Chinese Tallow Long-Term Impact on Stand Dynamics in a Bottomland Hardwood Forest Following Vegetation Management

Chinese tallow (Triadica sebifera (L.) Small) is an invasive tree known to have negative impacts on bottomland hardwood forests. Chinese tallow is proficient in reproducing and surviving in bottomland hardwood forests because it is more flood and shade tolerant than most native trees. The long-term effectiveness of Chinese tallow control is an important topic to research as only a few long-term studies of Chinese tallow control have been conducted. A 10-year re-measurement of 22 paired plots installed in 2012 on the Pineywoods Mitigation Bank near Diboll, Texas following herbicide treatments examined Chinese tallow and native tree stand structure. Each native and Chinese tallow plot included three sets of nested subplots. The nested subplots include one overstory, one sapling, and four seedling subplots. R (v4.3.3) software was used to run repeated measures analysis of variance (ANOVA), simple linear regressions, and other summary statistics. Chinese tallow density did not differ between years and herbicide treated and non-herbicide treated plots (p = 0.414). Relative density of Chinese tallow had a correlation between years and herbicide treated and non-herbicide treated plots (p = 0.040). Chinese tallow had a significant decrease in herbicide treated plots from 2012-2013 to 2022 (81.46% decrease from 2012). Native stand structure metrics did not differ between years and herbicide treated and non-herbicide treated plots (density p = 0.883, basal area p = 0.843, quadratic mean diameter p = 0.851, relative density p = 0.901). Diversity and evenness differed in all sapling plots from 2022 data to 2012-2013 data (diversity 2022-2012 p = p = p = 0.797, basal area p = 0.335, quadratic mean diameter p = 0.544, stand density index p = 0.272). This suggests that there is no difference in stand metrics between 6 and 10 years. Observational study on top-killed Chinese tallow likely killed from the 2021 freeze that occurred in East Texas showed that only 12.4% of Chinese tallow were top-killed from the freeze. All of the top-killed Chinese tallow were observed having sprouts growing out of the snag. There was no correlation between snag diameter at breast height and maximum sprout height (p = 0.172). I found no support for my hypotheses that Chinese tallow and native tree densities responded to herbicide application. Overall forest structure also did not respond to herbicide, although I found that relative density of only Chinese tallow was affected in Chinese tallow plots treated with herbicide. It was also found that the number of years since the last herbicide treatment did not correlate to the magnitude of changes in Chinese tallow and native tree densities. The observational study on top killed Chinese tallow from the 2021 freeze showed no correlation between maximum seedling sprout height relating to tree diameter at breast height (dbh). With updated 10-year maximum stand density index results we found that stand density index in bottomland hardwoods is substantially greater than upland hardwood stands reported in the central hardwoods region (Schnur 1937). Our results follow the pattern of another long-term Chinese tallow control study (Norman 2020), where change in Chinese tallow density was not correlated with the treatment type. Reduction in diversity metrics followed the same trend from invasive plant studies (Hart and Holmes 2013; Hejda et al. 2009; Zedler and Kercher 2004) that biodiversity will reduce over time when an area is invaded. Future long-term studies are needed to provide important information regarding biodiversity reduction to land managers.