Over the last century many of the world's grassland and savanna communities have experienced significant changes in the abundance of shrubs and trees. In light of this there is a need to better understand the influences of trees on the herbaceous community. In particular, the potential for trees to exert long-term impacts on communities following their death and the role of spatial factors such canopy size or isolation in determining species diversity have rarely been evaluated in a single system. I used observational and experimental approaches to explore the overarching question: how do isolated Quercus in California grasslands affect the surrounding community? I specifically tested whether co-occurring deciduous and evergreen oaks influence understory composition differently and how this influence varies along a latitudinal gradient in California using an extensive survey.

Because canopy oaks die often or have been actively removed I also evaluated the longevity of tree impact by surveying dead individuals. Using this survey data I examined how spatial heterogeneity in resources created by savanna trees as well as their relative arrangement within the grassland matrix contributes to landscape patterns of species richness. Lastly, I performed an experiment manipulating the mechanisms by which trees might create gradients in vegetation, focusing on the response of two species of native perennial grass that are potential restoration targets. I found a regionally consistent influence of oaks on understory species composition; all oaks were associated with increased abundance of non-native annual grasses. This pattern continued long after the death of the tree. Community composition outside the canopy did differ between evergreen and deciduous trees. These differences were correlated with the density of shallow oak roots and not environmental differences.

Oak trees also added species to the landscape pool, especially those with large canopy areas. Experimental manipulation of possible mechanisms suggested that for some native perennial grasses, a major limiting factor under oaks is interference from non-native annual grasses, particularly under trees. Together, these results suggest the influence of Quercus on California grasslands is multi-faceted, and may contribute to the distribution of non-native species on the landscape after extant trees are gone.