Grassland resistance to climate change: an evaluation of processes that limit plant community response in a 17-year climate manipulation

Background/Question/Methods   The interaction of vegetation with climate is mediated by environmental properties like soil structure and biotic processes like competition and adaptation. There is as yet no general framework for understanding how such biotic and abiotic factors may decouple vegetation and climate dynamics. As part of a long-term manipulation of winter temperature and summer rainfall in species-rich limestone grassland in Derbyshire, UK, we are conducting a suite of studies to identify why this ecosystem has resisted large shifts in species composition in response to climate treatments. Here we describe 1) evidence indicating the relative stability of many species to chronic shifts in temperature and rainfall at the plot (3x3 m) scale; 2) vegetation data surveyed at the microsite (10x10 cm) scale that show strong affinity to substrate properties associated with fine-scale variation in soil depth; and 3) the response of species to climate treatments across the soil depth gradient. We further consider more recent evidence that some populations may be capable of local adaptation to new climates, and that dispersal constraints of more southerly species are also driving vegetation stability.

Results/Conclusions   There is considerable evidence that fine-scale soil heterogeneity provides micro-refugia that prevent local species extinction in response to shifts in ambient climate. For some species, decreases in abundance in shallow soils in response to drought or warming are offset by increases in abundance in deep soils; this may indicate that the combined constraints of environmental stress and competition intensity shift along the depth gradient as the climate changes. Overall, compositional shifts were largest at the ends of the depth gradient (very shallow and very deep soils) but relatively minor for microsites of intermediate depth. Fine-scale environmental surveys suggest that depth effects are the result of variation in water potential, nutrient availability, and surface acidity. We conclude that soil heterogeneity is a strong promoter of vegetation stability in the face of ambient climate change, but that additional contributions of population-level changes are also significant. Incipient invasions of species from southern England may eventually override the local capacity of these ecosystems to adapt to new climates.