Invasive Species in Louisiana Forests

Linda Benedict  |  6/28/2006 9:24:58 PM

Hallie Dozier

Over the past 300 years, tens of thousands of animal and plant species have been introduced in the United States. A small number have proved invasive. That is, they cause harm to natural and managed landscapes, and they are difficult to eradicate. Several species make their homes in Louisiana’s forests. Three plant species, two animal species and an insect are of particular concern because of the extensive impact they have on the integrity of our forests.

Chinese tallow tree  
The Chinese tallow tree is a small, short-lived tree well-known in Louisiana. Also called popcorn tree, chicken tree, Florida aspen and – locally in Washington Parish – soapberry tree, this Asian native was originally brought to North America in the 1700s as an alternate source of oil for lighting and soap making. Its desirability as a home garden plant – brilliant fall color, fast growth and small size – and its attractiveness to migrating birds as a food source have contributed to its spread. Today, the tallow is distributed across the southeastern states. In Louisiana, it grows in every parish, though it is particularly troublesome in bottomlands, old fields, coastal prairies and riparian areas. Until fairly recently, Chinese tallow was not considered a threat to forested lands because it grows poorly in the shade. Its appearance in the understory of closed canopy forests and undisturbed sites, however, raises concern because of its potential to dominate gaps created by management operations and to prevent regeneration of desirable species. Forest managers say tallow is the most important invasive plant in Louisiana forests.

Chinese privet
This plant was introduced to North America in the 1800s to replace common privet in landscapes. It is one of the worst forest invaders in the Southeast because of its ability to dominate the understory, midstory and edges of forests and to impede regeneration of desirable plants, including canopy trees. Its extensive use in landscaping creates a seed reservoir for continuous introduction to forests, and it is particularly troublesome along expanding wildland urban interface areas such as the growing communities along the northern shores of Lakes Pontchartrain and Maurepas and in St. Landry Parish.

The characteristics that make Chinese privet and other ornamentals-turned-invaders valuable landscape plants also contribute to their invasiveness. They are easy to grow under a variety of soil and light conditions. They have few pests, and they are hard to kill.

Cogon grass
Chinese privet and tallow tree are practically ubiquitous across Louisiana, but cogon grass is a relative newcomer and possibly the greatest emerging threat to forests, especially in the southern half of Louisiana. A rhizomatous grass native to Southeast Asia, cogon grass first entered the United States in the early 1900s as packing material, though it was later reintroduced at several sites in Florida as potential forage. Cogon grass is a cosmopolitan invader, occurring in agricultural areas, coastal lands, mining areas, forests, watersheds and shrub lands on almost every continent in the world. This aggressive invader forms dense colonies that displace other plant species and prevent their re-establishment. It can alter the pyrogenic characteristics of forested grasslands, such as longleaf pine savannas, increasing flame lengths and temperatures to the point of killing even fire-adapted species. It tolerates a wide range of soil types and soil and fertility gradients. Cogon grass reproduces sexually through flower and seed production and asexually through rhizome growth.

Cogon grass has made its appearance in the southeastern parishes of Louisiana, possibly entering our state through rail, road and pipeline rights-of-way. Rhizomes break away from the main plant easily and can stick in dirt and mud on the wheels of vehicles, enabling it to hitchhike from site to site. Recent hurricane clean-up activities have presented a unique opportunity for this species to spread farther into Louisiana. Thus far, infestations are restricted, but cogon grass’s history elsewhere almost guarantees it will become a major problem if not contained early.

All of these species require an integrated management approach for control that includes mechanical (cutting, mowing), cultural (burning) and chemical (herbicide applications) manipulation plus revegetation. Use of indiscriminate, broad spectrum herbicides may not produce desirable results because of non-target mortality. Research exploring the use of more selective chemicals shows promise for controlling these invaders, but more research on how populations respond to management is needed.

Nutria, wild hogs
Two exotic mammals, nutria and wild hogs are particularly destructive to plant life in Louisiana’s forests. Nutria are voracious herbivores whose populations have an enormous negative impact on wetland vegetation. In Louisiana in less than a century, these semi-aquatic rodents have managed to wreak considerable damage to coastal areas, including marshlands and regenerating coastal forests. Less pressure from trappers and alligators in the past several decades may have exacerbated their population growth.

Wild hogs also damage trees and reduce regeneration. Unfortunately, their populations are greatest in forested areas with dense understories. Wild hogs root for their food, destroying understory plants, including canopy seedlings, and disturbing the soil. They compete with other wildlife for food, and they are a reservoir for several serious diseases, including pseudorabies, which is fatal to some cat species, and swine brucellosis, which can be fatal in people.

Formosan termites
Exotic insects also play a key role in forest health and integrity. A potential concern in Louisiana forests is the Formosan subterranean termite. About 11 percent of urban trees in the greater New Orleans area and in Lake Charles are infested, and informal estimates in some forests are similar. Cypress, oaks and pines all are vulnerable to these termies. The ease with which people move this insect from site to site is alarming, and once populations are established, they are almost impossible to eradicate. Though not able to disperse long distances naturally, the Formosan termite can be transported in contaminated wood products such as building materials and equipment, railroad ties and utility poles.

Controlling invasive species
The LSU AgCenter has been an active member of groups across the state addressing the problem of invasive species. Research and extension faculty develop educational materials to inform the public about these invasive species and continue to study invasive organisms. Projects include exploring the use of more selective chemicals that show promise for controlling privet and many aquatic species and investigating methods for better assessing the impacts of Chinese privet on forest composition and regeneration.

Faculty also have been active on the Louisiana Aquatic Invasive Species Task Force, which includes 29 appointed members representing both public and government entities. In 2005, the task force published the State Management Plan for Aquatic Invasive Species in Louisiana along with the Louisiana Department of Wildlife and Fisheries, the Louisiana Department of Wildlife and Fisheries, the Louisiana Sea Grant, and the Center for Bioenvironmental Research at Tulane and Xavier Universities. The complete plan is available at http://is.cbr.tulane.edu/docs_IS/Louisiana-AIS-Mgt-Plan.pdf.
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