Long-term vs. transient plant responses to warming and nitrogen addition in a temperate old field

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Breanna Craig and Hugh Henry, Biology, University of Western Ontario, London, ON, Canada

Background/Question/Methods
Climate warming and atmospheric nitrogen (N) deposition are two global change factors expected to have strong effects on plant productivity and species composition in northern temperate ecosystems over the next century. While examining the physiological responses of individual plants to temperature variation and nutrient availability under controlled conditions provides important mechanistic information, there also is a need to investigate responses to these factors in the field, where greater contextual relevance is present. However, even field experiments are typically limited with respect to temporal scale, and the question arises as to what extent short-term findings can be extrapolated to the longer term. We hypothesized that that lags in plant species composition responses to treatments and treatment effects on litter accumulation result in a divergence of long-term versus short-term responses in productivity.
To test our hypothesis, we compared the short-term (1 year) vs. long-term (14 year) effects of warming (via overhead heaters) and N fertilizer addition on plant productivity, relative species abundances and litter accumulation in a field experiment conducted in a temperate old field. Litter subsamples were collected from each plot in the spring to quantify litter accumulation. The aboveground biomasses of the dominant grass species, Poa pratensis and Bromus inermis, were then destructively sampled at peak biomass, and aboveground biomass estimates for the forb species were obtained non-destructively based on cover measurements. Spectral data (normalized difference vegetation index) also were used to examine the treatment effects on the timing of plant green-up and senescence. Total belowground plant biomass estimates were obtained from root cores.

Results/Conclusions
While spring green-up was accelerated in the warmed plots and in the long-term N plots, by peak plant biomass in early July, only the short and long-term N treatments exhibited increased plant biomass. The latter was only significant for Poa pratensis, and there were no significant treatment effects on cover for the individual forb species. In the long-term plots, there was an increase in total root biomass. However, overall, there were few differences in treatment responses among the short- and long-term plots. We attribute this result to the continued competitive dominance of the two highly abundant non-native grass species. Our results suggest an important role of initial species composition in potentially delaying the emergence of long-term treatment effects in global change field experiments.