The Plant Communities and Environmental Gradients of Pitcairn Island ...

Annals of Botany 92: 31±40, 2003 doi:10.1093/aob/mcg106, available online at www.aob.oupjournals.org

The Plant Communities and Environmental Gradients of Pitcairn Island: The Signi®cance of Invasive Species and the Need for Conservation Management N . K I N G S T O N 1 , * and S . W A L D R E N 2 1Department of Botany, Trinity College, Dublin 2, Ireland and 2Trinity College Botanic Gardens, Dublin 6, Ireland Received: 20 November 2002 Returned for revision: 21 January 2003 Accepted: 7 March 2003

Quantitative surveys of the vegetation of south-east Polynesian Islands are rarely undertaken owing to time and logistical restrictions; however they are fundamental in determining the conservation status of fragile island ecosystems. The aim of the research was to document quantitatively the vegetation of Pitcairn Island by investigating whether clearly de®nable plant communities existed on the island, and the underlying environmental gradients in¯uencing these communities. Initially, 10 3 10 m quadrats were taken from all areas of the island, with environmental parameters recorded for each quadrat. The vegetation was then mapped from high altitude vantage points. Two-way indicator species analysis was used to identify distinct plant communities, and canonical correspondence analysis was used to determine the underlying environmental gradients. The vegetation consists of 14 plant communities: four coastal, six forest, two fernland and two scrub communities. Large areas are covered by non-native scrub vegetation, and by monospeci®c Syzygium jambos (rose-apple) plantations. Less than 30 % of the island is covered by native forest, and these areas are limited to remote valleys. Fernlands also cover large areas, including both eroding areas and ridge tops. Coastal vegetation comprises rock and cliff communities with limited strand vegetation. The major environmental gradient affecting the composition of the plant communities is altitude, but anthropogenic in¯uences also have a large effect, owing to forest clearance and introduced species. The light environment is affected by the canopy species, and determines what ground ¯ora can develop. Identi®cation of distinct plant communities has allowed for a system of nature reserves to be suggested, which conserve all of these plant communities and a signi®cant proportion of the threatened plant species. ã 2003 Annals of Botany Company Key words: CCA, canonical correspondence analysis, classi®cation, conservation, invasive species, islands, multivariate analyses, ordination, south-east Polynesia, TWINSPAN, two-way indicator species analysis, vegetation description, vegetation mapping.

INTRODUCTION In the past, south-eastern Polynesian islands, and tropical islands in general, have been surveyed primarily for their ¯ora, with relatively few quantitative vegetation surveys being undertaken. This is due to the comparatively large amount of time required for vegetation studies, in contrast to ¯oristic surveys that simply involve compiling species lists. The scale of this lack of knowledge on the plant communities is shown in Mueller-Dombois and Fosberg's (1998) informative work on the vegetation of the tropical Paci®c islands. Whilst these authors included information for most of the south-eastern Polynesian islands, in many cases this is now out of date and based on descriptions written by Fosberg following the Mangarevan Expedition of the Bishop Museum in 1934, without any quantitative analysis (e.g. Society Islands, Austral Islands, Pitcairn Islands). Other non-quantitative surveys include those by Paulay and Spencer (1989) for Henderson Island, Stoddart (1975) for Aitutaki mainland and motus, and Stoddart and Sachet (1969) include notes on the vegetation of Rangiroa in their survey of the island's geomorphology. Vegetation * For correspondence. Fax +353 1 6081147, e-mail nkingston@ duchas.ie.

maps for Polynesian islands, such as those found in Papy (1951±1955) and in Florence (1993), are small-scale and, in some cases, based on conjecture. Quantitative analyses of vegetation have been carried out more recently in the Cook Islands (Merlin, 1991; Franklin and Merlin, 1992; Franklin, 1993) and in the Pitcairn Islands (Waldren et al., 1995). Franklin (1993) used remote sensing in an attempt to determine vegetation types for Miti'aro in the Cook Islands, but concluded that this technique was inappropriate for oceanic islands as it did not delimit vegetation types to adequate levels and was too expensive. Merlin (1991) and Franklin and Merlin (1992) used transects through wooded makatea (platforms of raised coral limestone) to describe the plant communities based on the dominance of tree species. This successfully determined the plant communities and the relative importance of each tree species within the communities described, thus determining the dominance of native species in the makatea. However, this strategy would prove unsuccessful over more diverse habitats, where tree and shrub species are not always present, and where differing substrates also feature. Waldren et al. (1995) used 10 m2 quadrats in a partially strati®ed sampling procedure, sometimes along transect lines, but with the aim of sampling from all areas and from

Annals of Botany 92/1, ã Annals of Botany Company 2003; all rights reserved

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Kingston and Waldren Ð Plant Communities on Pitcairn Island

F I G . 1. Location of the Pitcairn group of islands. The detailed map of Pitcairn Island includes the locations of places mentioned in the text.

