Seasonality of feral horse grazing and invasion of Pinus halepensis in grasslands of the Austral Pampean Mountains (Argentina): management considerations

Abstract

The introduction of domestic ungulates has altered the natural grazing regime, promoting the invasion of exotic woody plants being seasonality or stoking season one of the key factors regulating the grazing regime. The Pampean Mountain grasslands of Central Argentina have been intensively transformed by the presence of feral horse populations that have increased the last 40 years, coinciding with an increased abundance of Pinus halepensis dispersed from cultivated stands. In this study, we evaluated the effect of the seasonal variation in perennial grasses defoliation (no defoliation; continuous defoliation; defoliation during autumn-winter and defoliation during spring-summer) on the establishment of P. halepensis seedlings and the response of the dominant perennial grasses (Nasella tenuis and Piptochaetium lejopodum) to defoliation in areas with different grazing histories, following a split-plot design. The establishment of P. halepensis was highest in the grazed areas and in treatments where the defoliation of the perennial grasses was practiced throughout the year or during autumn-winter. Perennial grasses respond differently to defoliation according to the grazing history. In the grazed areas, defoliation negatively affected the perennial grasses during all the study period, showing the scarce resilience after grazing while in ungrazed areas, the perennial grasses were little affected by the seasonality of defoliation. Both, above- and belowground competition affected the establishment of P. halepensis, according to defoliation treatment and grazing history. The information obtained allows the elaboration of management recommendations to avoid the propagation of P. halepensis; therefore, the seasonal management of the grazing is a recommended practice toprevent or reduce the intensity of feral horse grazing during the autumn and winter months.