Hosted by the Oak Woodlands & Forests Fire Science Consortium
Presented by John Kabrick, USFS Northern Research Station
Prescribed fire is commonly applied to meet a variety of forest management objectives, including the restoration and maintenance of Oak (Quercus spp.) woodlands. In the Ozark Highlands, private landowners, conservation organizations, and government agencies are increasingly applying fire to restore woodland sites after a period of fire suppression. Even though fire effects on vegetation, fuels loading, and wildlife habitats are often studied in the Ozark Highlands, few studies have measured fire effects on soil physical and chemical properties in this region. Since it is important to understand how prescribed burning affects short- and long-term forest soil productivity, we initiated this study to quantify prescribed fire effects on organic soil horizon measurements, physical and chemical properties of soil mineral horizons, and soil solution nutrient flux, and to resolve the time required for soil properties to return to pre-burn soil conditions. Study sites are located in the Oak-Pine Woodland/Forest Hills Land Type Association in the Black River Basin of southern Missouri. Weathered from Roubidoux sandstone and Gasconade dolomite, soils at the study sites contain large quantities of coarse fragments and reduced nutrient content.
In 2015, fire was applied to sites that had no documented occurrence of fire for at least 40 years prior and sites that had been burned twice since 2002. Sampling efforts were focused upon stands on exposed hillslopes. The percent cover, thickness, and dry weight of the Oi, Oe, and Oa horizons were measured prior to and immediately following fire, and annually for two years post-burn. Prescribed fire completely consumed the Oi horizon and partially reduced the Oe horizon, but both were recovered two years later. Mineral soil bulk density was measured at the 0-10, 10-20, and 20-30 cm depths before fire, and bulk density at the 0-10 cm depth was re-sampled two years post-burn. There was no change in bulk density of the whole soil and the fine soil fraction. Mineral soil samples were collected prior to fire treatment, immediately post-burn, and at six-month intervals for two post-burn years at depths of 0-10, 10-20, and 20-30 cm. Compared to pre-burn values, there was a significant increase in the water-stable microaggregate size fraction within stands burned for the first time in recent history. Fire did not significantly alter total mineral soil organic carbon stocks, the labile carbon pool, or the pyrogenic carbon pool for any sampling depth. There was also no fire effect on total nitrogen, soil pH, effective cation exchange capacity, base saturation, aluminum saturation, or exchangeable base cation concentrations. The soil solution was continuously monitored for one year pre-burn and two years post-burn at 10 cm and 30 cm depths using Plant Root Simulator (PRSTM) ion-exchange probes. Ammonium-nitrogen availability was significantly greater over the two post-burn growing seasons at the 10 cm depth within periodically burned stands. However, during the second post-burn growing season, phosphate-phosphorus availability at the 30 cm depth was significantly less within periodically burned stands relative to stands burned for the first time. Gravimetric soil moisture content monitored monthly at the 0-10 cm depth was not different between treatments for during the duration of the study.
The 2015 prescribed fires were generally within prescription designed for low-intensity, low-severity burning. Soils were moist at the time of fire and soil temperatures minimally increased. Study results indicate that when fuels and soils are moist at the time of fire, there is minimal immediate impact on organic and mineral soil properties. Overall, burning for the first time after a period of fire suppression did not significantly alter mineral soil physical and chemical properties. The lack of differences among treatments before fire and the minimal change in mineral soil properties following fire suggests that when given time to recover between recurring burn events, periodic burning also does not adversely alter mineral soil properties. Study results are informative and can assist Ozark land managers in developing a prescribed fire regime that meets sustainable forestry objectives.