Following the sustained exploration and colonization of North America by Europeans over the past 500 years, exotic plant species have been introduced accidently through international trade and have grown at an unprecedented rate in the United States [1
]. Some plant species are beneficial while others are harmful. Over 98% of the United States food system comes from introduced food crops such as corn, wheat, rice and livestock species including cattle and poultry with a monetary value of nearly $
800 billion every year [2
]. However, some introduced plant species have caused ecological and economic losses in agriculture, forestry and other ecosystems. Invasions of non-native plant species can cause ecosystem degradation and biodiversity loss due to changes in nutrient cycling, energy budgets and fire regimes of native plant species [3
]. In addition, environmental damages and losses due to invasive exotic species in the United States have added up to $
120 billion dollars per year, out of which exotic plants alone account for $
25 billion dollars annually [2
Sixty-five percent of the land in Mississippi was covered by forests and the total value of Mississippi’s timber crop was $
1.4 billion in 2016, which was almost 20% of total production values from agriculture, forestry and animal husbandry [4
]. Mississippi forestlands have been invaded by numerous tree, shrub, vine, grass and fern species in different degrees [5
]. Considering that various invasion patterns and growth characteristics of these species may influence the accuracy of an analysis, this study focused on invasive tree species only. Therefore, identifying factors contributing to the presence and abundance of target invasive tree species and taking effective actions to combat invasions are meaningful and valuable in the state of Mississippi.
The specific objectives of the study are to: (1) identify ecological and socioeconomic factors that influence a set of invasive tree species’ presence and abundance on Mississippi forest plots and select the most relevant factors through the classification and regression trees (CART) analysis; and (2) use the appropriate spatial model to quantitatively analyze the contributions of these selected factors at the county level.
Ecological variables, identified in our literature review, influencing the presence and abundance of invasive tree species in Mississippi were elevation, stand age, forest type, type of natural disturbance, site productivity, growing stock and stand origin. Socioeconomic variables assumed to influence the presence and abundance of invasive tree species in Mississippi were road density, type of ownership, population density, per capita mean annual income, and proportion of wildland urban interface.
Elevation was found to have a significant negative effect on the number of invasive species, which indicated that invasions were less prone to occur at higher elevations [7
]. Elevation was also found to be negatively associated with the presence of Chinese tallow in the southern United States [8
]. This study also noted that nearly 80% of existing tallow invasions occurred at elevations lower than 165 feet. Young forests were believed to usually have higher light penetration, fewer competitors from external disturbance, and better sanitary conditions than old forests while mature forest interiors usually have lower temperatures, higher humidity, and darker environment than outside [9
]. In the study by Hussain et al. [13
], stand age had no effect on invasive trees’ presence but had a significant negative effect on the abundance of invasive trees on forestlands in Alabama.
Forest type including longleaf/slash pine group, loblolly/shortleaf pine group (reference group), oak/pine group, oak/hickory group, oak/gum/cypress group and elm/ash/cottonwood group was considered as a potential contributing factor and oak/gum/cypress group was proven to be a positive effect on the occurrence of Chinese tallow at the 5% significance level [14
]. Disturbance events such as harvesting or natural disasters could be a factor in promoting the spread of Chinese tallow tree because resources such as light and nutrients were released for colonization and establishment after disturbance [14
]. Bellingham et al. [15
] surveyed the exotic tree Pittosporum undulatum
in Jamaican mountain rain forests and concluded that invasion was accelerated after damage caused by hurricanes.
