Ecosystem structure and productivity of tropical rain forests along altitudinal gradients with contrasting soil phosphorus pools on Mount Kinabalu, Borneo

1 We measured above-ground net primary productivity (ANPP) and ecosystem structure and processes in eight rain forest stands at four elevations (700, 1700, 2700 and 3100 m) and on two geological substrates (sedimentary vs. ultrabasic rock) on Mount Kinabalu, Borneo. 2 All ultrabasic sites had smaller pools of total soil phosphorus (P) and of labile inorganic P than did the sedimentary sites at the same altitudes. We predicted that the magnitude of altitudinal changes in ANPP would be less on ultrabasic than on sedimentary substrates, reflecting lower temperature dependency of ANPP under stronger P limitation. 3 Although ANPP declined with increasing altitude on both substrates, the slopes of the two regression lines were similar. The intercept was, however, marginally greater on sedimentary than on ultrabasic substrate. 4 Stand-level nutrient-use efficiencies (the ratio of litterfall mass to nutrient return) for N and P were only affected by altitude on ultrabasic substrate where they increased exponentially. Mean foliar N and P contents per unit leaf area of the canopy species increased with altitude on both substrates, but differed between substrates at the same altitude only for P (lower on ultrabasic). 5 Leaf area index (LAI) decreased upslope on both substrates. We assumed that half of primary production was allocated below-ground in order to evaluate stand level net assimilation rate (NAR). This was nearly constant on sedimentary substrate, but declined linearly with increasing altitude on ultrabasic substrate, where it may have to be added to LAI to explain ANPP patterns. 6 We suggest that on sedimentary substrate trees may be able to maintain NAR under colder environments by increasing foliar N and P per unit leaf area, but P deficiency prevents them from adjusting on ultrabasic substrate.