Woody plant encroachment pervasive across three socially and ecologically diverse ecoregions

Abstract

Woody plant encroachment in grasslands represents one of the greatest challenges for global biodiversity conservation. Furthermore, this is a social-ecological problem, where human activity and behavior have resulted in significant changes in ecological processes that control woody plants, and failure to fully recognize the role of human activity has led to continued loss of grasslands worldwide. It is therefore critical that conservation professionals understand how ecological systems, settlement patterns, and fragmentation from anthropogenic development interact to influence rates of woody plant encroachment. Using annual estimates of tree cover derived from regionally available remote-sensing data, our objectives were, first, to describe rates of woody plant encroachment over the last 20 years (2000–2019) across three ecologically and socially diverse ecoregions in the Southern Great Plains of North America. Then, we examined how anthropogenic and biophysical variables influenced rates of encroachment (both directly and indirectly) within the region. Results indicate that, despite marked differences in social and ecological characteristics, all three ecoregions have experienced consistent increases in woody plant encroachment during the study period. This included the Flint Hills ecoregion of Kansas and Oklahoma, an area that experiences widespread and frequent fires. At the regional scale, rates of encroachment were directly and negatively related to the average area burned in a county, initial cover of trees, and fragmentation from row crops. Percent cover of development and row crop agriculture also indirectly alter rates of encroachment in a county by influencing initial tree cover and fire activity. The pervasive nature of woody plant encroachment, even in regions that experience frequent fires, suggests that many grasslands are being managed outside of critical ecological thresholds needed to maintain grasslands and limit woody encroachment, which can have significant implications for biodiversity. Our results show that anthropogenic or ecological factors do not act in isolation in their influence on woody plant encroachment and can form complex relationships that shape regional trends in woody plant encroachment.