5 5 Conocarpus erectus 30 G0rass 2 1 Commicarpus scandens 5515 0 G5555rass 3 1 Acacia tortuosa 350 106525201011 Pithecellobium unguis-cati 5030302520603 Sesuvium portulacastrum 305 303060306010304 Opuntia jamacinensis 1 Jatropha gossypifolia 1 Capparis ferruginea 5015 Canavalia maritima 15 Stenocereus hystrix 1115 0 1 Sporobolus virginicus 10211 15 Gomphrena 5 Pithecellobium s0p. 1 Calliandra pilosa 50101 Cyperus oxylepis 15 G5rass 4 Melocactus communis 1 Portulaca olereaceacea 1 THOMPSON AND WEBBER: SAND DUNE ECOLOGY OF THE PALISADOES 515

Table 4. Jaccard Correlation Coefficient, Species numbers and species numbers common to transects.

T3ransects 1 and 2 T3ransects 1 and Transects 2 and J4CC 424. 522. 29. S82pecies # common to transects 17

ENVIRONMENTAL/EDAPHIC DATA.—Humidity conditions at Palisadoes were low (65–70% R.H.) and constant at Pal 1 and Pal 2 with small fluctuation at Pal 3. Ambient temperature at Palisadoes values ranged between 31–35ºC at all sites confirming that xeric conditions prevailed at Palisadoes, but could not explain vegetation zonation observed. Salt spray 1 values decreased exponentially from the land/sea interface (31 mmho 20 ml- at 0 and 10 1 m) in the strand beach zone towards the strand thorn-scrub zone (1.5 mmho 20 ml- at 50 m) and appeared important in establishing the strand beach community. Wind speed was at a maximum of 4.5 m s-1 nearest to the coastline. Beyond 15 m, however, there was a sharp decline in wind speed at all transects. Light flux density values varied with distance, but with no pattern to the variation. Between 25–50 m, there was no measurable light flux density indicating the presence of a high percentage cover from vegetation with a developed canopy. Edaphic factors such as relative salinity, percentage water content and percentage organic matter changed along the transects. Although changes seemed small, they occurred at distances corresponding to the vegetation changes indicative of zonation discussed above. Generally, lower soil salinity values were recorded near the sea and soil salinity increased with distance from the sea at Pal 1 and Pal 3. Percentage organic matter increased along the transects especially at Pal 2 and Pal 3. Organic matter content is related closely to the vegetation present, in that it is generated by the presence of vegetation. Graphs of percentage cover and organic matter for the respective transects were similar. With increasing distance from the high water mark, sparse cover gave way to short shrubs and trees at the 10–15 m mark. These plants shed their leaves, which degrade to humus,

Table 5. Species list for the Palisadoes. Species numbers listed here represent the species occurrence on Figures 2, 3 and 4 (vegetation profiles).

Neumber Srpecies nam Neumbe Species nam 1 Acacia tortuosa 16 Ipomea pes-caprae 2 Calliandra pilosa 17 Jatropha gossypifolia 3 Canavalia maritime 18 Lippia alba 4 Capparis ferruginea 19 Melocactus communis 5 Cassia emarginata 20 Opuntia spinossisima 6 Commicarpus scandens 21 Pithecellobium unguis-cati 7 Conocarpus erectus 22 Portulaca oleraceacea 8 Cyperus oxylepis 23 Opunitia jamaicensis 9 Gomphrena sp. 24 Sesuvium portulacastrum 10 Gomphrena decumbens 25 Sporobolus virginicus 111 G6rass 2 Stenocereus hystrix 122 G7rass 2 Stigmaphyllon emarginatum 133 G8rass 2 Tephrosia purpurea 144 G9rass 2 Cephalocereus swartziii 15 Guiacum officionale 516 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 2, 2003

the main component of organic matter. From this point to the transect end, the organic matter content remained consistent, as did the vegetation cover. Soil nitrates and phos- phates were constant and low (0.5 mM nitrate and 0.03 mM phosphate), or just above or at the detection limit of the methods employed throughout the transects. Low nutrients and water content along with the high, but constant soil temperature (36–40ºC) placed em- phasis on the xerophytic harsh sand dune conditions.

