Direct and indirect effects of climate change on St John's wort, Hypericumperforatum L. (Hypericaceae)

We report results from a continuing, longterm field experiment addressing b iotic responses to climatic change in grasslands. We focus on effects of su mmer 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 rainfa ll regimes modified performance and interactions of H. perforatum, particul arly those with herbivorous insects. Winter warming had positive effects, w ith earlier initiation of plant growth and reduced damage by gall-forming a nd sucking insects in spring, but also had strong negative effects on plant height, flowering, and reproduction. Summer drought reduced reproductive s uccess, but even severe drought did not affect plant growth or flowering su ccess directly. Rather, summer drought acted indirectly by modifying intera ctions with herbivorous insects via increased vulnerability of the plants t o herbivory on flowers and capsules. Overall, the effects of summer precipi tation were expressed mainly through interactions that altered the response s to increased winter temperatures, particularly as summer drought increase d. The field site, in Oxfordshire, UK, is near the northern limit of distri bution of the species, and the experiment tested probable responses of H. p erforatum as climates shift towards those more typical of the current cente r of the distribution of the species. However, if climates do change accord ing to the projected scenarios, then H. perforatum is unlikely to fare well near its northern boundary. Increased winter temperatures, particularly if accompanied by increased summer drought, will probably render this species even less abundant in England than at present.