Fire and Shade Effects on Ground Cover Structure in Kirtland's Warbler Habitat

Abstract

Researchers and managers have suggested that a narrow range of ground-cover structure resulting from fire might be necessary for suitable Kirtland`s warbler nesting conditions. Yet, Kirtland`s warblers have bred successfully in numerous unburned stands and there is little direct evidence to indicate that ground cover structure is a limiting factor for nest sites or habitat suitability within appropriate landform-ecosystems. We documented the range of percent cover for dominant ground-cover structural components in burned and unburned habitat (stand ages 7-23 y) occupied by Kirtland?s warblers. The mean percent cover for the dominant ground-cover structural components was lichen/moss (12.1%), blueberry (Vaccinium angustifolium) (9.5%), bare ground and litter (5.6%), sedge/grass (5.2%), deadwood (4.3%), sand cherry (Prunus pumila) (3.3%), sweet fern (Comptonia peregrina) (2.3%), coarse grass (1.8%) and bearberry (Arctostaphylos uva-ursai) (1.2%). Burned sites had significantly more deadwood, sweet fern and lichen/moss cover, while unburned sites had significantly more bare ground and sedge/grass. We also investigated how fire, shade-history (i.e., pre-fire tree crown cover approximated by tree height and density) and succession influenced the percent cover of the dominant ground-cover structural components from 1 to 5-y after wildfire disturbance. The magnitude of differences in percent cover among shade-histories changed through time for the groundcover components sand cherry, deadwood, grass/sedge and coarse grass. The percent cover of sweet fern, bearberry and bare ground was significantly different between some shade histories. All dominant ground-cover components showed significant difference between at least one shade-history when compared to an unburned harvested reference stand. This suggests that more similarities exist among the three burned sites than between the burned sites and the unburned reference site. Our results suggest that fire, shade-history and succession influence ground-cover, but that various ground-cover components are affected differently by these factors. Because of the complex role disturbance history plays in maintaining ground-cover in Kirtland`s warbler habitat, optimal management prescriptions are difficult to specify, especially when aspects of Kirtland`s warbler ecology other than nest location are also considered. Although suitable ground cover structure can result without fire, maintaining prescribed fire is still desirable because this is a historically fire-regulated system. However, the range of ground-cover structures accepted by the Kirtland?s Warbler and its resilience to disturbance suggests that suitable ground-cover for Kirtland`s warbler could be maintained in some stands without burning after every timber harvest.