Spatial and temporal understory species turnover in a California foothill oak woodland

Background/Question/Methods:

California’s oak dominated woodlands form complex spatial and temporal patterns on foothill landscapes. A key conservation goal is to enhance plant species diversity in the understory, often using fire and managed grazing. Better understanding of the spatial and temporal patterns of species turnover can determine efficacy of management and restoration activities and is critical to predicting the effects of disturbance and climate change. We used a continuous long term dataset of fine scale oak woodland understory vegetation composition in the Sierra Foothills Research and Extension Center collected from 1998-2008. Data were collected from 66 permanent plots located in three watersheds across four categories of tree canopy cover. Each watershed was exposed to a treatment including a control moderate grazing treatment throughout the eleven years, a fencing treatment to exclude grazing after three years of initial data collection, and a fire and grazing treatment also applied after three years of initial data collection. We examined spatial turnover at small and medium scales, comparing spatial to temporal turnover in understory vegetation composition using the Jaccard index of dissimilarity. We examined the effects of grazing and fire treatments on the species composition and on the cover and richness of introduced and native vegetation. 

Results/Conclusions:

Spatial patterns in vegetation structure were not consistent, with most plots showing a high and unpredictable degree of dissimilarity at small scales. However, plots showed a more consistent decline in similarity over time, with plots predictably more similar when sampled more closely in time. Grazing and fire treatments influenced the cover of some but not all species, and patterns correlating with native and introduced status were also not consistent. Native species richness was higher in the understory of oaks compared to open grasslands with low canopy cover. The high level of temporal consistency and spatial inconsistency suggest that conservation goals should be spatially explicit and management of oak woodlands will be most effective when targeted at small spatial scales.