Journal of Vegetation Science 17: 233-244, 2006 © IAVS; Opulus Press Uppsala. - Environment, disturbance history and rain forest composition across the islands of Tonga - 233 Environment, disturbance history and rain forest composition across the islands of Tonga, Western Polynesia Franklin, Janet1*; Wiser, Susan K.2; Drake, Donald R.3; Burrows, Larry E.2,4 & Sykes, William R.2,5 1Department of Geography, San Diego State University, San Diego, CA 92182-4614, USA; 2Landcare Research, Christchurch, New Zealand; E-mail wisers@landcareresearch.co.nz; 3Botany Department, University of Hawai`i at Manoa, Honolulu, HI 96822; E-mail dondrake@hawaii.edu; 4E-mail burrowsl@landcareresearch.co.nz; 5E-mail sykesw@landcareresearch.co.nz; *Corresponding author; Fax +1 6195945676; E-mail janet@sciences.sdsu.edu Abstract Questions: How do forest types differ in their distinctiveness among islands in relation to environmental and anthropogenic disturbance gradients? Are biogeographic factors also involved? Location: Tonga, ca. 170 oceanic islands totalling 700 km2 spread across 8° of latitude in Western Polynesia. Method: Relative basal area was analysed for 134 species of woody plants in 187 plots. We used clustering, indirect gradient analysis, and indicator species analysis to identify continuous and discontinuous variation in species composition across geographical, environmental and disturbance gradients. Partial DCA related environmental to compositional gradients for each major forest type after accounting for locality. CCA and partial CCA partitioned observed compositional variation into components explained by environment/disturbance, locality and covariation between them. Results: Differences among forest types are related to environment and degree of anthropogenic disturbance. After accounting for inter-island differences, compositional variation (1) in coastal forest types is related to substrate, steepness and proximity to coast; (2) in early-successional, lowland rain forest to proximity to the coast, steepness and cultivation disturbance; (3) in late-successional, lowland forest types to elevation. For coastal/littoral forests, most of the compositional variation (71%) is explained by disturbance and environmental variables that do not covary with island while for both early and late-successional forests there is a higher degree of compositional variation reflecting covariation between disturbance/environment and island. Conclusions: There are regional similarities, across islands, among littoral/coastal forest types dominated by widespread seawater-dispersed species. The early-successional species that dominate secondary forests are distributed broadly across islands and environmental gradients, consistent with the gradient-in-time model of succession. Among-island differences in early-successional forest may reflect differences in land-use practices rather than environmental differences or biogeographical history. In late-successional forests, variation in composition among islands can be partly explained by differences among islands and hypothesized tight links between species and environment. Disentangling the effects of anthropogenic disturbance history versus biogeographic history on late-successional forest in this region awaits further study. Keywords: Coastal forest; Human impact; Limestone; Littoral forest; Lowland rain forest; Pacific; Succession; Tropical forest; Volcanic; Zonation. Abbreviations: GA = Group averaging; MRPP = Multiresponse Permutation Procedure; NMS = Non-metric Multidimensional Scaling; pCCA = Partial CCA. Nomenclature: Smith (1979, 1981, 1985, 1988, 1991); for species not treated by Smith: Yuncker (1959), Whistler (1991), Wagner et al. (1999). Introduction Island studies have been fundamental to understanding influences of geographic and ecological processes on community composition and structure (Carlquist 1974; Vitousek et al. 1995; Whittaker 1998; MuellerDombois 2002; Drake et al. 2002). This requires integrating (1) biogeography, which creates distinctive patterns through isolation and evolutionary divergence and extinction; (2) natural and anthropogenic disturbance; (3) gradients in distance from the sea, elevation and edaphic controls (Mueller-Dombois & Fosberg 1998). As model systems, tropical islands can also increase our understanding of processes that influence rain forest composition worldwide. Studies of continental tropical rain forests have emphasized the importance of environmental gradients such as topography, soils, and climate (e.g. Gentry 1988; Pyke et al. 2001; Tuomisto et al. 2003), natural disturbance (Whitmore 1989; Burslem et al. 2000), anthropogenic disturbance (Horn & Kennedy 2001; van Gemerden et al. 2003), and interactions among these factors (e.g. Svenning et al. 2004). Tropical island systems allow these influences to be examined across larger geographic scales, and because the forest communities tend to be simpler, the entire tree community can be studied (cf. Pyke et al. 2001; Svenning et al. 2004). 234 Franklin, J. et al. Western Polynesian rain forests occur on islands with a long history of human impacts (Hughes et al. 1979; Kirch & Ellison 1994). They are influenced by cyclones (e.g. Woodroffe 1984; Elmqvist et al. 1994; Franklin et al. 2004) and environmental gradients of coastal influence, elevation and substrate (Franklin et al. 1999; Wiser et al. 2002). They are ideal systems to explore how interactions among environment, disturbance history and biogeography influence rain forest composition, yet quantitative studies of these forests have thus far been limited to individual islands or island groups (e.g. Garnock-Jones 1978; Whistler 1980; Kirkpatrick & Hassall 1985; Webb & Fa’aumu 1999; Keppel et al. 2005). We assembled data from four previous forest surveys conducted in Tonga to address three questions: 1. How do forest types differ in distinctiveness among islands? For example, littoral and coastal forests worldwide harbour wide-ranging, sea-dispersed species (Guppy 1906; Ridley 1930; Whitmore 1985), so we expect coastal/littoral forests on different island groups to be less distinctive from one another than inland forests. We also expect early-successional forest to be dominated by a few widespread species with broad ecological tolerances and therefore to be more similar on different islands than late-successional forest. 2. Do environmental gradients of coastal influence and elevation and gradients of anthropogenic disturbance have consistent impacts on forests across island groups? 