Drought-killed Douglas-fir. USDA Forest Service photo by Connie Harrington.
Efforts to predict species-specific drought performance and survival have intensified because of recent episodes of widespread tree mortality provoked by extreme droughts. However, metrics that accurately and consistently quantify species physiological responses to drying soil have been elusive. To address this, a team of scientists led by Rick Meinzer and David Woodruff with the PNW Research Station monitored responses to a soil drying cycle in eight woody species representative of riparian, forest, oak woodland, chaparral, and sagebrush scrub vegetation types.
The scientists quantified the species’ response to drought by measuring available soil water and the point at which stomata in the leaf closed, preventing net uptake of carbon dioxide from the air. In moist soil, semiarid species with a larger range for functioning in drier soils had higher leaf turgor than mesic species; this was also associated with faster stomatal opening and activation of photosynthesis during the transition from darkness to light in the semiarid species. Leaf turgor loss point, an easily determined metric, proved to be a robust proxy for the link between soil water status and drought-induced stomatal closure.
The ability to predict the point at which a plant species begins to shut down owing to drought stress is essential for projecting likely response of vegetation across the landscape to anticipated environmental changes and for planning accordingly to deal with these changes.