The Shifting Importance of Competition and Facilitation Along Diversity, Environmental Severity, and Plant Ontogenetic Gradients

Encroachment of woody vegetation into grasslands is a widespread phenomenon that alters plant community composition and ecosystem function. Woody encroachment is often the result of fire suppression, but may also be related to changes in resource availability associated with global environmental change. We tested the relative strength of three important global change factors (CO2 enrichment, nitrogen deposition, and loss of herbaceous plant diversity) on the first three years of Bur Oak (Quercus macrocarpa) seedling performance in a field experiment in central Minnesota, USA. We found that loss of plant diversity decreased initial oak survival but increased overall oak growth. Conversely, elevated CO2 increased initial oak seedling survival and reduced overall growth, especially at low levels of diversity. Nitrogen deposition surprisingly had no net effect on survival or growth. The magnitude of these effects indicates that long-term woody encroachment trends may be most strongly associated with those few individuals that survive, but grow much larger in lower diversity patches. Further, while the CO2