3. Are there patterns of species composition related to locality (island group) that may suggest the importance of biogeographic factors instead? At this extensive geographic scale we expect space (island locality) to represent biogeographic factors (large-scale dispersal limitations affecting the regional species pool, local extirpation, speciation), rather than recruitment limitation (Harms et al. 2000) from local species pools (Zobel 1997), although it could represent unmeasured environmental or disturbance factors that are spatially correlated with island locality. Fig. 1. Western Polynesia Biogeographical Region, showing the islands and island groups of Tonga mentioned in the text (redrawn from Mueller-Dombois & Fosberg 1998). Material and Methods Study area and the data sets Tonga is the easternmost island group on the IndoAustralian plate, and comprises about 170 islands totalling 700 km2 of land spread across 600 000 km2 of ocean (Fig. 1). They are mainly low limestone islands from less than 1 to 5 - 10 million years old, with active volcanic islands to the north and west (Tappin 1993; Dickinson et al. 1999). Eruptions of the volcanic outliers have covered the limestone islands with andesite tephra that has weathered to rich soils. Tonga is within the western Polynesian biogeographic region (Stoddart 1992) (Fig. 1), where rain forests are species-poor compared to those of Australasia from which their floras are derived by long-distance dispersal (Raven & Axelrod 1972; Carlquist 1996; MuellerDombois & Fosberg 1998; Turner et al. 2001). Tonga, for example, has about 450 native flowering plant species (Park & Whistler 2001), Samoa 550 (Whistler 1992) and Fiji 1769 (Ash 1992). Non-native species approximately double the sizes of the floras. Endemism is low within Tonga (3% of the native flora), but fairly high within Western Polynesia as a whole (van Balgooy et al. 1996). Previous studies surveyed rain forest composition in three major island groups in Tonga (App. 1). The extensive, relatively undisturbed forests on ’Eua (Drake et al. 1996), Kao and Tofua (Park & Whistler 2001) (Fig. 1) were surveyed for conservation planning. On Tongatapu, the largest and most populous island, where forest fragments and very early successional forest cover about 11% of the island, plots were allocated to capture mapped variation in soil type and coastal influence and evaluate consequent composition, regeneration, and alien species occurrence (Wiser et al. 2002). Data from the small - Environment, disturbance history and rain forest composition across the islands of Tonga - Data analysis Classification and ordination were used to detect discontinuous and continuous variation in forest composition and relate this to variation in environmental and geographical factors and human disturbance history. Clustering (Greig-Smith 1983), based on Bray-Curtis distance calculated from relative abundances, and using group averaging (GA) as the linkage method, was carried out for the species by plot matrix. For comparison, we used TWINSPAN, Two-way Indicator Species Analysis (Hill 1979), and examined the two sets of results for congruence. Classification was followed by a multi-response permutation procedure (MRPP; Mielke 1984) to test the differences among groups of plots defined by the classification, and by indicator species analysis (Dufrêne & Legendre 1997) to determine the significance (by simulation) of indicator values (the average of relative abundance and relative frequency) for species associated with groups. The difference among groups in the disturbance-related cultivation index was tested. Indirect gradient analysis was carried out using Detrended Correspondence Analysis (DCA; ter Braak 1995). Non-metric Multidimensional Scaling (NMS; Clarke 1993) was run in parallel with the DCA and the results examined for congruence (Økland 1996; e.g. Pyke et al. 2001; Rydgren et al. 2003). In the DCA, rare species were downweighted and axes rescaled. NMS was carried out using default parameters with PC-Ord software (McCune & Mefford 1999). Patterns of groups identified by the classification were examined graphically with respect to the ordination axes. Spearman’s rank correlations between environmental variables and the ordination axes derived from the indirect gradient analysis were calculated. To describe consistent relationships between vegetation composition and both major environmental gradients and anthropogenic disturbance we used partial indirect ordination (pDCA) (ter Braak & Prentice 1988) with island designated as a covariable. We performed these analyses for each of three major forest types identified by clustering. We then used canonical correspondence analysis (CCA) and partial CCA (pCCA) to partition observed compositional variation into components explained by environment and disturbance alone, island locality (island group, representing biogeographical factors) alone, and covariation between the two (cf. Borcard et al. 1992; Økland 2003). Plots were grouped as: Tongatapu, ’Eua, ’Euaiki, Vava’u, and Kao plus Tofua (the two Ha’apai plots were excluded). To avoid over-fitting, we used forward selection to determine the set of environmental variables to include in the analysis. Again, we performed these analyses for each forest type, applying Monte Carlo tests to determine if relationships between supplied explanatory variables and composition were statistically significant (ter Braak & Smilauer 1988; Crowley 1992). Our interpretation followed Økland (1999), focusing on the relative amounts of variation explained by ‹ nearby island of ’Euaiki, collected during the Tongatapu survey, are analysed here for the first time. In Vava’u, survey objectives included characterizing a small forest (1 km2) for conservation (Bolick 1995), contrasting paired early- and late-successional forest fragments throughout the island group (Franklin et al. 1999), and assessing the composition and status of littoral/coastal forest (Steadman et al. 1999). We follow Whistler (1992) in using ‘littoral’ to refer to forests adjacent to the coast, i.e. within the spray zone, and ‘coastal’ for forests transitional between littoral and lowland. Data from the low islands of eastern Ha’apai (JF, unpubl. data) are also analysed here for the first time. In Vava’u, ’Eua and Tongatapu air photos and soil maps were used to stratify sampling; in the other surveys plots were sited subjectively to capture