Mapping pasture and rangeland degradation in the Southern Cone of South America using Landsat

Pastures and rangelands occupy roughly 33% of all land area in South America and are critical to beef production, providing 22% of ruminant food in the region. Pastures and rangelands comprise a critical component of rural livelihoods, but are vulnerable to ecological changes. Specifically, degradation of pastures and rangelands via expansion of woody vegetation and desertification decreases livestock accessibility to forage grasses and increases competition for light and nutrients. Despite this, there is a lack of understanding regarding the magnitude, geography, and rate of pasture and rangeland degradation in South America. To address this, we developed a remote sensing-based methodology to assess pasture and rangeland degradation by quantifying the proportional cover and transitions through time among grasses, woody plants, and bare ground in the Southern Cone of South America. To do this, we used field observations of land cover and land use in association with high resolution imagery to estimate a model that predicts sub-pixel proportions of vegetation cover from Landsat imagery from 2000 to 2018. Specifically, using a stratified random sample of high resolution imagery, we created a spectral library representative of pastureland cover types: bare ground, grasses, and woody cover. We then trained a machine learning regression model using these sub-pixel estimates, and predicted annual sub-pixel percent cover over all pastures and rangelands in the Southern Cone at 30-meter spatial resolution. Annual cover fraction maps provide temporal information regarding vegetation dynamics in pastures and rangelands that complement a categorical classification of land cover in the Southern Cone (authors' ongoing work), and provide critical baseline information necessary for adaptive pasture and rangeland management at regional scale. Our results improve understanding of land use change and pasture and rangeland degradation in the Southern Cone region, which is critically important to rural livelihoods and food security.