Understanding the controls over forest carbon use efficiency on small spatial scales: Effects of forest disturbance and tree diversity

Abstract

Tropical forests are characterized by their high photosynthetic activity. Results from pantropical studies show that on average only 30% of the products from photosynthesis are allocated to new biomass compared to on average 50% in temperate forest systems. Why those forests have a low carbon use efficiency (CUE) remains unknown. We present a synthesis of studies from two tropical sites with a similar methodological setup allowing the evaluation of the ecophysiological responses of tree communities to diversity effects and forest disturbance. Here we use xylem sap flux derived gross primary productivity (GPP) that was modeled with eddy covariance data for a seven-year-old planted forest with plots of varying tree species diversity in Panama and an old-growth forest with a distinct disturbance gradient in an Amazonian moist lowland forest. Additionally, net primary productivity (NPP) was calculated from inventory data. We found evidence that GPP is relatively stable with forest disturbance but GPP increases with tree species richness. Lowest CUE with only 35% of GPP allocated to growth was present in undisturbed forest patches. Light forest disturbance had a positive effect on CUE and 62% of GPP was allocated to grow new biomass. Monospecific stands had a higher CUE than diverse tree stands. We conclude that two factors are controlling CUE in tropical forests leading to low CUE. First, forest disturbance leads to a higher allocation of carbon towards growth. Second, a great fraction of GPP is used for processes dedicated to species complementarity. This improves the overall performance of diverse forest stands compared to monocultures.