variation in forest composition observed during reconnaissance. The data analysed in this study comprise the relative basal area (RBA) of woody plants ≥ 5 cm DBH of 134 non-cultivated species (App. 2) in 187 plots. Plots were 600 m2 except for those on Kao and Tofua (1000 m2). Environmental variables on record for all plots reflect indirect topographic gradients correlated with maritime influence and exposure, substrate characteristics, and disturbance history. These included elevation and distance to the coast (determined from topographic maps), and slope and aspect (measured in the field). We derived an index of windwardness by calculating how similar plot-aspect is to the windward (SE) aspect, with cos(aspect –135°) equals 1 for SE aspects, –1 for NW (leeward) aspects, and 0 for NE or SW aspects (modified from Beers et al. 1966). Substrate was recorded as sand, limestone, volcanic, swamp, coral or soil. Average percent exposed rock was converted to an ordinal scale to minimize observer differences between studies (0 = 0, 1-10 = 1, 11-40 = 2, 41-70 = 3, 71-100 = 4). Finally, because Tonga lacks detailed land-use records and synoptic historical air photos, we calculated an anthropogenic disturbance or ‘cultivation’ index as the sum of relative basal areas of six cultivated species (Cocos nucifera, Artocarpus altilis, three Citrus spp., and Mangifera indica) that were then excluded as dependent variables (cf. Wiser et al. 2002). For Tofua and Kao, elevation, aspect and substrate were determined from plot descriptions and slope was estimated from maps. Percent rock was not recorded so the ‘group’ average was used. 235 236 Franklin, J. et al. each analysis, rather than the ratio between variation explained and total inertia (e.g. Borcard et al. 1992; Økland & Eilertsen 1994). For each analysis, the sum of the canonical eigenvalues represents the variation explained by the constraining variables (after removing variation explained by any covariables). The percentage accounted for by each analysis was obtained by dividing the sum of canonical eigenvalues for that analysis by the total variation explained. This was then partitioned as that accounted for: (1) solely by environment and disturbance; (2) solely by island group; and (3) by covariation between the two. To aid our interpretation of the variation partitioning results, we determined how islands differed by applying MANOVA to continuously distributed variables describing environment and disturbance and comparing percentage frequency of occurrence of plots in each substrate category on different islands. To determine whether the variables used in these models were indeed adequate to explain the observed compositional variation, we compared eigenvalues and stand score positions between a DCA and DCCA (constrained by ‘island’ and selected environmental/disturbance variables) conducted for each forest type data set. Clustering was carried out using PC-Ord (McCune & Mefford 1999), ordination using CANOCO (ter Braak & Smilauer 1998), and other statistical analyses using S-Plus 6.1 (Anon. 1999) and R (Anon. 2004). Results Classification ‹ Clustering using GA as the linkage method and retaining ten groups of plots (< 15% similarity) yielded groups similar to TWINSPAN’s groupings (ca. 70% correspondence), so only the GA results are shown. Four littoral/coastal groups and six lowland groups were identified. One inland group represents early-successional stands, and five represent differences in species dominance among late-successional stands (Table 1). These groups are distinct based on MRPP (p < 0.0001), and within-group homogeneity (A = 0.23) is typical of community data. Plots characterizing littoral/coastal forest groups were well distributed among islands (Table 1). Littoral/coastal forests typically occur below 10 m elevation, except on more exposed windward aspects where they reach 25 m (Fig. 2a). Exceptions were one plot dominated by earlysuccessional Casuarina equisetifolia on young volcanic substrate, and two that were transitional between coastal and lowland on small windward islands. The littoral groups, Hernandia/Barringtonia and Casuarina, occur primarily within 40 m of the shore, while the coastal groups, Pisonia and Excoecaria, may occur further inland (Fig. 2b) and sometimes in association with rain forest species (and also with mangroves for Excoecaria). Table 1. Groups of plots based on classification using Group Averaging (see text). Indicator species are shown (p < 0.01). Forest group names are based on species dominance; BA = basal area; Number of plots by location (island[s]) is given as: Tof = Kao and Tofua; Eu = ’Eua; Va = Vava’u; Ton = Tongatapu; Ik = ’Euaiki near Tongatapu; Ha = eastern Ha’apai. Forest type/ Group Elevation (m) BA (m2 ha–1) Density (trees ha–1) 0.5-10 2-40 68 ± 29 32 ± 10 960 ± 366 892 ± 540 7-30 0-25 38 ± 20 32 ± 13 1037 ± 405 1313 ± 469 Early-successional lowland rain forest Rhus 5-250 29 ± 10 Late-successional lowland rain forest Myristica 8-180 Maniltoa 5-180 Calophyllum neo-ebudicum 100-400 Littoral Hernandia/ Barringtonia Casuarina Coastal Pisonia Excoecaria Inocarpus Erythrina 1-35 1 Tof Eu Va Ton 4 13 1 6 1 1 1 11 2 15 1215 ± 310 5 17 14 47 ± 18 40 ± 11 1024 ± 299 1373 ± 410 12 2 39 55 ± 18 1224 ± 344 16 1 60 ± 13 75 992 ± 366 999 1 5 Ik 1 1 P. grandis E. agallocha 2 1 8 1 Indicator species H. nymphaeifolia, B. asiatica C. equisetifolia 5 1 Ha R. taitensis, Cocos nucifera, Grewia crenata, Alphitonia zizyphoides, Pometia pinnata M. hypargyraea M. grandiflora, Cryptocarya turbinata, Zanthoxylum pinnatum, Pleiogynium timoriense, Garuga floribunda C. neo-ebudicum, Garcinia myrtifolia, Neonauclea forsteri, Dysoxylum tongense, Citronella samoensis, Podocarpus pallidus I. fagifer E. fusca - Environment, disturbance history and rain forest composition across the islands of Tonga - 237 Fig. 2. (a) Elevation versus the windwardness (southeastness) of aspect for littoral/coastal plots, and (b) frequency of plots versus log(distance to coast in meters) for littoral/coastal types. Fig. 3. Ordination of plots on first two DCA axes, with plots labeled (a) by groups (Erythrina excluded), and (b) by island (Table 1). Correlations with environmental variables (absolute value > 0.40, Spearman rank correlation p < 0.001) illustrated with vectors. Early-successional lowland forest, dominated by Rhus taitensis, was sampled on all islands except Kao and Tofua, although it probably occurs there (Park & Whistler 2001), as it is