Exploring the effects of emerald ash borer-induced ash tree decline and invasive shrub treatment on understory plant communities in an Illinois forest ecosystem

The emerald ash borer ( Agrilus planipennis ) is an invasive insect in the United States that preferentially feeds on ash ( Fraxinus spp.) trees, causing widespread ash decline. Resulting canopy gaps may facilitate understory invasion by non-native plants, such as Amur honeysuckle ( Lonicera maackii ). We investigated whether ash decline and invasive shrub management, which opens the shrub layer, interact to influence understory plant communities. We hypothesized that (1) ash tree decline facilitates L. maackii shrub layer growth, which suppresses understory plants, (2) invasive shrub treatment creates recruitment windows for understory plants, and (3) ash decline and invasive plant treatments interact to increase opportunities for co-occurring non-native species. We conducted our study at Homer Lake Forest Preserve, Champaign County, Illinois, where we sampled subplots in ash and non-ash understories across untreated regions (no invasive plant management) and treated regions ( L. maackii and Eleagnus umbellata treated one year prior). We constructed competing linear models to test for interactions between ash status and treatment status and constructed piecewise structural equation models to test the effects of ash status, treatment, and canopy cover on understory cover and diversity. We found that treatment and ash status significantly interacted to influence canopy cover. Structural equation models revealed few short-term direct effects of ash or treatment status on understory plants. However, decreasing L. maackii shrub layer cover was associated with increases in native diversity and abundance. Our results suggest that additional monitoring and management may be warranted in areas experiencing ash decline and/or invasive shrub treatment to increase native plant recovery.