all habitat types. In addition, quadrats were deliberately placed over rare species, so that the ¯oristic associations of rare species would be recorded. This strategy proved successful at delimiting vegetation groups for all plant communities on Henderson and Oeno islands, but as no environmental parameters were measured, only limited interpretation of ecological effects was possible. Study area

Pitcairn Island, the only inhabited island in the British Overseas Territory of the Pitcairn Islands, lies just south of the Tropic of Capricorn, about half way between New Zealand and South America (25°4¢S, 130°06¢W; Fig. 1). The island is very small, being only 4 3 2 km, and with the highest point at 347 m. Since the island is an oceanic high island (formed by volcanic activity at the sea-bed) its terrain is very rugged, with the soil and underlying rock being of volcanic origin. The coast is surrounded by cliffs formed by wave action and erosion, rising steeply to the highest point. Only 10 % of the island is level ground, an area of about 550 hectares; the remainder comprises slopes with a 20±45° angle, and much steeper in the remote valleys. The sub-

tropical climate provides a mean annual rainfall of 1716 mm, but with considerable annual variation. Mean temperatures range from 17 to 28 °C in the summer, and from 13 to 23 °C in the winter, with the winter being wetter and windier (Spencer, 1995; records published in Pitcairn Island Miscellany). There are no permanent springs on the island, but during periods of high rainfall ephemeral streams ¯ow in several valleys. The ¯ora is largely derived from that of south-eastern Polynesia, but is comparatively depauperate, due to the remoteness and the young geological age of the island (Florence et al., 1995; Kingston, 2001). The native ¯ora consists of 81 native species (including ten endemic taxa) and 250 introduced species, with 18 species threatened globally and 51 species threatened on Pitcairn Island itself (Kingston, 2001). Previous vegetation descriptions of Pitcairn Island are limited to brief notes made by scientists whose main interest was either plant collecting or studies of Henderson Island (St. John, 1987; Twyford, 1958; Waldren et al., 1995). All of these studies describe the highly disturbed nature of the island vegetation, with extensive areas of introduced species. Lintott, who visited the island in 1957, noted that the island was almost entirely covered in gardens or

Kingston and Waldren Ð Plant Communities on Pitcairn Island Syzygium jambos (L.) Alston (rose-apple) plantations, and any abandoned areas were choked with Lantana camara L. thicket, or were eroding badly (Lintott, pers. comm., 1998). Waldren et al. (1995) described several of the habitat types found, although they did not carry out a quantitative survey. They described poorly developed coastal communities, highly disturbed lowland vegetation with many non-native species, and mid-altitude slopes dominated by monospeci®c stands of S. jambos. Native forest was dominated by the endemic tree Homalium taypau H.St.John, with more diverse tree ¯oras in the remoter valleys. In addition, Waldren et al. (1995) suggested that the anthropogenically in¯uenced communities, such as Dicranopteris linearis (Burm.) Underw. fernlands, were spreading, as were scrub habitats. This was based on the fact that D. linearis was described as rare and local in 1934 (St. John, 1987). The aim of this study was to document quantitatively the vegetation of Pitcairn Island by investigating whether clearly de®nable plant communities exist on the island, and the underlying environmental gradients in¯uencing these communities. Knowledge of the plant communities and their species assemblages can then be utilized to: (1) determine the signi®cance of introduced species in de®ning the extant plant communities; (2) determine the most signi®cant threats to the island's native habitats; (3) assess the conservation status of the native vegetation; and (4) determine the habitat requirements of rare and endemic taxa, and thereby improve the success of future speciesspeci®c conservation activities MATERIALS AND METHODS Vascular plants were recorded from 10 m2 quadrats sampled across the island, using similar techniques to those used by Waldren et al. (1995). The sampling strategy had two main objectives, ®rst to describe the plant communities from across the island using randomly positioned quadrats, and secondly to determine the plant communities in which all endemic and threatened taxa occurred by using selectively positioned quadrats. In total, 83 quadrats were taken, and environmental data were recorded at each site. Fewer quadrats were taken from highly disturbed areas, as the diversity was low and the quantitative work was primarily for identi®cation of the plant communities in the areas of native vegetation. All species present in the quadrats were identi®ed and their associated abundance values were recorded using the DOMIN scale (Kent and Coker, 1992), with the approximate cover of tree canopies being projected vertically and estimated. Bare rock, litter and bryophyte cover were also recorded. Environmental parameters were recorded for each quadrat to determine putatively any underlying ecological controls on the vegetation; these included altitude, aspect (transformed using the technique of Beers et al., 1966) and slope. Photosynthetically active radiation (PAR) and red : far-red ratio (R : FR) were recorded from the centre of the quadrat using vertically positioned sensors (Skye Instruments, Powys, UK). A soil sample was taken also from the centre of the quadrat, and this was analysed for loss on ignition (LOI) and pH. It would have been desirable to