extensive on older lava flows of another volcanic outlier, Late (Sykes 1981). Late-successional lowland forest groups showed more uneven distributions among islands (Table 1). The average cultivation index was significantly higher for the early successional group (16 ± 17) than for all late-successional groups combined (1 ± 4; χ2 = 26.4, p < 0.0001, Kruskal–Wallis rank sum test). The ordination revealed compositional gradients linked to coastal influence (environment) and successional status (anthropogenic disturbance) that generally spanned island groups. Axis 1 divided littoral and coastal forest groups from lowland groups, and was most strongly correlated with elevation (Fig. 3). Littoral and coastal forests were distinguished from lowland forest by indicator species (Table 1), wide-ranging species (App. 3), and the occurrence of Calophyllum inophyllum, Terminalia catappa, Vitex trifoliata, Xylocarpus granatum, X. moluccensis, Scaevola sericea, Acacia simplex, Sophora tomentosa, and Tournefortia argentea. The compositional boundary between littoral/coastal forest and lowland forest, however, is not sharp (Fig. 3a). Species occurring in both groups include Elattostachys falcata, Syzygium clusiifolium, Pouteria grayana, Xylosma simulans, Ochrosia vitiensis, Morinda citrifolia, and Grewia crenata. Among the littoral and coastal types, strong island-by-island compositional patterns are not evident (Fig. 3b; Table 1). Ordination The first three DCA axes had gradient lengths of 6.8, 5.1 and 5.1, respectively. NMS yielded a relative orientation of plots (and groups) similar to DCA. Correlations (Spearman’s) between the first NMS axis and DCA axes 1 and 2 were 0.68 and 0.55, and between the second NMS axis and DCA axes 1 and 2 were – 0.65 and 0.59. 238 Franklin, J. et al. Fig. 4. Partial DCA of littoral/coastal forest types (58 plots), with ‘island’ treated as a covariable. Environmental variables are shown if the absolute value of their inter-set correlations with either Axis 1 or 2 of the ordination is > 0.2. Centroid positions of substrate categories are shown. Plots are coded by forest type, with symbols as in Fig. 3a. The early-successional forest group is found in the centre of the ordination diagram (Fig. 3a), because early-successional plots of all forest types tended to share the same pioneer species. Early-successional plots from a given island tended to be located on the ordination diagram near late-successional plots from that island (but this could result in part from the methods used to choose paired plots in the field in Vava’u). Lowland forest groups distinguished by species composition (Table 1) were arrayed on Axis 2, which was correlated with slope and percent rock (Fig. 3a). Islandby-island patterns were apparent among these groups (Fig. 3b). Maniltoa plots were primarily on steep rocky slopes in Vava’u where most forest remnants are now confined. Myristica plots occurred on flatter, less rocky slopes (Tongatapu and coastal ’Eua). The Calophyllum neo-ebudicum group occurred primarily above 100 m elevation on ’Eua, Kao and Tofua, while most Inocarpus plots were on poorly drained soils on Tongatapu, but not immediately adjacent to the coast. Based on the classification and ordination, plots were partitioned into three Fig. 5. Partial DCA of the early-successional lowland rain forest type (38 plots), with ‘island’ treated as a covariable. Environmental variables are shown if the absolute value of their inter-set correlations with either Axis 1 or 2 of the ordination is > 0.2. Because this type comprises one group (Table 1, Fig. 3a), some species that are characteristic of this group and mentioned in the text are shown. ALE MOL = Aleurites molucanna; ALP ZIZ = Alphitonia zizyphoides; ELA FAL = Elattostachys falcata; GRE CRE = Grewia crenata; HIB TIL = Hibiscus tiliaceus; RHU TAI = Rhus taitensis; SYZ CLU = Syzygium clusiifolium. types for further analysis: littoral/coastal forests, and early- and late-successional lowland forests. Partial DCA of littoral/coastal plots shows patterns of species composition and environmental correlations that emerge after accounting for inter-island differences (Fig. 4). Plots of the Excoecaria group occur on windward sides of islands, on limestone and swamp substrates. These stands have particularly high density and low basal area (Table 1). The Casuarina group on steep slopes on limestone or coral is differentiated on Axis 2 from the Hernandia/Barringtonia group on gentler slopes and sandy or soil substrates. Partial DCA of the early-successional lowland plots (Fig. 5) shows that variation in composition, after accounting for inter-island differences, is primarily re- Fig. 6. Partial DCA of the late-successional lowland rain forest type (81 plots), with ‘island’ treated as a covariable. Environmental variables are shown if the absolute value of their inter-set correlations with either Axis 1 or 2 of the ordination is > 0.2. Outliers from Inocarpus and Erythrina groups (Table 1) were excluded. Plots are coded by forest type, with symbols as in Fig. 3a. - Environment, disturbance history and rain forest composition across the islands of Tonga - 239 species composition. Axis 1 is positively related to elevation and distance from the coast and differentiated the Myristica (mean elevation = 70 m) and Maniltoa (61 m) groups from the C. neo-ebudicum group, found above 100 m elevation on ’Eua, Kao and Tofua (Table 1). The only Vava’u plot in this group was from one of the highest elevations in that island group (180 m). The second axis appears to differentiate rockier Maniltoa group plots, co-dominated by Garuga floribunda and Pleiogynium timoriense, from the remainder of this group, although rockiness was only weakly correlated with the second pDCA axis. Variation partitioning showed that for all three forest types, differences among islands unrelated to environment and anthropogenic disturbance account for similar levels of compositional variation (23-28%). For coastal forests, most of the remaining compositional variation (71%) is explained by disturbance and environmental variables that do not covary with island. This contrasts with both early and late-successional forests where there is a much higher degree of compositional variation reflecting covariation between disturbance/ environment and island (Fig. 7). And indeed, differences in environment among islands are primarily influencing inland, non-coastal areas. Plots from different island groups differ significantly