Forest degradation in Africa

West African tropical forests are among the most vulnerable of the Earth’s tropical ecosystems to human-driven environmental change. They have the highest temperatures and longest dry seasons of all tropical forest systems worldwide. The region is also under enormous pressure from a rapidly growing human population and its demands for agricultural production as well as wood and other forest products. Much of the original native forest cover has already been lost, and the remnants are at risk from logging, agricultural encroachment, and wildfires. The EcoGRAPH group uses satellite Earth observations to explore the effects of these disturbances on forest vegetation in Africa.

We used MODIS vegetation indices to identify general patterns of decreasing woody vegetation and increases in herbaceous vegetation cover across the Upper Guinean Region of West Africa (Liu et al. 2017). In a smaller-scale study focused on several forest reserves in Ghana, we used Landsat data from 1984-2015 to examine patterns and drivers of forest change (Dwomoh and Wimberly 2017). Forest degradation occurred through a positive feedback loop of declining canopy cover and increasing fire frequency, leading to the establishment of fire-prone vegetation dominated by grasses and invasive shrubs. Using a time series analysis of Landsat imagery, we developed a historical database of disturbances across all forest reserves of Ghana from 2003-2019 (Wimberly et al. 2022). Rates of forest degradation, loss, and recovery were relatively low from 2003-2015, followed by a sharp increase associated with drought and fires in 2016. The resulting decreases in open and closed forests from 2016-2019 were as great as those that occurred over the preceding 15 years. We are currently studying the implications of these changing disturbance regimes for carbon storage in African tropical forests through a NASA Carbon Cycle Science project.


Agricultural expansion in the United States

The prairies of North America are among the most threatened terrestrial ecosystem on the continent. Increasing corn and soybean production has led to substantial conversion of grasslands to row crop agriculture at the western edge of the U.S. Corn Belt in North and South Dakota. Much of this change is occurring in the Prairie Pothole Region, which has a high density of shallow wetlands and provides critical habitat for numerous migratory bird species. Grasslands provide multiple ecosystem services, including the maintenance of pollinator communities, long-term carbon sequestration, prevention of soil and nutrient loss, and protection of water quality. Loss and fragmentation of grassland cover reduces the amount and quality of habitat for a variety of insects, amphibians, and birds. The EcoGRAPH group uses satellite remote sensing and geospatial analysis to quantify patterns of grassland loss throughout the region as well as the drivers and ecological impacts of these changes.

We documented more than 500,000 ha of grassland loss in the western Corn Belt between 2007-2011 (Wright and Wimberly, 2013). Although most of the converted grasslands were former croplands enrolled in the conservation reserve program, nearly 20% were undisturbed native grasslands (Wimberly et al. 2017). Larger farms, younger farm operators, higher levels of farm income, higher proportions of rented croplands, and marginal yields were associated with higher rates of cropland expansion and grassland conversion. In eastern South Dakota, the remaining native grasslands are highly fragmented, with major habitat concentrations connected by narrow corridors of “stepping stone” patches (Wimberly et al. 2018). Our current research efforts are aimed at developing improved methods for monitoring native grassland loss and supporting grassland conservation planning.