The interpretation of vegetation change in a spatially and temporally diverse arid Australian landscape

Abstract

We investigated the relationships between the vegetation and soil surfaces on two active floodplains and associated stable landforms in a grazed arid landscape, over a period of seven years. Our intention was to distentangle the effects of processes which lead to variability at the landscape scale, using a large data set, as a prerequisite to better interpretation of vegetation changes.Long-lived trees were established on stable soil surfaces laid down in 700-year-old superfloods and on older landscapes. Less stable surfaces supported shorter-lived trees and shrubs, while the most frequently disturbed surfaces were entirely treeless.Many herbage species showed preference for particular soil surface types. For example, the presence of Sclerolaena bicornis distinguished active floodplains from stable sand sheets, while S. cuneata separated severely eroded from less damaged floodplains. The dynamics of herbage composition over the seven-year period were also different for contrasting surfaces, enabling the identification of a number of erosional, depositional and stable surface types. However, erosion of previously stable surfaces did not change herbage dynamics until the subsoil became exposed. Regardless of surface type, changes in species composition brought about by major rainfall events persisted for several years.The impact of cattle grazing on herbage composition over time was not detected despite independent evidence of grazing effects over the area. We attributed this to the confounding effect of a topographic and soil gradient, the lack of sufficient sites to stratify for all relevant factors despite a large initial data set and, possibly, an inadequate rating method for species abundance.