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.

The impact of hurricanes on tropical forests has been well documented in recent decades, with hurricane disturbance hypothesized to be a leading contributor to maintenance of the high diversity of trees in lowland tropical rain forests (Frangi & Lugo 1991, Vandermeer et al. 2000). Hurricanes have a heterogeneous impact both on landscapes and tree species (Liu & Fearn 2000, Walker et al. 1996). Damage to trees can take many forms, from leaf loss to stem snapping to uprooting, and is variable across the landscape due to topography, wind speed, direction and tree density (Walker 1995).

Current literature on the response of tropical tree seedlings to fertilization was analysed. Fertilization effects on biomass accumulation, relative growth rate and tissue nitrogen (N) and phosphorus (P) were examined for seedlings grown under relatively high light. Higher light conditions in gaps are expected as human disturbance expands in tropical landscapes. Two questions were asked: (1) Does nutrient addition enhance growth or result in nutrient accumulation in the tissues of tropical tree seedlings grown in pots under high light? (2) Are these responses conditional on their adaptations to light availability, seed mass or growth form? Most of the species (73% of light-demanding and 60% of shade-tolerant) demonstrated a positive growth response to fertilization. The magnitude of the response of light-demanding species was more than twice that of shade-tolerant species. A higher proportion of shade-tolerant species accumulated nutrients in their tissues (up to 95% for P), but over half the light-demanding species did so as well. For both types of species, a higher percentage accumulated P than N. Responses in growth and in P accumulation were negatively correlated with seed mass. Shrubs responded more than trees in biomass accumulation, but there was a trend toward greater response by trees in relative growth rate and N and P accumulation. The results suggest that variation in nutrient availability could affect the structure of tropical forests regenerating from large-scale disturbance. In fertile sites, competitive exclusion may occur within the light-demanding species, resulting in a decline in local tree diversity. Depending on the effects of human disturbance on nutrient availability, loss of tree diversity at a local scale may compound diversity loss due to habitat destruction at the landscape scale.