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New Study Suggests Frequent Burning May Hinder, Not Help, Future Oaks

Two men light and monitor fire near the edge of the woods.
A long-accepted practice in ecological restoration, controlled burning is increasingly used to control invasive plants.
March 1, 2018

One of the most common management tools used in restoring oak woodlands may have negative consequences when conducted annually, according to a new study from researchers at The Morton Arboretum in Lisle, Illinois.

A long-accepted practice in ecological restoration, controlled burning is increasingly used to control invasive plants such as buckthorn and promote the growth of native plants in natural areas throughout Illinois. But while the frequency and scale of burning in Illinois is projected to increase, the study found that annual burning within oak woodlands is altering soils in unexpected ways that could be detrimental to oak regeneration.

The research, published today in the journal Forest Ecology and Management, examined the impacts of annual, high-frequency, low-intensity controlled burning on soil carbon, nitrogen, and phosphorus dynamics in an oak-dominated forest in northern Illinois over a 30-year period. Researchers found that burning increased the amount of nitrogen in the soil, but as oaks in eastern U.S. forests have adapted to low levels of nitrogen, this increase may deter oak seedling growth relative to other trees, like maples. Additionally, increased levels of nitrogen in the soil have previously been shown to increase ecosystem invasion by non-native plants, such as buckthorn, one of the most common invasive tree species in northern Illinois.

The findings don’t indicate that land managers should stop using controlled burning as a restoration tactic; however they do suggest that there are trade-offs for native plants and that more research is needed to understand them, according to study co-author Meghan Midgley, PhD, soil ecologist at The Morton Arboretum.

“As fire has significant impacts on soil carbon and nutrients, continuing to evaluate the impacts of repeated, low-intensity prescribed burning on oak forest soil will be critical to understanding and predicting burning effects on vegetation dynamics and ecosystem functions in the future,” she says.

The site of the research, The Morton Arboretum, has been practicing prescribed burning annually for 30 years, which made the study unique.

“Annual burning of a woodland for three decades is highly unusual, but gave us a rich data set for this experiment,” adds Midgley. “We have the opportunity to conduct research in our living laboratory that will help inform not only Arboretum practices going forward, but can help other organizations and agencies with their controlled burning methods.”

Prescribed Burning at The Morton Arboretum
Prescribed burning began in The Morton Arboretum’s Schulenberg Prairie in the 1960s and has been a regular practice in its natural areas, including woodlands, since the 1980s. Decades of research, at the Arboretum and elsewhere, have conclusively demonstrated how important fire is to native ecosystems and have improved our understanding of its benefits. Read more about prescribed burns at The Morton Arboretum.