Invasive Exotic Plants

Invasive nonnative forest pests are multiplying and spreading in every forest type in the southern United States. The costs of controlling these pests have become extremely high, and the damage they cause to ecosystem composition, structure, and function continues to increase. Plants imported for potential release for forage, crops, soil reclamation, and ornamental purposes are not evaluated for invasiveness. Insect pests and diseases arrive in infested nursery stock, wood products, pallets, and dunnage, in spite of our regulatory system, which has been overburdened by the rapid increase in international trade.

Nonnative species usually outcompete native species due to their ability to grow and reproduce more rapidly, often taking advantage of unused resources to thrive. For example, after a stand-replacing fire many native species may take years to re-establish themselves whereas nonnative invasives take advantage of the new landscape and usurp the resources before the natives regain control. Lespedezas are extremely aggressive invaders of open areas. Dense monocultural thickets are formed and subsequently block any sunlight from any new growth from the soil. Conversely, garlic mustard is highly shade-tolerant and can gain foothold without any habitat disturbance.  

Allelopathy, or the release of chemicals that are toxic to other species, is one way the tree-of-heaven outcompetes native vegetation. Some species such as tallowtrees and autumn olives, though thought to be some of the most rapidly invading tree species in the region, are still widely sold and planted for wildlife habitat or reclamation of strip mines. Oriental bittersweet has been shown to hybridize with American bittersweet, potentially leading to a loss of genetic identity.

Education may be the key to reducing the threat of invasive exotic species to our native forests. Many land managers and private homeowners alike are unaware of the dangers regarding exotics invasion, or are not concerned with the potential impacts that intentionally planting exotic species may have. It is important to not only educate but also to institute a program of invasive plant control.

Other forest health sections of the encyclopedia, including insect and disease stressors, have covered many nonnative threats.  This section focuses on exotic invasive plants. In this section, exotic plants are expanded to include important tree and shrub species, vine and grass species, and forb and subshrub species that invade the southern United States.  Legalities, economic and ecological effects, and pathways of exotic invasives are introduced.  Ideas relating to invasive species control and methods of rehabilitation are also discussed.