Woody encroachment into grassland ecosystems has been documented in many parts of the world. Understanding the mechanisms that drive this successional pattern is important for our understanding of grassland community composition as well as global projections of species ranges. Experimental studies of competition for resources between herbaceous and woody species have suggested that competition is strongest for soil water resources. Abiotic factors (CO2 and nitrogen) as well as biotic factors (species richness) have indirect effects on soil water content and direct effects on woody survival and growth. We examined survival and growth of planted bur oak (Quercus macrocarpa) seedlings over a three-year time period under varying levels of CO2, N, and herbaceous species diversity and biomass.
Using a mixed effects model, we found that herbaceous species diversity had a significantly positive effect on oak survival but a negative effect on growth of surviving individuals. The observed relationship between herbaceous species diversity and oak performance may be due to correlated changes in the resource environment. We found a strong positive correlation between species richness and total herbaceous biomass as well as a weak negative correlation between herbaceous biomass and percent light transmission. We found no evidence for the direct effects of nitrogen addition or CO2 enrichment on oak survival or growth. We suggest that facilitative effects of herbaceous species diversity on oak establishment may be due to increased shading and consequent protection from seedling water stress in species rich plots. Following establishment, oak seedlings in species rich plots are exposed to intense competition for resources which leads to decreased oak growth rates. Further exploration of the mechanisms that control woody encroachment into grassland ecosystems needs to focus on the cascade of interactions between herbaceous species diversity, the local resource environment, and oak performance at different life history stages.