Age years BP

(Inferred age at Sirurki)

Figure 4.13. Selected vegetation records derived from pollen diagrams from tropical Southeast Asia and the West Pacific. Only the last 30,000 years are shown. Human impact is omitted. References to individual sites are as follows: Summit, Brass, Imbuka, Komanimambuno (Hope, 1976), Inim (Flenley, 1972; Walker and Flenley, 1979), Sirunki (Walker and Flenley, 1979), Draepi (Powell et al., 1975), Di-Atas (Newsome and Flenley, 1988), Sipinggan (Maloney, 1981), Bayongbong (Stuijts, 1984; Stuijts et al., 1988). For other sites see Flenley (1998). After Flenley (1998).

At the LGM, climates cooler than now by as much as 7-11 °C can be suggested from the pollen results, but only c. 6°C from the geomorphology. How can this be? Possibly, snow lines were kept artificially high by the reduced precipitation that probably occurred at the LGM. This suggestion has been advanced by Walker and Flenley (1979). But, the precipitation in the mountains cannot have been too reduced, or rainforests would have disappeared there. Late Quaternary vegetational changes are summarized in Figure 4.13.

This whole question has been reviewed by Pickett et al. (2004) in a reconstruction of Quaternary biomes for the Southeast Asian region, Australia, and the Pacific. They conclude that the evidence from the Southeast Asian tropics indicates an LGM cooling of 1-2°C at sea level and 6-9°C at high elevation sites. This discrepancy was first noted by Walker and Flenley (1979), who attributed it to a steeper lapse rate, which was itself related to the generally drier conditions at the LGM, leading to a lapse

Rates The Tropical Rainforest

rate closer to the dry adiabatic lapse rate. Such a steeper lapse rate was however criticized as impossible in an environment where the pollen evidence clearly showed the persistence of rainforests (Webster and Streten, 1972; Kutzbach and Guetter, 1986).

The idea has however been revived by Farrera et al. (1999) who give a range of plausible mechanisms by which a steeper lapse rate can occur on tropical mountains. One among these was the observation that the moist adiabatic lapse rate steepens anyway as temperature is lowered (Hartmann, 1994). Another relevant point is the possible impact of reduced concentrations of carbon dioxide at the LGM (Street-Perrott, 1994), which would favour C4 plants (grasses) at the expense of trees.

A steeper lapse rate alone is however unable to explain all aspects of observed vegetation changes. Hope (1976) demonstrated—regarding Mt. Wilhelm—the curious fact that the Upper Montane Rain Forest (UMRF) (cloud forest) did not simply migrate downhill at the LGM: it virtually disappeared. This is not a Late Pleistocene anomaly, for Upper Montane taxa were greatly reduced in each glacial period in the Banda Sea record (see Section 4.5). To understand this phenomenon we must consider the physiognomy and environment of the UMRF at the present time (Flenley, 1992, 1993). The trees are stunted, with short internodes and small thick leaves which possess a hypodermis (an extra layer of cells below the epidermis). Often extra pigments are present as well as chlorophyll: usually flavonoids and/or antho-cyanins. These attributes are typical of plants experiencing stress of various kinds, including high ultraviolet-B and temperature extremes. The soils are unusual in the thickness of their litter layer. The temperature environment shows extreme variations on a diurnal basis: from very cold nights and early mornings, to sunny mornings and misty afternoons with 100% humidity. The morning insolation is high in ultraviolet-B because of the altitude. It is known that high UV-B can produce experimentally in crop plants exactly the same physiognomic peculiarities as the Upper Montane Rain Forest (Teramura, 1983), and can also inhibit insect activity, leading to thick litter layers in soils (Day, 2001). It therefore seems possible that UV-B is involved in the ecology of these forests. It may well be that the extreme diurnal variation of the temperature regime is also involved.

How does all this help to explain the decline of the UMRF in cold phases of the Pleistocene? Presumably, when cooler temperatures forced taxa downhill, they found themselves in an environment where the diurnal extremes of temperature and UV-B no longer existed to the same extent. The tropical lowlands are in fact usually lacking in such extremes. Assuming the UMRF taxa are genetically adapted to their present environment, the disappearance or great restriction of that environment at the LGM would have led to their reduction in the pollen record (Flenley, 1996, 1998). This argument will be elaborated in Chapter 8.

4.6.2 The Pleistocene-Holocene transition

Marine records generally show abrupt pollen shifts from glacial to interglacial conditions, suggesting strong Milankovitch forcing of climate and rapid response of the vegetation. However, altered boundary conditions, including those related to coastal landscapes and oceanic and atmospheric circulation, may have played a part in producing this degree of synchroneity. A major exception related to the Coral Sea record of ODP 820 where an increase in rainforest lags the marine isotope change from MIS 2 to 1 by several thousand years. As this lag is also evident in the Lynch's Crater record (Kershaw, 1986), it cannot easily be attributed to global climate forcing. Possible explanations are: the time taken for rainforest patches to expand from glacial "refugia" (unlikely considering the regional extent of rainforest during the last glacial period); the influence of southern hemisphere insolation forcing including ENSO on the record; and the continuing impact of Aboriginal burning; all of which slowed the rainforest advance. More detailed analysis of the earlier part of the ODP 820 record may help resolve this question.