in mean elevation (Table 2), reflecting the increasing maximum elevation of the islands from Tongatapu (80 m) to Kao (1046 m; App. 1). Differences in distance from the coast (Table 2) reflect the larger size of Tongatapu (App. 1) and the distribution of forest remnants on different islands or island groups (e.g. on small islands and on escarpments near the coast in Vava’u). Islands also differ in slope and rockiness (Table 2). This both reflects true differences, i.e. Tongatapu and Ha’apai are largely flat with little Fig. 7. Relative percentage of compositional variation that can be explained by island (or island group) alone, covariation between island and environment/disturbance and environment/disturbance alone for the three major forest types of Tonga. Littoral/coastal forest: n = 58; Early-successional forest: n = 38; Late-successional forest: n = 91. lated to proximity to the coast, disturbance by former cultivation, and slope steepness, but not substrate type (unlike littoral/coastal forest) or elevation (unlike latesuccessional forest). Those occurring inland on less disturbed sites are characterized by species such as Elattostachys falcata and Syzygium clusiifolium. Plots on less disturbed sites but closer to the coast have more coastal species (e.g. Hibiscus tiliaceus). The most disturbed sites also occur near the coast and are characterized by Grewia crenata and Aleurites moluccana. Partial DCA of late-successional lowland rain forest plots (Fig. 6) excluded the ten Inocarpus and Erythrina plots (Table 1) which were extreme outliers in terms of Table 2. Values of environmental variables (described in text) that differ among plots representing the major islands or island groups (Table 1). (a) Mean ± SD values demonstrated by Scheffe’s multiple comparison test in a MANOVA to differ significantly between islands; those not sharing the same letter differ at p < 0.01. (b) Percentage frequency of occurrences of plots in different substrate classes. Summary statistics are shown for the islands of ’Euaiki and Ha’apai, but they were excluded from the MANOVA because of the small numbers of plots sampled on each. Variable (a) Continuous variables Elevation (m) Slope (º) Rockiness index Distance to Coast (m) (b) Substrate classes Volcanic Limestone Soil Coral Sand Swamp Tofua, Kao (n = 6) ’Eua (n = 40) Vava’u (n = 84) Tongatapu (n = 52) ’Euaiki (n = 3) Ha’apai (n = 2) 248 ± 138 a 14 ± 6 ab — 1232 ± 878 a 132 ± 85 b 15 ± 14 a 0.87 ± 1.14 a 690 ± 836 a 43 ± 51 c 13 ± 10 a 1.62 ± 1.05 b 177 ± 318 c 12 ± 9 c 1±3b 0.25 ± 0.59 a 1364 ± 1320 b 25 ± 9 18 ± 13 1.67 ± 0.58 162 ± 95 7±2 1±1 0±0 35 ± 21 100 0 0 0 0 0 2.5 35 50 2.5 10 0 0 83 0 3 14 0 0 17 67 0 0 15 0 100 0 0 0 0 0 0 0 0 100 0 240 Franklin, J. et al. exposed rock, and that sampling in other areas focused on less disturbed forests, which occur mainly on steeper, often rockier, slopes that are unsuitable for cultivation. Island, including contrasts in environment and anthropogenic disturbance, explains proportionally the most variation in late-successional forests (55%), followed by early successional forests (51%) and the least in littoral and coastal forests (29%). To further understand the source of these patterns we compared species that are wide-ranging across islands (≥ 5% frequency in 3-4 island groups; App. 3) with those that are indicators for islands (App. 4). A high proportion of littoral/coastal (Guettarda speciosa, Hibiscus tiliaceus, Pandanus tectorius) and early-successional (Rhus taitensis, Morinda citrifolia, Grewia crenata) species are wide ranging relative to lowland forest species (the most common being Elaeocarpus tonganus, Ellattostachys falcata, Ficus scabra, Pouteria grayana and Vavaea amicorum). Indicator species for Vava’u (Zanthoxylum pinnatum, Cryptocarya turbinata), Kao and Tofua (Psychotria insularum, Canarium vitiense), and ’Eua (Garcinia myrtifolia, Dysoxylum tongense; App. 4) are typically associated with late-successional forest (Table 1). A comparison of eigenvalues of DCA and DCCA showed that the variables used for variation partitioning accounted for most of the explainable variation in earlyand late-successional forests (ratio of DCA and DCCA axis 1 and axis 2 eigenvalues ranges from 0.78 to 0.97), but less in coastal forests (ratio of DCA and DCCA axis 1 and axis 2 are 0.70 and 0.54 respectively). Discussion Our synthesis of forest vegetation data for Tonga in Western Polynesia both confirms the generality of compositional patterns and their environmental correlates noted in previous studies (Whistler 1992; Drake et al. 1996; Franklin et al. 1999; Wiser et al. 2002), and extends their interpretation with quantitative archipelagowide analysis of the influence of environment, anthropogenic disturbance, and biogeography on forest composition. Littoral and coastal forests had characteristic assemblages of widespread species known to be seawater dispersed (Guppy 1906), such as Barringtonia asiatica, Calophyllum inophyllum, Excoecaria agallocha, Hernandia nymphaeifolia, and Terminalia catappa. Worldwide, disturbance strongly influences tropical forest composition, typically favouring widely-distributed species (Connell 1978; Whitmore 1985; Brown & Lugo 1990). Our results are consistent with this. Earlysuccessional rain forest composition in Tonga varies with a cultivation index but is broadly similar across environmental gradients, and throughout the region (Straatmans 1964; Schmid 1975; Garnock-Jones 1978; Whistler 1980; Morat & Veillon 1985; Whitmore 1985; Webb & Fa’aumu 1999). Our data are also consistent with the gradient-in-time model of succession that predicts that early-successional species have broader niches than late-successional species (Peet 1992). Late-successional lowland rain forest composition varied along environmental gradients, particularly elevation. These patterns are again consistent with the gradient-in-time model where niche breadth should decrease during succession (Peet 1992). Calophyllum neoebudicum forest was found at higher elevations than Myristica and Maniltoa-dominated forests, although elevations in Tonga do not extend high enough to support montane rain forest found elsewhere in Western Polynesia (Whistler 1980; Kirkpatrick & Hassall 1985; Ash 1992; Keppel 2005; Tuiwawa 2005). The distribution of C. neo-ebudicum forest seems to be determined by elevation rather than substrate because it was found on both limestone and volcanic substrates. There are also patterns of species