Drought stress and tree size determine stem CO2 efflux in a tropical forest

Abstract

1. CO2 efflux from stems (CO2_stem) accounts for a substantial fraction of tropical forest gross primary productivity, but the climate sensitivity of this flux remains poorly understood. 2. We present a study of tropical forest CO2_stem from 215 trees across wet and dry seasons, at the world’s longest running tropical forest drought experiment site. 3. We show a 27% increase in wet season CO2_stem in the droughted forest relative to a control forest. This was driven by increasing CO2_stem in trees 10-40 cm diameter. Furthermore, we show that drought increases the proportion of maintenance to growth respiration in trees >20 cm diameter, including large increases in maintenance respiration in the largest droughted trees, >40 cm diameter. However, we found no clear taxonomic influence on CO2_stem and were unable to accurately predict how drought sensitivity altered ecosystem scale CO2_stem, due to substantial uncertainty introduced by contrasting methods previously employed to scale CO2_stem fluxes. 4. Our findings indicate that under future scenarios of elevated drought, increases in CO2_stem may augment carbon losses, weakening or potentially reversing the tropical forest carbon sink. However, due to substantial uncertainties in scaling CO2_stem fluxes, stand-scale future estimates of changes in stem CO2 emissions remain highly uncertain.