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investigate a wider range of environmental parameters, but this was constrained by the problems of transporting equipment and the requirements of other expedition activities. As aerial photographs were unavailable, an overall vegetation map for the island was drawn by simply recording the vegetation types by eye from high altitude vantage points, when all areas had been explored in the ®eld and a detailed knowledge of what was present had been obtained. The vegetation types were chosen from the dominant species present, as suggested by Kuchler (1967), and this resulted in a basic map showing the dominant species of each vegetation type being compiled. This map was then adjusted, where necessary, once analysis of the quadrats was complete. The area under each habitat type was then calculated from the vegetation map. Data analyses employed multivariate techniques including two-way indicator species analysis (TWINSPAN) for community classi®cation (Hill, 1979), and canonical correspondence analysis (CCA; ter Braak, 1986) for ordination. For TWINSPAN, nine pseudospecies were de®ned using the DOMIN scores of 0, 1, 2, 3, 4, 5, 6, 7 and 8. The environmental parameters were tested using Spearman rank correlations, but none were found to show any signi®cant intercorrelations. Preliminary analyses of the data included all samples and species. However, it was found that if rare species (here de®ned as species that occurred only once in the data set) were removed, the resulting analyses were more readily interpreted and gave higher Eigenvalues in the CCA. According to Legendre and Legendre (1998), rare species have little in¯uence on the analysis, do not determine the vegetation structure, but may cause ordinations to be arti®cially aligned. PC-ORD for windows version 4´01 (McCune and Mefford, 1999), Data Desk (Data Description Inc., Ithaca, New York) and Microsoft Excel 97 were used for the data analyses. Maps were compiled using ArcView GIS version 3´1. RESULTS Vegetation classi®cation

TWINSPAN analysis revealed eight interpretable vegetation groups (Table 1). The ®rst major division separated inland quadrats (0; 67 quadrats) from all coastal quadrats (1; 16 quadrats). Group 00 contained quadrats taken in S. jambos-dominated forest. This group had a high incidence of native species (over 80 %) in the understorey, although S. jambos formed an almost monospeci®c canopy. These quadrats had the highest LOI (26´9 %), litter cover (median DOMIN value 9) and moss cover (median DOMIN value 3) values for the island, indicating a high organic component. The light environment in group 00 quadrats was also extreme, with the lowest mean PAR levels (7´4 mmol m±2 s±1) and mean R : FR (0´34). These values are the result of a dense canopy casting heavy shade. Group 01 contained a total of 62 quadrats, which could be subdivided into ®ve major groups. The ®rst division separated inland forest (010) from inland scrub (011) habitats. Group 01000 described forest dominated by H.

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TA B L E 1. Summary of the percentage occurrence of the main indicator species referred to in the text within each of the TWINSPAN groups

* Introduced species. Percentage occurrence of species in quadrats within the TWINSPAN group: d, 81±100; d, 61±80; d, 41±60; d, 21±40; d, 1±20.

taypau, containing a high proportion of ferns (59 % of species recorded) and epiphytes. Quadrats from this division were spread across the island, and H. taypau was the main indicator species for this group. Group 01001, which was distributed primarily across the north and east of the island, contained quadrats with species-rich native mixed forest, usually dominated by Pandanus tectorius Parkinson ex Z or Hibiscus tiliaceus L. Group 0101 contained native, species-rich, Metrosideros collina (J.R.Forst. & G.Forst) A.Gray forest quadrats, containing species such as the trees M. collina, H. taypau and Glochidion spp. and the ferns Davallia solida (G.Forst) Sw., Doodia media R.Br. and Arachnoides aristata (G.Forst.) Tindale. Quadrats in this group were all from high altitudes, always above 100 m and generally above 200 m. This group also had the lowest bare rock cover for

the island (median DOMIN value 0), and the highest R : FR values for forest recorded on the island (median 0´84), suggesting an open canopy with much light penetration allowing a diverse ground ¯ora to develop. Cluster 011 described all of the inland low scrub communities and subdivided into two main categories, one containing all of the ridge quadrats and the other de®ning quadrats from scrub and eroding slopes. All of the quadrats were at mid-altitudes, and were characterized by high numbers of non-native species and open unshaded conditions (mean R : FR of 1´41, mean PAR of 1317´4 mmol m±2 s±1, both of which are the highest for the island). The ridge quadrats were de®ned by very high numbers of nonnative grasses and shrubs (74´6 % of species recorded were non-native), in particular invasive species such as Sorghum sudanense (Piper) Stapf. and Conyza bonariensis (L.)

