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

Abstract

The 1988 fires in Yellowstone National Park providedan opportunity to study effects of a large infrequent disturbance on a natural community. This study addressed two questions: (1) How does prefire heterogeneity of the landscape affect postfire patterns of fire severity? and (2) How do postfire patterns of burn severity influence plant reestablishment? At three sites, 100 sampling points were distributed regularly in a 1-km x 1-km grid and sampled annually from 1989 to 1992. Information was recorded on fire severity (damage to trees, depth of ash and soil charring, and percent mineral soil exposed); pre-fire forest structure (forest successional stage; tree density; tree species; tree size; and evidence of pre-fire disturbance by mountain pine beetle [Dendroctonus ponderosae Hopk.] or mistletoe [Arceuthobium americanum Nutt. ex Engelm.]); post-fire 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). Fire severity was influenced by successional stage, with older stands more likely 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 likelihood of crown fire, but intermediate prefire damage reduced the likelihood of crown fire. Fire severity was not influenced by slope, aspect, or tree density. Postfire percent vegetative cover and density of lodgepole pine seedlings varied with burn severity. In lightly burned areas, percent cover returned to unburned levels by 1991. In severely burned areas, total percent cover was about half that of unburned areas by 1992, and shrub cover remained 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 forests will contribute to our understanding of successional processes following a disturbance that was exceptional in its size and severity.