Photosynthetic seasonality of global tropical forests constrained by hydroclimate

Abstract

The response of tropical forests to droughts is highly uncertain 1 . 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 season 2 . However, it remains uncertain to what extent water availability is responsible for productivity declines of tropical forests during the dry season 2,3 . 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.Photosynthetic metabolism in tropical forests controls ecosystem carbon uptake from the atmosphere, and it also influences critical ecosystem services, including carbon storage 4 , freshwater delivery 5 , maintenance of biodiversity 5 , and regulation of regional and global climate 6 . The photosynthetic metabolism of many tropical forests exhibits a recurring seasonality 2,7 . Understanding how climate influences these seasonal dynamics is an essential prerequisite for realistically predicting tropical forest responses to inter-annual and longer-term climate variation and change 3 . In particular, with a wide spectrum of varying total annual precipitation and dry-season length in the tropics ( Supplementary Fig. 7), the extent to which seasonality of vegetation productivity in tropical forests responds to water limitation remains unclear 2,3 . Although tropical forest seasonal dynamics have been studied at site and regional scales using eddy flux-tower networks and/or satellite remote sensing in Amazonia 2,8,9 , Insular Southeast (SE) Asia 7 and Africa 10 , a globally consistent functional inter-comparison of tropical forests is lacking. Thus, in this paper we address the following questions: What is the extent to which the seasonality of vegetation photosynthesis is limited by water availability in global tropical forests? Are there critical environmental thresholds that explain these seasonal variations? If so, what are the underlying physical mechanisms? What are the implications of such mechanisms on the future of tropical forests under cli...

Land-use and climate change risks in the Amazon and the need of a novel sustainable development paradigm

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“…The seasonally asynchronous nature of phenologymediated LUE establishes a middle ground in debates over whether the eastern Amazon canopy is enhanced or "greens up" during the dry season (Huete et al, 2006;Myneni et al, 2007;Samanta et al, 2012;Morton et al, 2014;Bi et al, 2015;Guan et al, 2015;Saleska et al, 2016). Changes to the canopy's LUE do indeed occur, but not synchronously with the dry season at our site (Fig.…”

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“…SIF has a small‐amplitude signal, so it was not possible to observe it until very recently. SIF observations have been shown to be directly pertinent to estimate crop photosynthesis (Guanter et al, 2014) and yield (Guan et al, 2016), GPP across ecosystems (Lee et al, 2015; Yang et al, 2015; Zhang, Xiao, Jin, et al, 2016), water stress (Guan et al, 2015; Konings et al, 2017; Sun et al, 2015; Zhang, Xiao, Guanter, et al, 2016), biosphere‐atmosphere interactions (Green et al, 2017), surface turbulent fluxes (Alemohammad et al, 2017), and phenology, especially in northern latitudes where vegetation indices and their seasonality are polluted by the snow albedo (Jeong et al, 2017). One other advantage of SIF is that it responds to only the PAR absorbed by chlorophyll of the canopy, whereas typical optical (absorbed photosynthetic active radiation) APAR or fPAR products reflect the PAR absorbed by the entire canopy (nonphotosynthetic and photosynthetic; Song et al, 2013).…”