Relationships between an invasive shrub Amur honeysuckle (Lonicera maackii, Caprifoliaceae) and environmental factors on recruitment of sugar maple trees (Ater saccharum, Aceraceae) in southwestern Ohio

Invasion of deciduous forests by woody shrubs such as Lonicera maackii is known to negatively impact the abundance and richness of native herbs, but effects on recruitment of seedlings and saplings of native trees, such as Acer saccharum, are less well known. Ultimately, these impacts could alter the species composition of forest trees. Our study was designed to evaluate the impact of L. maackii and environmental factors on recruitment of A. saccharum seedlings and on the transition of seedlings to saplings and saplings to mature trees. We selected four study sites in southwestern Ohio where we censused L. maackii and A.. saccharum seedlings, saplings, and trees in 16 plots (eight with and eight without L. maackii) at each site. We measured L. maackii abundance and maximum diameter of the primary stem (an indication of shrub size), edaphic factors (soil density, pH, soil percentage of nitrogen [%N], soil percentage of carbon [%C]), topographic factors (elevation, slope, aspect), biotic factors (overall tree abundance), and human influences (proximity to roadways). Using a generalized linear mixed model with model comparison techniques, we found that in plots with larger L. maackii, seedling recruitment was lower, the transition from seedlings to saplings was greater, and the transition from saplings to trees was unaffected. Seedling recruitment increased with increasing soil %C, but decreased with increasing soil AN. Slope was positively associated with a greater transition from seedlings to saplings, and soil density and soil %C negatively affected the transition to trees. The transition to trees was higher with greater tree abundance, and the transition to saplings and trees was greater away from roadways. Overall, these results indicate that environmental factors have positive and negative impacts on recruitment of A. saccharum, and large L. maackii have both a negative and positive relationship to recruitment. Further studies will be necessary to uncover the mechanisms involved in the negative and positive relationships of L. maackii to recruitment of A. saccharum.