Predicting the spread of Chinese tallow (Triadica sebifera) in the southeastern United States forestland: Mechanism and risk factors at the regional scale

Abstract

In this study, we analyzed the regional spread patterns of Chinese tallow (Triadica sebifera) and associated factors using the remeasured U.S. Forest Service’s Forest Inventory and Analysis (FIA) data. Each mapped FIA plot was classified into one of the four categories based on its invasion status during the two consecutive inventory cycles (2000–2009 and 2006–2015): a non-invaded plot in both cycles, a newly invaded plot in the second cycle, a previously invaded plot in the first cycle, but tallow disappeared in the second cycle, and a previously invaded plot with tallow existence in the second measurement cycle. The smoothed presence probability and rate of spread (annual invasion probability) were strongly linearly correlated (r = 0.90, p = 0.001). Risk factors contributing to Chinese tallow spread varied among the three sub-regions (invasion stages) of varying propagule pressure levels, however, elevation and distance to the nearest road were the two variables statistically significant across all sub-regions (the entire region), indicating the pivotal role of the interplay between geographic characteristics and landscape (dispersal) factors in the regional spread of Chinese tallow. Natural disturbances including hurricanes and tropical storms significantly facilitated the spread of Chinese tallow from lower coastal areas to the inland. Among all forest type groups, bottomland oak-gum-cypress forests were more susceptible to Chinese tallow invasion in the low and moderate propagule pressure sub-regions, while oak-pine forests had higher Chinese tallow coverage (degree of invasion) in the high propagule pressure sub-regions.