Does leaf size predict herbaceous plant water use strategy?

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Caroline R. Erickson and Kevin A Simonin, Department of Biology, San Francisco State University, San Francisco, CA, Sara A. Baguskas, Department of Geography and Environment, San Francisco State University, San Francisco, CA

Background/Question/Methods
The predicted future warmer climate of the Western U.S. will have dramatic impacts on plant community composition, productivity, and ecosystem function. Variation in leaf size is strongly associated with leaf energy balance, evaporative demand, and water use strategies of woody plants, but less is known about the relationship between leaf size and water use strategies of herbaceous plants. Our study examines leaf size as an ecologically relevant trait that can inform how herbaceous plant communities of the Northern Sierra Nevada respond to a future altered climate. We hypothesize that given the constraints leaf size has on leaf energy and water balance, large herbaceous leaves require relatively stable leaf water potentials (Ψ) (i.e. an index of water content) in order to maintain stomatal conductance (gs), and therefore evaporative cooling, so as to avoid lethal temperatures. Additionally, we hypothesize that small leaves can cool more efficiently and may experience a wider range of gs over time as Ψ decreases with drying soil. By measuring gs and Ψ for a wide range of herbaceous leaf sizes over the course of the growing season, we hope to elucidate the hypotheses above.
Results/Conclusions
Preliminary results suggest that in the Northern Sierra Nevada, large-leaved herbaceous plants maintain relatively stable predawn water potentials (Ψpd, a proxy for soil water content) and stable gs for a majority of the growing season. In contrast, small-leaved species show dramatic declines in both Ψpd and gs. The observed stability in Ψpd and gs throughout the relatively rain-free growing season suggests that large-leaved species employ a drought avoidance strategy, and maintain deep roots to access stable water supply in this winter precipitation dominated system. Our data will indicate whether leaf size is a reliable predictor of a plant’s position along the isohydric - anisohydric continuum (i.e. the degree of stability in midday water potential in response to drying soils). Though larger-leaved herbaceous plants may operate with a drought avoidance strategy, this may not be sufficient for the imminent climatic changes. Our results could suggest that larger-leaved herbaceous plant species will be more sensitive to a warmer future climate.