1. Department of Integrative Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada
2. Center for Conservation Biology, University of California, Riverside, California 92521, USA
3. Division of Biological Sciences, The University of Montana, Missoula, Montana 59812, USA

Correspondence to: John N. Klironomos¹ Correspondence and requests for materials should be addressed to J.N.K. (Email: jklirono@uoguelph.ca).

Attempts to understand the ecological effect of increasing atmospheric CO₂ concentration, [CO₂], usually involve exposing today's ecosystems to expected future [CO₂] levels^{1, 2}. However, a major assumption of these approaches has not been tested—that exposing ecosystems to a single-step increase in [CO₂] will yield similar responses to those of a gradual increase over several decades³. We tested this assumption on a mycorrhizal fungal community over a period of six years. [CO₂] was either increased abruptly, as is typical of most [CO₂] experiments, or more gradually over 21 generations. The two approaches resulted in different structural and functional community responses to increased [CO₂]. Some fungi were sensitive to the carbon pulse of the abrupt [CO₂] treatment. This resulted in an immediate decline in fungal species richness and a significant change in mycorrhizal functioning. The magnitude of changes in fungal diversity and functioning in response to gradually increasing [CO₂] was smaller, and not significantly different to those with ambient [CO₂]. Our results suggest that studies may overestimate some community responses to increasing [CO₂] because biota may be sensitive to ecosystem changes that occur as a result of abrupt increases.

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