COS 67-5 - Light and soil and fire, oh my: Investigating the environmental gradients driving plant communities in oak woodland restoration

Abstract: Once widespread, Quercus-dominated woodlands have been significantly degraded due to land use changes, fire suppression, and invasive species. While research has shown that gradients of light, soil, and fire structured diversity in predegraded woodlands, we know less about the processes structuring diversity in woodlands under restoration. Additionally, while seeding is often a component of restoration, few studies have addressed how different assembly drivers may be controlling the sown and non-sown species in a community. This study addresses how the gradients of canopy openness, soil conditions, and fire frequency influence plant communities in restored woodlands. Specifically, we assess how these gradients drive the taxonomic and functional trait diversity and composition of the sown and non-sown components of the plant community. I surveyed plant community composition, canopy openness, and soil properties, and determined burn rate (total burns/years since first burn) and seeding history in 210 m² plots in seven woodland restorations in Cook County, IL. Using field collections and databases, I also collected functional trait data for all species. I found that canopy openness (r² = 0.05, p = 0.009*) and burn rate (r² = 0.09, p = 0.001*) drive patterns of community composition. There were significant positive relationships between canopy openness and the abundance of native forbs (r² = 0.42, p < 0.0001*) and a significant negative relationship between burn rate and non-native abundances (r² = 0.23, p = 0.04*). The community composition also shifted from species with large seeds to species with small seeds as canopy openness increased (r² = 0.19, p = 0.007*). When considering the diversity of the entire community (sown and non-sown species), I found no effect of canopy openness, burn rate, or soils on species richness or Shannon-Weiner Index. However, the seeded status of a plot (i.e., whether that plot had received seed addition) and canopy openness interacted in their effect on species richness, such that there was a significant relationship between canopy openness and species richness in unsown plots, but not sown plots (r² = 0.53, p= 0.03*). These findings suggest that canopy openness and fire gradients are important for structuring plant diversity and composition in this system. However, the differing responses of sown and non-sown species are important to consider when understanding patterns of diversity and community assembly in restored woodlands. A better understanding of how these plant communities respond to environmental gradients will help guide restoration practices, helping land managers achieve restoration goals.