Hydrological Observations in Montane Rain Forests on Gunung Silam, Sabah, Malaysia, with Special Reference to the `Massenerhebung' Effect

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Abstract
1. Mount Silam is a coastal mountain in Sabah, Malaysia, showing a telescoping of vegetation zones and stunting of the montane forest near its summit (884 m a.s.l.). The causes of the stunting were investigated. 2. Hydrological observations were made in the forest at 680 m a.s.l. (just below the cloud cap; trees up to 32 m) and at 870 m (within the clouds; trees up to 15 m). At 870 m, rainfall was c. 25% above that at sealevel; radiation was 21% and 33% below values observed at 680 m and at sealevel, whereas Penman open-water evaporation E(o) was reduced by 19% and 31%, respectively. 3. Short-term water balance calculations suggested average actual evapotranspiration rates (mm day-1) of 1.9 (870 m) and 2-5 (680 m; 2.0 during a dry spell of 2 weeks) and average transpiration (E(t)) rates of 0-85 (870 m) and 2-1 (680 m; 1.7 during dry spell). The ratio E(t)/E(o) was decidedly low for the smaller forest, and well below the potential rate made possible by incoming radiation. 4. The soil near the summit remained wetter than that at 680 m. During spells with little or no rainfall soil water tensions rose sharply at 680 m but those at 870 m were usually close to saturation. 5. Mineral nutrient concentrations increased in the order: rainfall, throughfall, litter percolate. Both litter percolate and soil moisture were more acid at 870 m. Concentrations of potassium and calcium were low in subsoil moisture and in streamflow, suggesting tight cycling of these elements. 6. Topsoil concentrations of organic carbon and total nitrogen were c. six times higher at 870 m. Mineral nitrogen, mostly ammonium, was c. 2.2 kg ha-1 (0.06% of total nitrogen) at 870 m. Corresponding values at 680 m were 4-1 kg ha-1 and c. 0.6%. Numbers of nitrogen-mineralizing and nitrifying bacteria were low, particularly at 870 m. Nitrogen and carbon concentrations in fresh leaf litter were almost identical but concentrations of total phenols were much higher at 870 m. 7. Estimated annual amounts of mineral nutrients in rain and throughfall greatly exceeded estimated amounts of net uptake by the vegetation at 870 m. From the above sources, nitrogen may be rapidly immobilized in the soil and rendered unavailable to plants. 8. Possible causes of forest stunting are discussed and it is hypothesized that the stunting of the forest at 870 m is caused by a high carboxylation resistance which results in low photosynthetic rates and transpiration. The high carboxylation resistance may be related to a reduced soil supply of nitrogen but this is not manifest by lowered nitrogen concentrations in the leaves or litterfall. Alternatively, the high phenolic concentrations found in fresh leaf litter in the stunted forest could, upon leaching into the soil, interfere with photosynthesis, cell division in the fine roots, transpiration, and ion uptake.