PARENT SESSION
Oral Session # 45: Plant Ecology III: Plant - Water Relations I.
Presiding: R Giuliani
Wednesday, August 6. 8:00 AM to 11:30 AM, SITCC Meeting Room 200.

Summer water use by California coastal prairie grasses: Fog, drought, and community composition.

Corbin, Jeffrey^*,1, Dawson, Todd¹, Thomsen, Meredith¹, D'Antonio, Carla^{1, 2}, ¹ University of California, Berkeley, CA² USDA-ARS, Reno, NV

ABSTRACT- Plant species in arid and semiarid habitats have been shown to rely on a variety of character traits to survive prolonged drought conditions. Recent advances in the use of stable isotopes have shown that, while many perennial plants in dry conditions access water deep in the soil profile, many species also support shallow roots to take advantage of sporadic precipitation events. Coastal fog can be an important moisture source for California coastal vegetation, though its role in influencing species distributions is poorly understood. We sampled the stable H and O isotope ratios of water in the soil and roots of a variety of perennial grass species in four sites in northern California in order to determine the relative importance of fog and rain throughout the year. The relationship between H and O provided evidence that evaporation of water in the soil had taken place prior to uptake of water by vegetation. To account for this, we employed a novel correction for evaporative fractionation, and then applied a two-source mixing model. Our results indicated that 20-50% of the water in vegetation during the spring and summer came from fog rather than from residual water from winter rain. Fog use decreased as distance from the coast increased, and species differed in their use of each water source. We did not find consistent differences between species whose distributions are limited to coastal habitats versus species with broader distributions, but rather found that species responded individualistically to summer fog and exhibited varying degrees of dependence on fog. Native perennial grasses in coastal habitats can be strong competitors against exotic annual grass species. We believe that summer moisture inputs from fog may lengthen the growing seasons for these perennial grasses, giving them an important advantage over annual grasses.

Key words: california grasslands, stable isotopes, coastal fog, native perennial grasses