Deep Soil Water Buffers Drought Response in an Amazonian Forest.

The Amazon Basin experiences severe droughts that may become more common in the future as global and regional climate changes.  Little is known of the effects of such droughts on Amazon forest productivity, carbon allocation, and trace gas emissions.  We conducted a seven-year, partial throughfall exclusion experiment in east-central Amazonia to examine forest responses to a 35-41% effective reduction in annual rainfall.  We quantified soil water content to 11m depth, LAI, litterfall, wood production, and soil trace gas emissions in one-hectare treatment and control plots.  The treatment provoked a decrease of about 400 mm of volumetric water content, mostly between 2m and 11m depths.  Wood production declined by 13% the first year and up to 62% thereafter.  Litterfall declined only in the third year of drought, with a maximum difference of 25% below the control plot.  Deep roots that access deep water storage buffer this ecosystem from seasonal and interannual drought, but after 3 consecutive years of drought treatment, live aboveground carbon declined by 32 Mg ha^-1 through mortality of large trees.  Soil CO₂ efflux and its ¹⁴C signature showed no significant treatment response, suggesting similar amounts and sources of belowground production.  Soil efflux of N₂O decreased and CH₄ uptake increased in the drought plots, but effects on trace gas production and consumption were confined to the top 2 m of soil.  While trace gas emissions responded rapidly to changes in soil water content, changes in forest structure due to persistent drought may have a longer-term impact on carbon and nutrient dynamics.