Introduced plants threaten biodiversity and ecosystem processes, including carbon (C) and nitrogen (N) cycles, but little is known about the threshold at which such effects occur. We examined the impact of the invasive shrub Amur honeysuckle on soil organic carbon (SOC) and N density at study sites that varied in invasion history. In plots with and without honeysuckle, we measured honeysuckle abundance and size (basal area) and extracted soil cores. SOC and N densities were highest at the site with the longest invasion history and highest invasion intensity (i.e., greatest abundance and basal area of honeysuckle). Basal area of honeysuckle positively affected SOC and N densities likely because of increased litter decomposition and altered microbial communities. Because honeysuckle increases forest net primary productivity (NPP) and SOC, it also may play a role in C sequestration. Our results demonstrate the need to consider the influence of invasion history and intensity when evaluating the potential impact of invasive species.
Nomenclature: Amur honeysuckle; Lonicera maackii (Rupr.) Herder LOMA6.
Management Implications: Amur honeysuckle, an invasive shrub in the eastern United States and Canada, forms dense thickets that negatively affect tree seedlings and ground herbaceous vegetation in eastern deciduous forests. Honeysuckle also is allelopathic but other belowground dynamics remain largely unknown, such as whether the shrub affects C and N cycles, which can in turn affect forest productivity. Unfortunately, little is known about how invasion history (e.g., time since invasion) or invasion intensity (e.g., abundance and size of honeysuckle) affects these processes. We established plots with and without honeysuckle at four study sites that varied in invasion history and measured honeysuckle abundance and size as well as SOC and N density within each plot. SOC, N, and abundance and size (basal area) of honeysuckle were highest in plots invaded by honeysuckle at the site with the longest time since invasion. Basal area of honeysuckle positively affected SOC and N densities at this site and likely contributed to increased soil pH. These effects likely were related to increased decomposition of litter and alteration of microbial communities under honeysuckle. Because the effect of honeysuckle on SOC and N density was greatest in stands with the highest basal area, the alteration in density of SOC and N and in the C : N ratio also could positively or negatively affect native herbs or could intensify invasion by other nonnative herbaceous or woody plants. In addition, because honeysuckle affects soil chemistry and increases forest NPP, it may play a role in C sequestration. Resource managers should be attuned to these effects in areas with high abundance of large honeysuckle shrubs (i.e., those with large basal area) and the longest time since invasion. To maximize the limited resources often available for invasive plant control and to reduce ecosystem-level effects, management intervention should be directed toward stands with those characteristics. Our results demonstrate that knowledge of the history and intensity of invasion are important to fully understand the impact of Amur honeysuckle in native forests, especially with respect to belowground dynamics.