Effects of Non-Native Annual Plant Removal on Native Species in Mediterranean-Climate Shrub Communities

Removal of non-native plants is known to increase overall native cover within degraded communities that contain at least a small percentage of native plant cover. We investigated the mechanisms behind this pattern, asking whether removal of non-native annual species increases the density and species richness of the native community through increased seedling recruitment or through the growth of established native shrubs. We also investigated whether the effectiveness of non-native removal was influenced by region (coastal versus inland) and whether there was a threshold of native cover required for invasive removal to be effective. We established 13 study sites (7 coastal and 6 inland) located throughout the Nature Reserve of Orange County, CA, USA. Each degraded site contained four paired plots corresponding to a range of existing native plant cover: low 20-29%, medium-low 30-39%, medium-high 40-49%, and high cover 50-59% with one plot per pair subjected to non-native removal. We collected plant density, species richness, and established native shrub volume measurements to clarify the effectiveness of non-native removal. Non-native plant removal reduced non-native annual recruitment, increased that of native shrub seedlings, but had no impact on native forb recruitment. Non-native removal increased the number and reduced mortality of established native shrubs but did not influence shrub size. Native seedling density, species richness, and established native shrub number were highest inland, but coastal sites had larger adult shrubs. We found that non-native removal was most effective for increasing native density and species richness for degraded inland sites with less than 40% of existing native cover. The initial native cover did not affect established shrub volume or number. Our results confirm the importance of non-native plant removal in areas with medium-low or low native cover to increase native recruitment, species richness, adult shrub number, and to reduce established shrub mortality, especially during extreme drought.