Scorched shrubs sprout anew, researchers find in controlled fire study

UNIVERSITY PARK, Pa. — Invasive woody plants present a major ecological challenge in eastern U.S. forests by outcompeting native species and spreading quickly, forming dense thickets that crowd out native plants and disrupt ecosystems. Land managers have tried, with some success, to use prescribed fire to kill them, but is it effective? Researchers at Penn State reported mixed results when it comes to two of the most aggressive colonizers across North American landscapes, burning bush and border privet.

The researchers, who applied three different experimental burning regimens to the invasive plants, recently published their findings in Ecosphere.

Instead of setting forest fires, the researchers simulated fire using a propane torch and tested different ways of burning the plants. They applied basal heating — heating the base of the stem — for five, 20 and 35 seconds; crown heating — burning the leaves and upper parts; and a combination of both.

They found that crown heating had no effect, as the plants survived. Basal heating, however, was more effective, especially with longer exposure. The 35-second burns worked best for killing plants. Plant size made a difference, the researchers said.

Those with thicker stems were harder to kill, according to study first author Arun Regmi, postdoctoral scholar in the Department of Ecosystem Science and Management, College of Agricultural Sciences. But when the researchers burned both the base and crown of the invasive shrubs, they saw a response they didn’t expect.

“When we combined burning the leaves and the base, we found that the plants were less likely to die — instead, they resprouted and grew back,” said Regmi. “As far as we can tell that result has never been documented before. It’s counterintuitive and may explain why prescribed fire often has not been successful eliminating invasive shrubs.”

The effects of burning often didn’t show up immediately in the researchers’ experiments. Some plants didn’t die right away but died years later. The study spanned parts of two years, but the researchers were able to observe treated plants years after experiments concluded. Border privet was more sensitive to initial heat but demonstrated stronger post-fire resilience through vigorous resprouting, Regmi noted. Burning bush tolerated fire better initially but was more susceptible to delayed mortality

“A better understanding of the physiological mechanisms underlying the response of shrubs to varying heat damage would enable fire managers to more effectively utilize prescribed fire,” he said. “Follow-up monitoring, however, is essential, as delayed mortality can occur.”

Burning bush and border privet species were brought from East Asia to be planted as urban and suburban ornamentals almost two centuries ago and now are spreading aggressively in North American forests. Different methods have been tried to eliminate them, Regmi explained, but mechanical removal is expensive and herbicides are toxic in the environment and unacceptable in sensitive areas. Prescribed fire is seen as a potentially effective tool.

The experiment was conducted in a mature, 40-acre mixed-hardwood stand within the Arboretum at Penn State in State College. The site, locally known as Hartley Wood, has long been subject to invasion pressure from nearby ornamental plantings, with historical records indicating the introduction of invasive species since at least the early 20th century. By around 2010, the forest understory was heavily invaded. These conditions provided an ideal setting for testing the effects of experimental fire treatments on invasive shrub species under varying heating exposure, according to Regmi.

“Fire can help control these invasive shrubs, but it needs to be intense and sustained at the base — quick or light burns won’t work,” he said. “Burning the whole plant — leaves and stem — might actually help it recover. Managers need to monitor for years, not just immediately after treatment. Plants have complex survival responses to damage, and effective invasive species control requires understanding those biological responses."

The findings reveal that burning could be a plausible non-chemical method for controlling invasive shrubs, said study senior author Jesse Kreye, associate research professor of fire and natural resources management. However, he added, fire managers may be more successful in reducing shrub persistence by employing a low-intensity prescribed burn technique with longer ground-level heating and higher controllability called backing fire, where fire is intentionally set to move slowly against the wind or downhill.

This study was supported by the McIntire-Stennis Cooperative Forestry Research Program in the U.S. Department of Agriculture’s National Institute of Food and Agriculture.