Plant responses to experimental warming in a montane meadow

We studied the effects of a seven-year warming experiment on 11 forb species in the Rocky Mountains of Colorado in 1996 and 1997. Previous work on this experiment focused on ecosystem and community responses to warming. Our purpose here is to report on species responses. We found significant positive responses to warming for two species and negative responses for four species in terms of abundance, size, flowering, or frost damage. Because previous results from the warming experiment showed that artificial warming decreases soil moisture and increases nitrogen mineralization, we used nitrogen and water addition experiments on the two dominant forbs to determine whether species responses in the warming experiment could be due to shifts in resource availability. We found that Erigeron speciosus was limited more clearly by water than by nitrogen and Helianthella quinquenervis was limited by both nitrogen and water. These responses are consistent with the hypothesis that a primary effect of warming on plants occurs via changes in soil resource availability, but more complicated factors including competition are likely to be important to warming effects as well. Because previous work on this experiment indicated that annual forb detrital production is a key component of the carbon cycle of this system, we also asked which species responded to warming with changes in aboveground biomass. Over 1996 and 1997, four of nine perennial species had significantly lower biomass in the warmed plots, and in 1997 one species had significantly higher biomass. The biomass differences of Erigeron and Helianthella were almost equal and opposite, but while the decline in Erigeron was statistically significant the increase in Helianthella was smaller and not significant. In one year, a major effect of warming was to protect Helianthella from frost damage, which illustrates the importance of extreme weather events. Our study points to the potential importance of understanding ecosystem responses to climate change in terms of species responses.