Woody Vegetation in the Upland Region of Rarotonga, Cook Islands!
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Pacific Science (1985), vol. 39, no. 1 © 1985 by the University ofHawaii Press. All rights reserved Woody Vegetation in the Upland Region of Rarotonga, Cook Islands! MARK D. MERLIN2 2 ABSTRACT: Rarotonga is the largest (64 km ) and by far the highest (652 m) of the Cook Islands. The native coastal and lowland vegetation of this high volcanic, tropical island has been either completely removed or heavily disturbed. Numerous exotic plant species have been introduced and many ofthese are now naturalized in the lower elevation habitats ofthe island. The results ofthis initial, quantitative study in the upland forests ofRarotonga indicate, however, that the plant life ofthe rugged interior is still largely dominated by native species. Over 92 percent of all the woody plants (dbh > 2.5 cm) sampled in the 19 upland forest transects are either indigenous or endemic to Rarotonga. Native plants also accounted for more than 95 percent ofthe basal area covered by the woody vegetation in the upland study area. Three basic native plant associations have been recognized by dendrogram analysis: (1) the Homalium montane forest; (2) the Fagraea-Fitchia ridge forest; and (3) the Metrosideros cloud forest. The first two associations develop under subtropical climatic conditions, while the cloud forest is adapted to warm temperate conditions. Some aspects of the biogeo graphical significance ofthis unique forest region and the ecological implications of human disturbance in the uplands are also discussed. THE COOK ISLANDS INCLUDE fifteen small coral that both vegetation and floristic research in and volcanic islands with a total land area of the island group have been quite limited. approximately 238 km2 scattered over about Rarotonga is the only island in the Cook Ar 2,176,000 km2 of the South Pacific Ocean chipelago with two published floras (Cheese between 9° and 23° S latitude and 156° and man 1903, Wilder 1931); but the most recent 167° W longitude (see Figure 1). Rarotonga, of these, over 50 years old, has a number of located near the southern end of the archi inaccuracies and needs considerable updating pelago at 21° 12' S latitude and 158° 46' W (Sykes 1980, W. Authur Whistler, pers. 2 longitude, is the largest (64 km ) and by far comm. 1984). In addition to Sykes' literature the highest (652 m) of the Cook Islands (Fig review, the only recent botanical publications ure 1). This island is the subaerial portion of dealing with Rarotonga include a brief flo an extinct shield volcano typical of the ig ristic discussion emphasizing phytogeography neous formations commonly found in the (Philipson 1971), a taxonomic account of the deep Pacific Ocean Basin. ferns and fern allies found in the Southern In his annotated bibliographic study of Cook Islands (Brownlie and Philipson 1971), botanical research carried out in the Cook and a list of the vascular flora on the reef Islands, Sykes (1980) briefly described the veg islands of Rarotonga (Fosberg 1972). etation of the Cook Islands and presented an In an unpublished, descriptive report based exhaustive survey of the references to the ter on field work carried out in 1974 and 1975, restrial plant life in the archipelago. He notes Sykes (1976, revised in 1983) divided the veg etation of Rarotonga into coastal, lowland, and upland zones. The coastal zone, which 1 Manuscript accepted 15 August 1984. 2 University of Hawaii at Manoa, General Science contains strand, scrub, and small makatea Department, Honolulu, Hawaii 96822. (raised limestone) components, still has some 81 82 PACIFIC SCIENCE, Volume 39, January 1985 VEGETATION SAMPLING SITES ON RAROTONGA 21 15' tienrhyn o M.~nihikj Pukapuk.t c;;) Nass.u 0 NORTHERN GROUP Suwarrow Is. o 160 W 180 140 W Hawaii 20 N COOK ISLANDS • Sampling Siles PACIFIC Equ.lor.J. + --J-.oPa~~rlston Paved Roads OCEAN Aitutaki 0 ~bnU~itiIIrO ----0 Sireams Til ulea 0 l) 20 Aliu ~auke L!. Mln.,Peaks Q NGAo Coral Reef Mangaia "'",w SOUTHE N GROUP Towns Tropic f C.pricorn L-_'-'.!_4---o'{,,2__--" Mile Q 1{2 1 Kilometer Miles 0 100 200 ]90 Kilometers 0 100 200 ]00 400500 FIGURE 1. Map of the Cook Islands and vegetation sampling sites on Rarotonga. Woody Vegetation in,Upland Rarotonga-MERLIN 83 FIGURE 2. The northern coast, lowlands, and lower uplands of Rarotonga, areas (e,g., the Muri Sand) where native vege Numerous introduced species have become tation can be found. However, the majority of naturalized along the coast orin the lowlands. this zone has been greatly modified by human Some of these nonnative plants were brought disturbance. The native plants that can still be in during the prehistoric Polynesian period, found in this zone, such as Ipomoea pes but many more were introduced after the caprae, Vigna marina, Scaevola taccada, Bar initial European contact in the early part of ringtonia asiatica, Hernandia nymphaeifolia, the 19th century. and Pisonia grandis, are almost all organisms Weedy plants that have become abundant commonly found in the coastal areas of trop in some parts of the