Contrasting nutrient strategies and thresholds in tropical forests revealed by whole ecosystem nutrient flux

Soil nutrients can limit productivity on highly weathered soils, but vegetation can adopt a range of strategies to maintain productivity under low nutrient supply. Using a full nutrient flux approach, we examine nutrient use strategies across nine old-growth and logged lowland moist tropical forests in Malaysian Borneo. Soil nutrient availability was a weak predictor of productivity. We explored the reasons for this by examining the vegetation biogeochemical cycles of five key macro- and micronutrients. For nitrogen (N), we found very little evidence of nutrient limitation. Four nutrients showed evidence of shifting strategies under limitation, with clear evidence for quantifiable thresholds below which compensation strategies were invoked and contrasting resource optimization strategies employed for each nutrient. For potassium (K), enhanced leaf resorption was the primary strategy for coping with supply limitation. For calcium, shifting stoichiometry was the primary strategy. For phosphorus (P), a combination of both enhanced resorption and shifting stoichiometry was observed. The strongest relationships were found for P and K, with old-growth forests at this site showing some limitation and logged forests having sufficient nutrient supply. This study reveals the potential of nutrient flux approaches to describe the multifaceted and non-linear relationship between soil nutrient supply and uptake, and biomass productivity.