Modeling spatial variations of the invasibility of slash pine flatwoods to Chinese tallow (Triadica sebifera) invasion: mechanisms and key factors at the microscale.
Protecting endangered slash pine (Pinus elliottii) flatwood ecosystems from the invasion of Chinese tallow (Triadica sebifera (L.) Small) a community-transforming invasive tree species is of critical, regional importance to the biota of the Gulf Coastal Plain. We developed a conceptual model to decipher the mechanism of tallow invasion at the microscale based on the community assembly theory, modeling the spatial variations of invasibility of slash pine flatwoods to Chinese tallow using a spatially-explicit point process model. A set of idenitified factors (filters) including the nearest distance to roads and seed sources (dispersal filters), canopy closure and pine: hardwood ratio (overstory filters), and grass cover and microtopography (understory filters) as well as tallow abundance and distribution (input for the model) were measured within 281 contiguous 30 m2 quadrats with a total area of 0.86-ha in a slash pine flatwood. The identified filters vary spatially and affect tallow invasion in a nonlinear "concave down" manner. Additive polynomial functions of these identified filters provide a viable approach to model the spatial variations of tallow invasion probability (invasibility). The full models which include all filters, as well as the sub-models which include only the dispersal filters and either of the overstory and understory filters, appear to have an adequate predictive power based on R2, RSE and skewness as well as the scatter- and qq-plots. In contrast, the sub-models that include only one of the filters failed to precisely predict the observed invasion outcomes. The results support the importance of dispersal filters and reducing propagule pressure levels for effective tallow control and slowing of spread.