Effects of local site characteristics on abundance of Lonicera maackii in Southwest Ohio

Background/Question/Methods

Lonicera maackii was introduced to North America in 1898 and has now invaded forests and old fields in eastern and Midwest U.S.   As part of a larger project on patterns and processes in this invasion, we examined the impact of surrounding land cover and forest stand history on L. maackii percent cover in southwest Ohio.  Our study area is near the expanding front of the L. maackii invasion, so some woodlots have high levels of L. maackii and others have not been invaded. We used historical aerial photos and land cover data to assess the relative importance of propagule pressure and stand invasibility in determining L. maackii abundance.  The goals for this research were to determine the impact of 1) surrounding land cover and 2) forest stand history on L. maackii invasion of woodlots in southwest Ohio.  We measured L. maackii percent cover in 70 plots (100m x 100m) in Preble and Darke Counties (southwest Ohio) between 2007 and 2010. We obtained aerial photos of the study area to determine the condition of each woodlot in the late 1930s and early 1960s as either forested, mostly forested, partially forested, or nonforested. These categories were combined to assign a composite ranking related to stand age (forested for the last 80 years or more recently colonized by native trees). We also created 1500m buffers around each woodlot to analyze landcover such as row crops, water, pasture, forest, and development.  We removed woodlots with buffer areas that overlapped more than 25%, leaving 40 woodlots for final study.  Correlation and regression analyses were used to relate L. maackii cover to the above variables and determine the strongest predictor.

Results/Conclusions

Among woodlots, L. maackii cover was best predicted by the proportion of row crops within 1500m of woodlots (R² = 0.46, p = 0.0012), but also significantly related to the proportion of forest (R² = 0.36, p = 0.0053) and pasture (R² = 0.39, p = 0.0003).  L. maackii cover was negatively associated with proportion of row crops and positively associated with proportions of forest and pasture, which suggests propagule pressure is important (source populations can occur in forests and pastures, but not cropland) Lonicera maackii cover was negatively correlated with stand age (nonparametric p < 0.01), indicating younger stands are more invasible.