lower habitats of Raro ical South Pacific islands. tonga include herbaceous species such as The lowland vegetation zone of Raro Bidens pilosa, Elephantopus mollis, Euphorbia tonga comprises the low lying, generally level hirta, Mimosa pudica, Sida rhombifolia, area located between the coastal coral sands Mikania micrantha, Momordica charantia, and the steep hills a few hundred meters in Panicum maximum, Chrysopogon aciculatus, land. This zone also includes the lower, mode Cenchrus echinatus, Sorghum bicolor, and rately inclined, hilly areas near the coast and Stenotaphrum secundatum. Exotic woody the valley bottoms which, in some places, species that are commonly naturalized in penetrate far into the interior of the island some coastal or lowland habitats include (Figure 2). Although this zone contains the Leucaena leucocephala, Lantana camara, most fertile soils on Rarotonga, almost all of Aleurites moluccana, Inocarpus fagifer, Man the original lowland plant life of Rarotonga gifera indica, Cecropia palmata, Inga edulis, has been replaced by exotic associations of Syzygium sp., Psidium guajava, and more re cultivated and/or weedy plants (Figure 3). cently Ardisia humilis. It should also be noted 84 PACIFIC SCIENCE, Volume 39, January 1985 FIGURE 3. Cultivated pondfields and plantations in lower Takuvaine Valley, with Ikurangi Peak in the far background. that Hibiscus tiliaceus, an aggressive, woody both start atlower elevations on the windward species which is commonly found along some side of the island. coastal areas, in abandoned plantations and These upland vegetation subzones of taro (Colocasia esculenta) pondfields, on Rarotonga are botanically unique and of stream banks (Figure 4), and even in some of special scientific interest for two reasons: (I) the more recently disturbed lower forest as noted above and supported in the present slopes, is here considered to be an early Poly study, these subzones are still almost entirely nesian introduction in the Cook Islands (cf. covered by native species; and (2) since the Fosberg 1975). summit ofMangaia, the second highest island Unlike the vegetation in the coastal and in the Cook group, is only 168 m (550 ft) lowland zones on Rarotonga that is predomi above sea level, the native vegetation above nately exotic, the plant life found in the rugged about 200 m on Rarotonga is not, or in many uplands is overwhelmingly made up ofnative cases may never have been, found anywhere species (Cheeseman 1903, Philipson 1971, else in the Cook Islands. Consequently, the Sykes 1980). This upland zone includes all of native upland forests of Rarotonga can only the interior area above about 50 m to 200 m be compared with similar native subtropical (Figure 5). Itcan be separated into subtropical and warm temperate ecosystems found on and warm temperate subzones which have dis other high volcanic islands in: (1) the Society, tributions that vary widely according to the Marquesas, and Austral islands located altitude and aspect ofthe narrow ridges, steep hundreds of kilometers to the east; (2) the slopes, and deep valleys. Thus, for example, Samoan, Tongan, and Fijian islands located the subtropical and warm temperate subzones hundreds of kilometers to the west; or (3) in Woody Vegetation in Upland Rarotonga-MERLIN 85 FIGURE 4. Hibiscus tiliaceus and native ferns on the banks ofPapua Stream. FIGURE 5. The uplands in the interior of Rarotonga, with Maungaroa (509 m) the highest peak in this east-facing view. 86 PACIFIC SCIENCE, Volume 39, January 1985 the Kermadec Islands and part of the North posits. These are bordered by a small fringing Island of New Zealand farther to the reef that encloses a narrow lagoon. southwest. Although Rarotonga lies in the path of the In spite of the unique environmental situa generally mild, prevailing southeast trade tion and nearly original condition ofthe native winds, it is periodically affected by severe tro upland forests of Rarotonga, there have been pical storm or cyclone conditions during the no previous detailed ecological studies of this warmer, more humid hurricane season that region (Sykes 1980). The nearly pristine eco usually lasts from November to April. In the logy, exceptional phytogeography, and lack lowlands ofthe island, the mean monthly tem of any prior quantitative vegetation research perature ranges from 22° C in July to 26° C in in the central part ofthe island have prompted January, a difference of only 4° C (Kennedy this study ofthe woody vegetation in the sub 1974). The average annual rainfall in the tropical and warm temperate uplands of coastal lowlands is 2ll0mm. A large percen Rarotonga. The major aims of this research tage ofthe yearly precipitation usually occurs are to determine what kinds of woody plant during the summer months of the Southern associations occur in these uplands, to mea Hemisphere (especially December-March) sure the relative dominance and frequency of when the tradewinds are more variable and the individual species found in these vege the island may experience squalls and north tation types, and to quantify the extent to erly winds (Stoddart 1975).