Southeastern Naturalist T. Suriyamongkol, E. McGrew, L. Culpepper, Kacy Beck, H.-H. Wang, and W.E. Grant 2016 68 Vol. 15, Special Issue 9 Recent Range Expansions by Chinese Tallow (Triadica sebifera (L.) Small), the Most Prevalent Invasive Tree in the Forestlands of Eastern Texas Thanchira Suriyamongkol1,†, Erin McGrew1, †, Lela Culpepper2, Kacy Beck3, Hsiao-Hsuan Wang1,*, and William E. Grant1 Abstract - We documented range expansion of Triadica sebifera (Chinese Tallow) within forestlands of eastern Texas based on field data collected by the US Forest Service from 2001 to 2012. Chinese Tallow generally spread northward, with the number of sample plots in which Chinese Tallow was detected approximately doubling and mean percent coverage of Chinese Tallow in sample plots increasing significantly (t = -3.93, P < 0.05) during this period. Number of sample plots in each of 5 percent-coverage categories (<10, 10–20, 20–30, 30–40, >40) increased in each of 3 latitudinal (°N) bands (29–30, 30–31, >31) from the first to the second survey. Our empirical results support the general trend of northward expansion predicted by existing models, which were based on less-recent data. Introduction Invasive species cause economic losses in forestry and agriculture, which can hinder endangered and threatened species, diminish ecosystem biodiversity and productivity, and alter habitats of native wildlife (Moser et al. 2009). In the United States alone, invasive species cause environmental damage estimated at $120 billion annually (Pimentel 2005). Invasive species also are responsible for a reduction in species richness and abundance of native flora and fauna (Nemec et al. 2011). The introduction of non-native invasive species is second only to habitat destruction among the leading causes of species extinction (Lowe et al. 2000). A prerequisite for the development of effective strategies to control non-native plant invasions is the identification of historical trends in range expansion (L odge et al. 2006). Triadica sebifera (L.) Small (Chinese Tallow) is one of the most successful woody invaders in eastern Texas (USDA 2013). Chinese Tallow was introduced into the United States in the late 1700s and quickly naturalized (Bruce et al. 1997). Because of the large amount of vegetable tallow found in the seed, the Foreign Plant Introduction Division of the USDA promoted Chinese Tallow planting in Gulf Coast states to establish a local soap and candle industry from 1920 to 1940 (De- Walt et al. 2011). However, since termination of the USDA project, Chinese Tallow has escaped from cultivation and spread aggressively (DeWalt et al. 2011). It has 1Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843. † Contributed equally. 2Department of Ecosystem Science and Management, Texas A&M University, College Station, TX 77843. 3Department of Bioenvironmental Science, Texas A&M University, College Station, TX 77843. *Corresponding author - hsuan006@ tamu.edu. Manuscript Editor: Jerry Cook Proceedings of the 6th Big Thicket Science Conference: Watersheds and Waterflow 2016 Southeastern Naturalist 15(Special Issue 9):68–75 Southeastern Naturalist 69 T. Suriyamongkol, E. McGrew, L. Culpepper, Kacy Beck, H.-H. Wang, and W.E. Grant 2016 Vol. 15, Special Issue 9 been classified as an invasive species from North Carolina to Florida and westward into Arkansas and Texas, with parts of California also experiencing invasion (Bruce et al. 1997). Chinese Tallow is a shade-tolerant (Lin et al. 2004) invader that aggressively disrupts native ecosystem structure by forming monotypic stands (Bruce et al. 1997). The high number of moderately small-sized, relatively quickly maturing seeds promotes survival in disturbed environments (Gabler and Siemann 2013), including forest areas impacted by Sus scrofa L. (Feral Hog; Siemann et al. 2009). The seeds are spread efficiently by birds (Renne et al. 2002), can be transported by rivers and streams (Pattison and Mack 2008), and can germinate and grow in both fresh- and saltwater wetlands (Yang et al. 2015a), which facilitates the invasion of riverbanks, lakesides, and grassland prairies (Bruce et al. 1995). Compared to many native species, Chinese Tallow is more tolerant of herbivory (Hartley et al. 2010), exhibits faster aboveground growth after herbivore consumption (Huang et al. 2012), and interacts more efficiently with arbuscular mycorrhizae (Yang et al. 2015b). A variety of statistical models have been developed to estimate the probability of occurrence of Chinese Tallow: within the entire US (Pattison and Mack 2008); within the forestlands of Alabama, Mississippi, Louisiana, and eastern Texas (Gan et al. 2009); and within the forestlands of eastern Texas (Wang et al. 2014). A simulation model projecting possible routes and rates of expansion of Chinese Tallow, both within and beyond its current range in eastern Texas and Louisiana, also