Effects of forest windstorm disturbance on invasive plants in protected areas of southern Illinois, USA

Abstract

Abstract Invasive plant populations respond positively to light increase from windstorm‐caused canopy damage, but are typically out‐competed over time as the tree canopy recovers. Some invasive plants have the ability to slow or even completely prevent canopy regrowth. It is important to understand invasive plant dynamics in windstorm blowdowns to inform effective forest management. We hypothesized that blowdown areas are more invaded than unaffected forest and that invadedness of a blowdown will decrease with increasing time since disturbance and increase with disturbance magnitude. We used Landsat imagery to identify 31 blowdowns caused by one of three storms in southern Illinois of the United States: 2006 tornado, 2009 derecho or 2017 tornado. We statistically matched these blowdowns to areas of unaffected forest based on slope, aspect, elevation, distance to a road and distance to a trail. We surveyed blowdown and unaffected units for invasive plants in summer 2018. Disturbance magnitude was measured in two ways: per cent change in normalized difference vegetation index (NDVI) and area of blowdown. Overall, blowdowns were more invaded than their statistically matched areas. Invadedness of blowdowns decreased with increasing time since disturbance, indicating that the blowdowns are recovering as tree canopy regrowth shades out invasive plants. Per cent change in NDVI was not a supported predictor of blowdown invadedness, but invadedness was strongly positively related to blowdown size. Larger blowdowns recover more slowly than smaller disturbances due to different recovery pathways, providing more time and opportunity for the spread and establishment of invasive plants. Synthesis. Our findings suggest that although blowdowns contained higher cover of invasive plants than forest unaffected by windstorms, invasive plants were responding negatively to canopy regrowth. Land managers should prioritize treatment of invasive plants in larger and more recent blowdowns.