DISCUSSION

Zonation of species was evident at the Palisadoes. Sand dune zonation of coastal communities has been described by Davis (1942), Asprey and Robbins (1953), and Oosting (1954). The transect data serve to corroborate the zonal distribution of species and to show the relationship of these zones to edaphic and topographic features. STRAND BEACH ZONE OR PIONEER COMMUNITY.—The pioneer zone occurs above the high water mark, and the vegetation is characterized by halophytes and psammophytes (Asprey and Robbins, 1953) at sandy shores. Other terms attributed to this zone at sandy shores are strand beach association (after Davis, 1942), sand strand (Oosting, 1954), coastal strand (Beard, 1944) and sandy ephemeral tide mark communities, (Doing, 1985). Here, perennial trailing herbs and grasses exhibit stoloniferous habit, which is important in promoting embryonic dune formation. Colonies act as a barrier to intercept and hold water and wind transported sand. Plants of this zone also have the ability to be propa- gated by vegetative fragments and tolerance of these fragments to salt water. The success of such plants as pioneer species is also attributable to tolerance to salt spray, sand scour- ing and burial, high substrate temperature and low levels of soil nutrients (Leonard and Judd, 1997). Randall (1970a,b) supported the view that salt spray is the major factor controlling coastal vegetation. Clayton (1972) alludes to this as well, noting the contribution to mineral influxes to the coastal ecosystem being sufficient enough to affect vegetation distribution as well as form and function. The term ‘aerohalophyte’ has been coined to accommodate salt spray-affected vegetation. This term could apply to some of the vegetation at Palisadoes. Studies done by Boyce (1954) confirm that coastal dune sands, especially silicate sands (as that found at Palisadoes), typically possess little available nutrient ions, and proposed that the chief source of nutrients is salt spray. Plants common to this zone indicate that zonation of dunes is seral. It is important to note that even though percentage cover of species in the pioneer zone is relatively sparse compared with other zones on the dune, their presence is no less important in maintaining the integrity of this most unstable area of the dune. Therefore, even without the few species at Pal 0 the integrity of the tombolo, and by extension, the Harbour, could be compromised. It has been argued that the distinctive appearance of a community is due to the preva- lent leaf type of its component species. Therefore, leaf characteristics are important in distinguishing one community from another. It is generally accepted that there is a greater incidence of succulence of pioneer species on sandy shores like the Palisdoes, for example Sesuvium portulacastrum. Smaller leaf size classes predominate this close to the shoreline (Asprey and Robbins, 1953) and this was noted. On the other hand, it is also possible for the leaf size class to decrease with distance as evidenced at the Palisadoes THOMPSON AND WEBBER: SAND DUNE ECOLOGY OF THE PALISADOES 517

