Topographical Considerations in Prescribed Burning

In the flat terrain of the Coastal Plain, topography is a relatively minor issue in conducting prescribed burns. However, topographical features become increasingly more important in the hilly or mountainous areas of the Piedmont, Ozark/Ouachita Highlands, and Southern Appalachians. In these areas, fire behavior is strongly influenced by two topography-related variables: aspect and slope. Aspect and slope are important determinants of fire behavior due to their relationship with soil moisture and relative humidity. Due to differences in the duration and time of day sunlight is received, the most xeric sites are found on southwest slopes and the most mesic sites on northeast slopes. On all aspects, sites tend to become more xeric moving upslope.

Slope and aspect also influence another important factor in determining fire behavior: vegetation. On most lower-slope positions and throughout north- and east-facing slopes, forest cover generally consists of hardwood species whose fuels have relatively low combustibility. Combined with higher fuel moisture and relative humidity levels, these slopes burn less frequently and with less intensity than upper-slope positions. In contrast, south- to west-facing slopes have open canopies and generally fire-tolerant vegetation. Combined with dry fuels and low relative humidity, these slopes burn more frequently and with greater intensity. There is also a change in fuel-types with elevation, from pure hardwoods on foot-slopes to mixed pine-hardwoods on mid-slopes to pure yellow pine stands on upper slope positions.

Slope steepness affects the rate of spread of a fire by influencing flame length and preheating of fuels. Flames more easily detach from a shallow slope than a steep slope. As slopes increase, radiant heating can reach more fuels and both ambient and convectional winds may force the convection column to move along the slope adding convectional heat to the preheating of fuel upslope from the fire. More rapid heating of the upslope fuels increases flame lengths and rate of spread (Rothermel 1985). See also: Effects of Topography on Rate of Spread.

Topography can also affect fire behavior indirectly by influencing surface winds, such as in the formation of slope and valley winds. See also: Effects of Mountain Topography on Surface Winds and Slope and Valley Winds.