Invasion of Chinese tallow tree: Causes, consequences and forecast

Chinese tallow tree (Sapium sebiferum (L.) Roxb.) is a nonnative plant that has invaded coastal habitats of southeastern and Gulf Coast United States. I studied the ecological causes and consequences of tallow tree invasion and assessed its current population growth. Seeds defecated by birds germinated at rates higher than those unhandled by birds. In South Carolina, seedlings emerged throughout the growing season, and establishment differed among coastal forest types, tallow tree stands and years. Establishment was seed-limited. Data suggest a persistent, multi-year seed bank exists. Avian seed dispersal of tallow tree was assessed in South Carolina and Louisiana during the 1997--1999 fruiting seasons. The importance of specific dispersal agents (primarily winter residents) differed among regions, suggesting the generalized dispersal syndrome of tallow tree contributes to effective regional seed dispersal. During spring migration in southwestern Louisiana, the species richness, relative abundance and diversity of breeding, short-distance and transient migrant landbirds did not differ between tallow tree woodlot and bottomland hardwood forest stopover sites. Because tallow trees lack significant insect loads and displace native species, tallow tree may adversely affect the migratory success of many landbirds by providing abundant, poor-quality habitat. Projection matrices were used to estimate the annual population growth rate (lambda) of five tallow tree stands in common forest types in coastal South Carolina from 1997--2000. lambda differed among populations (range: 1.206 +/- 1.084) and the average of the 15 population-by-year matrices was positive (1.122 +/- 0.054, 95% confidence intervals). Simulations indicated that management strategies differed in effectiveness of reducing lambda within and among populations, suggesting that the same strategy applied to all populations can result in differential control. However, two strategies were particularly effective in reducing lambda in all populations. If environmental conditions persist, management will be needed to reduce current population growth of tallow tree in many coastal forests of South Carolina. Effective regional seed dispersal by birds, persistent seed banks, broad germination and establishment requirements, and high rates of survival among many size classes contribute to tallow tree invasion.