Drought in a Mature Amazonian Forest

15 Large areas of Amazonian evergreen forests experience seasonal droughts extending for 16 three or more months, and show maximum rates of photosynthesis and evapotranspiration 17 during dry intervals. This apparent resilience is belied by disproportionate mortality of the 18 large trees in manipulations that reduce wet season rainfall, occurring after 2-3 years of 19 treatment. The goal of this study is to characterize the mechanisms that produce these 20 contrasting ecosystem responses. A mechanistic vegetation-hydrology model is developed to 21 test the roles of deep roots and of soil capillary flux to provide water to the forest during the 22 dry season. Also examined is the importance of “root niche separation,” in which roots of 23 overstory trees extend to depth, where during the dry season they use water stored from wet 24 season precipitation, while roots of understory trees are concentrated in shallow layers that 25 access dry season precipitation directly. Observational data on canopy phenology, energy 26 fluxes, soil moisture, and soil and root structure from the Tapajós National Forest, Brazil, 27 provided comprehensive observational constraints on the model. Results strongly suggest 28 that deep roots with root niche separation adaptations explain both the observed resilience 29 during seasonal drought and the vulnerability of canopy-dominant trees to extended deficits 30 of wet season rainfall. These mechanisms appear to provide an adaptive strategy that en31 hances productivity of the largest trees in the face of their disproportionate heat loads and 32 water demand in the dry season. A sensitivity analysis exploring how wet season rainfall 33 affects the stability of the rainforest system is presented. The model can be used to quanti34 tatively predict ecosystem water balances and explore ecosystem tipping points under future 35 climate change. 36