Print PageGrassland systems, particularly those in arid and semiarid regions, exhibit complex dynamics in response to disturbances such as grazing. State and transition models, which provide a framework describing these complex non-linear vegetation changes, have been well-developed for rangeland management in California. However, they are traditionally descriptive, and empirical evidence of model predictions is rare. Our study tests the generality and predictability of grazing intensity as a driver of transitions among exotic and native California grasslands. We focused on three grassland types: perennial natives (e.g. Nassella pulchra), annual exotics that are acceptable forage species (e.g. Avena fatua), and an annual problematic exotic (Taeniatherum caput-medusae). In 2006, we planted the vegetation types in replicated northern California pastures, allowed the vegetation to establish and then in 2008, we initiated a combination of cattle trampling and mowing treatments to establish a gradient of six grazing intensity levels. After another two growing seasons, we conducted planned reciprocal invasions to overcome seed limitation and to test how grazing intensity influences invasion resistance and thresholds in the vegetation types.
Results/Conclusions
All vegetation types were resistant to invasion by the other species groups when not grazed. Native grasses and the problematic exotic, Medusahead, established much better in their own vegetation than in others, suggesting the presence of reinforcing positive feedback mechanisms. In contrast, annual forage grasses were better able to invade other vegetations; their invasion success depended more on grazing intensity rather than grassland type. All types shifted into forb dominated vegetation due to increasing grazing intensity. However, the level at which transitions occurred differed: native vegetation was most susceptible to forb invasion with forbs becoming dominant at very low grazing intensity. Medusahead vegetation was most resistant to forb invasion, with forbs becoming only co-dominant at the highest grazing intensity.
We show that species effects moderate invasions in the absence of grazing: all grassland types resisted forb invasion and the native and Medusahead grasslands resisted invasion of other grass types. Annual forage grasses were the only group able to invade other grassland types in absence of grazing. We also show that grazing, regardless of vegetation type, facilitated the invasion of weedy forbs. Importantly, plant species effects also moderated grazing effects, leading to invasion thresholds at different levels of grazing intensity. Native grasslands were less resilient to grazing facilitated invasion compared to grasslands dominated by annual exotic or noxious grasses, highlighting the importance of management strategies to maintain native grasslands.
