HETEROGENEITY OF SOIL MORPHOLOGY AND HYDROLOGY ON THE 50 HA LONG-TERM ECOLOGICAL RESEARCH PLOT AT PASOH, PENINSULAR MALAYSIA

The soils of the 50 ha long-term Centre for Tropical Forest Science (CTFS) research plot at Pasoh, Peninsular Malaysia were surveyed in detail in 1994–1996 and briefly re-examined in 2005–2006. Pasoh is rain shadowed by mountains to both east and west, and the aseasonal 1800 mm annual rainfall is marginal for an evergreen forest. There is a low ridge of Triassic shale in the north-eastern corner but two thirds of the plot consists of alluvial flats. The ridge is capped by remnants of a laterite (ferricrete) sheet, and the well-drained, brown fine loam over clay soils range from 50 cm over a sheet of dense gravel laterite on the crest, through moderately dense gravels on midslopes, to deep clays with few gravels on lower slopes. The alluvial sediments are derived from variable mixtures of granite, sandstone and shale. Drainage varies from moderately free on a low (1–3 m) terrace to perennially saturated in low parts of a floodplain. Many of the poorly drained soils are stagnogleys, with mucky surfaces, and very wet, mottled grey upper mineral horizons, overlying moist and less mottled brown subsoils. The ridge soils are moderately dystrophic, and soil depth and available moisture reserves for dry spells appear to be major edaphic constraints. The alluvial soils are also moderately dystrophic, and vary substantially with respect to aeration of root zones. Stagnogleying imposes a particular combination of aeration stresses on young plants, as young seedlings have to cope initially with the saturated upper horizons, but encounter better aerated lower subsoils if they survive and grow. Nutrients are also important, but it is the extent and type of drainage constraints that make Pasoh unusual among CTFS plots.