In the past few centuries, many remaining grassland and savanna ecosystems have undergone a directional shift towards increased woody plant species. Woody plant encroachment (WPE) is occurring in many bioclimatic zones globally, with tree proliferation occurring in more humid regions and shrub invasion in more arid ones. Simultaneously, these same ecoregions have undergone significant greening trends which may alter ecosystem nutrient dynamics and biogeochemical cycling. While WPE may contribute to this greening trend, the degree to which these two phenomena are linked has not yet been comprehensively studied. Here, we use an ensemble of high-resolution satellite-based products to better examine drivers of WPE and greening in the Northern Great Plains (NGP) to better understand how vegetation has changed in the early 21st century. We ask: 1) what are the abiotic factors (climate, edaphic conditions, disturbance) that govern these two phenomena? 2) How does land-use or land-cover change influence these observed changes? 3) How are these two phenomena coupled and to what degree do they exhibit synchrony?
Results/Conclusions
Our understanding of the drivers and consequences of WPE and greening are hindered by uncertainty regarding the relative strength of climatological and ecological drivers. The NGP is one of the last remaining intact prairie ecosystems in the world, with grass-and shrublands covering ~25% of the region (160,000 sq km). This provides a unique opportunity to understand the connections between WPE, greening, and their associated drivers, as both phenomena are widespread throughout the region. Here, we report that NDVI (normalized difference vegetative index) has increased across 79% of the NGP while only 21% of all cells experienced either zero or decreased greening trends. Of the areas that have shown greening, 25.7% of those have increased by >25% over the 18-year period. Further, we document that 20.9% of the grass- and shrublands (~33,500 sq. km) has increased in percent tree cover over the study period. These changes point to large vegetative shifts over the 21st century with implications for ecosystem function and land-climate feedbacks. We discuss both the abiotic and biotic drivers of such changes, as well as the synchrony of these two phenomena.