Nutrient cycling of Bornean tropical forests

The thesis addresses a novel topic of nutrient cycling of tropical forests and substantially adds to the small amount of currently available data in the area. While nutrients like nitrogen (N) and phosphorus (P) are considered to limit net primary productivity (NPP) in lowland tropical forests, direct evidence of nutrient limitation of NPP is scarce. The thesis focuses on humid lowland tropical forests along a disturbance gradient in Malaysian Borneo and quantifies key biological processes of nutrient cycling: nutrient requirement, uptake, use efficiency, resorption and allocation. The first empirical chapter examines wood-specific nutrient concentrations and their radial and vertical allocation within a whole tree. The second and third empirical chapters examine the effects of logging disturbance and soil availability on net primary productivity and nutrient cycling. Five macronutrients of N, P, potassium (K), calcium (Ca), magnesium (Mg), and carbon (C) are considered. I developed and employed a novel approach of coupling data on nutrient stoichiometry and NPP to provide direct estimates of fluxes and allocation of nutrients within forest ecosystems. In logged forests, the contribution of leaf resorption towards meeting nutrient requirement and the allocation of nutrients to canopy were lower, while allocation of nutrients to wood was higher than in old-growth forest. Contrary to expectation, NPP did not correlate with soil nutrient availability of any elements, suggesting soil availability is a weak predictor of NPP. Depending on the nutrient, different strategies, such as changes in resorption rates and changes in nutrient use efficiency, were adopted to compensate for low supply of that nutrient. Overall, this study demonstrates the value of coupling NPP with stochiometric data to gain insights into the nutrient cycle of intact and disturbed forests. These insights explain how limited supply of P, K and Ca is a challenge in lowland tropical forest ecosystems. The study found higher concentrations in sapwood relative to heartwood for Dipterocarp species, which provide empirical support for wood resorption during sapwood senescence. It also showed exceptionally high nutrient concentrations in the bark, which suggest debarking during logging operations could substantially contribute to maintaining nutrients within forest ecosystems.