Changes in Biomass Turnover Times in Tropical Forests and Their Environmental Drivers From 2001 to 2012

Tropical forests store about 70% of the total living biomass on land and yet very little is known about changes in this vital carbon reservoir. Changes in their biomass stock, determined by changes in carbon input (i.e., net primary production [NPP]) and carbon turnover time (τ), are critical to the global carbon sink. In this study, we calculated transient τ in tropical forest biomass using satellite-based biomass and moderate-resolution imaging spectroradiometer (MODIS) NPP and analyzed the trends of τ and NPP from 2001 to 2012. Results show that τ and NPP generally have opposite trends across the tropics. Increasing NPP and decreasing τ (“N+T−”) mainly distribute in central Africa and the northeast region of South America, while decreasing NPP and increasing τ (“N−T+”) prevail in Southeast Asia and western Amazon forests. Most of the N+T− tropical forest areas are associated with mean annual precipitation (MAP) below 2,000 mm·y⁻¹ and most N−T+ tropical forests with MAP above 2,000 mm·y⁻¹. The τ and NPP trends in the N+T− region are statistically associated with radiation, precipitation and vapor pressure deficit (VPD), while the τ and NPP trends in the N−T+ region are mainly associated with temperature and VPD. Our results inherit the uncertainties from the satellite-based datasets and largely depend on the carbon use efficiency from MODIS. We thus systematically assessed the robustness of the findings. Our study reveals regional patterns and potential drivers of biomass turnover time and NPP changes and provides valuable insights into the tropical forest carbon dynamics.