Experimental warming outside the growing season and exclusion of grazing has a mild effect on upland grassland plant communities in the short term

Winters are expected to warm more than summers in central and northern Europe, with largely unknown effects on grassland plant communities. By studying the interactions between winter warming and summer grazing, we aimed to disentangle their effects and give recommendations for future grassland management. Our study area Upper Teesdale, England has winter temperatures close to 0°C and a well-studied vegetation, known for its arctic-alpine species growing at their climatic warm range limits. We set up a winter warming experiment using open top chambers (ca. +0.5°C) from mid-September until mid-May 2019 to 2022 and excluded sheep grazing during summer in a fully factorial design. Graminoid biomass increased, and bryophyte biomass decreased with winter warming. There was little to no evidence that winter warming affected any of the other plant response variables we measured, neither did grazing nor the interaction between winter warming and grazing. Our experiment was relatively short in duration and treatments were realistic in magnitude, therefore the plant communities responded only slightly. Nevertheless, our data suggest a change towards more dominant vascular species and less bryophytes with winter warming, which might lead to lasting changes in the plant communities in the longer-term if not buffered by suitable grazing management.