ABSTRACT Question In addition to altering ecosystem states, anthropogenic changes may also alter the drivers of community dynamics within these ecosystems. Previous research has shown nitrogen as a key driver of community dynamics in grassland ecosystems, including those present at our study site. We sought to test whether these nutrient responses will shift as changes to disturbance regimes facilitate woody encroachment. Location Successional grassland at Cedar Creek Ecosystem Science Reserve (Minnesota, USA). Methods As part of an investigation into the drivers and consequences of Eastern White Pine ( Pinus strobus ) encroachment at this site, we surveyed pine abundance and herbaceous community composition in 32 treatment plots, following 18 years of experimental fire, nitrogen, and herbivore manipulation. Results Although pine abundance varied widely in unburned plots, it did not significantly respond to nitrogen addition or herbivory. As expected, pine encroachment was dramatically inhibited in burned plots. Species richness in the herbaceous community did not differ significantly between treatments. The Shannon diversity index responded interactively to fire and nitrogen, with nitrogen addition decreasing diversity in unburned plots but increasing diversity in burned plots. Nitrogen's effects on the overall composition of the herbaceous plant community were contingent upon fire. Within the burned treatment, nitrogen addition led to an increase in the cover of invasive C3 grasses. Within the unburned treatment, nitrogen had no consistent effect on herbaceous species composition. Conclusions Despite research showing nitrogen as a key driver of community dynamics in the grasslands of our study site, we found that this effect is contingent on the presence of fire and absence of woody encroachment. With this variation in nitrogen effects, we see that a factor playing a major role in structuring a community can cease to play that role as disturbance regimes and ecosystem states are altered.
Wheeler et al. (Fri,) studied this question.