Ecosystem carbon fluxes are tree size-dependent in an Amazonian old-growth forest

Tropical forests are important carbon sinks as they store huge amounts of carbon and are thus essential players within the global carbon cycle. While good estimates of the gross and net primary productivity (GPP and NPP, respectively) of tropical forests exist, we still lack a cohesive study on the contribution of different tree sizes to the overall forest carbon fluxes and their seasonal variation. Here, we model GPP based on xylem sap flux and eddy covariance data for an old-growth moist lowland forest in Central Amazon for the year 2013 (Jan-Dec). The model uses canopy transpiration and vapor pressure deficit as covariates and enables GPP partitioning into fluxes from canopy, subcanopy, and understory trees. Net primary productivity was calculated from forest inventory data. Carbon use efficiency (CUE) was computed as the ratio between NPP and GPP. GPP was 28.46 MgC ha−2 yr−1 at our study site. Canopy trees (diameter > 30 cm; average height of 28 m) were responsible for 21.47 MgC ha−2 yr−1 of the overall GPP, whereas subcanopy and understory trees contributed 3.95 MgC ha−2 yr−1 and 3.04 MgC ha−2 yr−1, respectively. Canopy trees allocated only 23% of their photosynthetic products towards growth and were responsible for more than 50% of NPP. Subcanopy and understory trees were more efficient by allocating 67% and 59% of their carbon assimilates towards biomass growth, respectively. GPP showed distinct seasonal patterns, with canopy trees doubling monthly GPP with the progressing dry season, whereas understory and subcanopy trees had a 60% higher GPP than in the wet season. This study provides evidence for the importance of large trees in tropical forests and highlights their crucial role in forest carbon cycling. Due to high drought-related mortality, large trees make up a critical component of the response of tropical forests to climate change.