Woody plant encroachment is widespread in grassland ecosystems and can have substantial impacts on conservation efforts, economic activities and carbon cycling dynamics. It is thus essential that we understand both the causes of such encroachment and its impacts on affected herbaceous communities. Due to the long lifespans of woody plants and grassland perennials, short term studies may present an incomplete picture of these dynamics. Here, we draw on a two decade manipulative experiment to explain the effects of fire, nutrient availability, and herbivory on woody plant encroachment and herbaceous community dynamics in a Minnesota old field grassland.
Our experiment was established in 2000 at Cedar Creek Ecosystem Science Reserve and placed within a successional grassland undergoing invasion by Pinus strobus (Eastern White Pine). Thirty-two 20x20m treatment plots feature fully factorial combinations of prescribed burning, nitrogen addition and large herbivore exclosure. During the 2018 field season, we resurveyed these plots, measuring percent cover of herbaceous vegetation in designated subplots and DBH of all woody plants of sufficient height. We additionally collected soil, aboveground biomass and belowground biomass samples at several points in each plot for use in future C-N analysis.
Results/Conclusions
As expected, fire dramatically inhibited pine encroachment. No burned plot showed more than 4% estimated pine canopy cover, while unburned plots spanned a range from 0-95% (mean 57%). Nitrogen addition had weaker effects on pine encroachment but substantially altered the herbaceous vegetation community. Herbivore exclusion had only minimal effects (likely due to changes in the broader site’s management regime) and was excluded from most analyses.
Initial analysis of the herbaceous community, based on the predictor variables alone, suggested positive effects of fire on cover of forbs and both C3 and C4 grasses, though with a negative interaction between fire and nitrogen addition for C4 grasses. However, a preliminary version of a Bayesian model incorporating pine cover as an additional predictor suggests additional nuance. While C3 grasses benefit from the absence of pine canopy cover, the direct effect of fire on this functional group is a 58% cover reduction relative to the control plots. As we refine our model, we plan to continue exploring this contrast between direct and indirect effects of fire on herbaceous cover, gaining further insight into the dynamics of pine encroachment.