Fire and competition in a southern California grassland: impacts on the rare forb Erodium macrophyllum.
The use of off-season burns to control exotic vegetation shows promise for land managers. In California, wildfires tend to occur in the summer and autumn, when most grassland vegetation is dormant. The effects of spring fires on native bunchgrasses have been examined but their impacts on native forbs have received less attention. We introduced Erodium macrophyllum, a rare native annual forb, by seeding plots in 10 different areas in a California grassland. We tested the hypotheses that E. macrophyllum would perform better (increased fecundity and germination) when competing with native grasses than with a mixture of exotic and native grasses, and fire would alter subsequent demography of E. macrophyllum and other species' abundances. We monitored the demography of E. macrophyllum for two seasons in plots manually weeded so that they were free from exotics, and in areas that were burned or not burned the spring after seeding. Weeding increased E. macrophyllum seedling emergence, survival and fecundity during both seasons. When vegetation was burned in June 2001 (at the end of the first growing season) to kill exotic grass seeds before they dispersed, all E. macrophyllum plants had finished their life cycle and dispersed seeds, suggesting that burns at this time of year would not directly impact on fecundity. In the growing season after burning (2002), burned plots had less recruitment of E. macrophyllum but more establishment of native grass seedlings, suggesting burning may differentially affect seedling recruitment. At the end of the second growing season (June 2002), burned plots had less cover of exotic and native grasses but more cover of exotic forbs. Nevertheless, E. macrophyllum plants in burned plots had greater fecundity than in non-burned plots, suggesting that exotic grasses are more competitive than exotic forbs. A glasshouse study showed that exotic grasses competitively suppress E. macrophyllum to a greater extent than native grasses, indicating that the poor performance of E. macrophyllum in the non-burned plots was due to exotic grass competition. Synthesis and applications. This study illustrates that fire can alter the competitive environment in grasslands with differential effects on rare forbs, and that exotic grasses strongly interfere with E. macrophyllum. For land managers, the benefits of prescribed spring burns will probably outweigh the costs of decreased E. macrophyllum establishment. Land managers can use spring burns to cause a flush of native grass recruitment and to create an environment that is, although abundant with exotic forbs, ultimately less competitive compared with non-burned areas dominated by exotic grasses.