Background/Question/Methods Non-native invasive plants can negatively affect the abundance and survival of native plant species and alter ecosystem function.  Amur honeysuckle (Lonicera maackii) is an invasive shrub, introduced from Asia for horticultural purposes, that is an increasingly onerous problem for forest management in eastern North America.  While much is known about the effects of Amur honeysuckle on native plant communities, less is known about the invasion process and the factors that influence the encroachment of Amur honeysuckle into forested sites.  Factors that have been implicated in the spread of invasive shrubs include canopy openness, proximity to urban/suburban areas and roads, and historical disturbance.  We examined site-specific characteristics that correlated with the establishment of Amur honeysuckle and other non-native invasive plants in mature forests in central Kentucky.  To characterize individual sites, we measured frequency and cover of non-native invasive shrubs, presence of co-occurring native shrubs, basal area of overstory trees, forest floor litter depth, species composition of the leaf litter, and soil chemical characteristics.
Results/Conclusions Amur honeysuckle was found in 13 of 15 study sites, and its stem coverage ranged from 0.03 to 64%.  Of the variables measured, forest floor litter depth (P=0.01), and the percent of oak litter on the forest floor (P=0.004) were negatively related to Amur honeysuckle presence. Forest floor litter depth (P = 0.03) was also negatively related to Amur honeysuckle percent cover.  There is anecdotal evidence that mature forests are less permeable than more disturbed sites to invasion by exotic plants, and this idea has been supported in the literature.  Our data suggest that forest floor mass creates a barrier to invasion by exotic plants.  In particular, forests with higher dominance by oak species were ostensibly more resistant to invasion and establishment by Amur honeysuckle.  Experimental work is needed to test the mechanism(s) of this resistance, and to tease apart potentially correlated influences of species (e.g., oak), the physical barrier created by forest floor mass, and the direct effects of compounds (e.g., lignin, tannins) that also reduce decay, leading to the increased mass.  Even so, our findings could aid land managers in the allocation of resources for monitoring and eradication of Amur honeysuckle in mature forests in this region.