Paths to Tree Drought-Induced Mortality: Progress to Date

Abstract:

Drought-induced forest mortality is predicted to increase in warmer and drier climates. Despite very significant research efforts over the last fifteen years to try to understand and model the physiological mechanisms of forest drought-induced mortality, our ability to predict when and which species will succumb to drought remains limited. Like any plant, trees die under drought because they lose more water than they acquire from the environment and eventually fail to keep living cells sufficiently hydrated (turgor loss), and die. Drought-induced mortality is a dehydration process. Such a process is difficult to model because different species employ very different strategies to maintain water balance during drought and because tolerance to water deficit also varies significantly across species. A water balance approach that integrates the regulation of water uptake and transport, water retention, and water loss as the main mechanisms to prevent living cells from permanently losing turgor provides an integrative way to think about drought-induced mortality that encompasses a variety of interacting physiological mechanisms. When regulation of water loss though stomata is insufficient to cope with diminished water supply from roots, increasing xylem tension will eventually lead to xylem failure and to interrupted water supply to living cells. Likewise, roots often shrink in dry soils and become disconnected from the soil. If so, water supply is interrupted, eventually leading to xylem failure, desiccation and death. Drought can also decrease carbohydrate supply via decreases of photosynthesis, phloem transport and/or carbohydrate reserve mobilization. Reduced carbohydrate supply could decrease solute concentrations and water retention capacity in living cells, an effect that can potentially lead to permanent loss of cell function and death. Most likely, these different paths to drought mortality occur simultaneously and strongly interact. However, the relative importance of each path likely varies across species and environments. A water balance approach provides a unifying framework that is consistent with available data to date.