Kingston and Waldren Ð Plant Communities on Pitcairn Island Cronquist. In the scrub category, eroding areas that were being re-colonized were characterized by the ferns Dicranopteris linearis and Davallia solida, and the woody species M. collina and Psidium guajava L. All of the groups in Group 1 were coastal sites. Group 10 included all quadrats from the south coast of the island, with the indicator species being Asplenium shuttleworthianum Kunze, Asplenium obtusatum G.Forst. and Plantago major L. These were typical of the fern-dominated rocky coastal communities, although some had high numbers and abundances of grasses and weeds present, such as Eleusine indica (L.) Gaertn. This group showed the lowest mean LOI (5´44 %), mean litter cover (DOMIN value 2´6) and mean slope (68´7), but the highest mean pH (7´24). Group 11 contained all of the northern coastal sites, and separated with the indicator species Cocos nucifera L. and Crinum asiaticum L. This group showed the highest pH for any of the islands communities (6´99), but also the lowest mean altitude (only 9´2 m above sea level). Quadrats in this group contained low numbers of species, with an average of only six per quadrat and a high incidence of bare rock cover (median DOMIN value 9). Ordination of quadrats and environmental variables

The results of the CCA of the quadrat data are presented for both the site and species scores (Fig. 2), with the TWINSPAN clusters superimposed. The CCA analysis was carried out to optimize the site scores. The ®rst three axes were interpreted as follows: axis 1 Eigenvalue 0´626, 46´1 % variance explained; axis 2 Eigenvalue 0´442, 32´5 % variance explained; axis 3 Eigenvalue 0´291, 21´4 % variance explained. A Monte Carlo test of 100 randomizations was carried out to test the robustness of the analysis. Ordinations presented here use the linear combinations of variables (LC) scores as recommended by Palmer (1993) and McCune et al. (2002). The Monte Carlo analysis showed the CCA Eigenvalues to be extremely robust (P = 0´01 for all axes), but the species±environment correlation to be less robust, particularly for axis 3 (axis 1, P = 0´06; axis 2, P = 0´01; axis 3, P = 0´64). This suggests that no single environmental factor, but rather a combination thereof, is signi®cant in de®ning the variability within the species data. The plot of axis 1 against axis 2 (Fig. 2) shows distinct clusters for each of the TWINSPAN groups. The highest axis 1 values are for quadrats in the S. jambos-dominated forest, mid-values for the forest quadrats containing native tree species (Homalium taypau, Hibiscus tiliaceus, M. collina) and scrub communities, culminating with the lowest values for coastal communities. Mid-values also have many of the understorey species associated with the diverse native forest communities, such as Davallia solida, Psilotum nudum (L.) Beauv. and Asplenium shuttleworthianum. Bidens mathewsii Sherff, Phymatosorus scolopendria (Burm.) Pic.Serm. and Pandanus tectorius ordinate between the scrub and coastal communities as these are species that occur in both inland and coastal habitats. Typical scrub taxa such as L. camara, Psidium guajava and Conyza bonariensis also ordinate among the scrub communities. Axis 2

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shows a transition from forest and coastal communities at the lower and mid-scores to scrub habitats at the high scores. Several environmental parameters are highly correlated with each of the three axes. Axis 1 is highly positively correlated with LOI, altitude, and percentage litter cover, whilst R : FR, pH and percentage bare rock are highly negatively correlated with this axis (P < 0´001; Table 2). R : FR, PAR and altitude are highly positively correlated with axis 2 (P < 0´001), whilst pH, LOI and percentage litter cover are negatively correlated. Percentage bare rock cover is positively correlated, and pH and LOI are negatively correlated with axis 3 (P < 0´001). Thus, the forest communities have the highest LOI and percentage litter values, with values increasing with axis 1 score. The M. collina forest group occurs at the highest altitudes, and thus increases in abundance with increasing axis 1 score. The red : far-red ratio increases with decreasing axis 1 score but also with increasing axis 2 score, thus increasing towards the coastal, ridge and scrub communities. Lower R : FR values were found in the Homalium taypau, Syzygium jambos, Hibiscus tiliaceus and Pandanus tectorius-dominated forest communities, which have the higher axis 1 scores and lower axis 2 scores. Vegetation map

A vegetation map for the island is presented in Fig. 3. This map was compiled from the original visual mapping corrected to correspond with the plant communities identi®ed quantitatively. TWINSPAN separates M. collina and H. taypau forest, but these are grouped on the vegetation map. In most cases, these two species grew together, with M. collina becoming dominant at higher elevations. The map also delimits Ficus prolixa G.Forst. (banyan) trees and settlement areas, where no attempt was made to de®ne these cultivated plant communities. In many cases, coastal cliff areas can be considered as coastal rocky communities, with sparse coverage of vegetation, but we were unable to sample from these areas. Nearly 30 % of the island is covered by monospeci®c stands of the two invasive species L. camara and S. jambos. DISCUSSION Forest communities

Metrosideros collina forest is found at high altitudes in the southern and western valleys and especially around the highest peak of the island. This community is also characterized by a high epiphytic and epilithic cover, commonly Peperomia spp., Vittaria elongata Sw., Trichomanes spp. and mosses. Thus, the native cover in these forests is high (over 80 %). Around High Point (see Fig. 1 for locations of places mentioned in the text) these forests are regularly covered in cloud, and the M. collina trees here have aerial roots to trap moisture, equating the vegetation to the `cloud forest' and `montane rain forest' biomes described by Mueller-Dombois and Fosberg (1998). These forests have a low density canopy owing, in part, to the small leaves of M. collina, allowing the development of

