Plant community dynamics following non‐native shrub removal depend on invasion intensity and forest site characteristics

Abstract

Globally, temperate deciduous forests are threatened by invasion of non‐native (exotic) plant species. In the eastern United States, Rosa multiflora is a dominant shrub invader in forests, which often forms dense thickets that reduce sunlight availability in the understory, where decreased native plant diversity and abundance are observed. Management and restoration are difficult but desirable, especially when invasion intensity is still low. Few studies have examined the relative success of different management strategies under varying invasion intensities. Our study objectives were to conduct a R. multiflora removal experiment in three forest sites experiencing different invasion intensities and to restore native plant biodiversity while preventing secondary invasion. We utilized three management strategies: invasive plant removal, removal followed by native seed addition, and removal plus native seed and mulched invasive stem addition. We investigated the similarity between seed bank species composition and existing vegetation before and after removal to assess the potential for passive restoration. Two seasons after removal, we found that simply removing roses increased native species richness, native floristic quality assessment (FQAIN), and native shrub abundance in our medium invasion site, and total species richness in our low and medium invasion sites. Compared to removal alone, native seed addition, with and without mulch addition, resulted in larger native and total species richness and FQAIN increases at all sites, larger increases in native shrub abundance and exotic species richness in our medium invasion site, and larger reductions in exotic and total shrub abundance in our low and medium invasion sites. Following removal, species similarity between the seed bank and vegetation improved for all three sites. Our results indicate that removal of R. multiflora alone increased native plant biodiversity in the medium invasion scenario, but the seed bank may not provide a large native species pool. Additional management strategies lead to improved outcomes, especially in our most invaded forest, demonstrating the need to conduct multiple plant removal treatments across forests with varying site conditions and plant invasion intensity to improve management recommendations.

Soil Nitrogen Cycling Following Invasive Shrub Removal

“…In 2017, prior to invasive plant removal, we sampled all tree, shrub, and herbaceous vegetation in each control (n = 3 site -1 ) and treatment plot (n = 9 site -1 ) at each forest site (n = 3). All plots were revisited for post-removal vegetation sampling in 2018 and 2019 to assess plant density following invasive plant removal (Moore et al 2023). Trees, shrubs, and herbaceous species were counted as the number of stems across all individuals of a given species during peak growing season, beginning in July.…”

“…The seed mix was customized to include native plant species present in or near our forest sites, and excluded species known to be palatable to deer (e.g., Trillium spp.) The seed mix consisted of 29 herbaceous and 7 graminoid species in quantities based upon seed size (Moore et al 2023). Treatment plots (6 site -1 ) were seeded by hand with 26.96 g of seed (6.74 g/m 2 ).…”

“…We suggest that a single year of management may not be su cient to observe changes in N cycling among the various strategies, and continued management over subsequent years may be necessary to observe differences in N cycling. Over time, net changes in plant communities (Moore et al 2023) and soil amendments may increasingly in uence N cycling post invasive plant removal.…”

“…In the low invasion site, mulched stem amendment reduced soil N (%) as expected, yet also reduced soil C (%) unexpectedly (P < 0.10). The low invasion site primarily consisted of small rose shrubs that were hand pulled (Moore et al 2023), which could have caused the decreased soil C via belowground root removal. We cannot rule out the possibility there was not enough time for decomposition of mulch amendments to increase soil C content within one year.…”

“…Following invasive plant removal, we expected larger reductions in N cycling rates where invasion density was greatest, with smaller reductions as invasion density decreased due to fewer shrubs affecting soil N cycling. Furthermore, we expected the addition of seed mix would lead to increased plant N uptake (from new grass and sedge germinants; Moore et al 2023), thereby reducing nitri cation rate via lower soil NH 3 pools. However, invasive plant removal is itself a disturbance which will result in seasonal and short-, mid-, and long-term soil N cycling responses, and some responses could take more than one year for soil N cycling to respond to the addition of native seeds.…”