Grasslands in a changing climate : Summer drought and winter warming effects on grassland vegetation

Sammanfattning: Grasslands harbour a high biodiversity of both plants and animals, and they provide many ecosystem services such as fodder production, pollination, and carbon storage. Climate change is likely to alter grassland ecosystems, with the effects varying according to the exact nature and timing of changes. Hence, understanding of seasonal climate change effects on grasslands and how negative impacts can be reduced is important to maintain biodiversity and to ensure continued delivery of ecosystem services.In this thesis I explored how seasonally specific aspects of climate change, i.e. summer drought and winter warming, affect aboveground plant biomass, plant community composition, and floral resources for pollinating insects. Moreover, I aimed to outline ways to mitigate potential negative climate change effects by adapting conventional grazing and mowing regimes and/or by applying soil amendments (i.e. compost) as a novel management method. Soil amendments have been suggested as a method to increase carbon sequestration and they might mitigate negative drought effects. However, there is no empirical evidence of how European grassland ecosystems would be affected should such measures be applied.A literature review of climate change studies revealed that the terms ‘wetter’ and ‘drier’ can be defined by a variety of hydroclimatic variables, or are not defined at all, making it difficult to synthesise climate change effects on ecosystems and societies. In two in-situ experiments I investigated the effects of summer drought (using rain-out shelters), soil amendments and mowing on four Swedish grasslands, and the effects of winter warming (using open-top chambers) and sheep grazing on three British Upland grasslands. The experimental summer drought caused a non-significant decline in aboveground plant biomass (i.e. fodder production), plant species diversity, and floral resources. Applying soil amendments increased aboveground plant biomass and floral resources (in yearly mown plots), but these positive effects were reduced under drought. There were signs of negative soil amendment effects on legumes. Winter warming led to an increase in graminoid biomass and a decrease in bryophyte biomass. Sheep grazing buffered the growth of a competitive species under winter warming but had only minor effects overall.My thesis emphasizes that it is important to clearly define terms like ‘wetter’ and ‘drier’ when studying effects of climate change on ecosystems, since clarifying the effects of climate across habitats and management interventions will require the synthesis of results across a range of experimental and observational systems. My field experiments indicate that even relatively small climatic changes affect grassland plant biomass and biodiversity, and that these effects depend on the season and grassland site in question. Furthermore, soil amendments have mainly positive effects on the grassland vegetation, indicating that they have potential for broad-scale application as a method to increase carbon sequestration. Given that my experiments were set up in-situ in grasslands and the treatments were rather mild and realistic in magnitude according to local climate change predictions, the observed vegetation changes within only three years are quite remarkable. They therefore highlight the need for detailed empirical and mechanistic understanding of how climate change processes are likely to affect grassland ecosystems.