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F I G . 2. CCA (canonical correspondence analysis) axis 1 plotted against axis 2, with sites plotted in A and species plotted in B. Site TWINSPAN (two-way indicator species analysis) groupings and environmental variables are shown in A, while indicator species are labelled in B. Diamond, Syzygium jambos forest; black square, Homalium taypau forest; dark grey square, species-rich native forest; light grey square, Metrosideros collina forest; dark circle, scrub and eroding slopes; dark grey circle, ridge vegetation; black triangles, southern coast; dark grey triangles, northern coast; +, species; x, environmental variable.

the characteristically rich ground ¯ora and epiphyte cover. This is shown by the high R : FR ratio for a forest community, and the lowest bare rock cover. The large

number of tree species also allows for a more diverse habitat than in some of the monospeci®c forest communities. This community resembles the native forest on the island prior to

Kingston and Waldren Ð Plant Communities on Pitcairn Island human settlement, with a mixed forest dominated by M. collina and H. taypau, but with many other native tree species, such as Cyathea medullaris (G.Forst.) Sw.,

TA B L E 2. Correlation scores, from PC-Ord CCA output, for the ®rst three axes with the key environmental variables Correlations² Variable

Axis 1

Axis 2

Axis 3

pH LOI Aspect Altitude Slope R : FR PAR Moss cover Bare rock cover Litter cover

±0´411*** 0´798*** ±0´104n.s. 0´791*** ±0´006n.s. ±0´567*** ±0´266* 0´259* ±0´572*** 0´601***

±0´411*** ±0´373*** 0´169n.s. 0´480*** ±0´185n.s. 0´533*** 0´661*** 0´139n.s. 0´174n.s. ±0´396***

±0´509*** ±0´213n.s. ±0´406*** ±0´089n.s. 0´305** 0´002n.s. ±0´197n.s. 0´314** 0´401*** ±0´091n.s.

² Correlations are `intraset correlations' of ter Braak (1986), recommended for use with the LC scores by Palmer (1993). *** P < 0´001; ** P < 0´01; * P < 0´05; n.s. P > 0´05.

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Coprosma bene®ca W.R.B.Oliv., Xylosma suaveolens (J.R.Forst. & G.Forst.) G.Forst. and Glochidion spp. Homalium taypau forest may have been managed for timber in the past and so forests that are pure stands of H. taypau may not represent natural vegetation. The natural situation is more likely H. taypau mixed with M. collina, as in the Faute and McCoys valleys, to form `montane rain forest' and `cloud forest'. Homalium taypau forest is distributed in patches across the island and intergrades with M. collina forest in many parts of the south of the island. It tends to have a species-poor ground ¯ora with more of the common and weedy native ferns, such as Nephrolepis hirsutula (G.Forst.) C.Presl. No H. taypau seedlings were observed in any of the forests even though the species was regularly observed in ¯ower. The closely related Homalium acuminatum Cheeseman, from the Cook Islands, only ¯owers following a climatic disturbance, such as a cyclone, but many seedlings can be observed on the forest ¯oor (G. McCormack, pers. comm.; pers. obs.). Interestingly, seedlings of the dominant forest species on nearby Henderson Island [Pisonia grandis (J.R.Forst. & G.Forst.) Seem.] were not observed during 15 months of ®eld observations. Syzygium jambos forest covers the largest area of the island (0´17 km2), including one of the most important sites

F I G . 3. Map showing the distribution of plant communities across Pitcairn Island.

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on the island for numerous rare native fern species (Brown's Water). While this site is very rich and diverse, the fern populations here are in decline, and Angiopteris chauliodonta Copel. numbers have declined drastically in the last 30 years (B. Christian, pers comm.). This is due, in part, to the dense shade cast by the canopy cover, which has a high leaf area index, allowing low light transmission, and thus not supporting a ground ¯ora layer, and, in part, to erosion, as the roots do not bind the resulting bare soil. Before the introduction of S. jambos, introduced for ®rewood about two centuries ago, these areas would have been native montane rain and lowland forest with a rich understorey of native fern species. Mixed forest also covers large areas of the island, but with a varied species composition. On the north and east of the island it is typi®ed by the trees Pandanus tectorius and Hibiscus tiliaceus, with locally Glochidion spp., Pisonia umbellifera, Celtis paci®ca G.Planch., Hernandia sonora L. and Cyclophyllum barbatum (G.Forst.) N.HalleÂ & Florence. On the southern and western parts of the island, mixed forest areas are drier and typically epiphyte-poor, and found at low to mid-altitudes, below the level of cloud forest. They contain a large number of native species and are probably typical of much of the island's lowland valley and ravine forest prior to human habitation. It is therefore potentially a remnant of a previously more widespread native plant community type. Close to the settled areas, these forests have been planted with Musa spp., Citrus spp., Cocos nucifera and other cultivated species, and many of the native trees have been exploited for canoe making (Hernandia sonora) and house building (Glochidion spp.). Scrub communities

