Short-Term Plant Community Responses to Warming and Defoliation in a Northern Temperate Grassland

Little is known about the short-term impacts of warming on native plant community dynamics in the northern Canadian prairies. This study examined the immediate effects of elevated temperature and defoliation on plant community diversity, composition, and biomass within a native rough fescue (Festuca hallii (Vasey) Piper) grassland over two growing seasons. We used open-top chambers to simulate climate change and defoliated vegetation in midsummer of the first year to simulate biomass loss associated with periodic ungulate grazing. Warming marginally increased plant species diversity and changed community composition shortly after treatment, but effects were not apparent the second year, and there were no apparent impacts on plant biomass. Nonetheless, warming may have impacted community diversity indirectly through reduced soil moisture content, a pattern that persisted into the second year. Overall, this northern temperate grassland demonstrated limited community-level changes to warming even in the presence of defoliation. 1. Introduction Over long time scales, the plant community composition within savanna habitats is sensitive to climate change [1, 2]. Over the past 50 years, western Canadian savannas, such as the Aspen Parkland ecoregion, have experienced an increase in mean annual temperature of approximately 1.3°C [3]. Mean temperatures for this region are predicted to further increase over the next century, leading to higher evaporation rates and reduced soil moisture availabilities [4, 5], as well as a longer frost-free period [6]. The consequences for native vegetation within these regions are unclear. Under anticipated climate change scenarios, native savanna ecosystems, existing as a mosaic of community types, may be particularly prone to ecological change [7]. Moreover, land-use practices such as livestock grazing may exacerbate any climate-driven changes in vegetation [8]. Plant community changes arise when species vary in their fundamental response to environmental and management factors, including temperature and defoliation in the form of grazing. Warming can impact plants both directly by altering the potential for photosynthesis throughout the growing season [9] or indirectly by altering complex secondary biological processes in the ecosystem [10]. Collectively, warming and defoliation may lead to reductions in the diversity of native species or the invasion of undesirable species [11]. The Aspen Parkland of western Canada is historically a distinct savanna habitat comprised of a mosaic of forest and grassland communities. Currently,