Print PageDrought events associated with climate change are expected to become more common over the Amazon in the near future, particularly in the eastern portion of the basin. Despite this, the question of how much climate change is too much for the maintenance of tropical forest integrity remains largely unanswered. To identify thresholds in forest resistance to drought, we conducted two studies. First, we simulated an approximately 35–41% reduction in effective rainfall in a 1 ha plot in the Tapajos National Forest (Para, Brazil) from 2000 through 2004. We compared forest response from this “dry down” plot with a similar control plot by measuring litterfall, wood increment, tree mortality, and below carbon cycling during seven years in both plots. Second, in 2005, a severe drought provided an opportunity to evaluate forest resistance to drought over a greater spatial extent than the partial throughfall experiments. Using field-based data from a network of plots in Amazonia experiencing the impact of the 2005 drought, we derived a relationship between biomass reduction and cumulative water deficit (CWD), and evaluated the impacts of the 2005 and 2010 droughts on carbon stocks of primary forests of Amazonia.
Results/Conclusions
In the first study, we show that wood production was the most drought-sensitive component of above-ground net primary productivity (ANPP), declining by up to 62% relative to the control, mainly due to increased mortality of large trees. In contrast, litterfall and soil CO2 efflux showed minor or no response to throughfall exclusion, respectively. Aboveground carbon stocks declined by 32.5 Mg ha-1 over the 6 years of the treatment, demonstrating that a severe, multi-year drought like those expected to become more frequent with climate change can substantially reduce forest C stocks in the Amazon. In the second study, we used the relationship between biomass reduction and CWD, established in the field in 2005, to estimate that 1.6 Pg [95% confidence intervals (CI) 0.8 and 2.6] and 2.2 Pg (CI 1.2, 3.4) of carbon were committed to the atmosphere following the droughts of 2005 and 2010, respectively. Results from the plot network and rainfall exclusion experiment apparently corroborate the hypothesis that severe droughts can cause major alterations in the carbon stocks of Amazonian forests.
