Non-native invasive plants are one of the biggest threats to forest ecosystems worldwide. Invasive shrubs in particular can suppress native plant biodiversity and alter important ecosystem processes, such as soil-N cycling. Eastern U.S. forests are dominated by Rosa multiflora, which suppresses herb and tree species and accelerates soil-N cycling. In 2017, we selected three urban forest sites for restoration based on intensity of R. multiflora invasion (as number of stems) within each forest. We established a 400-m2 restoration area in each site, quantified abundance of understory (height ≤ 1 m) and sapling (height > 1 m; DBH < 2.54 cm) plant species, and measured soil N pools before and after restoration. Our restoration approach included invasive plant removal and post-removal treatments to suppress secondary invasion and N losses. In each site, we established a control (no removal) and three replicates of three treatments: 1) invasive plant removal, 2) removal + native seed mix application, and 3) removal + seed mix + carbon-rich soil addition. In 2017 and 2018, soils were collected and analyzed for nitrate and ammonium to determine inorganic soil N pools. We performed a lab incubation study to measure potential soil N-mineralization rates in each treatment plot.
Results/Conclusions
Following invasive plant removal, mean understory canopy cover (measured at 0.5 m above ground) significantly decreased by 1.52 ± 0.19% across all sites (P<0.001) suggesting greater light levels to the forest floor. Preliminary soil N results show that early summer nitrate pools declined significantly (-18.9 ± 3.7 μg NO3- g-1 soil; P<0.001) in all plots where invasive plants were removed. No significant differences were observed between treatment groups or in soil ammonium pools. This suggests that removal alone can lead to a significant reduction in soil N pools at the beginning of the summer season. As expected, some invasive shrub species re-emerged in the understory in 2018, with Rosa multiflora being the most common; however, there were significantly fewer invasive shrub stems in the removal + seed mix + soil amendment plots (1.56 ± 0.56 stems plot-1) than in the removal only treatment plots (5.56 ± 1.14 stems plot-1; P=0.03). These results suggest competition with additional native plants and/or an inhibitory effect of the soil amendment reduced secondary invasion post-removal. Further work on soil N pools later in the growing season, soil N mineralization rates, and plant communities will provide evidence about the long-term effects of our restoration efforts.