Impacts of invasive honeysuckle removal on forest-breeding birds in southern Iowa

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Fernald, Katrina
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Curzon, Miranda
Klaver, Robert
Wolter, Peter
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Natural Resource Ecology and Management
Amur honeysuckle (Lonicera maackii), a non-native, invasive shrub, has established in woodlands across much of the eastern United States, altering understory dynamics and forest structure. Land managers in midwestern states have made efforts to remove honeysuckle from forests using multiple methods, including aerial treatment with glyphosate, a technique that can be used to treat large areas at low cost with high effectiveness. This study aims to quantify the response of forest vegetation and the breeding bird community four to five years after the rapid removal of honeysuckle from the shrub layer. To assess the response of the plant and forest bird communities, we established 65 randomly distributed sampling points across a 216-hectare study area that included three sprayed stands (103 ha total) and five unsprayed control stands (113 ha total) in upland oak-dominated forest in southern Iowa, USA. Vegetation was sampled in two ways at these points. First, we identified and measured herbaceous and woody vegetation in 5-m radius (78.5 m2) plots using standard inventory methods. Then, we measured the three-dimensional forest structure (vertical complexity and leaf area density at different heights) with a terrestrial laser scanner (TLS). Lastly, two point counts were conducted at each point to quantify the composition and abundance of forest birds during the breeding season (June-July, 2020). Unsprayed stands contained significantly more live honeysuckle in terms of stem density and estimated biomass four to five years post-treatment (F=22.99, p<0.001). Corresponding with the reduced abundance of honeysuckle, we observed lower leaf area density between one and three meters height in sprayed stands, as measured with TLS, and an increase in species richness and mean cover for herbaceous plants. While responses varied among individual species and functional groups, the total density of forest birds was greater in unsprayed stands with more honeysuckle. Ground-nesting and aerial-foraging groups, specifically, were less abundant in sprayed stands. Community composition also differed between the treatment conditions, and while no functional groups or individual species increased in density following aerial treatment of honeysuckle at the stand-scale, NMDS ordination results suggest the shrub-foraging group, tree-foraging group, Indigo Bunting (Passerina cyanea), and Red-eyed Vireo (Vireo olivaceus) were all more closely associated with sprayed conditions (less honeysuckle). Overall, these results suggest that while honeysuckle removal is clearly beneficial to the native forest plant community, it may cause decreases in understory-associated bird species, at least in the short term. We recommend pairing honeysuckle removal with native shrub restoration. We also observed honeysuckle regeneration in the sprayed plots four to five years post-treatment, indicating follow-up treatment is likely necessary to prevent reestablishment.
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