![]() ![]() These soils limit plant growth because they are extremely nutrient poor they currently support peat swamp on poorly drained sites and heath forests on well-drained sites, both with a distinct species composition, generally low productivity, and poor diversity compared with the richer lowland forests on fine-textured, more nutrient-rich and better-drained soils ( 22). The current topsoil texture map of the region ( 21) shows that coarse-textured, often poorly drained soils are a common feature of the central part of the region ( Fig. This situation existed for only 17% of time during the last 250,000 y ( 2), making it unlikely that it is solely responsible for the observed biogeographic pattern in Sundaland.Īnother explanation for the biogeographic boundary in central Sundaland relates to the soil conditions of the exposed sea floor ( 12). Furthermore, the savanna-corridor hypothesis is based on the climatic conditions during glacial maxima when land area was maximal. Although there is strong evidence for drier conditions within the region during the last glacial period ( 10, 12– 15), the presence of a continuous north-south savanna corridor through the center of Sundaland remains controversial, and most coupled vegetation–climate reconstructions contradict this possibility ( 4, 16– 20). These differences have been explained by a hypothesized north-south–oriented savanna corridor through the center of Sundaland that blocked dispersal of wet forest species ( 7– 13). Despite this long history of land connections, there exists a marked biogeographic boundary between western (Malay Peninsula and Sumatra) and eastern (Borneo) Sundaland ( 3, 5, 6). The present-day insular nature of this region is unrepresentative of the historical situation because most of the time the area formed a single landmass as a result of lowered sea levels associated with global cooling events ( 2– 5). Together with the Amazon Basin, Congo Basin, and New Guinea, Southeast Asia's Sundaland forms one of the world's largest equatorial tropical forests ( 1). This finding makes it clear that proposed biogeographic explanations for plant and animal distributions within Sundaland, including possible migration routes for early humans, need to be reevaluated. However, we could not confirm the presence of a savanna corridor. These results strongly suggest that exposed sandy sea-bed soils acted as a dispersal barrier in central Sundaland. ![]() ![]() For drought tolerance, no such pattern was detected. Indicator taxa of clusters that occurred across Sundaland had significantly higher coarse-soil tolerance than did those from clusters that occurred east or west of central Sundaland. We found 11 terminal floristic clusters, 10 occurring in Borneo, 5 in Sumatra, and 3 in Peninsular Malaysia. We then identified the indicator genera for clusters that crossed the central Sundaland biogeographic boundary and those that did not cross and tested whether drought and coarse-soil tolerance of the indicator genera differed between them. To test these two nonexclusive hypotheses, we performed a floristic cluster analysis based on 111 tree inventories from Peninsular Malaysia, Sumatra, and Borneo. An additional explanation might be related to the coarse sandy soils of central Sundaland. However, the short duration of these dry savanna conditions make it an unlikely sole cause for the biogeographic pattern. A dispersal barrier in the form of a dry savanna corridor during glacial maxima has been proposed to explain this disparity. ![]() The marked biogeographic difference between western (Malay Peninsula and Sumatra) and eastern (Borneo) Sundaland is surprising given the long time that these areas have formed a single landmass. ![]()
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