Effectiveness of passive restoration at increasing cover of native shrubs

Background/Question/Methods

Significant resources are invested in the restoration of Coastal Sage Scrub in Southern California due to conservation challenges facing this unique habitat. The presence of invasive annual grass species can have a detrimental impact on these efforts by inhibiting the growth of native species. Because funds supporting restoration are limited, finding the most effective and efficient methods are crucial to the long-term goal of habitat conservation across the region. One question is whether cover of native shrubs will increase with “passive” restoration, involving removing non-native species without adding any natives to the landscape. Another question is whether the effectiveness of this type of restoration varies depending on the system status, such as initial native cover. We established 13 sites across a 50 km south-to-north transect in 2009. At each site, eight 5 m x 5 m plots were paired by existing native cover: 20-30%, 30-40%, 40-50%, or 50-60%, and then randomly assigned to be either an invasive species removal treatment or a control without any manipulation. All non-native species in the removal plots were mowed with a hand mower each spring and sprayed with low-dose glyphosate herbicide each fall. Total cover of native perennials was measured each spring. In 2012, we recorded the size of all shrubs in both control and removal plots.

Results/Conclusions

Over the past several years, overall native perennial vegetation has increased in total cover in both the invasive removal and control plots. However, native cover in the weeded plots increased to a greater extent than in the control plots, consistent with release from competition by non-native grasses. This increase in cover occurred due to both increases in existing plant size (changes in volume of plants originally existing in plots), and due to the establishment of new individuals in plots. Lower-cover class plots had greater responses to weeding as compared to plots with the greatest initial native cover. Native shrubs were more dense and smaller in plots at the Northern end of our study transect compared to the Southern end of the transect. Weeding increased shrub establishment across the transect. These results not only provide insight into patterns of plant cover in time and their biotic and abiotic drivers, but also provide land managers the ability to selectively place efforts at weed control to maximize restoration impact.