Weedy scrub is very common across the island, covering approx. 15 % of the island area, caused by clearance, goat trampling and invasion by scrubby non-native species. Many of these areas were cultivated when the island had a larger human population, but have since reverted to scrub and grassland. The resulting community varies in species composition, depending on how and where it has formed. Trampled areas, and especially ridges, tend to have a low number of native species and are typi®ed by scrub of Lantana camara, Psidium guava, Conyza bonariensis and the grasses Paspalum conjugatum B.Berguis and Sorghum sudanense. Originally, these ridges would probably have been covered with a low forest of species such as Cerbera manghas L., Coprosma bene®ca, Xylosma suaveolens and Pandanus tectorius, or Dicranopteris linearis fernlands on eroded slopes. The area around Tautama is also covered in weedy scrub but has many native and coastal species owing to its low-lying nature. Typical species here are the endemic Bidens mathewsii, and ferns, such as Nephrolepis hirsutula, as well as non-native species such as Conyza bonariensis and Lantana camara. Fernlands are common in areas that have suffered recent erosion and landslides, as is typical across the Paci®c region. They are dominated by Dicranopteris linearis, but also contain many other species, depending on how recently they were laid bare. There is an obvious cycle of species that

invade after these periods, and this succession can be seen in detail in areas such as The Hollow. Dicranopteris linearis and Lycopodium cernuum L.Ðitself probably a recent colonist as it was not recorded prior to 1994Ðare the ®rst species to invade, with Ophioglossum spp., lichen and bryophyte species in damp areas. These bind the soil suf®ciently, allowing other fern species, such as Davallia solida and Cyathea medullaris, to invade, followed by trees and shrubs, such as Pandanus tectorius and M. collina. These are the areas that are unfortunately now open to invasion by the introduced species Lantana camara, Sorghum sudanense, Psidium guajava and Syzygium jambos, and fernlands containing these non-native invasive species are now more common than native areas. Coastal communities

Coastal rock and scrub habitats are easily distinguished by the species present and by the geographical location. On the south of the island the species may require a wetter and more shaded climate than those on the north of the island. Rocky habitats include taxa such as Peperomia spp., Dianella intermedia Endl., Asplenium shuttleworthianum, Cocculus ferrandianus Gaudich, Haloragis sp. and Bidens mathewsii, while scrubby habitats tend to be dominated by non-native species such as Sonchus oleraceus L. and Eleusine indica, in addition to the larger native coastal species, such as Bidens mathewsii, and Haloragis sp. The north coast is drier and, owing to its location next to the main settlement of Adamstown, tends to be more heavily invaded and disturbed. Pandanus tectorius and Cocos nucifera form a tall coastal forest with virtually no ground ¯ora, except for the large invasive species Crinum asiaticum, along the stretch north of Adamstown, in a vegetation formation that has been common since preEuropean times right across the Paci®c. However, rocky areas on the north of the island also house the best examples of typical Paci®c island strand vegetation, with Eugenia reinwardtianna (Blume) DC, Ipomoea macrantha Roem. & Schult., Portulaca lutea Sol. ex Seem., Scaevola sericea Vahl, Chamaesyce sparmannii (Boiss.) Hurus. and Lepturus repens (G.Forst.) R.Br. These species are typical of dry coastal and sandy shores across the Paci®c, but are very limited in extent on Pitcairn Island in comparison with neighbouring Oeno and Henderson Islands, which have a fringing reef and thus a more protected and sandy shoreline. Comparison with other Polynesian Islands

The present vegetation of Pitcairn Island is comparable with that of other Polynesian high islands, but with a more depauperate ¯ora, and lacking several of the vegetation types that occur on the very high volcanic islands in the other island groups. The low altitude of the island also reduces the distinction between the leeward and windward communities, so preventing the development of dry forest, high altitude grassland and montane bog communities. Due to its age and subtropical location, Pitcairn Island has no fringing reef and therefore no terrestrial coral substrates as with the high islands of the Australs, two of which have

