Efficiently and effectively controlling invasive plants while promoting native species is a growing challenge in managed forests. In eastern hardwood forests, invasive shrubs reduce herbaceous cover and diversity, suppress native tree regeneration, and may alter soil chemistry. Standard removal practices for invasive shrubs, such as cutting and treating stumps with herbicide (cut-stump method), can be prohibitively time-consuming and labor-intensive. Mechanized removal with a skidsteer-mounted mulching head (e.g. Fecon) may expedite the restoration of heavily invaded forests, but will disturb top soil and leave a layer of woody mulch, potentially hindering recovery of native understory communities. The widespread invasive shrub Amur honeysuckle (Lonicera maackii) forms dense thickets in forest understories, presenting an ideal system for testing the effects of mulching heads on understory communities. Our objectives were to compare the effectiveness of these two removal techniques in controlling invasive honeysuckle and promoting native species, to compare the resulting changes in soil chemistry, and to isolate the effects of mulch deposition from substrate disturbance. We removed shrubs from hardwood forests of central Indiana in late fall 2015 and used a factorial design with Fecon-produced residual woody debris removed from half of the Fecon plots and added to half of the cut-stump plots.
Removal treatments had similar positive effects on total herbaceous cover, mid-summer herbaceous diversity, and floristic quality, but differed in several other aspects. Richness, diversity, and evenness of spring ephemerals were negatively affected by mulch. Mid-summer herbaceous richness was increased by both removal methods, but significantly more so by Fecon removal. Honeysuckle seedling densities were significantly higher following Fecon removal than after cut-stump removal, whereas the opposite effect occurred for native seedlings. In addition to effects on understory plant communities, we observed several changes in soil chemistry, despite removing shrubs after leaf drop the previous fall. In general, honeysuckle removal reduced soil organic matter by 6.4%, increased soil C/N by 3.9%, and reduced total exchange capacity by 12.0%, but each of these changes were more pronounced in Fecon than cut-stump plots. Similarly, calcium and magnesium content were reduced in all removal plots, particularly following the Fecon treatment. Invasive plants commonly influence soil characteristics, but previous studies on honeysuckle removal have not found evidence for modifications to soil chemistry. While mechanical treatments are more efficient for large-scale shrub removal, they may perpetuate recruitment of the same woody invaders they are attempting to remove and inhibit restoration efforts by diminishing native seedling recruitment.