Quantifying soil moisture impacts on light use efficiency across biomes

Water availability limits ecosystem productivity across much of the Earth’s surface (Beer et al., 2010; Schwalm et al., 2010; Seneviratne et al., 2010; Ahlstr€om et al., 2015). In arid, semi-arid and Mediterranean ecosystems, limiting water availability is a recurrent phenomenon and governs plant growth and phenology (Reichstein et al., 2002). In addition, in temperate, boreal and tropical ecosystems, sporadic prolonged dry periods can lead to water-limited conditions and can have far-reaching impacts on ecosystem carbon (C) balance (Ciais et al., 2005; Granier et al., 2007; Doughty et al., 2015) and structure (Orth et al., 2016). Here, we investigate ‘droughts’, identified by their impact on vegetation productivity. This corresponds most closely to the definition of ‘agricultural droughts’ (Trenberth et al., 2007) and also includes seasonally recurring dry conditions. Most plants tightly co-regulate water loss and CO2 assimilation with the effect that, under conditions of low soil moisture and high atmospheric water vapour pressure deficit (VPD), stomatal conductance and hence assimilation and transpiration rates are reduced in order to prevent exceedingly low leaf water potentials and resulting plant tissue damage from cavitation (Cowan & Farquhar, 1977; McDowell et al., 2008; Sperry & Love, 2015). The CO2 assimilation rate at the leaf level, or gross primary productivity (GPP) – its integral at the ecosystem level – is the ‘engine’ of C cycling in terrestrial ecosystems. GPP emerges as the dominant driver of year-to-year variations in the global land C balance (Poulter et al., 2014; Ahlstr€om et al., 2015), and is closely controlled by water availability in the rooting zone across much of the Earth’s surface (Beer et al., 2010; Ahlstr€om et al., 2015).