Understanding responses of Amazon forests to seasonal and interannual water stress based on modeling studies and satellite observations
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As the largest tropical forest in the world, the Amazon rainforest plays key roles in regulating regional and global carbon and hydrological cycles. Water availability is a major controlling factor in this ecosystem, in terms of shaping the species composition and distribution as well as controlling the seasonal dynamics. Given the large uncertainty in precipitation projections and the consequent debates as to the fate of the forest, there is a pressing need to examine the response of the forest to water availability. Here, I first consider the role of two mechanisms that buffer plants from impact of water stress, hydraulic redistribution by plant roots and an internal water pool inside tree trunks, both of which are omitted in the National Center for Atmospheric Research (NCAR) Community Land Model (CLM). The former efficiently redistributes soil water to facilitate easier access by plants, and the latter provides a closer-to-leaf water pool that is more readily accessible than the soil water pool. CLM simulations show that the inclusion of both mechanisms helps fix its overestimation of dry-season water stress. Based on satellite/ground observations and reanalysis data, we also analyze how plant growth responds to El Niño events that result in below-normal precipitation over large areas across the Amazon basin. We find that the influence of El Niño events on vegetation differs between wet and dry seasons, with the former mainly controlled by radiation and the latter by water availability.