Effects of fire regime on physical soil properties
By Dr. Kevin Roberston, Associate Scientist, Fire Ecology
The effects of fire regime on soil structure are not very well known, especially in southern pine forests. To learn more, we used the Tall Timbers Fire Ecology Plots (Stoddard Plots), a set of half-acre (0.2 ha) research plots in old-field pinelands that have been burned at 1, 2, and 3 year intervals, or else unburned, since 1959. We measured soil bulk density, soil moisture, soil texture, and soil strength (resistance to penetration) to 20 cm depth, and water infiltration rate at the surface, within each fire interval. We found soil bulk density (see figure) and soil strength decreased with increasing fire return interval, with the lowest values in the unburned plots. Conversely, soil moisture and infiltration rate increased with fire return interval. There was a slightly higher clay and lower sand content near the soil surface in the more frequently burned plots.
To our knowledge, the areas in which the plots are located were farmed, in some places as late as the 1930s, and subsequently annually burned, until the establishment of the plots in 1959. Very frequent burning following agricultural abandonment may have slowed the rate of redevelopment of an active soil organism community and associated loosening of the soil. In contrast, the soils on the Wade Tract, a native pine forest that was never plowed, have very low bulk density and high infiltration rates (Levy 2007), despite being burned annually for decades.
Bulk density of soil at three depth increments in plots burned at different fire return intervals or unburned since 1959, average and standard error of three plot replicates.