THE EFFECT OF A DISTURBANCE CORRIDOR ON AN ECOLOGICAL RESERVE

The effect of a pipeline corridor constructed through an ecological re serve in Southern California was investigated by assessing plant speci es composition and soil chemistry. A homogeneous plant community compr ised primarily of exotic annuals was found along the entire length of the corridor. This community has low similarity to the adjacent native plant communities. Soil organic matter was significantly less on the disturbed corridor than in contiguous undisturbed areas, Both availabl e nitrogen and extractable phosphorus values were greater in the distu rbed corridor. By contrast, total nitrogen was significantly higher ou tside the pipeline. The more labile litter of the exotic annuals allow s increased mineralization along the corridor than does the more recal citrant litter of the native perennial shrubs in the undisturbed areas . Once established, the weedy exotic annual litter may completely turn over organic matter and nitrogen, favoring the persistence of the wee dy annuals. These exotic annuals appear to be moving into three of the native communities-grassland, coastal sage, and oak woodland-that hav e less organic matter and a more open plant canopy. Poor restoration e fforts can lead to the establishment of such exotics, subsequent invas ion into the surrounding undisturbed habitat, and degradation of the r eserve.