composition related to island that cannot be explained by environmental and disturbance variables measured in this study. We were surprised that so much variation in early-successional forest was explained by island and covariation between island and disturbance/environment (51%) given that many of the wide-ranging species (App. 3) are early successional. Because early-successional species tend to be widespread owing to dispersal ability and other life-history traits (e.g. long seed viability, Denslow 1996; Whitmore 1998; Gitay et al. 1999), we expected local and regional species pools (sensu Zobel 1997) to be similar for the early-successional forests. However, although they were not significantly associated with a particular island, Aleurites moluccana (a prehistoric introduction), Bischofia javanica (a preserved tree) and Pometia pinnata (commonly planted in villages and plantations) were important in early-successional forest on Tongatapu but infrequent elsewhere. This suggests that variation in species composition of secondary forest among islands could be related to cultivation practices and land-use history. Other gaps in the distributions of widespread, locally abundant early-successional native species (Polyscias multijuga and Dendrocnide harveyi for example) cannot clearly be linked to cultivation history, however. Differences in species composition among island groups were clearly seen among late-successional forests (e.g. Table 1, App. 4). This may be due to biogeographic history (dispersal limitations on a regional scale), lower dispersal ability of late- (relative to early-) successional species, and tighter links between species distributions and environment (where environmental gradients could not be characterized by our data) - Environment, disturbance history and rain forest composition across the islands of Tonga over the course of succession. For example, Myristicadominated forests occurred on ’Eua, and Tongatapu, and in Samoa (Whistler 1980) and the Lau Goup, Fiji (Garnock-Jones 1978), but not Vava’u. This pattern cannot be explained by latitude or eastern range limits (Fig. 1). Myristica is dispersed by large pigeons (Guppy 1906; Meehan et al. 2002), including several species of which have been lost from Tonga since the arrival of people (Steadman 1993, 1995), but also by Pacific pigeons (Ducula pacifica) (Steadman & Freifeld 1999; McConkey et al. 2004; Meehan et al. 2005) which are found throughout Tonga, but arrived after human colonization (Steadman 1997). Therefore, it is unknown if this distribution gap is due to human impacts on native dispersers or to biogeographical factors (dispersal, colonization). Other late-successional species found throughout Western Polynesia are common in one island group but uncommon or absent elsewhere in Tonga, e.g. Cryptocarya turbinata, Chionanthus vitiensis. Their absence from Tongatapu may be related to greater impacts of anthropogenic disturbance there. Zanthoxylum pinnatum shows a low-latitude (Fiji, Samoa, Vava’u, Niuatoputapu, Fig. 1) distribution in Western Polynesia, consistent with environmental (climatic) limits to its range, while regional endemics such as Maniltoa grandiflora, Cryptocarya hornei and Pouteria membranacea are only found in a portion of Western Polynesia, suggesting dispersal limitations. Island biogeography theory (MacArthur & Wilson 1967) suggests that a rare species could be absent from an island in an archipelago, especially a small or remote one, because it never colonized, or did not maintain a viable population when island areas and proximities varied as sea level fluctuated (Nunn 1994). When widespread and locally abundant species show gaps in distribution in a region with a long history of forest clearing for agriculture, however, and when those gaps are unrelated to island size or isolation, it may have been lost due to anthropogenic disturbance. This is especially compelling when, as in this study, the islands with the highest potential levels of natural disturbance, but lowest intensities of human land use, the active volcanoes Tofua and Kao, support species not found elsewhere in Tonga (App. 4), and, with the possible exception of Fagraea, they are not known to be particularly associated with volcanic substrates. Detailed palynological studies to elucidate long-term vegetation patterns would be particularly illuminating (Fall 2005). Our study furthers the qualitative synthesis of earlier work on Tongan vegetation provided by MuellerDombois & Fosberg (1998). We note however that, while extensive, this data set on Tongan forests is still far from comprehensive, and in particular, mangroves, 241 forest stands dominated by single species, e.g. Erythrina fusca, Pandanus tectorius, Hibiscus tiliaceus, and ecologically important (Janzen 1979; Terborgh 1986) banyans (Ficus obliqua, F. prolixa), have been observed by the authors but are not well represented. Further, the steep southeastern slopes of ’Eua have forests dominated by the palm Pritchardia pacifica, a species we recorded on only a single plot on ’Euaiki. Analyses of data from multiple surveys (cf. Knapp et al. 2004) spanning island groups allowed us to quantify regional similarities among littoral/coastal forests (particularly those related to seawater-dispersed species) and among early-successional forests whose species distributions indicates broad environmental tolerances. Some among-island differences in secondary forest composition involve preserved or planted species and seem to reflect differences in land-use practices, although these differences may also result from covariation between island and environment in our sample. In latesuccessional forests, the variation in species composition among islands can be partly explained by environmental differences that exist among islands and hypothesized tight links between species and environment, but in some cases may also result from a lack of representative sampling. Disentangling the effects of anthropogenic disturbance history versus biogeographic history on lowland rain forest in this region awaits further study. Acknowledgements. We thank W. A. Whistler for graciously providing the data from Kao and Tofua. Data collection and analysis were supported by the East-West Center, National Geographic Society, Georgia Southern University, San Diego State University, University of Hawaii, New Zealand Foundation for Research, Science and Technology (Contract CO9X004), and the New Zealand Ministry of Foreign Affairs and Trade. We acknowledge the use of data from Tongatapu drawn from New Zealand’s National Vegetation Survey Databank (NVS). We are grateful to the government of Tonga, especially the Office of the Prime Minister, the Ministry of Agriculture and Forestry, and the Ministry of Land and Survey, and to numerous individuals, especially S. Fotu, N. Prescott, M. Halefihi, T. Hoponoa, D. Steadman, L. Bolick, D. Smith, T. Motley, T. Fine, K. McConkey, F. Tonga, M. Pomelile, C. Imada, Aunofo, Ongo, V. Latu, M. Havea, ’I. Kamoloni, T. Faka’osi, T. Savage and M. Breach. C. Bezar, A. Whistler, R. 