Leaf specific mass as a predictor of water availability in riparian communities in Mediterranean Type Ecosystems in Southern France
Mentor:Cheryl Swift, James Irvine Professor of Biological Sciences Chair, Environmental Science Departments of Biology and Environmental Science, Whittier College
Patterns of species distribution in riparian areas in Mediterranean-Type Ecosystems depend on the ability of an individual to tolerate summer drought and to persist after winter flood events. We examined leaf specific mass, the ratio of dry mass to area, as an indicator of drought tolerance, and measured pre-dawn and midday water potentials across four streams in Southern. Leaf specific mass can be used as a measure of drought tolerance because we would expect that a greater mass to area ratio would result in lower rates of transpiration and water loss. The streams varied in summer water availability, and we used a total of eight species. Out of the eight species examined, four occurred in two or more streams with varying summer water availability: Fraxinus angustifolia Populus alba, Salix cinera and Salix elaeagnos, . We predicted that species would have increased leaf specific mass in streams with lower water availability. We found that leaf specific mass for Fraxinus angustifolia, Populus alba, and Salix elaeagnos did not increase with decreased summer water availability, but mid-day water potentials were lower where summer water availability was more limited. So, our prediction did not hold true for these species; however, the cost of constant leaf specific mass in streams with lower summer water availability appears to be lower mid-day water potentials. Salix cinerea showed a slightly different pattern than these three species. Leaf specific mass increased with decreased mid-day water potential. Our results suggest that different species typical of riparian communities in Southern France differ with respect to the way they respond to summer water availability, and these differences may dictate distribution of these species laterally with respect to the channel, or longitudinally with respect to position along river length.