Effects of a generalist, ungulate herbivore, white-tailed deer, and the invasive shrub, Lonicera maackii, on forest floor plant functional types and community composition
High density of white-tailed deer (Odocoileus virginianus), a generalist herbivore, is an important factor in driving forest compositional change. Amur honeysuckle (Lonicera maackii (Rupr.) Herder), an invasive shrub, negatively affects both tree and herb species. To assess the combined effects of Lonicera and deer on plant composition, five pairs of 20m-by-20m deer exclosures and access plots were established in 2010 at five forested sites near Oxford, Ohio. In half of each exclosure and access plot, Lonicera was removed, resulting in 20 plots.
We determined species identity and cover of plants <1 m in 18 0.1 m2 quadrats per plot in spring and summer of 2011 - 2014. Plot-level species richness was analyzed using repeated measures ANOVA. To examine effects on community composition, we used non-metric multidimensional scaling (NMDS) ordination and permutational multivariate analysis of variance (PERMANOVA). When composition differed among treatments, we analyzed treatment effects on each plant functional type using methods for split-plot design.
Repeated measures analysis revealed that plant species richness in spring was significantly (P < 0.05) affected by Lonicera removal and year: richness was higher in removal plots in 2012, 2013, and 2014. Species richness in summer was significantly affected by Lonicera treatment and deer by Lonicera interaction: Lonicera removal increased richness, but this effect was reduced where deer had access.
There were significant effects of deer exclusion on plant community composition in spring 2012, 2013 and 2014. Lonicera removal affected composition in summer 2013, but no interactions between treatments were found.
Functional type analysis revealed Lonicera intact plots had significantly higher cover of shrubs in spring 2012. In spring 2013, Lonicera removal plots had significantly higher cover of graminoids and spring perennials. In spring 2014, Lonicera removal plots had higher graminoid and tree cover. In summer 2013, Lonicera removal plots had higher cover of spring perennials and vines, and there was a significant interaction between treatments on tree cover; where deer were excluded, Lonicera removal resulted in higher tree cover, but where deer had access, cover did not differ between Lonicera treatment types.
These findings show that deer negatively impact forest communities, but also mitigate some negative effects of Lonicera.