Exploring patterns of the effectiveness of passive restoration on Coastal Sage Scrub

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 has an additional detrimental impact on these efforts by inhibiting the growth of native species, and changing fire and water regimes in the area. This study focuses on analyzing the effectiveness of “passive” restoration, involving the simple step of removing non-native species without actively planting or seeding any natives. Likely the success of such minimally interventional restoration depends highly on the initial system status, such as native cover, or the geographical position within a regional context. This experiment began in 2009, and includes 13 sites across a 50 km south-to-north transect. At each site, eight 25m²plots were paired by initial native cover and assigned to be either an invasive species removal treatment or a control. Cover of native perennials and invasive species is measured each spring, and sub-plots are used to record the presence of annual plants and perennial seedlings.

Results/Conclusions

As expected, native perennial cover increased in all plots, but to a greater extent in the weeded plots. However, in the last two years, the difference between the northern and southern sites was more pronounced, with the southern sites experiencing greater benefit from weeding. Two other aspects of the results raise interesting questions. First, we expected lower-initial-native-shrub-cover plots to have greater responses to weeding compared to plots with the greatest initial native cover, due to the larger effect on resource availability and the production of open space as compared to high-initial-cover plots. Instead, we found that overall, the sites experienced similar amounts of growth regardless of initial cover. However, preliminary analysis suggests that initial native cover may affect the change in cover of non-native species. Secondly, the change in the number of native species present was not significantly different between the weeded and control plots in previous years, but recently, the southern sites had more diversity in the weeded plots, whereas the northern plots had more similarity between weeded and control plots.  This suggests that increasing native species diversity may be a longer-term process, may be dependent on climate, or that passive restoration may be less effective at improving native species diversity in all areas. Understanding such mechanisms of recovery of native cover and diversity will help to construct habitat that resists the spread of invasive species.