Current Investigations of Fungal Ectomycorrhizal Communities in the Sierra National Forest1

Progress on two main studies on fungal ectomycorrhizal communities in the Sierra National Forest is discussed. One study examined the short-term effects of ground fire on the ectomycorrhizal community and the other examined the ectomycorrhizal associates of snow plant (Sarcodes sanguinea). In the ground-fire study we found that a large initial reduction in ectomycorrhizal biomass is caused primarily by combustion of the upper organic layers; prefire dominants in the Russulaceae and Thelephoraceae are dramatically reduced; and species at greater depths appear to survive the fire. We speculate that fire causes a short-term increase in species evenness. In the field portion of the Sarcodes study, we found that Sarcodes is specialized on a single mycorrhizal associate, Rhizopogon ellenae; the presence of Sarcodes is associated with dense islands of R. ellenae; R. ellenae appears to be a minor below-ground component of the red fir (Abies magnifica) community in locations near but not immediately adjacent to Sarcodes plants; and the Abies magnifica ectomycorrhizal community is dominated by members of the Russulaceae and Thelephoraceae. From studies of Sarcodes-R. ellenae interactions under laboratory conditions, we report that seed germination is stimulated by isolates of R. ellenae, isolates of R. ellenae derived from Sarcodes and Abies roots are capable of forming mycorrhizae on pine (Pinus) roots, and we have achieved initial success in establishing a tripartite Sarcodes-Rhizopogon-Pinus association under laboratory conditions. Ectomycorrhizae (EM) are dual organs composed of the fine roots of plants and fungal mycelia. They are the primary interface through which most temperate forest trees receive their mineral nutrients. All members of the Pinaceae require these mutualistic fungi for normal growth and survival (Smith and Read 1997). The diversity of EM fungi is extremely high. Over 6,000 species have been described (Molina and others 1992) and, because of the current state of our taxonomic knowledge, this probably represents a gross underestimation of the total number. Even at a local scale, diversity is very high. In single-species pine stands of approximately 0.1 ha, 15 to 35 species of EM fungi are typically reported, single soil cores often contain several species (Eberhart and others 1996), and even adjacent root tips are frequently colonized by different fungi (Bruns 1995). In spite of the importance of EM fungi, little is known about the structure of the complex communities that they form or the functional differences among the component species. Until recently, almost no quantitative descriptions of the abundance of EM species on roots were reported. This lack of information was largely caused by the difficulty in identifying fungal species in their vegetative states (that is, as mycorrhizae or mycelia). With the advent of the polymerase chain reaction (PCR), molecular-based methods now make identification much 1 An abbreviated version of this paper was presented at the Symposium on the Kings River Sustainable Forest Ecosystems Project: Progress and Current Status, January 26, 1998, Clovis, California. 2 Professor, Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720. 3 Adjunct Professor, Department of Environmental and Forest Biology, SUNY-ESF, Syracuse, NY 13210. 4 Assistant Professor, Department of Environmental and Forest Biology, SUNY-ESF, Syracuse, NY 13210. 5 Student, Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720. 6 Graduate Student, Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720. 7 Technician, Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720.