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The effects of seasonal prescribed fire on the belowground ectomycorrhizal community and live fine root biomass were investigated before, 1 year after, and 2 years after prescribed underburning. Ectomycorrhizas were sampled from four replications of three treatments (fall underburning, spring underburning, and a nonburned control) in a randomized complete block design. Samples were separated in two subsamples representing the upper 5 cm and lower 5 cm of a soil core. Molecular tools were used to distinguish 140 restriction fragment length polymorphism (RFLP) species of fungi directly from the ectomycorrhizas. Prior to underburning, the number of RFLP species and amount of live root biomass were similar among treatment units and between upper and lower core samples. Fall underburning largely removed live root biomass to a depth of 10 cm and significantly reduced ectomycorrhizal species richness compared with spring underburning and the nonburned control for at least 2 years. RFLP species richness and live root biomass following spring underburning were generally similar to the nonburned treatment. The successful reintroduction of fire to the ecosystem to retain high species diversity of ectomycorrhizal fungi and achieve the desired future condition of large-tree ponderosa pine retention with low fuel loads may require more than underburning in a single season.Résumé : Les effets du brûlage dirigé saisonnier sur la communauté ectomycorhizienne dans le sol et la biomasse de racines fines vivantes ont été étudiés avant, 1 an après et 2 ans après un brûlage dirigé superficiel. Les ectomycorhizes ont été échantillonnées dans les quatre répétitions de trois traitements (brûlage automnal, brûlage printanier et témoin non brûlé) établis selon un dispositif en blocs aléatoires complets. Les échantillons ont été divisés en deux sous-échantillons représentant les parties supérieure (5 cm) et inférieure (5 cm) d'une carotte de sol. Des outils moléculaires ont été utilisés pour distinguer 140 espèces de champignons sur la base des RFLP directement à partir des ectomycorhizes. Avant le brûlage, le nombre d'espèces et la biomasse de racines vivantes étaient similaires entre toutes les parcelles expérimenta-les et entre les parties supérieure et inférieure des carottes de sol. Le brûlage automnal a pratiquement éliminé la biomasse de racines vivantes jusqu'à une profondeur de 10 cm et significativement réduit la richesse en espèces ectomycorhiziennes comparativement au brûlage printanier et au témoin non brûlé pendant au moins 2 ans. La richesse en espèces basées sur les RFLP et la biomasse de racines vivantes à la suite du brûlage printanier étaient généralement semblables au traitement non brûlé. La réintroduction réussie du feu dans l'écosystème dans le but de conserver une grande diversité d'espèces de champignons ectomycorhiziens et de recréer les conditions futures souhaitées pour conserver des pins ponderosa de forte dimension avec une faible quantité de combustibles pourrait exiger plus qu'un brûlage superficiel au cours ...

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Abstract. Reestablishing historical fire regimes is a high priority for North American coniferous forests, particularly ponderosa pine (Pinus ponderosa) ecosystems. These forests are also used extensively for cattle (Bos spp.) grazing. Prescribed fires are being applied on or planned for millions of hectares of these forests to reduce fuel loads, alter forest structure, and maintain and enhance the productivity of native plant communities. However, cattle grazing is ubiquitous in ponderosa pine forests and the consequences of post-fire cattle grazing on plant communities are not well understood. We evaluated cattle grazing effects (grazing, no grazing) on upland bunchgrass and Carex geyeri dominated ponderosa pine plant communities over five growing seasons after prescribed fires (spring reburn, fall reburn, no burn). Vegetation was measured prior to a 5-year interval reburn and the subsequent exclusion of cattle, and in the second and fifth growing seasons thereafter. We found no interactions between reburning and grazing for the understory response variables. For all reburn treatments, including unburned areas, five growing seasons of cattle grazing exclusion significantly increased: (1) total vegetative cover, (2) native perennial forb cover, (3) grass stature, (4) grass flowering stem density, and (5) the cover of some shrub species and functional groups. Grazing exclusion did not strongly affect plant compositional patterns, although differences were detected. Compared to unburned areas, neither spring nor fall reburning increased perennial native species cover or richness, and reburning reduced sedge cover. Fall reburning increased cover of native colonizers, and exotic species cover and richness (largely Bromus tectorum and Cirsium vulgare), although overall exotic cover remains low (,1%). We document several potentially chronic impacts of cattle grazing in both burned and unburned areas, and show that the understory release from a long history of cattle grazing caused a greater degree of change than the initial reintroduction of fire. If a goal of ecological restoration in these forests is increased cover of native perennial plants, and the potential for increased native perennial grass reproduction, then cattle grazing exclusion, or a change in cattle management, could provide critically important options in restoration plans.