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Photosynthetic seasonality of global tropical forests constrained by hydroclimate


The response of tropical forests to droughts is highly uncertain1. During the dry season, canopy photosynthesis of some tropical forests can decline, whereas in others it can be maintained at the same or a higher level than during the wet season2. However, it remains uncertain to what extent water availability is responsible for productivity declines of tropical forests during the dry season2,3. Here we use global satellite observations of two independent measures of vegetation photosynthetic properties (enhanced vegetation index from 2002 to 2012 and solar-induced chlorophyll fluorescence from 2007 to 2012) to investigate links between hydroclimate and tropical forest productivity. We find that above an annual rainfall threshold of approximately 2,000 mm yr−1, the evergreen state is sustained during the dry season in tropical rainforests worldwide, whereas below that threshold, this is not the case. Through a water-budget analysis of precipitation, potential evapotranspiration and satellite measurements of water storage change, we demonstrate that this threshold determines whether the supply of seasonally redistributed subsurface water storage from the wet season can satisfy plant water demands in the subsequent dry season. We conclude that water availability exerts a first-order control on vegetation seasonality in tropical forests globally. Our framework can also help identify where tropical forests may be vulnerable or resilient to future hydroclimatic changes.

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Figure 1: Multi-year average of normalized wet-/dry-season difference of the photosynthetic properties for the global tropical evergreen forest regions.
Figure 2: Associations between total precipitation and wet-/dry-season difference in satellite-inferred photosynthetic properties.
Figure 3: Relationship between ‘dry-season total deficit’ and ‘maximum release from water storage’ in global tropical evergreen forests.


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K.G. and E.F.W. acknowledge financial support from the NASA Earth and Space Science Fellowship (NESSF). J.S.K.’s contribution is supported under the NASA Terra-Aqua Science program (NNX11AD46G). S.R.S. and J.W. acknowledge support by NASA Terra-Aqua Science program (NNX11AH24G) and by DOE Terrestrial Ecosystem Science (DE-SC0008383). We also acknowledge all the data providers for sharing the scientific data.

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K.G., M.P. and H.L. conceived the idea; K.G. conducted the analyses; J.W., M.L., S.R.S., J.J. and A.I.L. provided the data; all authors contributed to the writing and revisions.

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Correspondence to Kaiyu Guan.

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Guan, K., Pan, M., Li, H. et al. Photosynthetic seasonality of global tropical forests constrained by hydroclimate. Nature Geosci 8, 284–289 (2015).

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