Climate warming and increased atmospheric nitrogen (N) deposition are both predicted to influence plant biomass in grass-dominated ecosystems strongly over the next century. Nevertheless, in short-term warming and N addition field experiments, potential interactions of these factors with interannual variation in temperature and precipitation may limit the generality of results. We examined plant biomass responses to 7 years of warming and N addition in a grass-dominated temperate old field. Warming was administered using overhead infrared heaters both year-round and over-winter only, with the latter used to assess the contribution of winter warming to the yearround warming effect. Warming did not significantly affect total aboveground biomass in every year, but both year-round and over-winter warming increased biomass in a year when these treatments accelerated spring snow melt, and decreased biomass in a year that featured a severe late spring frost. N addition consistently increased aboveground plant biomass, with the exception of the first year, and there was no interaction between N addition and warming. There was a significant positive correlation among years between precipitation and aboveground biomass, but there was no consistent association between years with low precipitation and the warming and N addition responses. Overall, our results suggest that warming effects on plant biomass are not as consistent as those of N addition effects, but that warming in the late winter and early spring can play a key role in determining plant biomass responses to warming in temperate ecosystems.

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