has been developed (Wang et al. 2011, 2012). In the present paper, we document the recent range expansion of Chinese Tallow within the forestlands of eastern Texas based on analyses of an extensive set of field data collected by the US Forest Service on fixed plots during the period from 2001 to 2012. We then compare our empirical results with predictions of existing models which were based on less-recent data. Materials and Methods We obtained data for our study from the Forest Inventory and Analysis Program (FIA) of the US Forest Service using the Southern Nonnative Invasive Plant data Extraction Tool (SNIPET) (USDA 2013). The Southern Research Station of the US Forest Service began conducting an invasive plant survey on forest land in 13 southern states in early 2000. The survey is conducted on a state-by-state basis and is designed to survey one-fifth of the sample plots in each state annually, returning to each plot on a 5-year cycle (Rudis et al. 2005). Field data are collected from a lattice of 4047-m2 hexagons, with 1 sample plot located randomly within each hexagon (Bechtold and Patterson 2005). Each sample plot consists of 4 subplots of radius 7.32 m that form a cluster consisting of a central subplot and 3 peripheral subplots equidistant from each other arrayed in a circle of radius 36.58 m centered on the central plot (Bechtold and Patterson 2005). Currently, 2 invasive plant survey cycles (2001–2005 and 2006–2012) have been completed in Texas (USDA 2013). We summarized the data from each of the 2 surveys by (1) counting the number of sample plots in which Chinese Tallow had been detected, (2) noting the percent Southeastern Naturalist T. Suriyamongkol, E. McGrew, L. Culpepper, Kacy Beck, H.-H. Wang, and W.E. Grant 2016 70 Vol. 15, Special Issue 9 coverage of Chinese Tallow within each of these plots, and (3) mapping the spatial distribution of these plots (using ArcMapTM 9.1; ESRI, Redlands, CA). We documented range expansion by comparing the results from the 2 surveys with regard to (1) number of plots occupied, (2) mean percent coverage (using a paired t-test), and (3) spatial distribution. We compared spatial distributions in terms of the number of plots in each of 3 latitudinal (°N) bands (29–30, 30–31, >31) in which the mean percent coverage of Chinese Tallow was <10, 10–20, 20–30, 30–40, and >40, respectively. We also calculated an index of potential rate of spread based on the distance between each plot in which Chinese Tallow was first detected during the second survey and the nearest plot in which Chinese Tallow had been detected during the first survey, that is, distance to the nearest known propagule source. Results Chinese Tallow generally expanded northward, with both the number of sample plots occupied and the percent coverage of Chinese Tallow in sample plots increasing during the second survey both within and beyond the range documented during the first survey (Fig. 1). Chinese Tallow was present in 245 (~10%) and 471 (~19.5%) of the sample plots during the first and second surveys, respectively. The mean (± SE) percent coverage of Chinese Tallow in plots in which Chinese Tallow had been detected during the first survey increased significantly (t = -3.93, P < 0.05) from 6.9% (± 0.68%) during the first survey to 9.1% (± 0.59%) during the second survey. The number of sample plots in each of the 5 percent coverage categories increased in each of the 3 latitudinal bands from the first to the second survey (Fig. 2a, b). Distances to the nearest known propagule source ranged from 0.8 km to 82.0 km, with a mean (± SE) of 13.6 km (± 0.98 km). Discussion Several recent studies have used models to estimate probability of occurrence or rate of expansion of Chinese Tallow in the US (Gan et al. 2009; Pattison and Mack 2008; Wang et al. 2011, 2012, 2014). Pattison and Mack (2008) estimated range limits within the US using the CLIMEX model (Sutherst and Maywald 1985) to characterize the climatic conditions under which Chinese Tallow occur in Asia and identify the geographical extent of similar conditions in the US. They suggested that Chinese Tallow is capable of expanding 500 km northward from its current distribution in the southeastern US, with its northern range limits imposed by cold temperatures. Gan et al. (2009) estimated probabilities of invasion (probabilities of occurrence) within southern US forestlands by correlating land characteristics and climatic conditions with presence/absence of Chinese Tallow based primarily on analysis of data collected during the FIA inventory period that ended in 2006. Their study identified areas vulnerable to invasion to the north of the present range in eastern Texas, Louisiana, Mississippi, and Alabama, and suggested the estimated invasion probabilities could become higher with the assumption of increasing environmental temperatures. Wang et al. (2014) Southeastern Naturalist 71 T. Suriyamongkol, E. McGrew, L. Culpepper, Kacy Beck, H.