Native Plants Come Back When Invasives are Removed

By Marcus Schneck, Pennlive

When invasive shrubs are removed from the forest, native plants can rebound more strongly than expected, according to research conducted at Penn State University.

"We believe that's because invasive shrubs take up residence in the best spots in the forest," said researcher Erynn Maynard-Bean, who recently earned her doctoral degree in ecology. "They are most successful where there are the most resources — sunlight, soil nutrients and water." Fortunately, reports Maynard-Bean, "When invasive shrubs are removed, the growth of native plants in those locations beats expectations."

Invasives in forest understory

Invasive privet, honeysuckle, and barberry shrubs choke the understory of many woodlands in the East. Repeatedly removing those invasives can lead to a vigorous regrowth of native shrubs, boosting plant diversity and helping wildlife, report Penn State researchers./Penn State Arboretum

Maynard-Bean arrived at that conclusion through a long-term project that spanned seven years at Penn State’s arboretum. In an experiment initiated by associate professor of forest ecology Margot Kaye, the researchers repeatedly removed 18 species of invasive shrubs and then monitored the response by native plants.

With the removal of the invasives, plant diversity, native understory species abundance and regeneration of overstory tree species all increased.

The study was done in a 42-acre woodlot known as Hartley Wood. Oak, hickory and maple trees in the woodlot, including a gigantic white oak that died in 2000 and was determined to have germinated around 1673, had escaped logging operations that cleared most of the area.

But proximity to landscaped residential properties had led to a profusion of exotic plants establishing themselves in the woods.

"We found that seven years of invasive shrub removal boosted natural regeneration of native plants that exceeded the abundance measured in unmanaged forest understories with low levels of shrub invasion," Maynard-Bean said.

The research highlights the native forest plant community's ability to respond to invasive removal, pointed out Kaye, whose research group has been studying the impacts of woody shrub invasion on eastern forest dynamics for more than two decades.

The research is especially relevant because eastern deciduous forests are becoming more fragmented as urban and suburban areas extend into forests. Associated edges and open spaces have allowed invasive shrubs to make inroads.

As invasive shrubs have increased at the expense of native species, interest in invasive shrub removal to restore native habitats is growing. But forest managers are not sure about how much natural regeneration of native plants they can expect.

"A lot of people think that when you remove invasive shrubs you have to plant natives, and that is obviously helpful but difficult to afford on a large scale," Maynard-Bean said. "But there are native plants in the forest that are mixed with the invasives, and if you maintain the removal [effort], the natives will come back in and take over."