Current Investigations of Fungal Ectomycorrhizal Communities in the Sierra National Forest

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. USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 83 Bruns, Kretzer, Horton, Stendell, Bidartondo, Szaro Current Investigations of Fungal Ectomycorrhizal Communities in the Sierra National Forests more feasible. As a result, many EM community studies are beginning to be reported (Erland 1995, Gardes and Bruns 1996a, Horton and Bruns 1998, Horton and others 1998, Horton and others 1999, Kårén and Nylund 1996). The long-term goals of all our studies are to examine the structure of EM communities in diverse settings, to look for common themes in these structures, and to elucidate the functional differences among EM fungi. Our strategy has been to focus on a series of much smaller but more researchable questions and to assemble results of these studies into a general picture of fungal EM communities. Our prior work in coastal pine and Douglas-fir (Pseudotsuga menziesii) forests has shown that the species that dominate fruiting can be minor components of the below-ground community; mature communities are typically dominated by members of the Thelephoraceae and Russulaceae; where roots of Douglas-fir and pine or Douglas-fir and manzanita (Arctostaphylos spp.) intermingle, they usually are associated with the same fungal species; and a spore-bank guild, dominated by members of the Suillineae (primarily Rhizopogon) and members of the Ascomycota, is present in forest soils, and these species dominate pine seedlings in the first few years after stand-replacing fires (Baar and others 1999, Gardes and Bruns 1996a, Horton and Bruns 1998, Horton and others 1998, Horton and others in press, Taylor and Bruns 1999). In the Sierra National Forest (SNF) we focused on two studies that complemented our earlier or ongoing studies elsewhere. In the first, we examined the short-term effects of ground fire on the EM community associated with ponderosa pine (Stendell and others 1999). The second study focused on EM associates of snow plant and on the EM community on red fir (Abies magnifica) surrounding snow plants. Sarcodes (fig. 1) is a nonphotosynthetic member of the Ericaceae, subfamily Monotropoideae. It is “epiparasitic” in that it obtains all of its carbon from surrounding trees via an EM fungus. Early studies in our lab on other nonphotosynthetic epiparasitic plants showed that, in contrast to photosynthetic plants, virtually all of the nonphotosynthetic plants that we studied had highly specific fungal associations. The only apparent exception was Figure 1—Sarcodes sanguinea. 84 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. Current Investigations of Fungal Ectomycorrhizal Communities in the Sierra National Forests Bruns, Kretzer, Horton, Stendell, Bidartondo, Szaro Sarcodes, and because it is a common plant in SNF, we decided to sample it more intensively and more locally than in our previous study, which included only 12 Sarcodes plants (Cullings and others 1996). In the course of sampling we found a site where Sarcodes was unusually abundant. This “high-density” Sarcodes site, situated in a red fir monoculture, enabled us to examine the EM community adjacent to Sarcodes and compare it with that in the surrounding red fir forest. It also provided an ideal setting for studying the sizes of fungal clones associated with Sarcodes.