American Forests Are Being Overrun by Invasive Species


The study found that many native plants are being displaced by invasive species that have been brought to the United States during the last century.

A recent plant survey discovered that numerous invasive non-native species are thriving in Ohio.

Invasive species that have been introduced to the United States over the last century are displacing numerous native plants, according to a recent botanical assessment of southwest Ohio.

In order to determine how the Queen City’s plant diversity has altered over the last two centuries, biologists from the University of Cincinnati are retracing two extensive surveys that were carried out 100 years apart. They concentrated on sections of cemeteries, Mill Creek’s banks, and public parks that have been preserved from development for the last 200 years.

Denis Conover Plant Survey

A new plant survey of southwest Ohio found a rise in nonnative, invasive species such as English Ivy. Credit: Lisa Ventre/UC

The study was recently published in the journal Ecological Restoration.

Thomas G. Lea, a botanist from Cincinnati, did a plant survey in Cincinnati between 1834 and 1844, and the most recent study by UC continues his work. He gathered specimens for a herbarium during that period and donated them to the Philadelphia Academy of Natural Sciences. Before his death in 1844, Lea had categorized 714 different species. In 1849, his brother published a posthumous edition of his work.

A century later, renowned UC botanist E. Lucy Braun followed in Lea’s footsteps by performing a second plant survey in Cincinnati. Her 1934 research on her, which was published in The American Midland Naturalist, discovered more than 1,400 species. She relied on Lea’s precise notes from her to take her back to the locations he visited, many of which had been transformed over time into houses, streets, or apartment complexes.

In southwest Ohio, where urban expansion did not trample over natural areas, biologist Denis Conover of the University of California at Davis and his co-author Robert Bergstein followed in the footsteps of Braun and Lea. Numerous species that were intentionally planted as landscape plants were discovered to be thriving in the wild.

Denis Conover

UC biology professor Denis Conover holds porcelainberry, one of several nonnative and invasive species he found in a new plant survey in southwest Ohio. Credit: Lisa Ventre/UC

“The spread of nonnative invasive species into wooded natural areas in southwestern Ohio threatens the continued survival of native flora and fauna. Efforts by park managers and volunteers to control invasive plant species have become a major part of their duties. This effort will be required in perpetuity and will be at great expense both monetarily and timewise due to collateral damage to native plants, wildlife, and humans caused by the extensive use of herbicides, chainsaws, and other mechanical equipment,” the study concluded.

Horticulturists introduced most of the nonnative plants from Europe and Asia as ornamentals. Their seeds eventually spread in the wild.

The biggest culprit? Amur honeysuckle, a woody shrub that has taken over many eastern forests.

“It has escaped into the wild and is propagating on its own,” said Conover, a professor of biology at UC’s College of Arts and Sciences.

English Ivy

A new plant survey of southwest Ohio found a rise in nonnative, invasive species such as this English Ivy growing up a tree trunk. Credit:
Denis Conover

Not to be confused with native trumpet honeysuckle, which grows in southern states and is referenced in the works of American writers William Faulkner and Robert Frost, Amur honeysuckle is a shrub from Asia that has delicate white flowers in the spring and red berries in the fall .

“Amur honeysuckle is now the most abundant woody plant in Hamilton County,” he said. “One bush can produce thousands of seeds that get dispersed by birds and mammals.”

A survey by Braun in 1961 found Amur honeysuckle starting to grow in some parts of Hamilton County but not yet spreading in the wild in other Ohio counties. Today, it is a dominant woody plant found ubiquitously throughout the state, crowding out virtually all other low-lying vegetation, the study found.

“In some woodlands, the Amur honeysuckle layer is so dense that the only native species remaining are older trees whose canopy is already growing above the shrub layer,” the study said.

“It leafs out before native woody plants and holds its leaves longer into the fall,” Conover said.

Some invasive plants are successful because they produce chemicals that hinder the growth or germination of nearby competitors, an insidious weapon called allelopathy, he said.

Conover said where these introduced plants are found, there is often far less biodiversity to support wildlife and the food chain. Once they take hold, eradicating plants like Amur honeysuckle is labor-intensive, expensive, and time-consuming.

“Native plants just don’t have a chance. Everything that depends on the native plants — insects, birds — can be lost,” Conover said. “When they introduce nonnative plants to the United States, they can also import fungal diseases that can wipe out native trees, which is what happened with the American chestnut.”

Callery pear trees with their pretty spring flowers and quick growing times were a favorite tree to plant in front yards of new subdivisions. Today, they grow wild along highways and forests.

Ohio lawmakers plan to phase in a ban on the sale of Callery pear trees in 2023.

The UC survey found dozens of other examples of foreign species that have taken root in southwest Ohio’s woods, including porcelain berry, tree of heaven, winged euonymus, European buckthorn, Oriental bittersweet, common privet, and lesser periwinkle. It also found Norway maple, Amur cork tree, and white poplar along with herbaceous species such as lesser celandine, garlic mustard, Japanese knotweed, and Japanese stilt grass.

Reference: “The Rise of Non-Native Invasive Plants in Wooded Natural Areas in Southwestern Ohio” by Denis G. Conover and Robert D. Bergstein, June 2022, Ecological Restoration.
DOI: 10.3368/er.40.2.94

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