Prefire heterogeneity, fire severity, and early postfire plant reestablishment in subalpine forests of Yellowstone National Park, Wyoming

The 1988 fires in Yellowstone National Park provided an opportunity to stud y effects of a large infrequent disturbance on a natural community. This st udy addressed two questions: (1) How does prefire heterogeneity of the land scape affect postfire patterns of fire severity? and (2) How do postfire pa tterns of burn severity influence plant reestablishment? At three sites, 10 0 sampling points were distributed regularly in a 1-km x 1-km grid and samp led annually from 1989 to 1992. Information was recorded on fire severity ( damage to trees, depth of ash and soil charring, and percent mineral soil e xposed); pre-fire forest structure (forest successional stage; tree density ; toe species; tree size; and evidence of pre-fire disturbance by mountain pine beetle [Dendroctonus ponderosae Hopk.] or mistletoe [Arceuthobium amer icanum Nutt. ex Engelm.]); postfire percent cover of graminoids, forbs, and low shrubs; number of lodgepole pine (Pinus contorta var. latifolia Engelm .) seedlings; and general topographic characteristics (slope and aspect). F ire severity was influenced by successional stage, with older stands more l ikely to be in the more severe burn class, and by tree diameter, with tree damage diminishing with tree size. Prefire bark beetle and mistletoe damage also influenced fire severity: severe prefire damage increased the likelih ood of crown fire, but intermediate prefire damage reduced the likelihood o f crown fire. Fire severity was not influenced by slope, aspect, or tree de nsity. Postfire percent vegetative cover and density of lodgepole pine seed lings varied with burn severity. In lightly burned areas, percent cover ret urned to unburned levels by 1991. In severely burned areas, total percent c over was about half that of unburned areas by 1992, and shrub cover remaine d reduced. Recruitment of lodgepole pine seedlings was greatest during the second postfire year and in severe-surface burns rather than in crown fires . Continued monitoring of vegetation dynamics in Yellowstone's burned fores ts will contribute to our understanding of successional processes following a disturbance that was exceptional in its size and severity.