The ecological effects of slope and aspect in chalk grassland
The microclimate of plants growing close to the ground is strongly influenced by the orientation of a soil/vegetation surface with respect to the sun's rays (slope and aspect). In chalk grassland in the UK, slopes of contrasting aspect frequently have distinctive patterns of vegetation. A series of climatic and microclimatic measurements were made at field sites indifferent regions on the English chalk (North Dorset and the Yorkshire Wolds) during the period June 2000 to September 2002. Using digital terrain models (DTMs), process-based models of microclimatic variables at different points in the landscape were developed. The mechanisms through which topography may influence vegetation and species distribution were investigated with field experiments and measurements. Both existing vegetation data from Perring (1956) and new data collected from the field sites were analysed using detrended correspondence analysis (DCA) and generalised additive models (GAMs), to elucidate the relationships between vegetation and soil, topography and climate. A consistent gradient in chalk grassland vegetation was found across spatial scales, associated with the frequency of species with a "stress tolerant” strategy. This gradient in vegetation is apparently driven by species' responses to several separate, but often correlated, variables including soil moisture, maximum summer temperatures and soil fertility. Over the past 50 years, stress tolerant species have declined in frequency at the North Dorset field site, Ellenberg fertility indices have increased and light indices have decreased. The observed changes are consistent with fertilization from atmospheric N deposition and/or relaxation of rabbit grazing after the myxomatosis outbreak in the 1950s. Plots on sloping ground, and in particular, south-west facing slopes, were least affected by these changes, suggesting that high temperatures, phosphorus and water limitation have acted as a buffer against vegetation change, and that complex topography creates refuges for stress tolerant species in the landscape.