9408079 Allen Exotic annuals have been invading southern California shrublands at increasing rates over the past 40-50 years. A number of factors may be responsible for the invasion, including fore, anthropogenic nitrogen deposition, soil disturbance, grazing, and fragmentation. Of these potential causes, N deposition has received no attention, and the effects of frequent fire are just beginning to be realized. Southern California shrublands are subject to aerial N deposition rates as high rates as high as 30 kg/ha/yr, with lower rates at a distance from agricultural and urbanized areas. Many of the colonizing annuals are probably nitrophilous, e.g., they have high rates of growth with added N. By contrast, we hypothesize that the native shrubs have relatively little response to increased N. Fire alone can explain the increased weediness and decrease in shrub cover in some sites, but areas that have not been burned in 30-40 years have and increase in exotics. These include primarily annual grasses in the genera Bromus, Hordeum, and Avena. The native vegetation that is most subject to weed invasion is the coastal sage scrub, a type that is limited in extent, is threatened by urbanization, and harbors a number of sensitive species. %%% Our objective is to examine the interaction between fire and nitrogen deposition on plant growth. We request funding through the SGER Program to take advantage of the recent fires in California. We have located two sites that were burned in November, one with high and one with low N deposition. We propose to set up a factorial experiment at each site, with burned and unburned vs. fertilized and unfertilized plots. We hypothesize that annual grass growth will be increased more than shrub growth after N fertilization, and that fire hastens the conversion from native shrubland to annual grassland. This research has important implications for the conservation of coastal sage shrub. A reduction in the fire cycle will be important to prevent a rapid conv ersion to weedy annuals, but we may also recommend a reduction in anthropogenic N deposition.
