Abstract:

Tropical forests play a major role in the global terrestrial carbon cycle; however, substantial uncertainties remain regarding their capacity to continue acting as a carbon sink under changing climate conditions. Using an eddy covariance record from a mature lowland wet tropical rainforest in Costa Rica, we evaluated partitioning methods of measured net ecosystem exchange to separate gross primary production (GPP) and ecosystem respiration. Random forest modeling revealed timescale-dependent environmental controls on light use efficiency on carbon uptake, indicating vulnerability to increasing atmospheric dryness. We further investigated the observed diurnal hysteresis in GPP and the dynamics of carbon-water decoupling at the half-hourly timescale and identified that progressive stomatal closure under rising afternoon atmospheric demand was the primary driver of diurnally asymmetric carbon uptake. Our findings demonstrate that partitioning method choice and temporal scale strongly shape inferred GPP magnitude and drivers in wet tropical forests, and that land surface models assuming symmetric diurnal GPP will systematically overestimate afternoon tropical carbon uptake, with implications for projecting forest carbon balance under continued warming.