Kingston and Waldren Ð Plant Communities on Pitcairn Island areas of elevated limestone as well as volcanic soils, and two of which have large lagoon areas with extensive coastal sandy substrates. This lack of coral substrate also differentiates the vegetation of Pitcairn from Polynesian atoll vegetation such as is found in the Tuamotus. The lack of a protective fringing reef also means that the coast is less gradually sloping, as in the Society islands, but is more similar to the steep cliffs found in the Marquesas and parts of Rapa. The Marquesas, however, differ in that they are located further into the tropical zone and so sustain quite different species. Other Polynesian islands have also been more greatly altered and affected by a longer history of human habitation. Pitcairn Island does not have the large areas cleared for Cocos nucifera and Coffea arabica L. plantations that are common in the Society, Austral and Gambier Islands. Instead, due to less intensive agriculture, such plantations are con®ned to the areas around habitation and in some of the more accessible valleys. Other vegetation types that are absent from Pitcairn Island include wetland and coastal communities such as mangrove swamps, saltmarshes and wetland agriculture. Marsh areas are especially common in the Austral islands, resulting in an aquatic habitat and the suite of associated species (e.g. Nymphacea cf. lotus L., Lepironia articulata) (HalleÂ, 1980; pers. obs., 2000). Native forests on Pitcairn Island fall into the category of montane rain forests, with small areas of cloud forest as de®ned by Mueller-Dombois and Fosberg (1998) for Paci®c islands. Pitcairn Island is too low to have large areas of cloud forest as found in the Society Islands, but has areas with features of cloud forest at the highest altitudes, such as extensive epiphyte cover, aerial roots on M. collina trees and continuous cloud cover during wet periods. On Pitcairn Island these forests occur with abundant pteridophyte growth in the understorey. This feature has also been noted for the Society Islands by Fosberg (1992), and personally observed on Tubuai in the Austral Islands. Tubuai is similarly a relatively low island (max. 422 m), but has an area of cloud forest on the highest peak, with M. collina with aerial roots, and dense moss and epiphyte cover (pers. obs., 2000). Unfortunately, although Tubuai is a much larger island than Pitcairn (45 km2), it has a much smaller area under native cloud forest, as the highest peaks are very heavily invaded with Psidium cattleianum Sabine, or have been burned and replaced by fernlands. Non-native secondary forest and scrub is common at lower elevations and in valleys. This is common across the Paci®c in areas where non-native crops and trees were planted amongst native trees, and is referred to as the `tree garden' biome by Mueller-Dombois and Fosberg (1998). The dense tangles of Hibiscus tiliaceus that develop in these valley bases once they are abandoned were observed especially in the Water Valley area of Pitcairn Island (an area of Polynesian settlement; M. Weisler, pers. comm.) and, to a lesser extent, in Tautama. Other valleys are still maintained for fruit gardens, even if only sporadically visited (e.g. St Pauls and valleys around Adamstown). As with all Paci®c high islands, fernlands are a common feature of the landscape. These are native communities that

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would have always existed on Paci®c islands. However, this plant community spreads as a result of anthropogenic activities, especially the burning of leeward slopes. Woody species are also present in older fernlands, and non-native species commonly use these areas for increasing their range, such as Psidium guajava and Lantana camara scrub referred to earlier as invading ridge-tops on Pitcairn Island. However, fernland is relatively uncommon on Pitcairn (only 2´8 % of total cover) in comparison with the other more heavily disturbed Polynesian islands, such as Tubuai which has more than 15 % fernland cover (pers. obs. 2000). However, the area under fernland on Pitcairn Island seems to have increased since the 1930s (see St. John, 1987). Coastal vegetation on the north side of the island is typical of Polynesia, with species such as Scaevola sericea and Lepturus repens occurring on the rocky shore, and Pandanus tectorius forests occurring further back. The south coast is especially similar to the rocky shores of the uplifted Austral islands (Rurutu and Rimatara) with Lycium sp., Bidens sp. and Chamaesyce sp. (pers. obs., 2000). Coastal areas differ from other coastal regions of Polynesia in that they are less disturbed by plantations and human habitations, and also have poorly developed strand communities lacking typical species such as Pemphis acidula J.R.Forst. & G.Forst, Suriana maritima L. and Timonius polygamus (G.Forst) C.B.Robinson. Conservation requirements

Clearance of invasive species has been identi®ed as one of the most important conservation requirements for both species and habitat conservation on the island (Kingston, 2001). A large number of species (approx. 250) have been introduced to Pitcairn Island, both accidentally and intentionally, ten of which are listed in Cronk and Fuller (1995) as problem taxa (including Psidium cattleianum which has virtually wiped out native cloud forest on nearby Tubuai). Some have become widespread and troublesome, and include both animals (e.g. rats, mice, wasps, ants and fruit ¯ies) and plants (e.g. Lantana camara, Sorghum sudanense). The number of introductions appears to have increased with time, and the problems associated with these species have become acute only over the past ®ve decades. The pace of introductions does not appear to have decreased in recent years, despite a growing awareness of the problems caused by introduced species. Other threats to the native vegetation have been through historical clearance of native forest and, more recently, through erosion. Identi®cation of the eight distinct plant communities has allowed for a system of nature reserves to be recommended. Using the technique of complimentarity, these reserves have been located in order to conserve all of the plant communities present on the island, as well as a signi®cant proportion of the threatened plant species (Kingston, 2001). CONCLUSIONS Pitcairn Island has eight distinct plant communities, but is mostly covered by a mix of scrub and forest. All plant

