Soil Changes Associated with Forest Fires in the Longleaf Pine Region of the South

The longleaf pine region is one of the most extensive and important forest regions in the United States. It extends in an uninterrupted belt from North Carolina to Texas. Within this region, during past generations, lt has been customary for the inhabitants to intentionally spread fire through the woods during the winter months. Fires may largely be attributed to cattle owners, who attempt to improve the native range, and to naval store operators, who burn the woods at such a time that weather factors prevent the fire from burning fiercely; and, as the ground cover and undergrowth have been consumed, the woods are, therefore, largely safeguarded from a disastrous accidental fire until the following year. The effect of these annual fires on the soil is unknown. An understanding of the influence of such fires on the soil is necessary in the general problem of forest management in the southern pine region, particularly as lt is now believed by many that controlled fires have a legitimate use In forest management. The greater percentage of present day forests of longleaf pine are open grown and have little or no tall undergrowth. They hava instead & luxuriant ground cover of grasses and other herbaceous plants. Dry sandy soils have a much less dense ground cover than heavier textured soils. The absence of undergrowth may definitely be attributed to the occurrence of fires in past years. On areas protected from fires for ten years or longer numerous species of hardwoods occur as an under-story. Under closed stands on unburned areas herbaceous plants are usually totally excluded by the shading effect of the over-story and the smothering effect of pine litter. A forest floor from 2 to 3 inches in thickness, consisting of litter and Flayer, is present. In openings in the unburned stand, dense, pure stands of native grasses occur. The most common of these grasses are various species of Ariatida and Andropogon. locally called wire grass and broom straw. These plants frequently attain a length of two to three feet and the wire grass particularly tends to bend over and form a tangled, interwoven mass which forms an excellent mulch over the mineral soil surface. An-' nually burned areas, on the other hand, are characterized by an absence of underbrush and by the absence of a dense, tangled grass cover. Many grass plants are present, however, but because they are rarely more than one or two years old, due to frequently occurring fires, they are shorter in height and more erect than the grass vegetation of unburned areas. The ground cover consists, in addition to various grasses, of a wide variety of broad-leafed herbaceous plants, including many members of the Leguminoseae. After each winter fire the soil Is directly exposed to the Intense rains and insolation characteristic of the region. By early summer, however, the rapidly growing ground cover usually affords protection against the elements. These differences in ground cover are of the utmost importance in understanding differences in soil properties which will now be discussed. The areas studied in this work were widely spaced over the longleaf pine region from Georgia to Louisiana. On these study areas it was found that the soil underlying the heavy layer of organic material on unburned areas was strikingly different in certain physical properties from the soil of the burned areas. The unburned soil, to a depth of 3 to 4 inches, was riddled with holes and tunnels of earthworms and small burrowing animals. The soil was extremely permeable and could be scooped up with the fingers to a depth of 3 to 4 inches. In contrast, the soil of the burned area, which lacked a protective mulch of pine litter or matted grasses, was very compact and practically void of animal holes and tunnels. Penetrability measurements of the surface soil on six pairs of plots showed that the unburned soil was 69 percent more penetrable than the burned soil. Likewise, volume weight determinations for eight paired plots showed that, for the O to 3 inch soil depth, the unburned soil was 12 percent lighter per unit volume than the burned soil. Both of the above differences, when compared with their errors, were found to be highly significant. In these and all subsequent comparisons the burned soil was taken as 100 percent, and differences between mean values were tested for statistical significance. Moisture retention as an indication of organic matter was studied for seven paired plots by determining water-holding capacity, wilting percent, and hygroscopic coefficient of sifted soils. No difference was found in waterholding capacity of soils from burned and unburned areas. Soils classed as sands were found to have a higher wilting percent and a higher hygroscopic coefficient