Pacific Science (1992), vol. 46, no. 2: 159-178 © 1992 by University of Hawaii Press. All rights reserved Vegetation of Samoa and Tonga! W. ARTHUR WHISTLER2 ABSTRACT: Based on field studies and a previous review of the literature, 22 plant communities are recognized in the two adjacent South Pacific archipela­ goes of Samoa and Tonga. Because of similarities of climate and flora, most of the communities are similar in the two archipelagoes; the major differences result from the coralline nature of most of Tonga and the volcanic nature of Samoa. The communities are briefly described, the dominant species are listed, and the variation between the two archipelagoes is noted. THE TONGAN AND SAMOAN archipelagoes are unincorporated territory ofthe United States. situated in the tropical South Pacific to the It lies at a latitude of 13-15° S (ll° when east of Fiji (Figure I). Tonga, which lies at a Swains Island is included) and a longitude of latitude of 15-23° S and a longitude of 173­ 168-173° W. The archipelago comprises nine 177° W, comprises about 150 islands with a inhabited islands, plus Swains Island and 2 total area of 697 km , but only about 36 of uninhabited Rose Atoll (ca. 200 km beyond these islands are currently inhabited. The the easternmost volcanic island, Ta'u), and 2 archipelago is basically a double chain of has a total area of ca. 3100 km . It is a single islands running in a north-northeast direction chain of volcanic islands running in a west­ with small, high, volcanically active islands northwest direction. The four main islands, on the west, and lower, larger, raised coral going from west to east, are Savai'i (1820 km2 2 islands on the east. The main limestone is­ area, 1860 m elevation), 'Upolu (1110 km , 2 2 lands are Tongatapu (257 km in area, 80 m 1100 m), Tutuila (124 km , 650 m), and Ta'u 2 2 elevation), 'Eua (87 km , 330 m), and Vava'u (39 km , 930 m). 2 (90 km , 200 m). Numerous, small, raised­ Because both Tonga and Samoa are situat­ coralline islands known collectively as ed between the Tropic of Capricorn and the Ha'apai lie between Tongatapu and Vava'u. equator, their climate is tropical. There is In the volcanic chain, the main islands of the little seasonal or diurnal temperature varia­ southern part, going from south to north, are tion, although the winters in Tonga, which is 'Ata, Tofua, Kao, Late, and Fonualei; of farther from the equator than Samoa, are these, only Tofua is inhabited. Kao, which lies sometimes relatively cool. There is no pro­ adjacent to Tofua, has the highest elevation in nounced dry season and all areas except the Tonga (1046 m). The northern part of the Ha'apai Islands of Tonga receive at least 200 volcanic chain comprises three islands, cm of annual precipitation. Occasional Niuafo'ou, Niuatoputapu, and Tafahi, all of droughts, however, may occur, and hurri­ which are inhabited; these are actually closer canes are a threat in the summer season to Samoa than they are to the nearest in­ (December to April). habited island of Tonga (Vava'u). The two archipelagoes differ, however, in Samoa is an archipelago divided politically geology and soils. Samoa is volcanic in origin into Western Samoa, which is an independent and "oceanic" (i.e., it was formed from basalt country, and American Samoa, which is an rising from the ocean floor ofthe Pacific basin [from the Pacific Plate] beyond the continental islands to the west). The islands were born in I Manuscript accepted 2 May 1991. isolation and have never had a connection to 2 National Tropical Botanical Garden, and Depart­ ment of Botany, University of Hawaii at Manoa, 3190 any other land area. The islands of Tonga, Maile Way, Honolulu, Hawaii 96822. however, are mostly coralline and "continen- 159 160 PACIFIC SCIENCE, Volume 46, April 1992 SaVai'i. 'Upolu ~ WESTERN SAMJA ..... Tutuila­ Ta'u • Niuafo'ou •• Tafahi Niuatoputapu SOUTH PACIFIC OCEAN • Fonualei .." Vava'u Late Kao,• .• Tofua • Ha I apai Islands 20 0 KINGDC:M OF 'IDNGA 210 .., Tongatapu •'Eua 50 100 kIn o, e t • 'Ata 170 0 FIGURE 1. Map ofTonga and Samoa. Vegetation of Samoa and Tonga-WHIsTLER 161 tal," and lie on the eastern edge of the Asia­ TABLE I Australia Plate. Uplifting of the edge of this COMPARISON OF TIIE NATIVE FLORA AND GEOGRAPHY OF plate has been caused by subduction of the TONGA AND SAMOA Pacific Plate under it, which in turn has caused the volcanic activity on the western edge ofthe TONGA SAMOA island chain (Crane 1979). Area 797 km2 3,100 km2 Nearly the entire surface of Samoa is Highest elevation 1,046m 1,860m covered with soil derived from basalt (Wright Angiosperm families 83 95 1963), but in Tonga this can be said only of Angiosperm genera 248 300 the western chain ofislands. The most recent Angiosperm species 340 ca. 550 volcanic activity in Samoa occurred