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

Abstract

ALTRES AJUTS: NE/N014022/1, NE/J011002/1

- 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. - 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.