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We report results from a continuing, long-term field experiment addressing biotic responses to climatic change in grasslands. We focus on effects of summer precipitation (enhanced rainfall, drought, control) and winter ground temperatures (warming, control) on growth, reproduction and herbivory in St John's wort, Hypericum perforatum L. Both winter warming and summer rainfall regimes modified performance and interactions of H. perforatum, particularly those with herbivorous insects. Winter warming had positive effects, with earlier initiation of plant growth and reduced damage by gall-forming and sucking insects in spring, but also had strong negative effects on plant height, flowering, and reproduction. Summer drought reduced reproductive success, but even severe drought did not affect plant growth or flowering success directly. Rather, summer drought acted indirectly by modifying interactions with …