Invasion of closed canopy forests by shade-tolerant alien plants has the potential to modify species composition, stand structure, ecosystem function, and long-term forest development patterns. Ligustrum sinense is a shade-tolerant alien shrub that has invaded bottomland forests throughout the southeastern United States. This species has received comparatively little attention in the literature despite its potential to drastically alter invaded sites. The overarching goal of our study was to document the relationships between Ligustrum sinense invasion and woody plant biodiversity and development patterns in an intact southeastern U.S. bottomland forest. The forest was dominated by Quercus nigra and Liquidambar styraciflua. Ligustrum sinense ranked fifth in basal area contribution, occurred in 97% of our plots, and represented 95% of all understory stems. Spearman's rho for dominance (based on basal area of stems > 5 cm diameter at breast height [dbh]) of L. sinense and woody plant species richness for each plot revealed a significant negative relationship (rs = −0.69, P < 0.01). A similar relationship was revealed between L. sinense density and woody plant species diversity (rs = −0.78, P < 0.01) and evenness (rs = −0.82, P < 0.01). Spearman's rho for L. sinense density and native understory stem density (individuals ≥ 1 m height, < 5 cm dbh) also revealed a significant negative association (rs = −0.48, P < 0.01). Under the current disturbance regime and without active management, we projected the forest would shift to support a stronger component of L. sinense and that structure would transition from tree to shrub dominance for sites within the forest.
Nomenclature: Chinese privet, Ligustrum sinense Lour; sweetgum, Liquidambar styraciflua L.; water oak, Quercus nigra L.
Management Implications: Invasion of Ligustrum sinense on bottomland sites may inhibit forest regeneration and shift structure of invaded stands from tree to shrub dominance. Scatterplots of L. sinense density and woody plant diversity measures revealed that 0.05-ha plots with greater than ca. 200 L. sinense stems (4,000 stems ha−1) had relatively low evenness and Shannon diversity values. Although low biodiversity might not be the result of L. sinense invasion, if abiotic conditions of invaded sites are similar to nearby sites with higher biodiversity, then such severely invaded sites may be prioritized for L. sinense control. Restoration efforts might be best if focused on sites with mature L. sinense stems that have ascended to the uppermost forest stratum and lack the multi-stemmed growth form common for the species in smaller size classes. These areas may be those considered most impacted by L. sinense establishment and may be considered priority restoration areas by managers. Based on the potential density of L. sinense stems, even relatively small invaded forests might need to be compartmentalized by field crews.