Many terrestrial records are too coarse or insufficiently well-dated to detail local patterns of change during the last termination. However, at an altitude of 3,630 m in Irian Jaya, there is supporting evidence for a rapid replacement of grasslands and scattered shrubs by rainforest at the Pleistocene-Holocene boundary. Similar changes are evident at Lake Inim (Flenley, 1972) and in the elegant suite of sites on Mt. Wilhelm, the highest mountain in Papua New Guinea (Hope, 1976). With four sites at elevations from 2,750 m to 3,910 m, Hope was able to trace the deglaciation of the mountain and the rapid climb of the altitudinal forest limit to about 4,000 m in the early Holocene.

Walker and Flenley (1979) found a hint of a Late Pleistocene oscillation at Sirunki, though its age of c. 17kcal. yr bp does not correlate well with the Younger Dryas and is more consistent with the Antarctic Reversal. Support for such an oscillation has recently been demonstrated from a detailed analysis of the last termination at Rawa Danau in Java (Turney et al., 2006). Towards the end of the LGM (Turney et al., in press), high values for grass pollen—combined with the presence of the montane trees Dacrycarpus, Podocarpus, and Quercus—indicate much drier and cooler conditions than today. Initial increases in temperature and rainfall are recorded as early as 17 kcal. yr bp with increased representation of lowland rainforest taxa and reduction in Poaceae. There is then a reversal of this trend between 15.4 kcal. yr bp and 14.6 kcal. yr bp, prior to both the Antarctic Reversal and Younger Dryas, suggesting a regional tropical rather than hemispheric control over climate variation. Although rainforest became dominant at 14.6 kcal. yr bp, increased catchment erosion suggests rainfall further increased around 12.9 kcal. yr bp and that the summer monsoon may not have become fully established until the early Holocene.

4.6.3 The Holocene

Rainforest achieved its maximum areal extent in the Early-Middle Holocene under high levels of precipitation and temperature before it opened up again mainly within the last 5,000 years. Reasons for this rainforest reduction include climate factors, although these varied regionally. Hope (1976) attributes a reduction in the altitudinal treeline of about 200 m to a reduction in temperature in New Guinea, while season-ality or increased ENSO influence is considered to have been the major influence on both a change in the composition of rainforest and slight sclerophyll woodland expansion in northeast Queensland (Kershaw and Nix, 1989; McGlone et al., 1992; Haberle, 2005). However, the major impact on Holocene rainforest has been that of people.

Although people have been in the region for around 1.8 million years (Swisher et al., 1994; Huffman, 2001) and have had the ability to manage vegetation through the use of fire within the last 100 kyr, the ability of people to physically clear rainforest for agriculture is essentially a Holocene phenomenon within the region. There are indications of agriculture (for rice-growing) as early as 16 kcal. yr bp in the Yangtze Valley, China (Yasuda, 2002)—an area recognized as one of the cradles of crop domestication (Vavilov, 1951; Diamond, 1998)—and its spread into Southeast Asia, including those parts of the Sunda Platform which were then joined to the Asian Mainland. Several upland sites in Sumatra suggest that swiddening (slash-and-burn) was occurring for cultivation of dry (non-irrigated) rice or root crops as early as c. 10.3 kcal. yr bp at Danau-di-Atas (Newsome and Flenley, 1988), c. 10 kcal. yr bp at Pea Bullok (Maloney and McCormac, 1995), and c. 9 kcal. yr bp at Rawang Sikijang (Flenley and Butler, 2001). The general pattern of evidence for the region was reviewed by Maloney (1998) and Flenley (2000).

A separate center for the origin of agriculture is found in New Guinea, based on rootcrops—such as Colocasia (taro)—and palynological evidence of forest destruction presumably for agriculture dates back to c. 9 kcal. yr bp in the Baliem Valley (Haberle et al., 1991), to c. 6 kcal. yr bp or earlier at Draepi Swamp (Powell et al., 1975), and to 5 kcal. yr bp at Sirunki Swamp (Walker and Flenley, 1979). Early human activity in the New Guinea Highlands was confirmed by archeological finds at Kuk Swamp (Golson and Hughes, 1976; Golson, 1977; Denham et al., 2003). These included evidence of swamp drainage, presumably for the growing of taro, back to at least 6.8 kcal. yr bp and possibly c. 10 kcal. yr bp. The destruction of swamp forest on Lynch's Crater about 5 kcal. yr bp suggests that some form of cultivation may have spread into the rainforest areas of northeastern Australia at this time.

The progressive impact of these activities has led to the creation of permanent grasslands in many areas. These include the cogonales of the Philippines, the kunai of New Guinea, and smaller areas in Sumatra and elsewhere. In general, these areas have been maintained by frequent burning, and they tend to occur in regions where there is a more lengthy dry season (Thomas et al., 1956). Recently, agriculture and logging has of course still further diminished the area of surviving forest, but consideration ofthat is beyond the scope of this chapter.

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  • tom
    Is the moist adiabatic lapse rate greater near a tropical rainforest of on Antarctica?
    9 years ago

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