-H. Wang, and W.E. Grant 2016 Vol. 15, Special Issue 9 used a similar approach to estimate the probabilities of Chinese Tallow invasion (occupancy) in eastern Texas forestlands based on data collected during the FIA inventory period that ended in 2011. In addition to estimating invasion probabilities, their study focused on evaluating the possible effects of forest management practices on Chinese Tallow range expansion. They found that habitats most at risk occurred primarily in northeastern Texas and suggested that probabilities of further invasion could be reduced most by site preparation and artificial regeneration. Although the models of Pattison and Mack (2008), Gan et al. (2009), and Wang et al. (2014) all identified areas to the north of the present range that were vulnerable to invasion, they did not provide a time scale for range expansion. Wang et al. (2011) estimated the rate of range expansion by Chinese Tallow in eastern Texas and Louisiana using a spatially explicit simulation model parameterized primarily based on data collected during the FIA inventory period that ended in 2006. They projected annual range expansions that extended from the Gulf Coast of Texas and Louisiana northward and westward 300 km by the year 2023. Wang et al. (2012) subsequently modified the model of Wang et al. (2011) Figure 1. Spatial distributions of sample plots in which Chinese Tallow was detected during (a) the first survey (2001–2005) and (b) during the second survey (2006–2012), and mean percent coverage of Chinese Tallow in those plots. Based on information from the Forest Inventory and Analysis Program of the US Forest Service (USDA 2013). Southeastern Naturalist T. Suriyamongkol, E. McGrew, L. Culpepper, Kacy Beck, H.-H. Wang, and W.E. Grant 2016 72 Vol. 15, Special Issue 9 Figure 2. Comparison of the spatial distributions of Chinese Tallow during (a) the first survey (2001–2005) and (b) the second survey (2006–2012) conducted by the Forest Inventory and Analysis Program of the US Forest Service (USDA 2013), and (c) projected by the simulation model of Wang et al. (2011) for year of 2013. Results are summarized in terms of the number of plots in each of 3 latitudinal (°N) bands (29–30, 30–31, >31) in which the mean percent coverage of Chinese Tallow was <10, 10–20, 20–30, 30–40, and >40, respectively. Number of sample plots in the 3 latitudinal bands, from south to north, were 75, 839, and 1483, respectively. Southeastern Naturalist 73 T. Suriyamongkol, E. McGrew, L. Culpepper, Kacy Beck, H.-H. Wang, and W.E. Grant 2016 Vol. 15, Special Issue 9 to simulate the effectiveness of potential management schemes and estimated the economic costs associated Chinese Tallow invasions. They found that much less intensive control was required to decrease the physical extent of invasion than was required to reduce the annual economic impact associated with invasion. Our empirical results, which are based on more-recent data than were utilized in the models described above, support the general trend of northward expansion predicted by these models, e.g., Fig. 2(b) in Pattison and Mack (2008), Fig. 1 in Gan et al. (2009), Fig. 6(A) in Wang et al. (2014). Our index of potential rate of spread, assuming a mean interval of seven years between samples on any given plot, suggests a mean potential dispersal velocity of 1940 m/year (13.6 km/7 years), which is faster than that used in the model of Wang et al. (2011; 1231 m/year) and that estimated by Renne et al. (2000; 1000 m/year) based on field experiments involving Chinese Tallow seed dispersal by birds in coastal South Carolina. Almost surely there were propagule sources in areas un-sampled because field data are collected from a lattice of 4047-m2 hexagons, with 1 sample plot located randomly within each hexagon (Bechtold and Patterson 2005). However, it confirms the expected trend in range expansions of Chinese Tallow. Historical trends in range expansions of non-native species provide valuable information upon which to base effective control strategies and mitigation plans (Lodge et al. 2006). We would suggest that the field data generated via the national array of FIA plots not only has generated (and continues to generate) an excellent basis for monitoring range expansions by non-native species, but also provides a perhaps under-utilized opportunity to evaluate empirically the predictions of species distribution models and models simulating rates and routes of range expansion. Acknowledgments We would like to thank those participants at the Sixth Big Thicket and West Gulf Coastal Plain Science Conference in Nacogdoches, TX who provided comments on our paper. We also thank 2 anonymous reviewers for their time and effort. The manuscript is greatly improved as a result of their comments. 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