Plant physiological response to environmental stress in semiarid ecosystems

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The frequency and severity of drought is expected to increase in western Canada during the next century as a consequence of climate disruption. To maintain the vitality of natural ecosystems, we must understand how different functional groups of plants cope with restricted water availability. Drought tolerance and avoidance strategies vary among tree, shrub and grass species in semiarid ecosystems. These differences are related to water-use efficiency (WUE), the ratio of leaf photosynthetic carbon gain to water loss by transpiration. High WUE plants have lower photosynthesis rates in full sunshine (Amax) and ideal moisture conditions, but continue to grow throughout summer drought. They co-occur with low WUE plants that have higher Amax. Low WUE plants are more opportunistic, growing rapidly during brief periods of soil moisture availability. They tend to have thinner leaves with higher nitrogen content and shorter lifespan. High WUE plants are more common among long-lived trees and woody shrubs than among herbaceous shrubs and grasses, but there are exceptions and WUE even differs within-species, due to seasonal changes in moisture availability, leaf age, leaf position in the canopy and the sex of dioecious plants. I will use advanced ecophysiological techniques, including photosynthetic gas-exchange, leaf reflectance, chlorophyll fluorescence, and stable carbon and hydrogen isotopes, to measure differences in drought response among functional types, among species, between sexes and along a canopy light extinction gradient. Studies will be carried out in riparian woodlands, coniferous forests and grasslands of Alberta. A key focus of my research is how mesophyll conductance, the diffusion conductance of carbon dioxide between internal leaf cavities and chloroplasts, influences WUE. My research is important for Canada, because it reveals physiological impacts of drought in semiarid ecosystems. Research outcomes will advance understanding of sensitive physiological indicators of ecosystem health and will be applied to renewal and conservation. Students will receive training in ecophysiology. This research effectively addresses NSERC's "Healthy Environment and Ecosystems" theme, targeted for increased funding.