The effect of fungal pathogens on the water and carbon economy of trees: implications for drought-induced mortality.

Drought-induced forest mortality is emerging as a widespread phenomenon with potentially large 12 implications for forest function and dynamics (Allen et al., 2010; Anderegg et al., 2012; Martínez13 Vilalta et al., 2012). Although the physiological mechanisms underlying tree mortality are still not 14 completely understood, there is agreement that they involve the storage and transport systems of water 15 and carbohydrates (McDowell et al., 2008; Sala et al., 2010; McDowell, 2011). The xylem of plants is 16 susceptible to drought-induced embolism and severe water deficits may result in the complete loss of 17 xylem hydraulic conductivity and cause tree mortality (hydraulic failure; cf. Tyree & Sperry, 1988; 18 McDowell et al., 2008; Choat et al., 2012). Drought also has detrimental effects on the carbon 19 economy of plants, and it has been hypothesized that reduced assimilation due to stomatal closure 20 may lead to a depletion of stored carbon reserves and, eventually, to tree death due to carbon 21 starvation (Waring, 1987; Martínez-Vilalta et al., 2002; Bréda et al., 2006; McDowell et al., 2008). 22 However, only in recent studies has a direct link between reduced carbon reserves and tree mortality 23 been established (Adams et al., 2009; Galiano et al., 2011; Adams et al., 2013; Hartmann et al., 2013; 24 Mitchell et al., 2013; Quirk et al., 2013; Sevanto et al., 2014). Finally, phloem transport could also 25 become impaired due to the inability of plants to maintain phloem turgor under extremely low xylem 26 water potentials, limiting the local availability of carbohydrates for metabolic functions (Sala et al., 27 2010; Sevanto et al., 2014). 28