Field‐quantified responses of tropical rainforest aboveground productivity to increasing CO 2 and climatic stress, 1997–2009

A directional change in tropical‐forest productivity, a large component in the global carbon budget, would affect the rate of increase in atmospheric carbon dioxide ([CO 2 ]). One current hypothesis is that “CO 2 fertilization” has been increasing tropical forest productivity. Some lines of evidence instead suggest climate‐driven productivity declines. Relevant direct field observations remain extremely limited for this biome. Using a unique long‐term record of annual field measurements, we assessed annual aboveground net primary productivity (ANPP) and its relation to climatic factors and [CO 2 ] in a neotropical rainforest through 1997–2009. Over this 12 year period, annual productivity did not increase, as would be expected with a dominant CO 2 fertilization effect. Instead, the negative responses of ANPP components to climatic stress far exceeded the small positive responses associated with increasing [CO 2 ]. Annual aboveground biomass production was well explained (73%) by the independent negative effects of increasing minimum temperatures and greater dry‐season water stress. The long‐term records enable a first field‐based estimate of the [CO 2 ] response of tropical forest ANPP: 5.24 g m −2 yr −1 yr −1 (the summed [CO 2 ]‐associated increases in two of the four production components; the largest component, leaf litterfall, showed no [CO 2 ] association). If confirmed by longer data series, such a small response from a fertile tropical rainforest would indicate that current global models overestimate the benefits from CO 2 fertilization for this biome, where most forests' poorer nutrient status more strongly constrains productivity responses to increasing [CO 2 ]. Given the rapidly intensifying warming across tropical regions, tropical forest productivity could sharply decline through coming decades.