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95 Terrestrial Ecosystems Harley I. Manner, Dieter Mueller-Dombois, and Moshe Rapaport Prior to European contact, virtually all Pacific Islanders lived in rural locations, dependent on the natural environment for basic subsistence needs. This dependence inevitably resulted in largescale ecosystem conversion. Today, the pace and intensity of exploitation have accelerated. Areas under primary forest, comprising many rare, endemic species of plants and animals, are covered by secondary forest, savannas, agriculture, and urban development. The study of terrestrial ecosystems and their modification has thus become a critical issue in the Pacific Islands. Vegetation is used as the principal biological component by which terrestrial ecosystems are recognized. Strand and Atolls The strand is that portion of the beach or coastline of volcanic rock or coral limestone located adjacent to the seashore but above the high-tide line (Fosberg 1960). The vegetation of the strand is a relatively narrow band of halophytic (salt-tolerant) herbs, shrubs, and tree species, often characterized by fleshy leaves with a salty sap, as well as by stunted and prostrate growth forms along windswept coasts (Figure 8.1). These species are well adapted to the harsh conditions of physical disturbance, salinity, drought, desiccation, and rocky, calcareous soils. Most strand species are widespread and indigenous to the tropical and subtropical Pacific Islands. Given the large number of atolls in the Pacific and the high ratio of coastline to land area, this ecosystem is one of the most common in the region. Atoll ecosystems are strand-like in character and support the relatively few species tolerant of salinity (Figure 8.2). The individual motu on an atoll do not always support all species found throughout the atoll. The motu often support only fragmental communities consisting of very few species. But most are indigenous. 8 Figure 8.2. Profile diagram of vegetation on a coral atoll, showing depressions used for growing Cyrtosperma chamissonis. Figure 8.1. Strand vegetation on a Micronesian atoll (photo HIM). 96 ■ The Pacific Islands On large motu exceeding a few hectares, natural vegetation tends to be more luxuriant, particularly where the freshwater lens is better developed. Large motu usually contain several phreatophytic (reliant on groundwater) species in their interior. Such islets commonly display a zonation of vegetation that coincides with the availability of fresh groundwater. The following zones may be present: 1. A sparse halophytic cover of vines, grasses, and sedges on the upper foreshore of windward coasts. One of the most widespread plant species is the prostrate vine Ipomoea pescaprae (beach morning glory). Also found here are some grasses and shrubs (particularly Scaevola taccada). The small tree or arborescent shrub Pemphis acidula commonly forms thickets on limestone rock along atoll beaches. 2. A halophytic low forest fringe. This is often dominated by Tournefortia argentea (the tree heliotrope) and typically occurs on the beach crest. 3. Littoral forest composed of halophytic, low-statured species . This may include Pandanus tectorius and Casuarina equisetifolia (ironwood), now mostly displaced by coconut plantations. 4. Mixed forests composed of several phreatophytic species. They may form two layers, with Pisonia grandis or Calophyllum inophyllum in the upper canopy and Guettarda speciosa in a lower subcanopy. Here too, the indigenous species have now become largely displaced. 5. Central depression with marsh or swamp forest vegetation . On the larger islets, these depressions have usually been excavated and enlarged for the cultivation of Cyrtosperma chamissonis (swamp taro) and Colocasia esculenta (taro). Some depressions may be saline and consequently contain mangroves. 6. Halophytic forest on lagoon shore beaches. Mangroves, including Bruguiera gymnorrhyza and Rhizophora spp., sometimes form small stands fringing lagoon shores. On smaller motu this zonation is often reduced or absent. The vegetation then is composed only of low-lying shrubs, grasses, and sedges (Stoddart 1975). On dry atolls, even where large motu are present, the vegetation is likewise restricted and dominated by only a few salt-tolerant and drought-resistant species, often forming monospecific communities (Mueller-Dombois and Fosberg 1998). On Taongi Atoll in the Northern Marshall Islands, the forest and scrub vegetation is dominated by Tournefortia argentea. In the Phoenix Islands, pure stands of Cordia subcordata can be found (Hatheway 1955). In moister atolls of the Southern Marshall Islands, Pisonia grandis grows in species-pure dense stands, often accompanied by the presence and smell of guano (Hatheway 1953). These stands of Pisonia were formerly widespread features of atolls. Their monodominance as forest species may be related to the harsh, salty atoll environment and the very small total number of plant species available, as well...

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