Disturbance is considered a major factor in the promotion of non-native plant species. In chaparral and coastal sage scrub ecosystems of Southern California, fire is an important component of the disturbance regime. Non-native grasses become abundant after fire and can alter environmental conditions and resource availability during early post-fire years. Longer-term persistence of non-native grasses can occur and limit the establishment and growth of native species. An intense wildfire burned across the entire San Dimas Experimental Forest in the San Gabriel Mountains of Southern California in 2002, and the recovering community still contains large areas of non-native grasses. The objectives of this study were to describe the distribution and abundance of non-native grasses and possible impacts of non-native grasses on the native plant community eight years after the last fire. Questions were addressed by measuring microenvironmental factors and sampling vegetation during May and December 2010 on fifty 28-m2 plots across a 110-ha study area. Correlation analysis, two-sample tests, and classification and ordination procedures were used to analyze the data. A manipulative field experiment is being conducted to test the effects of non-native grasses on germination and establishment of native forbs. Results of that experiment will be available in April 2011.
During May 2010, 2799 individuals representing 33 species were observed in sample plots. During December 2010, 1988 individuals and 24 species were observed. Jackknife estimates of species richness declined from 38 in May to 32 in December. Simpson diversity (1-D) declined from 0.560 in May to 0.479 in December. Significant differences were found in soil K, pH, and water potential between the May and December 2010 sampling periods. In May soils had higher K, lower pH, and lower water potential than in December. Native shrub cover was negatively correlated with non-native grass cover during both sampling periods. Non-native grass cover was positively correlated with distance from fuel breaks. Plots with lower soil K and pH, cooler air temperatures, and lower soil moisture content had greater non-native grass cover during both sampling periods. Native shrub cover was positively correlated with soil K, soil pH, air temperatures, and soil moisture content. Results suggest that an understanding of the local microclimate and the possible effects of non-native grasses on microenvironmental factors will aid restoration efforts to improve degraded native habitat and restore ecosystem functionality.