Introduction Present setting

This region differs from those supporting tropical rainforest in other parts of the world in that it is less continental and geologically much more dynamic. It incorporates some major pieces of continental plate, but its center—the so-called "Maritime Continent" (Ramage, 1968)—is largely a complex interaction zone between the Asian and Australian Plates resulting from the continued movement of the Australian Plate into Southeast Asia (Metcalfe, 2002). The effects of tectonic and volcanic activity have resulted in mountain uplift, particularly in New Guinea, and formation of the volcanic island chain of Indonesia. Vulcanicity also occurs out into the Pacific beyond the "andesite line" where most "high" islands are volcanic and most "low" islands are coral islands developed on sunken volcanoes.

The extensive areas of continental shelf—particularly the Sunda and Sahul Shelves—but including the shelves along the east coast of northern Australia and around the South China Sea, combined with the impact of the Indonesian through-flow that restricts the movement of warm water from the Pacific to the Indian Ocean, have resulted in the highest sea surface temperatures on Earth in the form of the West Pacific Warm Pool. The enhanced convective activity associated with the warm pool results in high rainfall through much of the year in the heart of the Maritime Continent and dominance of the vegetation by evergreen rainforest. The area also provides the major source of heat release that drives the East Asian-Australasian summer monsoon system reflected in the strong summer rainfall patterns beyond the Intertropical Convergence Zone in each hemisphere, and resulting in the occurrence of seasonal, raingreen or "monsoon" semi-evergreen to deciduous rainforest over much of continental Southeast Asia and the very north of Australia (Figure 4.1). Additional influences on rainforest distribution are the warm northerly and southerly currents

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Figure 4.1. Distribution of rainforest vegetation in the far east and pollen-analyzed sites covering at least the last 6,000 years. Rainforest types have been simplified from distributions and descriptions of communities identified by Fedorova et al. (1993, 1994).

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Figure 4.1. Distribution of rainforest vegetation in the far east and pollen-analyzed sites covering at least the last 6,000 years. Rainforest types have been simplified from distributions and descriptions of communities identified by Fedorova et al. (1993, 1994).

emanating from the Pacific equatorial current that, in combination with the southeast and northeast trade winds from the Pacific, result in the production of high orographic rainfall and associated rainforest along mountainous eastern coastal areas of Southeast Asia and Australia.

Most of the region is subjected to high interannual rainfall variability that is also, to a large degree, a product of its particular geography and the dynamics of oceanic and atmospheric circulation systems. The energy provided by convective activity within the Maritime Continent is the major contributor to the operation of the east-west Walker circulation that breaks down periodically, resulting in the movement of the warm water banked up against the Indonesian throughflow eastwards and resulting in a substantial reduction in precipitation from all sources over most of the region. These El Nino phases of the so-called "El Nino-Southern Oscillation" (ENSO)—that have also been linked to a weakening of the monsoon (Soman and Slingo, 1997)—can cause severe droughts and fires, even within rainforest, especially where there is disturbance from human activity.

Although tropical influences dominate the climate of the region, the Tibetan Plateau is important in creating a strong winter monsoon influence. The height and extent of this plateau results in the production of cool dry air that exacerbates seasonal contrasts in the northern part of the region and has a push effect on summer monsoon development in the southern hemisphere.

4.1.2 Nature of the evidence

Most of the evidence for past vegetation and climate from the region is derived from palynological studies. Perceived problems of pollen analysis in the lowland tropics— due to the richness of the flora, dominance of effective animal pollination, and lack of strong winds within the core area—resulted in most early research being focused on highland communities (Flenley, 1979). In these per-humid areas, a major interest has been and continues to be on altitudinal variation in the changing position and composition of montane rainforest and alpine zones in relation to global climate influences. Studies have been restricted mainly to swamps and shallow lakes covering the latter part of the last glacial period and Holocene.

Ventures into the terrestrial lowlands have generally not proved particularly successful due not only to original perceptions but also to the dearth of continuous sediment sequences in both perennially and seasonally wet environments, and lack of differentiation of peatland, riparian and dryland forest communities in the extensive peatlands that are otherwise very suitable for pollen analysis. Notable exceptions are deep defined basins of volcanic origin that have revealed detailed records of both vegetation and climate change, sometimes covering long periods of time.

A major feature of the region—that has been exploited in recent years—is the maritime setting whereby ocean basins occur in close proximity to land areas. A number of sediment cores have provided long and fairly continuous regional records of vegetation and climate change, securely dated from associated oxygen isotope records. Nevertheless, none of these records yet covers the whole of the Quaternary and reliance is placed on geologically isolated glimpses of past environments for some indication of the nature of the early part of this period.

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