Effects of experimental throughfall exclusion on forest structure and the aboveground net primary productivity of an Amazonian forest

Global and regional climatic changes are predicted to reduce precipitation in Eastern Amazonia. Although increased droughts will likely have major impacts on forest dynamics in this region, our ability to predict these changes is still limited. We use the results of a partial-throughfall exclusion experiment in the east-central Amazon to assess the effects of drought on forest structure and above-ground net primary productivity (ANPP). The experiment consisted of two 1-ha plots: a droughted-plot where 50% of the incoming precipitation was diverted from the soil during the 6 mo wet season from 2000 to 2004; and, a control-plot that received natural rainfall inputs (~2000 mm/yr). We observed a consistent reduction in ANPP in the droughted-plot relative to the control-plot after the first exclusion period  through 2003:  While ANPP was nearly equal in the two plots in 2000 (1%), it was 13, 30, and 41% lower in the droughted-plot in 2001, 2002, and 2003, respectively. In the last year of the treatment (2004) and one after the drought treatment was removed (2005), by contrast, ANPP differences between plots rebounded to 30% and 10% of the control, respectively. The combined effects of increased mortality of large trees and reduced ANPP in the droughted-plot resulted in a net carbon loss of ~40 Mg/ha over the 5-yr study period. Mortality and ANPP were positively correlated with plant available water. These results suggest that increased drought has the potential to reduce substantially the carbon stocks of moist tropical forests.