Xi'an Jiaotong-Liverpool University, China (People's Republic)
Background/Question/Methods
Anthropogenic factors, including climate change, nutrient deposition, and land management changes, are driving dramatic shifts in ecosystems worldwide. In some cases, these shifts may entail dramatic alterations of ecosystem structure, as when a grassland transitions to savanna or forest due to the encroachment of woody species. Such changes are likely to alter not only the state of ecosystems, but also the factors to which they will respond in the future.
Previous research has shown that nitrogen is a key driver of productivity and competitive dynamics in the old-field grasslands of Cedar Creek Ecosystem Science Reserve (Minnesota, USA). Some of these fields are now subject to encroachment by Eastern White Pine (Pinus strobus). As part of a long-term investigation of pine encroachment’s drivers and consequences, we conducted a census of pine trees and a survey of herbaceous species cover. This sampling followed 18 years of experimental manipulation, during which factorial combinations of nitrogen addition and fire were applied to a set of 32 20x20m treatment plots. We then tested for treatment effects on pine encroachment and species diversity. To understand community composition changes, we used NMDS plotting for visualization and PERMANOVA for statistical support.
Results/Conclusions
As expected, we found that fire dramatically inhibited pine encroachment. In unburned plots, pine abundance varied dramatically, while most burned plots contained no living pines. Nitrogen addition did not significantly alter pine encroachment. Unexpectedly, we found that nitrogen effects on community composition were contingent upon burning treatment. Within the burned treatment, nitrogen addition plots, with substantial cover of introduced C3 grasses, were distinctly differentiated from C4 dominated control plots. In contrast, among unburned plots, which featured reduced but still substantial herbaceous cover, nitrogen had no discernable effect on species composition. Herbaceous species diversity also showed an interactive response to fire and nitrogen, though this effect was likely driven by abundance changes in a small number of common grass species.
Despite nitrogen’s key roles in influencing productivity and competition at Cedar Creek, its compositional effects appear dependent on fire and the associated suppression of woody plant cover. More broadly, we see that a factor playing major role in structuring a community can cease to play that role as disturbance regimes and ecosystem states are altered. Such patterns may play an important role in determining the implications of ongoing ecosystem changes.
