Plants

Millions of acres of forest land in the Southeast are being increasingly occupied by nonnative invasive plants. Their range, infestations, and damage are continually expanding. All Federal parks and forest lands in the Southeast have nonnative infestations (Hamel and Shade 1985, Hester 1991). The actual infested acreage, spread rates, and damage estimates are still unknown, although this information is essential for planning containment and eradication strategies and programs (U.S. Congress Office of Technology Assessment 1993). The Forest Service and State partners have initiated a cooperative survey of 42 invasive nonnative plants within the region and another 20 species in Florida; however, it will take several years to collect initial data (for a list, see “Nonnative Invasive Plants of Southern Forests”.

Invasive plants are able to outcompete native species. They reproduce rapidly because of the absence of predators from their native ecosystems, and eventually form dense infestations that exclude most other plants, except certain other nonnatives (Randall and Marinelli 1996). Other reasons for their invasiveness are that they are naturally robust plants or have been made so through plant breeding efforts; that most are perennials with tough roots or rhizomes; that many are still being sold as ornamentals and some are widely planted for wildlife use and soil stabilization; that most produce abundant seeds or spores that are spread widely by birds, wind, and water; and that their seeds or tubers persist in the soil (Randall and Marinelli 1996). It remains unclear what percentage of nonnative plants arriving in the Southeastern United States become invasive. One problem in determining this is the nature of invasive plant spread, which can be characterized by a short-to-lengthy lag phase preceding an exponential spread phase. In many species, e.g., kudzu, tallowtree, wisterias, etc., the lag phase can be very protracted and can mask eventual problems. This spread function also explains why eradication is most possible during the early lag phase.

Occupation and infestations by nonnative pest plants decrease forest productivity, threaten forest health and sustainability, and limit biodiversity and wildlife habitat in the Southeast (Wear and Greis 2002). Alterations to ecosystem structure, functions, and processes are occurring, but study of these effects has just begun (Ehrenfeld and others 2001). Some invasives, such as cogongrass [Imperata cylindrica (L.) Beauv.], can alter natural fire regimes and increase risk of wildfire occurrence and damage (Lippincott 2000). Nonnative plant “biological pollution” is one of the greatest threats to biodiversity across the southern landscape, attacking our highly valued nature preserves and recreational lands. Adjoining croplands, home sites, pastures, and wetlands contain invasive plant species that will eventually affect forests. These nonnative invaders (often called nonnative, nonindigenous, alien, or noxious weeds) include trees, shrubs, vines, grasses, and forbs. In all there are about 70 infestation-forming, terrestrial plant species invading forests and their edges in the temperate parts of the Southeast. Twenty-six of these are discussed here to provide a general sense of identifying characteristics, common pathways of introduction, mechanisms of invasiveness, ecosystem effects, and range of current occupation. Not discussed here are the approximately 70 tropical and subtropical nonnative species currently invading south Florida.