Diversity and composition of ground-layer vegetation in Indiana mixed-hardwood forests invaded by the non-native Amur honeysuckle (Lonicera maackii (Rupr.) Herder)

Background/Question/Methods

Amur honeysuckle (Lonicera maackii (Rupr.) Herder) is a non-native shrub that was introduced to the United States from Asia in the 1890s.  Since its introduction, it has aggressively colonized many forests throughout the central U.S., leading to decreased diversity and richness in native plant communities.  We examined ground-layer vegetation in five Indiana mixed-hardwood forests that contained a gradient of Amur honeysuckle densities to determine which ecological gradients related to honeysuckle dominance influence native species composition.  During the summer of 2010, we sampled ground-layer vegetation and environmental variables using a series of fixed-area plots placed along transects.  We used percent cover estimates of native herbaceous vegetation and seedlings (woody stems <1.37 m tall) to calculate Shannon-Weiner diversity (H'), taxonomic richness (S), and taxonomic evenness (E) for each study site.  Nonmetric multidimensional scaling (NMS) and a linear mixed-effects model were used to examine variation in the composition of native ground-layer vegetation in relation to environmental variables including percent cover, density, diameter, and height of Amur honeysuckle, percent cover of bare soil, percent cover of leaf litter and woody debris, percent cover and density of non-native vegetation excluding Amur honeysuckle, and leaf litter depth.        

Results/Conclusions

The study site with the lowest mean H' (1.0 ± 0.1), S (4.5 ± 0.6), and E (0.7 ± 0.1) also contained the highest densities of Amur honeysuckle (3,097 ± 200 individuals/ha ≥1.37 m tall and 95 individuals/100 m² <1.37 m tall).  Conversely, the site with the highest mean H' (2.2 ± 0.1), S (12.2 ± 0.7), and E (0.9 ± 0.01) contained the second-lowest mean density of Amur honeysuckle individuals ≥1.37 m tall (1678 ± 304 individuals/ha) and the second-highest mean density of individuals <1.37 m tall (56 ± 12 individuals/100 m²).  The three-axis solution of the NMS ordination explained 65% of the variation, with three environmental variables strongly correlated (r² ≥ 0.2) with at least one of the axes.  Sites with lower native diversity, richness, and evenness were associated with greater cover and larger mean diameter of Amur honeysuckle, as well as greater cover of bare soil.  According to the mixed-effects model, native plant diversity was negatively influenced by greater cover of Amur honeysuckle (p = 0.002) and bare soil (p < 0.001).  Future work in this study will incorporate more study sites as well as the influence of both the age and spatial distribution of Amur honeysuckle.