High and medium site productivity levels positively affected the occurrence of Chinese privet and Nepalese browntop [16
]. Gan et al. [8
] noted that site productivity was not crucial in the early stages of Chinese tallow invasion but higher productivity furthered expansion after initial establishment. Compared to fully stocked, poorly and medium growing stocked plots influenced occurrence of Chinese privet and Nepalese browntop positively [16
Roadsides are ideal habitats for invasive plant species because these areas provide corridors or create disturbances for invasions [17
]. Flory and Clay [9
] found that densities of exotic shrubs were higher at 0 meter (m) than 10, 20, 30 m from the roads and there were 46% fewer stems of invasive tree species per square meter at 10 m from the road than at 0 m from the road. Gelbard and Harrison [19
] measured the populations and coverage of non-native and native plant species on 92 sites at distances of 10 m, 100 m and over 1000 m to road and found that exotic forb species were least abundant in the most distant sites. Private ownership was proved to have a positive effect on tallow tree invasion in southern U.S. forestlands [8
]. Private lands might be less closely monitored and managed, more frequently harvested and covered with younger trees [20
]. The possibility existed that policy incentives for private landowners to conduct prevention and mitigation were lacking [21
Human population density has been shown to be a good predictor of exotic species richness [3
]. McKinney [22
] incorporated population density as a derivative variable (population/area) to study the effects of human activities on species richness within the United States. Multiple regression analysis showed that population density was the best predictor for net plant species richness gain. Similarly, Qian and Ricklefs [24
] discovered that the number of exotic species was more closely related with the size of human population than with ecological conditions. Income was found to be positively related to invasive exotic plant richness while road density and the area of agriculture were not significant [3
]. They suggested that income functioned as a proxy measure of socioeconomic activities that favor plant invasions. Hope et al. [25
] found neighborhoods with incomes above median family income had twice the invasive plant diversity than those from less wealthy areas.
The wildland-urban interface (WUI) is the place where natural wildland vegetation meets housing. WUI includes two distinct housing patterns: intermix housing, where housing and vegetation co-occur, and interface housing, where housing is close to vegetation [3
]. Gavier-Pizarro et al. [3
] found that housing variables could better explain invasion than environmental and other human variables. Besides, among the housing variables, the amount of low-density residential area and change in housing units between 1940 and 2000 were more associated with exotic tree species at the 5% significance level. Besides, the growth rate of urban land use was greater than parks due to an increase of suburban housing and it opened new habitats for nonnative plant and animal species [27
Among the variables evaluated, elevation had a negative effect on the presence and abundance of invasive tree species in Mississippi at the county level. The negative relationship between elevation and the number of exotic plants has been documented in some studies. Sax [36
] found that the distribution of naturalized exotic species in Chile was highly associated with elevation and species richness. Gan et al. [8
] also found that the occurrence of Chinese tallow in the southern United States was negatively related to elevation. A possible explanation might be that there are fewer appropriate plant species that could be introduced to higher elevation areas [37
]. Lower elevation usually comes with warmer temperatures and less water. In Mississippi, the elevation ranges only from sea level at Gulfport to 806 feet (246 m) in the northeast hills, which doesn’t have a lot of variation. With higher elevations, temperature decreases and precipitation increases so fewer invasive tree species are found. Besides, the range of elevation change is small, which supports the idea that elevation didn’t play a substantial role even though it is significant.
It was not surprising that stand age had negatively influenced the abundance of invasive tree species at the county level. The negative relation indicated that invasive trees were prone to be abundant in young forests. Older forest communities were believed to be more resistant to exotic plant establishment while younger communities were more vulnerable to invasion in old fields in Minnesota [38
]. Light availability might be the limiting factor because it was reported to vary with stand age [9
]. In young forests, trees have not yet grown to be fully stocked, which creates sufficient sunlight for shade intolerant species. In this research, most of these invasive tree species are shade intolerant. The shade intolerance (i.e., light loving) characteristic of tree-of-heaven predicts that tree-of-heaven grows best in full sunlight, and at times, partial sunlight, and demonstrates best growth on open, disturbed sites. Mimosa grows best in full sun but tolerates some shade [39
]. Princesstree is an early successional species that is strongly intolerant of shade [40
]. One review indicated chinaberry is not shade tolerant, but it is unclear if chinaberry occurs as a shade intolerant or shade tolerant. Although Chinese tallow survives and grows in shade, it generally performs better in sunlight. In a floodplain forest in eastern Texas, Chinese tallow mortality increased from about 0.05% at 10% full sunlight to 5.3% at 0.1% full sunlight [43
Per capita mean annual income was found to have a positive effect on the presence of invasive tree species at the county level. The original introduction purposes of invasive trees might explain the positive relation between income and presence. Most invasive species including tree of heaven, silktree were introduced to the United States as ornamentals while some were introduced accidentally through international trade such as princesstree. The possibility also existed that there was more gardening activity in higher income areas than lower income area. Besides, there was usually more transportation and commerce in higher income areas, which contributed to the spread of invasion. Due to the geographical advantages, the counties in southern Mississippi are generally more populated and have more commercial activities than those in the north.
