RESPONSE OF RIPARIAN WILLOWS TO GROUNDWATER FLUCTUATIONS IN SOUTHERN CALIFORNIA COASTAL DRAINAGES

Groundwater extracted from coastal streams is an important resource for Marine Corps Base Camp Pendleton, San Diego County, California. It is also vital to the riparian habitat associated with the streams, which in turn is important for several species of endangered animals, most notably the least Bell's vireo. Willows are the dominant woody species in these habitats, and we studied their response to varying groundwater over a seven year period that included one of the wettest and one of the driest episodes on record. A five year study of seasonal drought effects on Salix lasiolepis as assessed by xylem potential found that xylem potential of established trees was significantly related to the change in groundwater depth over the season, but only weakly to the absolute depth. Despite large climatic variation, the pre-dawn measures showed that almost all trees had a moisture status well above the danger zone for permanent damage. Groundwater was measured at monitoring wells established above and below sewage treatment plants that ceased discharge of treated effluent at different times during the course of the study. Permanent vegetation transects adjacent to the wells provided data on the change in species composition and cover. With one possible exception, we found no evidence for significant changes in the vegetation or the groundwater associated with the cessation of discharge. The exception was a patch of dieback along San Onofre Creek. Although the dieback was correlated with a sharp drop in groundwater, this amplitude and the associated dieback appear to be within the historical range of variation in this highly variable ephemeral stream. A study of seedling establishment following a year of extreme flooding in 2005, showed substantial levels of early survival, but with pronounced groundwater-related differences between the smaller (San Onofre Creek) and larger (Santa Margarita River) streams. We conclude that despite large interannual variation and the cessation of discharge, the riparian systems of Camp Pendleton proved to be highly resilient to groundwater fluctuations.