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

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. 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. 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. 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.