site. Leaf size is severely reduced in the Thornscrub zone characterized by the Acacia- cacti association, compared to the preceding pioneer and dune zones. STRAND DUNE ZONE OR SERAL COMMUNITY.—Asprey and Robbins (1953) argued that the series of sand communities postulated for the Florida Keys by Davis (1942) could not be found in Jamaica due to alteration. On the south coast of Jamaica, for example, the coastline is modified by the invading thornscrub. This is exemplified at the Palisadoes site. The dune plant community is characterized by thickets of both herbaceous and woody plants, namely that have largely replaced coastal pioneers. Since the dune is where soil accumulates, it provides a more stable environment than that of the shifting sands of the beach. As a result of the stability of the dune, larger shrubs and trees are found. Most distinctive about the dune community is the littoral hedge or dune thicket. Here, plants are predominantly woody ranging from 0.5–3 m high and clustered in a hedge running parallel to the sea. This hedge may consist of a single species or several species combined. Organic matter content also tends to be low near the water’s edge. Generally, high salinity and soil temperatures combined with low nutrient levels severely inhibit the growth of plants, without which, there can be no organic matter. With increasing distance inland, the percentage organic matter increases as vegetation cover increases as reflected in the results. Another consideration to be made is that, many of the plants in the xeric coastal habitat respond to unfavorable seasons by leaf shedding. This leads to accumulation of leaf litter, which takes a relatively longer time to decompose. Pal 1 and Pal 3 showed similar patterns of low organic matter up to 10 or 15 m (pioneer zone) and rising to maximum levels thereafter. However, organic matter content at Pal 2 remained fairly constant throughout the transect. Species composition at this transect also indicated the presence of Gomphrena, sp. Calliandra sp. and Capparis sp. distributed throughout. This pattern was not evident at the other sites and it could be said that organic matter content is one factor responsible for distribution of species, and hence zonation. THORN-SCRUB ZONE OR CLIMAX COMMUNITY.—This climax community consisting of an assemblage of Acacia-cacti, Capparis sp. and Calliandra sp. species represented the dominant the vegetation type. The soil salinity, temperature and water values reflect the harsh conditions that exist and ability of plants to adapt to those conditions. It is known that Acacia sp. and Calliandra sp. species have root systems that reach deep layers of the soil. Thus, plants with shallow root systems would not be expected to dominate such a habitat unless they are ephemerals and are able only to survive a short life span during the rainy season. Of the three zones, the thorn-scrub community occupied the greatest area at transects Pal 1–3. The difference from the preceding zone is that hardly any herbaceous species were found here. Climax communities are tremendously valuable to ecosystems. They represent stability and complexity, and if disturbed, take the longest time to regen- erate. This fact has implications for the continued stability of the Palisadoes tombolo, because of activities such as road widening and the harvesting of cacti species without efforts at replanting. These activities effectively ensure the removal of species that char- acterize this zone, or alter species composition by removal of select species. From the floristic analysis, general characteristics of the tombolo’s vegetation and habitat were observed. This includes the xeric habitat type and vegetation expected to be found there due to successful adaptation strategies. Therefore, plants are mostly woody, have small or no leaves, do not achieve extensive height and for the most part are nanophanerophytes. In a specific sense, however, differences in local conditions give rise 518 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 2, 2003 to the zones observed. The species list for the area includes two endemic species (Opun- tia jamaicensis and Calliandra pilosa). All plants on the dune are affected by anthropo- genic activities, but more so the cacti. Only one individual of the species was observed. Asprey and Robbins (1953) had reported five endemic species characteristic of sand dunes, which suggests that species diversity on the dunes has been adversely affected through the loss of species. It is not known how many, or if any, other species have disappeared since the 1953 assessment. In general, Asprey and Robbins’ observations nearly five decades ago still stand. The beach and strand dune are still characterized by herbaceous dominants and some woody species merging into the climax community of the thorn- scrub assemblage. The pioneer zone did not extend beyond 15–20 m along the Palisadoes. SALINITY, NUTRIENTS AND COMPETITIONS.—The question as to whether beach plants are xerophytes or halophytes has been debated. Kearny (1904) argued that beach plants are xerophytic and not halophytic. Recent authors (Barbour, 1970; De Jong, 1979) reviewed the definition of the term ‘halophyte’. It could be said that even though coastal plants exhibit xeromorphic characteristics in general, they may be specialized as halophytes, or more specifically ‘aerohalophytes’ (Waisel, 1972). Contradictions have arisen (Barbour, 1970), which led him to conclude that strand species such as those along the Palisadoes appear to be intolerant halophytes. The coastal habitat investigated in this study, could be what Grime (1977) classed as ‘nutrient deficient habitats’ that manifests in a reduction in growth form of herbaceous species and tendency of xeromorphy in woody species. Generally, nutrient levels of dune soils are very low (Lee and Ignaciuk, 1985) and there is debate as to the most significant source of nutrients to the coastal vegetation. Another contributing factor to the low nutrient status of beach and dune sand is the low water holding capacity of the medium. Nutri- ents can be available to plants only when dissolved in solution. If water retention is low, then sands are left dry most of the time, and nutrients, even if present are not in the available form. The role of competition in zonation has been discussed (Wilson and Keddy, 1986). Competitive interactions between annuals and perennials also need to be regarded in the role in species distribution. Boorman’s investigations on sand dunes (1982) reveals that growth strategies of annuals, biennials and perennials differ and it is still being debated whether species coexist and compete, or coexist because they do not compete. Pioneer species at Palisadoes, for example, were mostly annuals that have relatively higher growth rates and completed their life cycles before the unfavorable season. However, responses are seen to cover a wide range and the competitive strategies can hardly be predicted for any community, although a few generalizations can be made. It is clear that vegetation type is influenced by the complex interaction of environmental and edaphic factors. These factors can interact in such a way as to reveal distinct zonations in plant communities. This is evident in the sand dune vegetation at Palisadoes. Attempts have been made to correlate edaphic and climatic factors to changes in vegetation and to identify possible gradients (Wilson and Keddy, 1986). Vegetation gradients usually reflect environmental gradients and zonation represents a static gradient. Two obvious trends at the Palisadoes are the decreased influence of salt spray with distance and the increase of organic matter content with distance. It could be that salt spray is more responsible for providing nutrients to the pioneer community than is organic matter. The pioneer zone is most influenced by salt spray, while the strand dune and thorn-cacti THOMPSON AND WEBBER: SAND DUNE ECOLOGY OF THE PALISADOES 519 association are influenced by a combination of soil water content and organic matter content. Therefore, it may be argued that the harbor, being partly bounded by the tombolo, is dependent on the stability of the tombolo for its very existence. Should the tombolo be breached by natural and or man-induced causes, it is reasonable to expect changes in the dynamics of the harbor. The tendency has been to focus on mangrove species when con- sidering vegetation associated with the harbor. However, due recognition should also be given to vegetation on the windward side. It has been noted that westwards along the tombolo, the dunes get larger, species number increases, and vegetation height and density increases. Their presence is, therefore, critical to the stability of the tombolo. As such, more studies are needed to investigate the dynamics of this and other tropical sand dunes. The entire Palisadoes strip forms part of the Port Royal-Palisadoes Protected Area declared by the Natural Resources Conservation Authority in 1998. It is hoped that with this designation, the importance of the vegetation to the stability of the tombolo, as well as the importance of preserving its contribution to the overall aesthetics will be given priority.

LITERATURE CITED

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ADDRESSES: (H.P.T.,D.F.W.) University of the West Indies, Department of Life Sciences Kingston 7, Jamaica.