61 Rapidly Increasing Invasion of Eastern Texas Forestlands by the Most Prevalent Invasive Shrubs: Chinese and European Privets Aminta Arevilca1, Luis Elissetche2, Michael Scanlin3, Sara Stephens1, Hsiao-Hsuan Wang1,*, and William Grant1 Abstract - Based on analyses of an extensive set of field data collected by the US Forest Service, we identified trends in range expansion of Ligustrim sinense (Chinese Privet) and L. vulgare (European Privet), the most prevalent invasive shrubs within forestlands of eastern Texas. Chinese and European privets increased markedly in northeastern Texas from 2004 and 2012. The number of sample plots in which Chinese and European privets were detected increased from 79 (in 25 counties) to 300 (in 40 counties), and the mean percent coverage within those sample plots already invaded in 2004 increased significantly by 2012. The number of sample plots in each of 3 percent-coverage categories (<10, 10–20, >20) increased in each of 2 longitudinal bands (>95°W and ≤95°W) from the first to the second survey. Our empirical results support the general trend of westward and southeastward expansions predicted by existing models, which were based on less -recent data. Introduction Invasive plants reduce forest productivity (Wang et al. 2012), degrade forest health (Pejchar and Mooney 2009), and pose a particularly severe threat to the economically and ecologically important forestlands in the southeastern US. Southeastern forests contain about 81,341,814 ha (201 million ac) of timberland, corresponding to about 58% of US timber production and about 16% of timber production world-wide (McNulty et al. 2000), and they also support a high level of biodiversity, with over 3000 native plant species documented in these habitats (Linder 2004). In the forestlands of eastern Texas, where Pinus taeda L. (Loblolly Pine) is of both commercial and ecological importance (Wear and Greis 2002), Ligustrum sinense Lour (Chinese Privet) and Ligustrum vulgare L. (European Privet) are the most prevalent invasive shrubs in some counties (USDA 2013). There are ~50 species of Ligustrum, or privet shrubs, native to Europe, Asia, and North Africa (Miller 2003). Due to the availability of their preferred warm, humid climate in Texas—average annual temperature of 19.5 °C (67.1°F), with an average of 240 frost-free days per year, and average annual precipitation is 1.32 m (52 in) (Streng et al. 1989)—Chinese and European privets have become some of the most invasive plants recorded in the US (USDA 2013). 1Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843. 2Department of Bioenvironmental Science, Texas A&M University, College Station, TX 77843. 3Department of Civil Engineering, 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):61–67 Southeastern Naturalist A. Arevilca, L. Elissetche, M. Scanlin, S. Stephens, H.-H. Wang, and W. Grant 2016 62 Vol. 15, Special Issue 9 Even though Chinese and European Privets come from different parts of the world, they are difficult to distinguish except at flowering (Miller 2003). Both species are thicket-forming shrubs up to 9.1 m (30 ft) in height, multiple stemmed with long, leafy branches, and have opposite leaves less than 5.1 cm (2 in) long (Miller 2003). The European or Common Privet was first introduced to the US in the early to mid-1800s (Haragan 1996). Chinese Privet made its way to the US in 1852 from Asia (Haragan 1996). Both of these species have traditionally been used as ornamental shrubs in the eastern parts of the US (Miller 2003). After escaping cultivation, these species spread to 41 states (USDA 2015) and now appear on the Texas Department of Agriculture’s list of nuisance plants (Camilli 2005). Each mature Chinese or European Privet plant can produce hundreds of fruits containing millions of seeds (~2.7 million) annually (Haragan 1996), and seeds germinate promptly without a period of moist cold (Young and Young 1992). Seeds are dispersed widely by birds and other wildlife (Dirr 1998). In addition, privets can grow in low-nutrient soils, tolerate low light-levels (Miller 2003), and, hence, can invade under dense forest canopies (Harrington and Miller 2005, Merriam and Feil 2002), dominating the understory of mesic forests throughout the southern US (Haragan 1996). Chinese and European Privets continue to expand their