Plant susceptibility to drought has been hypothesized to be related to contrasting strategies for avoiding tissue desiccation and hydraulic failure (isohydry vs. anisohydry). Despite extensive research on the physiological processes associated with mortality during drought, mechanisms underlying drought-related mortality remain unresolved. The objective of this work was to quantitatively relate contrasting strategies for avoidance of tissue desiccation and hydraulic failure with observed mortality in a range of species across an iso- anisohydric spectrum (i.e. hydric status). We examined photosynthetic gas exchange, plant and soil water status, shoot hydraulic vulnerability (P50), hydraulic safety margins, and foliar and stem non-structural carbohydrate (NSC) content in nine species exposed to drought and control treatments. Species were selected to represent a broad range of behavior along the iso- anisohydric continuum. Species’ positions along the iso- anisohydric continuum were evaluated based on the relationship between midday and predawn leaf water potentials, “hydroscape” (the range of leaf water potential (ψl) over which stomata control ψl) and turgor loss point (TLP). We developed a mortality index (MI) based upon amount of time elapsed between initiation of drought and occurrence of a set level of mortality that all but one of the study species experienced prior to study’s end.
Mortality index was significantly correlated with metrics of hydric status including hydroscape (p=0.0019, r2=0.54), the slope of the relationship between predawn ψl and midday ψl (p=0.033, r2=0.35), and TLP (p=0.05, r2=0.44). Physiological characteristics correlating with mortality included shoot hydraulic vulnerability (p=0.003, r2=0.56) and depletion of foliar NSC (p=0.036, r2=0.54). The three species most vulnerable to mortality: Betula occidentalis (MI=1), Salix scouleriana (MI=0.9), Alnus incana (MI=0.58), were highly isohydric and experienced substantial depletion of foliar NSC. The three species least vulnerable to mortality: Quercus garryana (MI=0.33), Cercocarpus ledifolius (MI=0.33) and Rhamnus illicifolia (MI=0), were highly anisohydric and experienced virtually no NSC depletion. The three species with intermediate levels of mortality Ceanothus cuneatus (MI =0.41) Quercus douglasii (MI=0.4) and Heteromeles arbutifolia (MI=0.37), behaved somewhat inconsistently with their hydric rankings . H. arbutifolia and Q. douglasii experienced surprisingly high levels of foliar NSC depletion and hydraulic vulnerability, respectively. Despite its relatively high mortality ranking, C. cuneatus had neither a high level of NSC depletion, nor a high level of hydraulic vulnerability, suggesting some cause of mortality unrelated to carbon starvation or hydraulic failure.