Shrub encroachment occurs near previously invaded areas in Patagonia (southern South America)

Abstract

During the last century shrub encroachment has occurred in many arid and semiarid areas of the world. Patagonia is a large xeric territory located in the southern tip of South America, which has been grazed by sheep for more than a century. In many areas overgrazing led to vegetation change (shrub encroachment), which in turn reduced sheep carrying capacity and caused ranch abandonment. However, these changes in physiognomy did not occur everywhere, and even some authors did not find evidence that shrub encroachment is currently occurring. Our objectives were to determine if encroachment really happened in NE Patagonia and, in the case of confirming shrub encroachment, to evaluate if the increase in shrub density was spatially homogeneous. We used object-based image analysis to evaluate shrub density and cover in aerial photographs taken in 1970 and 2018. In both dates, shrub density showed a clear contrast between two kinds of areas showing higher and lower density than was expected by chance. Shrub density and cover more than doubled between 1970 and 2018. This increase was concentrated in those areas with moderate or high densities of shrubs in 1970. Our results confirm the occurrence of shrub encroachment in NE Patagonia and demonstrate that it is currently concentrated near previously invaded areas. These findings have implications when defining protocols for monitoring shrub encroachment since monitors should focus on both areas that already have shrubs and areas that lack them.