When non-native plants become problem invasive species, they appear to push the entire ecosystem across a threshold, creating an alternative vegetation state that is difficult to restore. While this pattern is consistent with theoretical predictions concerning multiple stable states, there have been few empirical tests. The research will promote the understanding of restoration and multiple state dynamics in California grasslands, one of the most invaded ecosystems in the United States. Through observational studies and field experiments, it will investigate several factors that will aid in restoration and conservation: whether variation in soil nitrogen availability creates refugia for native and non-native grasses, whether intermediate levels of nitrogen supply can generate both non-native- and native-dominated states, whether interactions between vegetation and soil microbes create positive feedbacks and lead to the formation of multiple states, and whether regional colonization differences between non-native and native species overwhelm these local interactions.
The project will also train young scientists through year-long experiential science programs. Students from local high schools will be involved with data collection as well as with the design and implementation of a public demonstration experiment. The project will also encourage the participation of women, particularly Hispanic women, by recruitment and outreach efforts. The results will be disseminated broadly to applied practitioners, scientists, and the general public through directed publications, workshops, and seminars.
Native perennial grasslands in California are on a drastic decline due to invasion by exotic annual species. Our project focused on how abiotic environmental heterogeneity and different colonization abilities influence coexistence between exotic annual grasses and native perennial grasses. Environmental heterogeneity is important to diversity patterns as it can create local sites where either exotic or native species can dominate. For example, high soil nitrogen is often thought to facilitate dominance of exotic annual grasses, while native perennial grasses dominate at low nitrogen levels. To test these ideas, we created native (Nassella pulchra) and exotic (Bromus hordeaceus, Lolium multiflorum) grasslands – keeping all other factors constant -- along an experimental gradient of nitrogen availability. Under low nitrogen conditions the native grass dominated regardless of whether it was invading native or exotic vegetation. However, contrary to expectations, the exotic grasses were not always the superior competitors under high nitrogen conditions. Instead, the native grass Nassella was competitively superior compared to Bromus across a wide range of N availability. However, competitive interactions between Nassella and Lolium were dependent on N levels as well as what type of vegetation that they invaded. This last case supports our expectation that priority effects – that the species who establishes first can affect subsequent invasion success – can occur at intermediate nitrogen availabilities. Colonization is an important process because it influences what seeds can get where and how many seeds there are at given site. Even though Bromus was competitively inferior to Nassella in our experiments, it is a successful invader in California grasslands. Could it be that the superior colonization ability of Bromus – that it produces a lot of seeds -- might be the most important reason why annual exotic grasses are successful invaders and restoration of native grasslands is such a difficult task? To test this, we experimentally manipulated seed inputs in experimental "checkerboards" composed of Bromus and Nassella patches. We demonstrated that equalizing the amount of seeds produced by both species within a patch significantly increased recruitment of Nassella seedlings within the exotic patches. Thus, the overwhelming propagule pressure of the exotic may reverse any competitive advantage experienced by the native Nassella grasses. Our experimental results have several implications for the conservation and restoration of native grasslands. First, they suggest that natives should be able to naturally re-colonize areas with exotic grass vegetation, particularly in areas where nitrogen deposition is low and where seed input from nearby native grassland patches is high. They also suggest that activities to reduce nitrogen inputs and augment areas with native seed may be highly effective, particularly in areas with intermediate levels of nitrogen deposition. We have presented these results in a series of conferences and symposia aimed at land managers and conservation organizations to disseminate these important conclusions and aid restoration decision-making Our project also had a large training and outreach component, with the goal of teaching students interested in invasion and restoration ecology appropriate experimental and analytical techniques. Two postdocs, two PhD, and almost 25 undergraduate students and postgraduate students participated in the project. Of these, almost 85% were women, and 40% were underrepresented minorities. Of the undergraduates, twelve have gone on to pursue graduate degrees in ecology or related fields. We also established service learning program with a local high school and conducted field trips as part of a K-12 teacher education program.