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Evol. 12: 266-269. Received 24 August 2005; Accepted 15 December 2005. Co-ordinating Editor: R. Ejrnæs. For App. 1, see JVS/AVS Electronic Archives; www.opuluspress.se/ App. 1. Data sets used in this study. Group = Tonga’s main island groups (Fig. 1). Plots = no. of plots in each dataset and Spp. = total no. of trees and woody species (> 5 cm DBH) recorded in that group of plots including cultivated species. Date = year of survey. Lat(itude) and Long(itude) are approximate locations of center of island group or island sampled. #Isl = number of islands sampled; Area is of island(s) sampled; Elev is maximum elevation sampled (maximum elevation of island(s) in parentheses). Sources for island size, elevation and location: Steadman et al. (1999), Drake et al. (1996), Park & Whistler (2001), Wiser et al. (2002), topographic maps. Group Island(s) Description Plots Spp. Date Tongatapu Tongatapu Tongatapu Vava’u Tongatapu ‘Euaiki ‘Eua Vava‘u group Forest fragments Forest fragments & coastal forest Undisturbed forest Forest fragments 52 3 40 64 73 39 86 77 1997 1997 1990 1993,1995 Vava’u Ha’apai Ha’apai Ha’apai Vava‘u group Eastern, low, limestone Kao, volcanic Tofua, volcanic Coastal forest Forest fragments Undisturbed forest Undisturbed forest Total 20 2 1 5 187 50 20 17 35 140 1995 2004 1997 “ # Isl Area (km2) Elev (m) 175°10° W 174°50° W 174°55° W 174°02” W 1 1 1 13 261 1 81 119 30 (80) 30 (40) 280 (312) 180 (200) “ 174°30° W 174°00° W 174°05° W “ 2 1 1 “ 17 13 47 7 8 (60) 250 (1046) 400 (558) Source Lat Long Wiser et al. 2002 Wiser et al. (unpubl.) Drake et al. 1996 Bolick 1995; Franklin et al. 1999 Steadman et al. 1999 Franklin (unpubl.) Park & Whistler 2001 “ 21°10°S 21°05°S 21°25°S 18°40°S “ 20°00°S 19°40°S 19°45°S App. 1. Internet supplement to: Franklin, J.; Wiser, S.K.; Drake, D.R.; Burrows, L.E. & Sykes, W.R. 2006. Environment, disturbance history and rain forest composition across the islands of Tonga, Western Polynesia. J. Veg. Sci. 17: 233-244. 2 App. 2. Species list. A total of 140 taxa were used in the analysis including six cultivated species used in the cultivation index. Some species could not be distinguished in the field as the plants lacked flowers or fruits, and were grouped for analysis. Geniostoma rupestre and G. insulare were grouped as G. rupestre. Ficus obliqua and F. prolixa were grouped as ‘Banyan’. Two species in two plots from Vava’u remain unidentified and were included as unknown but separate taxa in the analysis (‘Pouteria species unknown’ and ‘Sapotaceae genus unknown’), and one species from Kao and Tofua is tentatively identified (Celtis harperi?). Tong1 = Wiser et al. (2002) and Wiser et al. unpubl. for ’Euaiki; Vav2 = Franklin et al. (1999); Vav3 = Steadman et al. (1999); Eua4 = Drake et al. (1996); KaoTofua5 = Park & Whistler (2001); EHap6 = JF unpubl. * This is really a Syzygium species but the combination in that genus has not yet been made from Eugenia, the genus in which most of these Pacific island trees were once treated. Family Species name Species authority Mimosaceae Mimosaceae Meliaceae Rubiaceae Euphorbiaceae Rhamnaceae Olacaceae Annonaceae Rubiaceae Moraceae Barringtoniaceae Euphorbiaceae Sapotaceae Clusiaceae Clusiaceae Annonaceae Burseraceae Burseraceae Caricaceae Casuarinaceae Ulmaceae Apocynaceae Oleaceae Rutaceae Rutaceae Rutaceae Icacinaceae Arecaceae Rubiaceae Boraginaceae Agavaceae Lauraceae Lauraceae Cyatheaceae Rubiaceae Cycadaceae Urticaceae Fabaceae Ebenaceae Ebenaceae Ebenaceae Euphorbiaceae Meliaceae Meliaceae Elaeocarpaceae Elaeocarpaceae Sapindaceae Apocynaceae Fabaceae Fabaceae Myrtaceae Euphorbiaceae Loganiaceae Acacia simplex Adenanthera pavonina Aglaia heterotricha Aidia racemosa Aleurites molucanna Alphitonia zizyphoides Anacolosa lutea Annona muricata Antirhea inconspicua Artocarpus altilis Barringtonia asiatica Bischofia javanica Burckella richii Calophyllum inophyllum Calophyllum neo-ebudicum Cananga odorata Canarium harveyi Canarium vitiense Carica papaya Casuarina equisetifolia Celtis harperi? Cerbera odollam Chionanthus vitiensis Citrus grandis Citrus maxima Citrus reticulata Citronella samoensis Cocos nucifera Coffea liberica Cordia subcordata Cordyline fruticosa Cryptocarya hornei Cryptocarya turbinata Cyathea lunulata Cyclophyllum barbatum Cycas rumphii Dendrocnide harveyi Desmodium heterocarpon Diospyros elliptica Diospyros major Diospyros samoensis Drypetes vitiensis Dysoxylum forsteri Dysoxylum tongense Elaeocarpus graeffei Elaeocarpus tonganus Elattostachys falcata Ervatamia orientalis Erythrina fusca Erythrina variegata Eugenia reinwardtiana Excoecaria agallocha Fagraea berteroana (Sparr.) Pedley L. A. C. Sm. (Cav.) D. D. Tirveng. (L.) Willd. (Spreng.) A. Gray Gillespie L. (Seem.) Christoph. (Parkinson) Fosberg (L.) Kurz Bl. (A. Gray) Lam L. Guillaumin (Lam.) Hooker f. & Thomps. Seem. A. Gray L. L. Horne Gaertn. (Seem.) A. C. Sm. (L.) Osbeck (Burm.) Merr. L. (A. Gray) Howard L. Bull. Lam. (L.) A. Chev. Gillespie Gillespie (G.Forst.) Copel. (G.Forst.) Hallé & Florence Miq. (Seem.) Chew (L.) DC. (J. R. & G. Forst.) P. S. Green (Forst. f.) Bakh. A. Gray Croizat (Juss.) C. DC. A. C. Sm. Seem. Burkill (A. Gray) Radlk. Markgr. Lour. L. (Bl.) Bl. L. A. Gray ex Benth. Tong 1 Vav 2 X X X Vav 3 Eua 4 Kao EHap Tofua 5 6 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X App. 2-4. Internet supplement to: Franklin, J.; Wiser, S.K.; Drake, D.R.; Burrows, L.E. & Sykes, W.R. 2006. Environment, disturbance history and rain forest composition across the islands of Tonga, Western Polynesia. J. Veg. Sci. 17: 233-244. X X X X X X X X X X X X X X X X 3 App. 2, cont. Family Species name Species authority Moraceae Moraceae Moraceae Moraceae Burseraceae Clusiaceae Loganiaceae Euphorbiaceae Tiliaceae Rubiaceae Sapindaceae Gyrocarpaceae Sapindaceae Monimiaceae Sterculiaceae Sterculiaceae Hernandiaceae