Invasibility of Fire-Managed Ecosystems to the Chinese Tallow Tree (triadica Sebifera) in the Lower Gulf Coastal Plain, USA: Mechanisms and Key Factors at the Landscape Level

The Chinese tallow tree (Triadica sebifera, hereafter, tallow) is a nonnative invasive species that has invaded diverse ecosystems including forests, prairies and wetlands in the Gulf Coastal Plain of the southeastern United States. We proposed a landscape (ecosystem)-level modeling framework of tallow invasion and applied it to a fire-managed landscape to evaluate the effect of prescribed fire on tallow invasion and ecosystem invasibility. A spatially random sample consisting of 55 0.04-ha circular plots was installed in a landscape of appropriately 2,900 ha in the Mississippi Sandhill Crane National Wildlife Refuge in 2015. These plots, plus ten additional tallow-invaded plots from another study were measured in May-June of 2015 and remeasured in November of 2018. Across the entire landscape, pine flatwoods (invasion probability = 0.52) were generally more susceptible to tallow invasion than pine savannas (invasion probability = 0.16) (p = 0.002), and the former had a greater number of tallow seedlings and saplings (p < 0.001), but fewer large tallow trees (p = 0.07) than the latter. The effect of fire on tallow invasion is two-fold and changes with fire return intervals and ecosystem conditions. Fire may promote seed germination and seedling recruitment, but recurrent fires top-kill or even completely kill young seedlings and saplings. Large proportions of invaded plots and abundant tallow seedlings and saplings that accumulate in pine flatwoods are essentially attributed to their high overstory tree basal area but relatively low shrub/grass coverage. On sites near roadways or with tallow seed trees, short fire intervals tend to increase invasion probability and the abundance of tallow seedlings and saplings, whereas long fire intervals increase the abundance of large tallow trees. Under current fire treatments, pine flatwoods are more susceptible to seedling colonization and sapling establishment, while pine savannas favor the growth and development of large tallow trees.