Abstract
Droughts in a warming climate have become more common and more extreme, making understanding forest responses to water stress increasingly pressing. Analysis of water stress in trees has long focused on water potential in xylem and leaves, which influences stomatal closure and water flow through the soil‐plant‐atmosphere continuum. At the same time, changes of vegetation water content (VWC) are linked to a range of tree responses, including fluxes of water and carbon, mortality, flammability, and more. Unlike water potential, which requires demanding in situ measurements, VWC can be retrieved from remote sensing measurements, particularly at microwave frequencies using radar and radiometry. Here, we highlight key frontiers through which VWC has the potential to significantly increase our understanding of forest responses to water stress. To validate remote sensing observations of VWC at landscape scale and to better relate them to data assimilation model parameters, we introduce an ecosystem‐scale analog of the pressure–volume curve, the non‐linear relationship between average leaf or branch water potential and water content commonly used in plant hydraulics. The sources of variability in these ecosystem‐scale pressure‐volume curves and their relationship to forest response to water stress are discussed. We further show to what extent diel, seasonal, and decadal dynamics of VWC reflect variations in different processes relating the tree response to water stress. VWC can also be used for inferring belowground conditions—which are difficult to impossible to observe directly. Lastly, we discuss how a dedicated geostationary spaceborne observational system for VWC, when combined with existing datasets, can capture diel and seasonal water dynamics to advance the science and applications of global forest vulnerability to future droughts.
Highlights
As the climate warms, droughts are getting hotter, more extreme, and more frequent (Dai, 2013; Touma et al, 2015; Trenberth et al, 2014)
While we focus on forests, many of the ideas in this paper apply to other natural biomes, and to detecting water stress in croplands (Steele-Dunne et al, 2017; Togliatti et al, 2019)
We argue that observations of ecosystem- scale vegetation water content (VWC) would inform a number of ecological applications and overcome several existing in situ measurement challenges
Summary
Droughts are getting hotter, more extreme, and more frequent (Dai, 2013; Touma et al, 2015; Trenberth et al, 2014). One approach to measuring canopy RWC is to normalize VWC by its annual or seasonal maximum (resulting in a relative vegetation water content at ecosystem-scale, VWCeco; Rao et al, 2019) while accounting for changes in aboveground biomass, for example, from leaf abscission during a drought event. This accounting requires ancillary information on phenology.
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