Inundation effects on small-scale plant distributions in San Diego, California vernal pools

California vernal pools were used to develop and test hypotheses related to species response patterns and distributions along environmental moisture and elevation gradients. Data from three rainfall years were examined for patterns of plant distribution in relation to length and frequency of inundation and relative elevation. Field distributions on elevation and inundation duration gradients were characteristic of each species, with species distributions overlapping along the gradient. Plants with limited inundation tolerance ('Non-Pool' species) responded to wet years by shifting their distributions toward the dry end of the moisture gradient and in the dry year toward the wetter end. These species have most of their distribution outside of pools. Species with limited tolerance to the gradient extremes had their peak frequency in intermediate positions ('Edge' species). 'Pool' species had substantial inundation tolerance. Several were near the limits of inundation tolerance in the deepest study pools, but others withstood longer inundation. 'Pool' species had static distributions on the elevation gradient, growing in whatever conditions prevailed in a particular year. A number of annual 'Pool' species had their highest frequency in the intermediate, moderately wet year. The dry year had a strong negative effect on the frequency of a few 'Pool' annuals, but established perennials persisted regardless. In general, results of the regression analyses can be understood given the other information available on pool hydrology and vegetation. They do not produce any obvious hypotheses to test. For a given species, variables have different predictive value, depending on the year.