ranges in the southern US (Wilcox and Beck 2007), and reliable predictions of habitats at risk are urgently needed (Williams and Minogue 2008). Statistical models have been developed to estimate the probability of occurrence of Chinese and European Privets within the forestlands of eastern Texas (Wang and Grant 2014) and throughout the southern US (Wang and Grant 2012). In this paper, we document the recent range expansion of Chinese and European Privets 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 also compare our empirical results with predictions of existing models, which were based on less-recent data. Materials and Methods We focused our investigation on eastern Texas, which has been invaded extensively by Chinese and European Privets in some counties (USDA 2015). We obtained invasion data from the Forest Inventory and Analysis Program (FIA) of the US Forest Service (USDA 2013) using the Southern Nonnative Invasive Plant data Extraction Tool (SNIPET). 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 1/5 of the sample plots in each state annually, returning to each plot on a 5-y cycle (Rudis et al. 2006). 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 with a radius of 7.32 m that form a cluster consisting of a central subplot and 3 peripheral subplots equidistant Southeastern Naturalist 63 A. Arevilca, L. Elissetche, M. Scanlin, S. Stephens, H.-H. Wang, and W. Grant 2016 Vol. 15, Special Issue 9 from each other and arrayed within a 36.58-m-radius circle cent ered on the central plot (Bechtold and Patterson 2005). As of this writing, 2 invasive plant surveys had been completed in Texas, ending in 2004 and 2012, respectively (USDA 2013). We summarized the data from each of the 2 surveys by (1) counting the number of sample plots in which Chinese and European Privets had been detected, (2) noting the percent coverage of Chinese and European Privets within each of these plots, and (3) mapping the spatial distribution of these plots (using ArcGIS 10.2, 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 (between invaded plots sampled during the 2005–2012 survey and their coverage during the 2001–2004 survey using a paired t-test), and (3) spatial distribution. We compared spatial distributions in terms of the number of plots in each of 2 longitudinal bands (>95°W and ≤95°W) in which the mean percent coverage Chinese and European Privets was <10, 10–20, and >20, respectively. Results Chinese and European Privets generally expanded westward in east Texas between 2004 and 2012 (Fig. 1). The number of sample plots in which Chinese and European Privets were detected increased from 79 (3% of sample plots) in 2004 to 300 (12%) by 2012, and the mean percent coverage of Chinese and European Privets within those sample plots already invaded during the 2001–2004 survey (1.7%) increased significantly (t = 1.97, df = 299, P < 0.05) to 6.8% during the 2005–2012 survey. From 2004 to 2012, the number of counties invaded by Chinese and European Privets increased from 25 to 40; the number of sample plots invaded increased most noticeably in Anderson, Cherokee, and Smith counties (Table 1). The number of sample plots in each of the 3 coverage categories increased in each of the 2 longitudinal bands between the first and second surveys (Fig. 2). Discussion Two recent studies (Wang and Grant 2012, 2014) used models to estimate the probability of occurrence of Chinese and European Privets in the southern US. Wang and Grant (2012) estimated the probability of invasion (probabilities of occurrence) within southern US forestlands by correlating land characteristics and climatic conditions with presence/absence of Chinese and European Privets based primarily on analysis of data collected during the FIA inventory period that ended in 2006. Their study identified areas vulnerable to invasion throughout Mississippi, with a band stretching eastward across south-central Alabama, eastern Texas, and western Louisiana. Wang and Grant (2014) used a similar approach to estimate the probability of invasion by Chinese and European Privets (occupancy) in eastern Texas forestlands based on data collected during the FIA inventory period that ended in 2011. In addition to estimating invasion probability, their study focused on evaluating the possible effects of forest-management practices on range expansion by Chinese and European Southeastern Naturalist A. Arevilca, L. Elissetche, M. Scanlin, S. Stephens, H.