Riverine wood-pasture responds to grazing decline

Abstract

There is insufficient available information on structural changes within wood-pastures including their relationship to abiotic influences such as livestock grazing, flooding and available soil nutrients. In this paper, we address the links between important environmental variables and different stages of the wood-pasture cycle, with the aim of understanding fluctuations in this relationship and processes that follow changes in wood-pasture condition. We used satellite and aerial image interpretation to identify structural vegetation shifts over 44 years under significantly declining livestock numbers. We used ground truthing of 24 plots to assess the current field scenario and employed canonical correspondence analysis (CCA) to evaluate the relationship between plant communities and environmental influences. Three dominant structural vegetation types grassland, transitional vegetation with thorny shrubs and woody encroachment were surveyed and the following set of variables was chosen: grazing intensity, inundation frequency, elevation, soil total nitrogen, soil available phosphorus, soil potassium, soil magnesium, soil calcium, soil pH and soil carbon to nitrogen ratio. Interpretation of satellite images revealed dominance of wood-pasture in the past, which alternated structurally between more open and more closed physiognomies. CCA with ground truthing data and forward selection revealed grazing intensity as the predominant ecological driver modifying vegetation structure, as well as transitioning vegetation patterns between open herbaceous and closed woody cover. Each structural vegetation type demonstrated a collective distribution pattern and a close relationship to certain abiotic drivers, indicating strong interactions between soil parameters, grazing pressure and vegetation composition.