1353 ERIC R. STENDELL 1 , THOMAS R. HORTON 2 *, THOMAS D. BRUNS Department of Plant and Microbial Biology, 111 Koshland Hall, University of California at Berkeley, Berkeley, CA 94720-3102, U.S.A.  Medical College of Wisconsin, Milwaukee, WI 53226, U.S.A.  Department of Forest Science, Oregon State University, Corvallis, OR 97331, U.S.A. The effects of a prescribed fire on the presence of ectomycorrhizal fungi on the root tips of ponderosa pine were investigated one year after a prescribed ground fire. Ectomycorrhizas were sampled within 1 m plots before and one year after the fire, and in nearby control plots that were not burned. The cores were divided into litterorganic, upper mineral, and lower mineral layers. The total ectomycorrhizal biomass in the control plots did not differ between year one and year two samples for any core layer, while in the fire plots the destruction of the litterorganic layer resulted in an eight-fold reduction in total ectomycorrhizal biomass. Mycorrhizal biomass in the two mineral layers was not significantly reduced by the fire. We used molecular tools to identify fungi directly from the ectomycorrhizas. In unburned plots members of the Russulaceae and Thelephoraceae were among the most frequent and abundant ectomycorrhizal types ; species of most other taxa were rare. In the control plots these two families were among the dominant species in both years, but patchiness on a fine spatio-temporal scale caused some major changes in ranking of individual species between sample years. Rhizopogon subcaerulescens was the most pronounced example ; its biomass in the control plot samples was seven times greater in year two than in year one because of an exceptionally large cluster of mycorrhizas encountered in a single core. The effect of fire on individual species was difficult to assess because of this patchiness and because all species were low in abundance after the fire. The most abundant pre-fire species were reduced to undetectable post-fire levels, while several less abundant species, including R. subcaerulescens, Cenococcum geophilum, and several unknown types were not substantially reduced by the fire. We speculate that the more abundant species, Martellia sp., thelephoroid 1, and Tomentella sublilacina, were differentially affected because their dominance was most prominent at the litter and organic layers. In any case, a short-term effect of this fire appears to be increased species evenness. Forest managers use prescribed fire to avoid catastrophic wildfire, maintain forest health, regulate stand composition, and reduce competition. Prescribed fires result in mortality of understorey vegetation, the consumption of litter and the reduction of organic material in the top layer of the soil (Ahlgren & Ahlgren, 1960). The effects of control burns go beyond the above ground portions of the forest ecosystem, affecting soil micro-organisms as well (Ahlgren & Ahlgren, 1965 ; Pietikainen & Fritz, 1995). After fire, bacteria are stimulated and fungi are depressed in soils (Va  zquez, Acea & Carballas, 1993). Ectomycorrhizal fungi are a particularly important component of the soil microbial community, directly providing plant hosts with enhanced water and nutrient uptake, extended root life and protection against root pathogens (Smith & Read, 1997) while influencing successional patterns and overall forest health (Perry et al., 1989). Our focus is on the effects of fire on the community structure of ectomycorrhizal fungi. Since ectomycorrhizas tend to proliferate in the litter and humus layers (Harvey Larsen & Jurgensen, 1976, 1979 ; Buchholz & Motto, 1981 ; Buchholz & Gallagher, 1982), * Corresponding author. consumption of organic materials by fire should decrease ectomycorrhizal root tips. Indeed, Buchholz & Gallagher (1982) reported that active ectomycorrhizal root tips were lower in burned plots than in unburned plots, but this difference was not significant for any soil depth tested (0–7, 7–14, 14–21 cm). Variability in the occurrence of mycorrhizal fungi can be high and Bellgard, Whelan & Mutson (1994) emphasized the need for temporal controls in fire ecology experiments. In this study we investigated the effects of an early summer prescribed burn on the abundance of ectomycorrhizal root tips on ponderosa pine (Pinus ponderosa Laws.) by comparing samples taken before and one year after the fire. We also sampled ectomycorrhizas in unburned control plots for comparison to data from the fire plots. By identifying fungi from the root tips, we were able to provide some insight into how their ectomycorrhizas were distributed in space and time. MATERIALS AND METHODS Description of study site The study site was at 1438 m above sea level in the Sierra National Forest, California (36 58 48 N, 119 8 13 W). Mycol. Res. 103 (10) : 1353–1359 (1999) Printed in the United Kingdom Early effects of prescribed fire on the structure of the ectomycorrhizal fungus community in a Sierra Nevada ponderosa pine forest