637 Ecology, 82(3), 2001, pp. 637–648  2001 by the Ecological Society of America PLANT RESPONSES TO EXPERIMENTAL WARMING IN A MONTANE MEADOW PERRY DE VALPINE 1,3 AND JOHN HARTE 2 1 Department of Environmental Science and Policy, Center for Population Biology, and Institute of Theoretical Dynamics, One Shields Avenue, University of California, Davis, California 95616 USA 2 Energy and Resources Group and Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720 USA Abstract. 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 warm- ing decreases soil moisture and increases nitrogen mineralization, we used nitrogen and water addition experiments on the two dominant forbs to determine whether species re- sponses 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. Key words: carbon cycle; climate change; Erigeron speciosus; Helianthella quinquenervis; mon- tane forbs; plant physiological ecology; resource limitation; Rocky Mountains, Colorado. INTRODUCTION Two related goals of studying biotic responses to climate change are to understand changes in species distributions and abundances (Field et al. 1992, Jensen et al. 1992, Pacala and Hurtt 1993, Bazzaz 1996, Chap- in and Shaver 1996) and to understand ecosystem feed- backs to climate change, particularly via carbon uptake or loss (Houghton et al. 1983, Pastor and Post 1988, Vitousek 1990, Ojima et al. 1991, Hobbie 1992, Oechel et al. 1993, 1994, Smith and Shugart 1993, Chapin et al. 1995, Woodwell 1995, Lashof et al. 1997). An im- portant link between these two topics concerns the level of detail of species responses necessary to predict eco- system responses. Climate models often use a ‘‘one big leaf’’ representation of ecosystems, but species re- sponses may not combine to produce such simple be- Manuscript received 11 June 1999; revised 28 December 1999; accepted 30 December 1999. 3 Present address: National Center for Ecological Analysis and Synthesis, 735 State Street, Suite 300, Santa Barbara, California 93101-3351 USA. havior under climate perturbations (Chapin and Shaver 1985, 1996, Hobbie 1992, Bazzaz 1996). Indications of the complex role of species in ecosystems come from studies showing that particular species characteristics can change ecosystem dynamics (Vitousek 1990, John- son and Damman 1991, Hobbie 1992, Oechel et al. 1994, Chapin et al. 1996, Lauenroth et al. 1997) and that range shifts during past climate change were spe- cies specific (Davis 1989, Graham 1992, Webb 1992). Species responses to climate change will be difficult to understand both because correlational studies of spe- cies ranges may not be predictive (Pacala and Hurtt 1993) and because species may respond to the distri- bution rather than just the mean of climate conditions (Clark 1988, Overpeck et al. 1990, Bazzaz et al. 1996). An ongoing artificial warming experiment in the Rocky Mountains of Colorado has allowed investiga- tion of ecosystem responses to one of the most direct aspects of predicted climate change, increased radiative heat flux. Work to date on this experiment has focused on community and ecosystem responses including