Limitation of native grass seedling establishment in California coastal prairie: Seed availability versus competitive interactions

Background/Question/Methods

Recent efforts to restore native grass populations in California grasslands have focused on ways to increase the rate of establishment of native grass seedlings. Whether due to the low supply of native seeds, and/or due to the intense competitive environment that newly germinating individuals experience, relatively few native grass individuals survive the first growing season. Yet, research indicates that in more mesic habitats, this period of establishment is a critical bottleneck in efforts to establish native grass populations.  In a restoration context, the relative importance of low native seed availability versus mortality due to competitive interactions offers very different prescriptions for efforts to increase native grass populations. If establishment of native grasses is limited by seed availability, then augmentation of natural seed rain should successfully increase population sizes. On the other hand, if competition from non-native grass and forb species prevents establishment, then seed augmentation would fail without treatments to reduce the competitive environment.  We established a series of plots in two highly invaded coastal California grasslands in which experimental addition of native seeds was combined with treatments designed to reduce biomass of exotic competitors – addition of labile carbon to reduce nitrogen availability, and spring mowing to reduce standing biomass. The number of native species in each treatment were sampled in Spring 2006 and 2007. We tested whether seed addition, carbon addition, mowing, or a combination of treatments, increased establishment of native grasses.

Results/Conclusions

Seed addition had no significant effect on the establishment of native grass species at either grassland site. At one of the sites, the number of Bromus carinatus individuals was four times higher in plots that received spring mowing as compared to plots that were not mowed. However, there was no interaction between seed addition and mowing treatments. Our results indicate that seed addition, by itself, is not sufficient to restore native grass populations in a highly invaded community. So far, we have found no evidence that treatments that reduce competitiveness of exotic competitors can make seed addition more effective. These results indicate that treatments that restoration strategies such as mowing, grazing, and burning that are designed to reduce the standing crop of exotic species are still likely the most effective ways of increasing establishment of native species.