Editing IPCC:AR6/SRCCL/Chapter-2 (section)

=== 2.7.2 Water transport through soil-plant-atmosphere continuum and drought mortality ===

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How climate change, especially changes of precipitation patterns, influence water transport through the soil-plant-atmosphere continuum, is a key element in projecting the future of water vapour flux from land and cooling via latent heat flux ( ''high confidence'' ) (Sellers et al. 1996 <sup>[[#fn:r1976|1976]]</sup> ; Bonan 2008 <sup>[[#fn:r1977|1977]]</sup> ; Brodribb 2009 <sup>[[#fn:r1978|1978]]</sup> ; Choat et al. 2012 <sup>[[#fn:r1979|1979]]</sup> ; Sperry and Love 2015 <sup>[[#fn:r1980|1980]]</sup> ; Novick et al. 2016 <sup>[[#fn:r1981|1981]]</sup> ; Sulman et al. 2016 <sup>[[#fn:r1982|1982]]</sup> ). Even without changes in leaf area per unit area of land, when plants close stomata in response to water shortage, dry atmosphere or soil moisture deficit, the stand-level fluxes of water (and associated latent heat flux) decrease (Seneviratne et al. 2018 <sup>[[#fn:r1983|1983]]</sup> ). Closing stomata enhances drought survival at the cost of reduced photosynthetic production, while not closing stomata avoids loss of photosynthetic production at the cost of increased drought mortality (Sperry and Love 2015 <sup>[[#fn:r1984|1984]]</sup> ). Hence, species-specific responses to drought, in terms of whether they close stomata or not, have short- and long-term consequences (Anderegg et al. 2018a <sup>[[#fn:r1985|1985]]</sup> ; Buotte et al. 2019 <sup>[[#fn:r1986|1986]]</sup> ). Increased drought-induced mortality of forest trees, often exacerbated by insect outbreak and fire (e.g., Breshears et al. (2005) <sup>[[#fn:r1987|1987]]</sup> , Kurz et al. (2008) <sup>[[#fn:r1988|1988]]</sup> , Allen et al. (2010) <sup>[[#fn:r1989|1989]]</sup> ) (Section 2.2.4), have long-term impact on hydrological interactions between land and atmosphere (Anderegg et al. 2018b <sup>[[#fn:r1990|1990]]</sup> ).

New models linking plant water transport with canopy gas exchange and energy fluxes are expected to improve projections of climate change impacts on forests and land-atmosphere interactions ( ''medium confidence'' ) (Bohrer et al., 2005 <sup>[[#fn:r1991|1991]]</sup> ; Anderegg et al., 2016 <sup>[[#fn:r1992|1992]]</sup> ; Sperry and Love, 2015 <sup>[[#fn:r1993|1993]]</sup> ; Wolf et al., 2016 <sup>[[#fn:r1994|1994]]</sup> ). Yet, there is much uncertainty in the ability of current vegetation and land surface models to adequately capture tree mortality and the response of forests to climate extremes like drought (Rogers et al. 2017 <sup>[[#fn:r1995|1995]]</sup> ; Hartmann et al. 2018 <sup>[[#fn:r1996|1996]]</sup> ). Most vegetation models use climate stress envelopes or vegetation carbon balance estimations to project climate-driven mortality and loss of forests (McDowell et al. 2011 <sup>[[#fn:r1997|1997]]</sup> ); these may not adequately project biome shifts and impacts of disturbance in future climates. For example, a suite of vegetation models was compared to a field drought experiment in the Amazon on mature rainforest trees and all models performed poorly in projecting the timing and magnitude of biomass loss due to drought (Powell et al. 2013 <sup>[[#fn:r1998|1998]]</sup> ). More recently, the loss of water transport due to embolism (disruption of xylem water continuity) (Sperry and Love 2015 <sup>[[#fn:r1999|1999]]</sup> ), rather than carbon starvation (Rowland et al. 2015 <sup>[[#fn:r2000|2000]]</sup> ), is receiving attention as a key physiological process relevant for drought-induced tree mortality (Hartmann et al. 2018 <sup>[[#fn:r2001|2001]]</sup> ). A key challenge to modelling efforts is to consider differences among plant species and vegetation types in their drought responses. One approach is to classify plant species to ‘functional types’ that exhibit similar responses to environmental variations (Anderegg et al. 2016 <sup>[[#fn:r2002|2002]]</sup> ). Certain traits of species, such as tree height, is shown to be predictive of growth decline and mortality in response to drought (Xu et al. 2016a <sup>[[#fn:r2003|2003]]</sup> ). Similarly, tree rooting depth is positively related to mortality, contrary to expectation, during prolonged droughts in tropical dry forest (Chitra-Tarak et al. 2017 <sup>[[#fn:r2004|2004]]</sup> ).

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