Shrub and grassland communities coexist in a mosaic in Southern California and other Mediterranean-climate systems. Increases in fire frequency, anthropogenic nitrogen deposition, and a legacy of cattle grazing have all contributed to vegetation-type conversions, in which shrub-dominated communities can convert to annual grasslands, dominated by non-native invasive annuals. It is often not known whether an area currently dominated by non-native invasive annuals was previously a shrub or a native grassland community. Which abiotic conditions are most important in determining plant community composition? We analyzed long-term vegetation monitoring data in the shrub and grassland communities of Orange County, paired with analyses of soil samples and other abiotic features to determine whether common species have preferences for some abiotic features. Data collection methods were initiated in 2007 and consisted of a combination of point intercept andquadrat sampling along 50 m transects. From 2017 through 2019, we collected data on the abiotic conditions of each transect. We collected slope aspect, slope grade and soil samples to be able to examine differences in soil composition and bulk density amongst transects.
Results/Conclusions
Several abiotic factors were significantly different depending on the plant community assigned to that transect. For example, chaparral and coastal sage scrub transects were located on significantly steeper slopes than grassland transects. Soil texture and bulk density were significantly smaller (lower % sand by weight and lower weight per volume) for grassland transects than for chaparral and coastal sage scrub transects. Fire frequency did not vary based on plant community. Our analyses of abundant species confirmed that, within a single functional group, the distribution of individual species was significantly influenced by several abiotic variables. Each species had a different set of abiotic variables that more strongly determined their abundance. We observed that native shrub cover continued to be fairly consistent over time, despite some regional differences and extreme shifts in individual locations post-fire. In contrast, native forb cover varied wildly from year to year, exhibiting responses to precipitation, including lag effects from precipitation in the previous year. We have also demonstrated that non-native species increased post-fire and that some species, such as Erodium cicutarium, were most likely to occur in areas with high fire frequency. Our results clarify the role of abiotic factors in determining the abundance of major functional groups and of individual species in the shrub-grass mosaic of Southern California.