Grasslands in California have been highly altered by the introduction of numerous herbaceous species from the Mediterranean, including the winter annual grasses which now dominate most sites. Although these species arrived more than two centuries ago, unanswered questions remain about these novel communities, including what controls the patchy distribution of annual grasses within many landscapes, and how remnant native populations decline or persist in response to disturbance. Quantitative vegetation surveys and repeated mapping efforts have allowed us to examine some of these questions at several sites in California. First, we explored how the presence of isolated oak trees in savanna grasslands affects the surrounding plant community composition with a set of systematic vegetation surveys across four sites. Paired soil cores were taken associated with surveyed quadrats to assess numerous soil attributes, as well as the abundance of fine oak roots in shallow soil. Second, we intensively mapped populations of several species of native perennial grass to address how livestock grazing might affect boundaries between native- and exotic-dominated patches. This mapping took place inside and outside of grazing exclosures at two locations with different soil conditions, and at two time points five years apart.
Results/Conclusions
Across a large precipitation gradient in California, our surveys illustrate that plant communities under the canopy of oak trees belonging to several species were more homogeneous than those in surrounding open grassland. This pattern was due to striking similarity in dominant plant species beneath trees, mostly exotic annual grasses (Bromus and Hordeum sp.) and thistles (Sisymbrium and Carduus sp.). Outside the tree canopy (even up to 10 m from the edge), shallow oak roots also had large effects on plant communities; where roots were more numerous cover of exotic grasses was lower, and species diversity was higher. Thus, interactions between oak trees and neighboring herbaceous plants may alter the distribution of native and exotic species within grasslands. The boundaries of mapped native grass populations also fluctuated over five years, depending on the site and grazing treatment. At the lower fertility site where native grasses were initially less abundant, populations were spreading, especially in the presence of livestock grazing. Population boundaries were more static in the high fertility site, where native grasses occupied more area initially, although they had shrunk in grazing exclosures. Together, these studies demonstrate the significant spatial and temporal heterogeneity in species composition within these novel communities in California and their varied responses to management.