Long-term CO_2 fertilization increases vegetation productivity and has little effect on hydrological partitioning in tropical rainforests

摘要

Understanding how tropical rainforests respond to elevated atmospheric CO_2 concentration (eCO_2) is essential for predicting Earth’s carbon, water, and energy budgets under future climate change. Here we use long-term (1982–2010) precipitation (P) and runoff (Q) measurements to infer runoff coefficient (Q/P) and evapotranspiration (E) trends across 18 unimpaired tropical rainforest catchments. We complement that analysis by using satellite observations coupled with ecosystem process modeling (using both “top-down” and “bottom-up” perspectives) to examine trends in carbon uptake and relate that to the observed changes in Q/P and E. Our results show there have been only minor changes in the satelliteobserved canopy leaf area over 1982–2010, suggesting that eCO_2 has not increased vegetation leaf area in tropical rainforests and therefore any plant response to eCO_2 occurs at the leaf level. Meanwhile, observed Q/P and E also remained relatively constant in the 18 catchments, implying an unchanged hydrological partitioning and thus approximately conserved transpiration under eCO_2. For the same period, using a top-down model based on gas exchange theory, we predict increases in plant assimilation (A) and light use efficiency (ε) at the leaf level under eCO_2, the magnitude of which is essentially that of eCO_2 (i.e., ~12% over 1982–2010). Simulations from 10 state-of-the-art bottom-up ecosystem models over the same catchments also show that the direct effect of eCO_2 is to mostly increase A and ε with little impact on E. Our findings add to the current limited pool of knowledge regarding the long-term eCO_2 impacts in tropical rainforests.