Fire Effects on Water and Soils In Loblolly Pine Forests

The layer of organic matter on the forest floor is an important component of the surface fuelbed. Thus, the depth and structure the soil O horizon influences fire behavior, and fire in turn inhibits the buildup of organic materials on the forest floor. Loblolly pine forests typically have “mor” type structures with a clear separation between organic material in the O horizon and mineral soil in the A horizon (Fisher and Binkley 2000). The O horizon can be further grouped into three states of decay from non-decomposed material (Oe), to still recognizable but partly decomposed (Oi), to unrecognizable humus (Oa).  Low-intensity prescribed fires often will consume the Oe and Oi layers but leave the more humid Oa layer intact (Brender and Copper 1968, Waldrop et al. 1987). Thus, older, well-developed O horizons are well suited to carry fire but retain a protective organic layer on the soil surface. In fact, such fires are often beneficial in releasing nutrients retained in the O horizon and accelerating the process of recycling nutrients back to the mineral soil.

Higher intensity fires that consume the entire O horizon will have greater impacts on the mineral soil below. If organic material in the mineral soil is also consumed, it can affect soil structure by increasing bulk density, reducing porosity, and reducing infiltration. However, these impacts are atypical of most prescribed fires. Instead, repeated low-intensity burning typically increases organic matter concentrations in the upper layers of mineral soil (McKee 1982). Regular burning also raises soil pH, leading to increases in base saturation and cation exchange capacity. These effects can result in higher availability of phosphorus (P), Calcium (Ca) and magnesium (Mg) in the surface soil layer. Nitrogen (N) availability may also increase with burning. However, volatilization and loss of N in gaseous form typically lead to a reduction in total soil N (Schoch and Binkley 1986, Bell and Binkley 1989). Following a fire, increases in N resulting from N fixation by understory herbs (Hendricks and Boring 1999) and decomposition of the forest floor (Schoch and Binkley 1986) can compensate for these losses. In situations where soil fertility is inherently low, or where high-intensity fires are combined with short-rotation timber harvesting over multiple rotations, frequent burning may ultimately lead to a loss in site productivity (Carter and Foster 2004).

As with any disturbance, fire has the potential to increase erosion. Removal of the O horizon exposes mineral soil to the direct impact of rain, potentially accelerating erosion on steeper slopes. Mortality of trees and other vegetation reduces the stabilizing effects of their root systems. These effects appear to be relatively minor, however, for low-intensity understory fires in Loblolly pine forests. In ephemeral Piedmont streams, watersheds subjected to low-intensity prescribed fire produced the same amount of sediment as undisturbed pine watersheds (Van Lear et al. 1985). Sites treated with a series of low intensity prescribed fires, rather than mechanical thinning, yielded lower levels of sediment load over the long term (Wade et al. 2000). Nutrient losses that occur during burning, other than as N gases, may impact nearby streams. However, leaching losses of nutrients to stream waters after low-intensity surface fires are generally small.  In the case of more intense fires that accelerate erosion, losses of nutrients bound to sediments may increase.

See also: Fire Effects on Soil and Fire Effects on Water.