Interspecific and Intraspecific Microhabitat Temperature Variations during a Fire

A savanna codominated by the deciduous oak Quercus stellata and the evergreen oak Q. fusiformis in the Edwards Plateau region of Texas was burned to document variations in fire temperature beneath the canopies and in the grasslands surrounding these trees. Higher temperatures were recorded beneath canopies of Q. stellata than beneath those of Q fusiformis. Beneath the canopies of Q stellata, a rapid increase in temperature occurred as the fire burned from canopy edge to midcanopy, whereas beneath the canopies of Q. fusiformis there was a gradual decline in temperature along this gradient. Microhabitat variations in fire-generated temperatures were correlated with interand intraspecific differences in fuel load and fuel moisture content. INTRODUCTION High temperatures associated with fires have both direct and indirect effects on vegetation which may be negative or positive. A negative direct effect is heat-caused mortality (the thermal death point of most plant tissue is 60 C (Hare, 1961; Kayll, 1966); positive direct effects include stimulation of seed germination and opening of serotinous cones (Stone and Juhren, 1951; Hadley, 1961; Lotan, 1974). Negative indirect effects, which usually occur at high temperatures, include soil sterilization (Ahlgren and Ahlgren, 1965), mineral volatilization (White et al., 1973; Klemmedson, 1976; Tiedemann and Anderson, 1980) and reduction in soil moisture (Viro, 1974; Christensen and Muller, 1975); positive indirect effects, which usually occur at low temperatures, include increased soil nutrients through ash fall (Christensen and Muller, 1975; Christensen, 1977) and increased microbial activity (Jorgensen and Wells, 1971; Christensen and Muller, 1975). When a fire burns through an area, some microhabitats are subjected to higher temperatures than others. Since these temperature differences may be partly responsible for differential vegetational responses among microhabitats, it is desirable that they be recorded not only to achieve a fuller understanding of within-habitat vegetational response to fire, but also to determine what environmental factors cause these variations in microhabitat temperatures so that predictive models of fire-generated temperatures may be formulated. Few studies have systematically documented microhabitat variations in temperature or the factors responsible for these temperature variations. Most studies present fire-generated temperature data from only a few randomly selected locations within a habitat (such as a grassland) or habitats within an area (grassland or adjacent shrub habitat). Here we document temperature variations among microhabitats during a fire in an oak savanna and evaluate the importance of differences in fuel load and moisture content in the microhabitats which may cause these variations. MATERIALS AND METHODS This investigation was conducted in a 36-ha pasture on the Kerr Wildlife Management Area, Kerr Co., Texas. The vegetation in this area is typical of the Edwards Plateau region of central Texas, consisting of savannas dominated primarily by the evergreen oak Quercus fusiformis (Small) and the deciduous oak Q stellata (Wang). (Nomenclature follows Correll and johnston, 1970.) In a 1-ha plot within this pasture, four Quercusfusiformis and four Q stellata were selected from a pool of trees which were, with few exceptions, encircled by grassland