Hernandiaceae Malvaceae Flacourtiaceae Fabaceae Rubiaceae Verbenaceae Fabaceae Lauraceae Euphorbiaceae Anacardiaceae Sapotaceae Caesalpiniaceae Rutaceae Melastomataceae Araliaceae Rutaceae Rubiaceae Rubiaceae Myristicaceae Apocynaceae Rubiaceae Apocynaceae Pandanaceae Lythraceae Thymelaeaceae Myrtaceae Nyctaginaceae Nyctaginaceae Pittosporaceae Sapotaceae Sapotaceae Sapotaceae Sapotaceae Anacardiaceae Podocarpaceae Araliaceae Sapindaceae Rubiaceae Verbenaceae Arecaceae Myrtaceae Rubiaceae Rubiaceae Anacardiaceae Santalaceae Ficus obliqua Ficus prolixa Ficus scabra Ficus tinctoria Garuga floribunda Garcinia myrtifolia Geniostoma rupestre Glochidion ramiflorum Grewia crenata Guettarda speciosa Guioa lentiscifolia Gyrocarpus americanus Harpullia arborea Hedycarya dorstenioides Heritiera littoralis Heritiera ornithocephala Hernandia moerenhoutiana Hernandia nymphaeifolia Hibiscus tiliaceus Homalium whitmeeanum Inocarpus fagifer Ixora calcicola Lantana camara Leucaena leucocophala Litsea mellifera Macaranga harveyana Mangifera indica Manilkara dissecta Maniltoa grandiflora Melicope retusa Memecylon vitiense Meryta macrophylla Micromelum minutum Morinda citrifolia Mussaenda raiateensis Myristica hypargyraea Neisosperma oppositifolium Neonauclea forsteri Ochrosia vitiensis Pandanus tectorius Pemphis acidula Phaleria disperma Pimenta dioica Pisonia grandis Pisonia umbellifera Pittosporum arborescens Pouteria garberi Pouteria grayana Pouteria membranacea Pouteria species unknown Pleiogynium timoriense Podocarpus pallidus Polyscias multijuga Pometia pinnata Porterandia crosbyi Premna serratifolia Pritchardia pacifica Psidium guajava Psychotria carnea Psychotria insularum Rhus taitensis Santalum yasi G. Forst. G. Forst. G. Forst. G. Forst. Decne. A. C. Sm. J. R. & G. Forst. G. Forst. (J. R. & G. Forst.) Schinz & Guillaumin L. Cav. Jacq. (Blanco) Radlk. A. Gray Ait. Kostermans Guill. (Presl) Kubitzki L. H. St. John (Parkinson) Fosberg A. C. Sm. L. (Lam.) de Wit A. C. Sm. (Muell. Arg.) Muell. Arg. L. (L. f.) Dubard (A. Gray) Scheff. A. Gray A. Gray (Rich ex A. Gray) Harms (G. Forst.) Seem. L. J. W. Moore A. Gray (Lam.) Fosberg & Sachet (Seem. ex Havil.) Merr. (Markgr.) Pichon Parkinson J. R. & G. Forst. (G.Forst.) Baill. (L.) Merr. R. Br. (G. Forst.) Seem. Rich ex A. Gray (Christoph.) Baehni (H. St. John) Fosberg (Lam) Baehni (DC.) Leenh. N. E. Gray (A. Gray) Harms J. R. & G. Forst. (Burk.) A. C. Sm. & S. Darwin L. Seem. & H. Wendl. L. (G.Forst.) A. C. Sm. A. Gray Guill. Seem. Tong 1 Vav 2 X X X X X X X X X Vav 3 X X X X X X X X X X X X X X Eua 4 Kao EHap Tofua 5 6 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X App. 2-4. Internet supplement to: Franklin, J.; Wiser, S.K.; Drake, D.R.; Burrows, L.E. & Sykes, W.R. 2006. Environment, disturbance history and rain forest composition across the islands of Tonga, Western Polynesia. J. Veg. Sci. 17: 233-244. X X X X 4 App. 2, cont. Family Species name Species authority Sapindaceae Goodeniaceae Mimosaceae Anacardiaceae Mimosaceae Solanaceae Fabaceae Moraceae Myrtaceae Myrtaceae Myrtaceae Myrtaceae Myrtaceae Rubiaceae Combretaceae Combretaceae Malvaceae Boraginaceae Ulmaceae Sapotaceae Meliaceae Verbenaceae Olacaceae Meliaceae Meliaceae Flacourtiaceae Rutaceae Sapindus vitiensis Scaevola sericea Schleinitzia insularum Semecarpus vitiensis Serianthes melanesica Solanum mauritianum Sophora tomentosa Streblus anthropophagorum Syzygium brackenridgei Syzygium clusiifolium Eugenia crosbyi* Syzygium dealatum Syzygium richii Tarenna sambucina Terminalia catappa Terminalia litoralis Thespesia populnea Tournefortia argentea Trema cannabina Genus unknown Vavaea amicorum Vitex trifolia Ximenia americana Xylocarpus granatum Xylocarpus moluccensis Xylosma simulans Zanthoxylum pinnatum A. Gray Vahl (Guill.) Burkhart (A. Gray) Engl. Fosberg Scop. L. (Seem). Corner (A. Gray) C. Muell. (A. Gray) C. Muell. Burk. (Burk.) A. C. Sm. (A. Gray) Merr. & Perry (G.Forst.) Durand ex Drake L. Seem. (L.) Sol. ex Correa (L. f.) Heine Lour. Benth. L. L. Koenig (Lam.) M. Roem. A.C. Sm. (J. R. & G. Forst.) W. R.B. Oliver Tong 1 Vav 2 X X Vav 3 Eua 4 Kao EHap Tofua 5 6 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X App. 2-4. Internet supplement to: Franklin, J.; Wiser, S.K.; Drake, D.R.; Burrows, L.E. & Sykes, W.R. 2006. Environment, disturbance history and rain forest composition across the islands of Tonga, Western Polynesia. J. Veg. Sci. 17: 233-244. 5 App. 3. Common (wide-ranging) species associated with forest types, frequency (% of plots in which species occurred in an island group) shown for island groups with > 5 plots. * Geniostoma rupestre and G. insulare could not be distinguished in the field, and were grouped as G. rupestre (see App. 1). Ton = Tongatapu; Vav = Vava’u group; Eua = ’Eua; KT = Kao and Tofua (no. of plots in parentheses). Ha’apai and ’Euaiki were excluded because of the small number of plots. Species name Ton (55) Vav (84) Eua (40) KT (6) Littoral/Coastal Guettarda speciosa Hibiscus tiliaceus Neisosperma oppositifolium Pandanus tectorius Terminalia litoralis 5 40 29 31 9 23 35 21 37 11 13 20 28 10 5 17 17 0 17 0 Early successional Alphitonia zizyphoides Cocos nucifera Geniostoma rupestre* Glochidion ramiflorum Grewia crenata Macaranga harveyana Morinda citrifolia Rhus taitensis 4 35 11 2 51 15 51 44 52 46 32 14 35 5 38 39 20 5 3 8 18 8 10 18 67 17 83 33 17 17 17 50 Late successional Diospyros elliptica Elaeocarpus tonganus Elattostachys falcata Ficus scabra Micromelum minutum Pittosporum arborescens Pouteria grayana Vavaea amicorum Xylosma simulans 9 22 31 20 16 7 16 40 33 30 25 60 19 44 24 57 55 57 0 18 50 25 5 3 25 25 38 17 100 100 50 0 17 17 50 0 App. 4. Importance value (IV, average of relative abundance and relative frequency) for species that are indicators for island groups. *Found only in plots on the associated island group. Island group Species name IV p Vava’u Zanthoxylum pinnatum* Cryptocarya turbinata 57 45 0.026 0.033 ’Eua Garcinia myrtifolia Dysoxylum tongense* Citronella samoensis 45 40 38 0.013 0.030 0.036 Kao, Tofua Psychotria insularum* Canarium vitiense* Heritiera ornithocephala* Neonauclea forsteri Fagraea berteroana Melicope retusa* Celtis cf. harperi* 100 83 83 49 40 33 17 0.001 0.001 0.001 0.005 0.035 0.027 0.035 App. 2-4. Internet supplement to: Franklin, J.; Wiser, S.K.; Drake, D.R.; Burrows, L.E. & Sykes, W.R. 2006. Environment, disturbance history and rain forest composition across the islands of Tonga, Western Polynesia. J. Veg. Sci. 17: 233-244.