The influence of wildfire on invasive plant abundance and spatial structure in eastern ponderosa pine savanna

Fire can alter the trajectory for plant invasions; however, invasive plant responses to fire vary among regions and species. It is unclear how recent increases in large, mixed-severity wildfires in eastern ponderosa pine forests of North America have influenced patterns in plant species invasion. We sampled invasive plant species across three mixed-severity wildfire perimeters over 3666 sample sites in eastern ponderosa pine of western Nebraska, USA. Our analysis sought to determine whether invasive species occurred more frequently and at greater abundance in burned areas, and whether either frequency of occurrence or abundance of invasive plants could be explained by fire severity. We found complex relationships between mixed-severity wildfire and invasive plant species. Cheatgrass ( Bromus tectorum ) was the most common invasive species sampled, followed by Kentucky bluegrass ( Poa pratensis ) and smooth brome ( Bromus inermis ). Cheatgrass and Kentucky bluegrass were more common in burned areas. Cheatgrass occurrence was higher in low and moderate severity burned areas than in unburned locations. While cheatgrass occurred at a similar number of sample sites in unburned and high severity burned areas, cheatgrass cover was lower at high severity burned sites compared to unburned sites. Our results emphasize variation in invasive plant species occurrence and cover in relation to fire in eastern ponderosa pine savannas and the importance of making ecosystem specific decisions regarding invasive species management in response to wildfire.