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communities contain high numbers of invasive species, leaving only remnants of undamaged native forest. Large areas are covered with scrub vegetation dominated by nonnative species, and by almost monospeci®c Syzygium jambos plantations. Fernlands also cover large areas, including both eroding areas and ridge tops. Coastal vegetation comprises largely rock and cliff communities with limited development of strand vegetation. The major environmental factor affecting the composition of the plant communities is altitude, related to the presence of coastal communities at low altitude, and forest or ridge crests at high altitudes. Anthropogenic factors also have a large effect, especially due to clearance and introduced species. The light levels penetrating and the resulting R : FR are affected by the species that form the canopy; LOI is affected by the litter cover and the microbes in the soil, as well as erosional factors, such as whether roots are present (and thus bind the soil) or not (the soil washes away). These factors also determine the type of ground ¯ora that develops. All of the plant communities contain large proportions of introduced species, and this has serious implications for the future conservation of both the vegetation types and rare species occurring therein. ACKNOWLEDGEMENTS We thank the people of Pitcairn Island for their hospitality, and the Island Council and Pitcairn Islands Commission for logistical support, especially Jay Warren and Leon Salt. Thanks to Graham Wragg and Ed Saul for marine transport, and Wildlife Management International for ®eld support. Support was received from UK Foreign & Commonwealth Of®ce, Linnaean Society of London, Royal Geographic Society, Trinity College Dublin Association & Trust, Royal Horticultural Society, Systematics Association, Merlin Trust, Percy Sladen Memorial Fund, Oleg Polunin Trust, Air New Zealand, Air Tahiti and Skye Instruments. L I TE R A T U R E C I T E D Beers TW, Dress PE, Wensel LC. 1966. Aspect transformation in site productivity research. Forestry 64: 691±692. Cronk QCB, Fuller JL. 1995. Plant invaders. `People and plants' conservation manuals. London: Chapman and Hall. Florence J. 1993. La veÂgeÁtation de quelques Ãõles de PolyneÂsie FrancËaise. In: ORSTOM, ed. Atlas of French Polynesia. Paris: ORSTOM. Florence J, Waldren S, Chepstow-Lusty AJ. 1995. The ¯ora of the

Pitcairn Islands: a review. Biological Journal of the Linnean Society 56: 79±119. Fosberg FR. 1992. Vegetation of the Society Islands. Paci®c Science 46: 232±250. Franklin J. 1993. Discrimination of tropical vegetation types using SPOT Multispectral Data. Geocarto International 8: 57±63. Franklin J, Merlin M. 1992. Species-environment patterns of forest vegetation on the uplifted reef limestone of Atiu, Mangaia, Ma'uke and Miti'aro, Cook Islands. Journal of Vegetation Science 3: 3±14. HalleÂ N. 1980. Les Orchidees de Tubuai (archipel des Australes, Sud PolyneÂsie). Cahiers de l'Indo-Paci®que 2: 69±130. Hill MO. 1979. TWINSPAN ± a FORTRAN program for arranging multivariate data in an ordered two-way table by classi®cation of the individuals and attributes. Ithaca: Cornell University. Kent M, Coker P. 1992. Vegetation description and analysis ± a practical approach. London: John Wiley & Sons. Kingston N. 2001. The ¯ora and vegetation of Pitcairn Island ± its phytogeography and conservation. PhD Thesis, University of Dublin, Ireland. Kuchler AW. 1967. Vegetation mapping. New York: Ronald Press. Legendre P, Legendre L. 1998. Numerical ecology. Amsterdam: Elsevier. McCune B, Mefford MJ. 1999. Multivariate analysis of ecological data. Oregon: MjM Software. McCune B, Grace JB, Urban D. 2002. Analysis of ecological communities. Oregon: MjM Software. Merlin MD. 1991. Woody vegetation on the raised coral limestone of Mangaia, southern Cook Islands. Paci®c Science 45: 131±151. Mueller-Dombois D, Fosberg FR. 1998. Vegetation of the tropical Paci®c islands. New York: Springer-Verlag. Palmer MW. 1993. Putting things in even better order: the advantages of Canonical Correspondence Analysis. Ecology 74: 2215±2230. Papy HR. 1951±1955. Tahiti et les Ãõles voisines. La veÂgeÁtation des Ãõles de la SocieÂteÂ et de Makatea (OceaÂnie FrancËaise). Traveaux du Laboratoire Forestier Toulouse. Tome V: 2(3). Paulay G, Spencer T. 1989. Vegetation of Henderson Island. Atoll Research Bulletin 328: 1±13. St. John H. 1987. An account of the ¯ora of Pitcairn Island with new Pandanus species. Honolulu: Paci®c Plant Studies 46. Spencer T. 1995. The Pitcairn Islands, South Paci®c Ocean: plate tectonic and climatic conditions. Biological Journal of the Linnean Society 56: 13±42. Stoddart DR. 1975. Scienti®c studies in the southern Cook Islands: background and biogeography. Atoll Research Bulletin 190: 1±30. Stoddart DR, Sachet M-H. 1969. Reconnaissance geomorphology of Rangiroa Atoll, Tuamotu Archipelago, with a list of vascular ¯ora of Rangiroa. Atoll Research Bulletin 125: 1±44. ter Braak CJF. 1986. Canonical correspondence analysis: a new eigenvector technique for multivariate direct gradient analysis. Ecology 67: 1167±1179. Twyford IT. 1958. The soil resources of Pitcairn Island. Suva, Fiji: Department of Agriculture. Waldren S, Florence J, Chepstow-Lusty AJ. 1995. A comparison of the vegetation communities from the islands of the Pitcairn Group. Biological Journal of the Linnean Society 56: 121±144.