Modeling spatial variations of the invasibility of slash pine flatwoods to Chinese tallow (<em>Triadica sebifera</em>) 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 conceputal 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.

保护濒危的湿地松（Pinus elliottii）低地森林生态系统免受乌桕（Triadica sebifera (L.) Small）这种改变群落的入侵树种的侵害，对墨西哥湾沿岸平原的生物区系具有至关重要的区域意义。我们基于群落组建理论开发了一个概念模型，以在微观尺度上解释乌桕入侵的机制，并使用空间显式点过程模型模拟湿地松低地森林对乌桕的可入侵性的空间变化。在一片湿地松低地森林中，在连续的281个30平方米的样方（总面积0.86公顷）内测量了一组已确定的因素（过滤器），包括到道路和种子源的最近距离（扩散过滤器）、林冠郁闭度和松树与阔叶树的比例（上层林冠过滤器）、草本覆盖和微地形（下层植被过滤器）以及乌桕的数量和分布（模型的输入）。所确定的过滤器在空间上存在差异，并以非线性的“下凹”方式影响乌桕的入侵。这些已确定过滤器的加法多项式函数为模拟乌桕入侵概率（可入侵性）的空间变化提供了一种可行的方法。包含所有过滤器的完整模型，以及仅包含扩散过滤器以及上层林冠过滤器和下层植被过滤器其中之一的子模型，基于R²、均方根误差（RSE）和偏度以及散点图和QQ图似乎具有足够的预测能力。相比之下，仅包含其中一个过滤器的子模型无法准确预测观察到的入侵结果。研究结果支持了扩散过滤器的重要性以及降低繁殖体压力水平对于有效控制乌桕和减缓其扩散的重要性。