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 it 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 3 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 results and the species richness results appear to describe opposing trends, this is due only to the fact that the natural drivers are moving in opposite directions (decreasing species richness and increasing CO2). Interestingly, the mechanisms that underlie both patterns are very similar, increased CO2 and increased species richness both increase herbaceous biomass which (1) increases belowground competition for resources and (2) increases facilitation of early plant survival under a more diverse plant canopy; in other words, both competition and facilitation help determine community composition in these grasslands.
Bibliographical noteFunding Information:
Acknowledgments This research was supported by the Department of Energy Program for Ecological Research Grant DE-FG02-96ER62291, the National Science Foundation Long-term Ecological Research Grant DEB-0080382, the NSF Long-term Research in Environmental Biology DEB-0716587, the University of Minnesota, the University of Wisconsin–Milwaukee (UWM) AOP program, the UWM Department of Biological Sciences, and the NSF Graduate Research Fellowship Program. IAD was additionally supported by the New Zealand Ministry of Science and Innovation (Ecosystem Resilience OBI). We thank Sarah Christman and all other Cedar Creek interns for field assistance, and Joe Mascaro and Ramesh Laungani for helpful comments during the writing process. The authors declare
- Global change
- Woody encroachment