Scale-dependent effects of restoration practices in Californian grasslands

          Californian coastal grasslands have undergone widespread conversion from perennial native communities to exotic-dominated annual communities. Attempts to restore such communities often involve mowing to reduce the above-ground dominance of exotic annual plants, or adding carbon to the soil to reduce available nitrogen and slow exotic growth. It is important to understand how these treatments affect plant diversity, both because of intrinsic interest in maintaining diversity and because altering patterns of diversity may have consequences for future invasions. Treatments may affect diversity by altering both the mean and variance of resources. As the variance of a resource within a plot depends on the size of that plot, the effect of a restoration treatment on diversity may be scale-dependent. We evaluated the effect of mowing and carbon addition on diversity in two heavily invaded Californian coastal grassland sites. In summer 2006, we surveyed each treatment plot and assessed plant diversity at five scales, ranging from 0.015 m² to 4m². One of the two sites showed scale-dependent responses of diversity to the treatments. Mowing significantly increased diversity at the smallest scale and significantly reduced it at the largest scale, while carbon addition reduced diversity at all scales. Pairwise subplot Bray-Curtis distance also showed scale-dependent treatment effects, with mowing significantly decreasing distance in intermediate plot sizes. Treatments at the other site had no effect on diversity or subplot distance. These results suggest that restoration strategies may affect diversity in complex ways, and that effects on diversity should be measured at multiple scales.