Singh et al., 2017




One low- to moderate-intensity prescribed fire, which left the duff layer intact, did not significantly increase soil erosion.

Rates of soil loss relate to how quickly groundcover is re-established post-burn, precipitation levels, and whether or not rainfall is intercepted by leaves in tree crowns.

In this study, researchers sought to quantify watershed-scale sediment movement related to prescribed fire in south central Illinois. The study took place in the 2000-hectare Trail of Tears State Forest, where mixed hardwood forests grow on steep, loess-capped slopes, with high erosion potential.

Oak and hickory trees historically dominated forest overstories here, but shade-tolerant, mesophytic species (e.g.,sugar maple, American beech, black gum) have increased in dominance during the past century due to fire suppression and reduced timber harvesting. On-site soils are classified in the Menfro-Clarksville complex, which consists of silt-loam soils, some of which are fine, loamy, siliceous, and/or gravelly.

For this experiment, researchers delineated and paired 10 small drainages ranging from .06 to .12 hectares in size. Pairs had similar sizes, slopes, aspects, species composition, litter depths, and soils. Within each pair, one drainage was burned and the other was not. One of the pairs was found to be unsuitable for study inclusion due to karst topography influence, leaving four for analyses.

Data were collected between September 21, 2009, and September 16, 2010, and the prescribed burn occurred on November 6, 2009.

Forest species composition was described through plot measurements. Pre-burn data included tree basal area and numbers of trees, saplings, and seedlings. Throughout the study period, researchers collected data on air temperatures and rainfall amounts. Litter depth and soil moisture were measured prior to and following the burn. Temperature sensors were used to assess soil surface temperature, fire severity, and fire intensity.

To determine the quantity of sediment carried in rainwater, researchers installed a large polyethylene collection tank just below the outlet of each drainage. A sheet metal flume with high sides was inserted to divert runoff into the collection tank.

After each rain event with measurable runoff, tank water was analyzed for total suspended solids and sediments.  To gauge soil erosion or accumulation, multiple rebar erosion pins were installed within each drainage and were measured one year after the burn. Total precipitation during the study period was about 20 percent higher than the average annual precipitation; 31 of 85 rainfall events produced measurable runoff. One especially heavy rainfall was not included in the data because all collection tanks overflowed. That event occurred in October, prior to the burn, the single month with the most rainfall. The least rain fell during August 2010. Taken together, the period including March, April and May (2010) received the most precipitation.

Photos: Erosion pin installed flush with the soil surface (a). Installation of galvanized metal flume (b) and completed set up of flume and tank design (c).

Prior to the fire, sediment losses after rainstorms were similar between control areas and burned areas. After the fire, sediment concentrations increased in burned areas. However, the difference between burned and control areas was not statistically significant. The projected annual overall soil loss in burned units ranged from 1.41 to 90.54 kg/ha/year, which is considered minimal.

Looking at data from all drainage areas burned and unburned – sediment loads were lower before the burn than afterward. Authors suggest that the postburn increase may be influenced by seasonal factors not directly related to the burn itself. For example, higher rainfall in spring likely contributes to increased runoff and higher rates of soil movement, particularly because dormant vegetation lacks leaves to consume rainwater or slow surface flow.

Variables that had no statistically significant influence on suspended solid loads or sediment loads in post-burn storm runoff included tree species composition, tree size, slope, aspect, and litter depth.

Authors said even the highest rate of soil erosion associated with prescribed fire measured in this study was minor compared to soil losses associated with urbanization, row cropping, and other agricultural practices, as documented in other studies. The authors concluded that very little soil is lost as a result of low intensity prescribed burning, provided that soil humus (duff layer) retains high moisture levels during the burning process, so that organic components are not consumed and thus rainfall can be absorbed instead of running off. Further, soil losses following a prescribed burn will be lower if groundcover is reestablished quickly, and if a leaf-on tree canopy is present to intercept rainfall.

In conclusion, prescribed fire did not significantly increase sediment loads or suspended solid loads in surface runoff in the mixed hardwood forests of southern Illinois, despite steep slopes and highly erodible loess soils. Assuming average annual precipitation and typical regional topography, regional land and fire managers who follow standard prescribed fire practices for mixed hardwood forests need not worry about serious soil loss or erosion risk.

Comparisons of mean ±95% confidence interval (CI) pre-burn and post-burn (a) total suspended solids concentration; (b) sediments concentration; (c) total suspended solids loads; and (d) sediment loads data by treatments combined over watersheds. Significant differences (p values <0.05) are noted by different letters at the top of each bar.

Gurbir Singh, Jon E. Schoonover, Kyle S. Monroe, Karl W. J. Williard, and Charles M. Ruffner
Forests 2017, 8(4), 112

The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, land owners and the public about the use, application, and effects of fire in the region. www.oakfirescience.com

This research brief was funded by The Joint Fire Science Program. www.firescience.gov