Context matters for warming: Interannual variation in grass biomass responses to seven years of warming and N addition

Background/Question/Methods

Climate warming and increased atmospheric nitrogen (N) deposition are both predicted to have strong influences on plant biomass in grass-dominated ecosystems over the next century.  Nevertheless, in short term warming and N addition field experiments, potential interactions of these factors with interannual variation in background temperature and precipitation may limit the generality of results.  In particular, the occurrence in some years of stresses such as drought and late spring frost could negate the potential stimulation of biomass production by warming, and reduce the responsiveness of plants to increased N.  We examined plant biomass responses to seven 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 year-round warming effect.  Aboveground biomass for each species was estimated at the peak of the growing season using non-destructive sampling based on allometric relationships between leaf height and leaf mass.  Root biomass pooled over all species was measured directly from soil cores collected to a depth of 0 to 15 cm. 

Results/Conclusions

Warming did not significantly affect total aboveground biomass in every year, but it increased biomass in a year when warming accelerated spring snow melt, and it decreased biomass in a year that featured a severe late spring frost.  There were no significant differences between biomass responses to year-round and winter-only warming.  N addition consistently increased aboveground plant biomass, with the exception of the first year, and there was not a significant interaction between N addition and warming.  There was a significant positive correlation among years between precipitation and aboveground biomass, but there were no consistent effects of drought on the warming and N addition responses.  Overall, our results suggest that warming effects on grass biomass are not as consistent as those of N addition effects, but warming in the late winter and early spring can play a key role in determining plant biomass responses to warming in temperate ecosystems.