Savanna and shrubland fire behavior modeling in South Texas

Abstract

The vegetation of South Texas has changed from mesquite savanna to mixed mesquite/acacia (Prosopis/Acacia) shrubland over the last hundred years. Fire reduction due to lack of fine fuel and suppression of fires is cited as reasons for this vegetation shift. There is not much known about fire behavior in these new communities. Two current fire behavior models were evaluated (BEHAVE and the CSIRO fire calculator) as well as the proper scale for fire behavior models in the mixed mesquite/acacia shrublands. There were three fires with two plots each in the study area located within the Harris Ranch, 32 km west of Uvalde, Texas. The average temperature during the burns was 38⁰C with a relative humidity of 29% and a wind speed of 7 km/hr. The CSIRO model under-predicted the rate of fire spread and over-predicted flame lengths and the BEHAVE model under-predicted both rate of fire spread and flame lengths. Net heat and temperature were related to the amount of debris on the site, but were not related to the amount of grasses, forbs, shrubs, or continuity of cover. Fuel loads were typical of South Texas shrubland, in that they were uneven and inconsistent. The temperatures needed to kill mesquite were reached only in isolated locations. Single summer fires, even under extreme conditions, cannot reverse the shrub encroachment in South Texas, though recurring summer fires may have more of an effect. A prescribed burning program with burns of various intensity and seasons would maintain species richness and diversity by creating a mixture of successional patches throughout the area.