Public ownership was negatively associated with the presence of invasive trees at the county level. The negative relationship indicated that invasions were more likely to occur on private forestlands. This finding was consistent with the research of the tallow tree in southern U.S. by Gan et al. [8
]. In Mississippi, 81% of forests are owned by nonindustrial private landowners. It is quite possible that most landowners in Mississippi have their jobs and inherit several acres of forest from their parents or grandparents. Their initial intention is likely not to get monetary benefits, neither do they have professional knowledge to manage.
A unique aspect to this study is that our models incorporated socio-economics information. Research studies identified in the literature often study only ecological influence factors of exotic species, while only a few address socioeconomic factors in a meaningful way. Besides incorporating ecological factors, this research considered more socioeconomic factors such as road density, type of ownership, population density, per capita mean annual income and wildland-urban interface in the analysis. Another advantage is the methods and techniques. Many variables were of mariginal relevance and the distribution of ecological data was not normal. Therefore, Classification and Regression Trees (CART) analysis was used to help select most relevant factors, and avoid collinearity without any prior knowledge or assumption of the data distribution.
Another unique aspect to our research was the choice of a spatial lag model. Some previous research incorporated ecoregion as an explanatory variable to minimize the effect of spatial dependency. This research improved on that. The spatial lag model enabled it to eliminate spatial autocorrelation and analyze influence factor spatially. At last, this study treated presence and abundance as two distinct phenomena. Seen from the results above, factors affecting the presence and abundance of invasive trees in Mississippi were not the same. Based on this research, we would expect it would be easier for policymakers to distinguish which counties were more susceptible to invasion and which counties were more suitable for the growth of invasive tree species in Mississippi. If more efforts were focused on these most vulnerable counties, the prevention and eradication of invasive tree species in Mississippi could be implemented in the most cost-effective way.
This study identified potential ecological and socioeconomic factors contributing to the presence and abundance of invasive tree species in Mississippi. The classification and regression tree (CART) analysis was conducted to select the most relevant factors and then the spatial lag model was employed to spatially analyze impacts of these factors. Three factors, elevation, ownership type and per capita mean annual income, contributed significantly to the presence of invasive tree species while stand age and elevation influenced abundance. This study concludes that the presence of invasive tree species in Mississippi is more likely to be found in counties with low elevation, higher per capita mean annual income and more private forestlands while greater abundance occurs in counties with lower elevation and younger forests.
Our results have several practical implications for monitoring and mitigation of invasive tree species in Mississippi. Since presence and abundance of invasive tree species are associated with distinct factors, management prescriptions should also differ. Efforts to monitor invasive trees should target counties with lower elevations, more private forestlands, and higher per capita mean annual incomes while efforts to mitigate should be dedicated to counties with low elevation and younger forests. When foresters apply management prescriptions in counties with lower elevation and higher income, they should try to avoid the introduction of invasive species. If these areas have been invaded, more intense and responsive mitigation actions should be taken. The involvement of private landowners in prevention and restoration of infested plots would benefit the ecosystems greatly. Increasing the amount of funds to educate private landowners on the negative impacts of invasive tree species may motivate them to engage in more prevention.
Future research should explore the role of the various categories within private ownership, which could be further divided into industrial and nonindustrial private ownership because industrial and nonindustrial forestlands usually have different management objectives. Future research can focus on individual invasive tree species instead of treating them as a group if policymakers plan to work on one specific species.