on the Gymnosperm species 2 o Pteridophyte species 77 215 island of Savai'i (ending in 1911), and in Endemism 3% ca. 30% Tonga. on several islands, most notably Niuafo'ou (which last erupted in 1946) and Tofua (which is still steaming). Most ofTonga the number of families and genera in the two has a coralline surface, although much ofthis archipelagoes are not very different. The num­ is now covered with a layer of volcanic ash ber ofspecies and level ofendemism, however, from an ancient volcanic eruption to the west are much higher in Samoa. ofthe main islands (Crane 1979). The exposed Although the geology ofthe two archipela­ edges ofthe uplifted coralline deposits that are goes is quite different, the similarity ofclimate characteristic of Tonga are not found in Sa­ and flora make the vegetation of the two moa, and consequently some plants adapted archipelagoes similar enough to be treated to this habitat are absent from there. together, and this has been done in this paper. Both Tonga and Samoa are floristically Like the rest ofPolynesia, the two archipel­ part ofthe "Fijian Region" that extends from agoes, which have been inhabited for over the Santa Cruz Islands and Vanuatu to Niue 3000 yr, have been extensively modified by (Takhtajan 1969). Lying in the eastern por­ human activity. This disturbance has led to tion ofthis region, the two archipelagoes have the loss ofmuch of the native vegetation that smaller native floras than the Melanesian once covered the islands. The loss was greatest islands to the west, which lie closer to the on the lowest, smallest, most fertile islands. Indo-Malaysian source region. Tongatapu, which is very flat and fertile, is Several floristic studies ofTonga have been now almost entirely devoid of native vegeta­ published (Hemsley 1894, Burkill 1901, tion, except for a narrow strand of littoral Yuncker 1959, Sykes 1978, 1981), and large forest along some of the coasts and areas of collections by M. Hotta, G. Buelow, W. R. mangrove on the north side. Vava'u is also Sykes, and W. A. Whistler have added many relatively flat and low, and its native vegeta­ unpublished records. Several floristic studies tion is now restricted mostly to the cliff areas ofSamoa have also been published (Reinecke on the northern coast. 'Eua, however, is some­ 1896, 1898, Setchell 1924, Christophersen what less disturbed, primarily because the 1935, 1938, Christensen 1943, Yuncker 1945, eastern portion is higher and more rugged and Whistler 1983b), and these have been aug­ population pressure is less. The volcanic is­ mented by a large collection by W. A. Whis­ lands are even less disturbed by human activi­ tler between 1972 and 1991. Based on a ty, particularly the uninhabited ones, but they compilation of these sources and specimens, are small in area and their vegetation (and the numbers in Table 1 were obtained. flora) is more similar to that ofSamoa than it The floras of Tonga and Samoa are very is to the rest of Tonga. similar to each other; over 70% of the native Samoa, with a larger area and higher eleva­ Tongan flowering plants are also found in tion, is somewhat less disturbed, although Samoa. A comparison ofthe two floras (Table probably over halfofits native vegetation has I) shows that Samoa has four times the area been severely altered by human activity and and nearly twice the elevation of Tonga, but natural catastrophies. Much of this has hap- 162 PACIFIC SCIENCE, Volume 46, April 1992 pened in the last few decades, as an increasing TABLE 2 population, an unregulated forestry industry, VEGETATION TyPES IN SAMOA AND TONGA and two destructive hurricanes have taken their toll. The best remaining areas of undis­ SAMOA TONGA turbed vegetation in Samoa are in the high­ lands of Savai'i. However, the vegetation of Littoral vegetation Western Samoa has not been evaluated since I. Herbaceous strand + + 2. Littoral shrubland + + the devastating hurricane of February 1990. a. Makatea + To make some kind of sense out of the 3. Pandanus scrub + + patterns of plant distribution, the vegetation 4. Littoral forest + + is divided here into 22 communities, as shown a. Mixed species + + in Table 2. These 22 are classed into six b. Barringtonia + c. Calophyllum + broader categories: littoral vegetation, wet­ d. Pisonia + + land vegetation, rainforest vegetation, upland e. Hernandia + scrub vegetation, volcanic vegetation, and f. Terminalia + + disturbed vegetation. g. Excoecaria + Wetland vegetation 5. Coastal marsh + + 6. Montane marsh + Littoral Vegetation 7. Montane bog + 8. Mangrove scrub + + The littoral vegetation category comprises 9. Mangrove forest + + four communities: herbaceous strand, littoral 10. Swamp forest + shrubland, Pandanus scrub, and littoral for­ a. Coastal + est. Several factors tend to blur the distinc­ b.