Prescribed fire in grassland butterfly habitat: targeting weather and fuel conditions to reduce risk to larvae and enhance post-burn habitat heterogeneity
Hill, Kathryn Courtney
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Prescribed burning is one of the primary tools used for habitat restoration in the fire-adapted prairies of the Pacific Northwest. Concerns about detrimental effects of burning on butterfly populations, however, can inhibit implementation of treatments. Burning in cool and humid conditions is likely to produce low-severity patches and result in lowered soil temperatures, which can be critical to survival of butterfly larvae. With these burning conditions, it may also be possible to enhance the heterogeneity of butterfly habitat, and thus achieve a secondary objective. In this study, twenty experimental plots were burned across a wide range of weather and fuel conditions to address the potential for meeting these two objectives. Overall risk to diapausing butterfly larvae – assessed by measuring subsurface soil temperatures and heat dosages – was lower when air temperature was less than 26 oC, dead fuel moisture was greater than 8%, and relative humidity was higher than 54%, providing threshold recommendations for habitat management using prescribed fire. Targeting moister burning conditions to leave some unburned or low-severity patches that can serve as insect refugia required at least a small amount of pre-existing fuel discontinuity. Post-burn habitat composition and structure were also influenced by burning conditions. In the first growing season post-burn, species richness increased slightly more when burns had occurred in mid-summer vs. late summer. Ordinations showed shifts in plant composition from perennials towards annuals, with somewhat larger shifts for plots burned in the afternoon. Geospatial analysis was used to assess the heterogeneity of vegetation height structure, which can help to buffer insect populations by producing fine-scale variability in thermal heat loading at ground level and providing a range of microclimates. This structural heterogeneity and microclimate availability increased following morning burns conducted in higher dead fuel moistures, and when burns occurred in little to no wind, highlighting the linkages between fuel continuity, fire spread, and vegetation response. This research provides regional recommendations for burning in sensitive butterfly habitat and contributes to a greater understanding of how fuel moisture and continuity during grassland burning can affect fine-scale heterogeneity of vegetation structure and microhabitat availability.
- Forestry