Clemson study: Prescribed fire doesn’t hinder forests from producing clean water

A new study by Clemson University researchers lends further credence to the effectiveness of prescribed fire as a forest management tool that does not adversely impact ecosystem health by increasing sediment or nutrient runoff.

With the largest mountain wildfire in South Carolina history still fresh in the memories of many in the Upstate, that distinction is important because a primary reason for the use of prescribed fire is reducing the likelihood and severity of wildfires by reducing the amount of litter such as pine needles and fallen leaves available to — quite literally — add fuel to the fire.

In a review entitled “Prescribed fire effects on sediment and nutrient exports in forested environments,” Clemson researchers Kipling Klimas, Patrick Hiesl, Donald Hagan and Dara Park concluded that though sediment in runoff increases after prescribed fire in certain situations, these erosion events are associated with intense precipitation shortly after the fire and do not impair ecosystem function.

“These prescribed fires are beneficial not only to the ecology here, but we’ve been able to show that lower-severity burns, which are typical of prescribed fires, do not cause the same amount of soil loss as higher-severity burns characteristic of wildfires,” Klimas said.

Klimas cited the wildfire that was ignited by an escaped campfire in November 2016 at Table Rock State Park as an example of the destruction wildfires can cause and evidence for the benefits of prescribed burning. After a dry fall season and with an ongoing drought in the Upstate, the Pinnacle Mountain Fire grew to 10,623 acres — making it the largest mountain wildfire on record in South Carolina — by the time it was controlled after burning for more than a month, according to the S.C. Forestry Commission.

“We hope this study can raise awareness to anyone who’s living in some of these long-burned areas in the Upstate,” said Klimas, who completed the research as part of his master’s studies. “Because there were private landowners who were affected by these wildfires in 2016 — who lost land or their property was damaged — so hopefully being aware that this practice (of prescribed fire) won’t harm their property, won’t cause their ponds or streams to get clogged up with silt, will be beneficial. There’s still a lot of public skepticism.”

Klimas, a native of Houston, Texas, completed the project as part of his Clemson master’s work and graduated in May 2020. He is currently working on his Ph.D. at Utah State University, as he continues to study wildfire.

“My research at Clemson uniquely qualified me for my current position,” he said.

Forestlands occupy approximately two-thirds of South Carolina’s land area and its $21 billion forestry industry represents the state’s top manufacturing industry. But while prescribed fire has long been considered a powerful and effective management tool, few previous studies had sought to quantify the impact of fire effects on forest health and water quality across varying environments and scales.

“Research has shown that prescribed fire is an effective tool for accomplishing various forest management goals, such as fuel reduction, ecological restoration or maintenance of ecosystem services,” said Hiesl, an assistant professor of forested operations who served as Klimas’ graduate advisor during the research. “But because this technique is an anthropogenic, or manmade, disturbance that is used annually on millions of acres of forestland around the world, it is also important to understand the extent to which prescribed fire accomplishes intended management goals without compromising the ability of the forest to produce clean water.”

Thus, the study sought to analyze sediment and nutrient runoff data across varying samples and scales, from a small-scale rainfall simulator assembled by the researchers outside Lehotsky Hall, which houses Clemson’s Department of Forestry and Environmental Conservation, to large-scale watershed analysis.

The assessment of nutrient and sediment runoff yields is important because while forests are an important source of clean water globally, and watersheds are often managed to provide clean water to urban populations, a primary concern associated with fire is elevated surface runoff and erosion caused by rainfall events following the fire, due to their ability to transport pollutants including sediment, macronutrients or other organic compounds into water systems.

“After fire, erosion and soil loss and soil mobility is the greatest concern associated with fire or any sort of forestry harvesting operation,” Klimas said. “So, that’s why we’re looking specifically at sediment. When you get sediment into streams — because we have trout waters up here in the Upstate, we’ve got streams that feed into municipal water supplies — it can reduce the productivity of the water body, it can kill off aquatic organisms and have other detrimental effects to that ecosystem. The streams coming out of the Blue Ridge Escarpment right here in the Upstate, around Table Rock Mountain and Caesar’s Head, they are all really important for downstream water quality.”

Filling those knowledge gaps about sediment and nutrient export from forested watersheds after prescribed fire is not only necessary to quantify the impact of prescribed fire on water quality, but the study also showed that high fuel consumption along the forest floor — which is uncommon during prescribed fire but rather indicative of a wildfire that prescribed fire could have helped prevent — increases the risk of erosion and export during precipitation.

“With increasing litter consumption, so as the fire burns more of that fuel along the forest floor, any sort of rain event afterwards will produce greater runoff and erosion,” Klimas said. “So, we saw greater sediment in the runoff as the burn severity — or the amount of litter fuel consumed — increased across all forest types.”

The study analyzed nutrient and sediment yields across varying burn severities — from very low to moderate to high-severity burns — in order to analyze not only the benefit of prescribed fire on forest and watershed health, but also to analyze any potential drawbacks, as well. Along with using burn severity as a variable, the study also analyzed three types of forest: pine, oak and a mixed pine/hardwood forest.

Existing studies that considered the effects of prescribed fire on sediment yield in surface runoff have cited high burn severity or a significant reduction in understory vegetation as the factor responsible for increased erosion. High‐severity burns, which consume a significant amount of the protective litter layer, expose underlying forest soils to the full heat of the fire, which, at temperatures as low as 200 degrees Celsius, can actually impact soil structure such that it causes the development of a water‐repellent, hydrophobic soil layer.

“What that means is less water is penetrating the soil and is being carried over land as runoff, which is largely attributed to that litter,” Klimas said. “Because when you’ve got leaves, you’ve got pine needles, that slows down water and allows it to percolate down into the soil.”

The study suggests that although prescribed fire can significantly increase sediment yield and erosion, these increases are ecologically negligible in many events, particularly when compared with the erosion impact of other forest management practices. Forest managers typically target lower burn severity, as it has been shown to reduce surface runoff velocity and sediment volume inputs into water systems.

“The results of this study are encouraging, as they demonstrate that prescribed fire likely does not detrimentally impact soil and water quality in the region,” said Hagan, assistant professor of forest ecology. “So land managers — many of whom are interested in expanding their prescribed fire programs — can burn without fear of compromising ecosystem services and environmental quality.

The study also concluded that future research on water quality from forested environments after prescribed fire should be broadened to include better understanding of fire regime interactions at watershed and small‐plot spatial scales that determine burn severity and landscape‐scale documentation of post‐fire erosion events and subsurface nutrient pool response.

“Part of the rationale for the study is that prescribed fire is, obviously, a human-induced disturbance and we want to make sure that something we are applying to the landscape is going to be as beneficial as possible,” Klimas said.