-H. Wang, and W. Grant 2016 64 Vol. 15, Special Issue 9 Privets. They found that habitats most at risk to further invasion under current conditions occurred primarily in northeastern Texas and that site preparation and artificial regeneration reduced the estimated probability of further invasion. Our empirical results, which are based on more-recent data than were utilized in the models described above, support the general trend of westward and southeastward expansion predicted by these models, e.g., Fig. 2a in Wang and Grant (2012) and Fig. 3a in Wang and Grant (2014). There were likely undetected privet seed sources in unsampled areas adjacent to the lattice of 4047-m2 hexagons in which field data were collected (Bechtold and Patterson 2005); thus, privet ranges may have been slightly larger than the data indicate. However, our findings document Chinese and European Privet range expansion. Historical trends in range expansions of non-native species provide valuable information upon which to base effective control strategies and mitigation plans (Walther et al. 2009). We suggest Figure 1. Spatial distribution of sample plots in which Chinese and European Privets were detected during (a) the 1st survey (2001–2004) and (b) the 2nd survey (2005–2012), and mean percent coverage of Chinese and European Privets in those plots. Based on information from the Forest Inventory and Analysis Program of the US Forest Service (USDA 2013). Southeastern Naturalist 65 A. Arevilca, L. Elissetche, M. Scanlin, S. Stephens, H.-H. Wang, and W. Grant 2016 Vol. 15, Special Issue 9 Table 1. Number of sample plots in the indicated counties in which Chinese and European Privets were detected during the indicated survey cycle. * indicates counties that had a >10-plot increase in privet occurrence between 2004 and 2012. Analysis is based on the information from the Forest Inventory and Analysis Program of the US Forest Service (USDA 2013). County 2004 2012 County 2004 2012 Anderson* 1 12 Morris 2 4 Angelina 7 13 Nacogdoches* 4 25 Bowie 15 19 Orange 0 1 Cass* 11 26 Panola* 2 14 Chambers 0 1 Polk 3 9 Cherokee* 1 16 Red River 1 4 Franklin 0 4 Rusk* 5 17 Gregg 2 6 Sabine 0 1 Hardin 1 2 San Augustine 2 5 Harris 2 8 San Jacinto 2 2 Harrison* 4 17 Shelby* 3 14 Henderson 0 6 Smith* 1 12 Houston 0 2 Titus 2 4 Jasper 0 5 Trinity 1 4 Jefferson 1 5 Tyler 0 2 Leon 0 1 Upshur* 0 10 Liberty 1 6 Van Zandt 0 7 Madison 0 1 Walker 0 1 Marion 2 3 Waller 0 1 Montgomery 0 3 Wood 3 7 Figure 2. Comparison of the spatial distribution of Chinese and European Privets during (a) the 1st survey (2001–2004) and (b) the 2nd survey (2005–2012) conducted by the Forest Inventory and Analysis Program of the US Forest Service (USDA 2013). Results are summarized in terms of the number of plots in each of 2 longitudinal bands (>95°W and ≤95°W) in which the mean percent privet coverage was <10, 10–20, and >20, respectively. The number of sample plots in the 2 longitudinal bands, from east to west, was 1491 and 1009, respectively. Southeastern Naturalist A. Arevilca, L. Elissetche, M. Scanlin, S. Stephens, H.-H. Wang, and W. Grant 2016 66 Vol. 15, Special Issue 9 that the field data generated by surveys of the national array of FIA plots represents a resource in terms of providing the opportunity to identify recent broad-scale trends in invasive plant species distribution. Together with the increasing availability of geo-referenced climatic and landscape-use databases, the FIA data provide the opportunity to correlate these trends with potential causal factors, and, as field sampling continues on the FIA plots, to empirically test the predictions of range expansion based on these correlations, and make appropriate adjustments to the models upon which these predictions were based. Acknowledgments We would like to thank those participants at the 6th 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 was greatly improved as a result of their comments. Financial support for our work was provided by the Undergraduate Research Fund of the Department of Wildlife and Fisheries Sciences, Texas A&M University. Literature Cited Bechtold, W.A., and P.L. Patterson. 2005. 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