Vegetation change along a Mediterranean to arid zone bioclimatic gradient reveals scale-dependent ecotone patterning

Abstract

The drivers and rate of vegetation change across spatial gradients can give critical insights into the compositional and structural change we can expect under climate change. Spatial ecotones are of particular interest as they represent heterogeneity in the patterning of vegetation that may reflect how temporal environmental change will manifest in more abrupt step changes in plant composition and/or structure. Another dimension of interest is the degree to which survey methodology impacts the detectability of thresholds in vegetation. We surveyed a Mediterranean to arid zone gradient in South Australia with nested and non-nested transect designs and related the observed vegetation change to soil, landscape and climate to determine the strongest environmental associations. Ordination, principal components analysis (PCA) and threshold indicator taxa analysis (TITAN) were used to detect potential ecotones associated with environmental thresholds. Results from the two transects were compared with test the effects of survey method and spatial sampling on pattern detection. Ordinations and regressions for both transects indicated vegetation changed linearly along the environmental gradient. Species richness and total cover increased with rainfall. Species turnover was very high, with low nestedness, indicating high susceptibility to environmental change. Climate is the major driver of broad-scale vegetation change on our gradient and at this scale vegetation trends are detectable with a range of survey methodologies. TITAN identification of a threshold within the shorter, nested transect (but not the longer transect which extended into the arid zone) indicated that survey methodology influences ecotone detectability, and that although smaller-scale vegetation disjunctions may be present, change spanning the entire mesic to arid zone is largely monotonic.