Disentangling the invasion process of subtropical native forests of Uruguay by the exotic tree Ligustrum lucidum: establishment and dominance determinants

What factors, processes and mechanisms regulate invasive processes and their effects? This is one of the main questions addressed by the ecology of biological invasions. Ligustrum lucidum, a tree species native to East Asia, became an aggressive invader of subtropical and temperate forests around the world. We analyzed here the L. lucidum invasion in Uruguayan forests to determine the factors controlling two stages of the invasive process, the establishment, and the dominance. Establishment was assessed by the occurrence, measured in 1525, 1 × 1 km-cells, and dominance by remotely measuring the L. lucidum coverage at the forest canopy in 5554, 1 × 1 km-cells. The occurrence and dominance were modeled using Generalized Linear Models in function of independent environmental and geographic variables. Ligustrum lucidum has become established in 13.4% of the Uruguayan forests and has dominated the forest canopy in 1.2%. Our models explained 45% and 35% of the occurrence and dominance spatial variance respectively and detected in both cases strong diffusion patterns from the S-SW region to rest of Uruguay. Occurrence increased mainly in function of urban areas, and with the proximity to towns, probably because L. lucidum trees planted in gardens are seed sources, and near railways and highways, that could function as biological corridors. Occurrence also increased in loamy soils and near rivers, suggesting moisture conditions are favorable for establishment. Dominance increased with reduced forest area, in high productive soils and at higher altitudes. Moreover, dominance increased near urban areas, roads, and railways, as well as in highly afforested landscapes, and in loamy and low-rockiness soils. The invasion of Uruguayan forests by L. lucidum is in the spread and impact stages, currently in expansion from the invasion focus on the S-SW region, where the oldest urbanizations are settled, towards the rest of the country. The geographic proximity to the invasion focus is currently the main predictor of both L. lucidum establishment and dominance. Additionally, whereas establishment is manly facilitated by human infrastructures improving propagule pressure and dispersion, dominance is enhanced in small or fragmented forest patches, in rich-nutrient soils, and at higher altitudes, suggesting ecosystem resistance is also operating.