The Effect of Fire Severity on Bee Nesting Habitats

Increased fire severity in western United States forests due to climate change and accumulation of fuels can have lasting impacts on vegetation structure that especially impact pollinators. Fire severity has been found to strongly influence bee abundance, with increasing fire severity often correlated with increasing bee abundance. One mechanism for this relationship is altered nesting habitat due to the effects of fire on forest vegetation. The availability of nesting resources varies with burn severity as well as time post-burn. Previous research has mainly been conducted within 5 years post-burn, but here we examine sites more than 20 years post-burn to test if burn severity affects bee nesting resources longer term. We quantified the habitat available to ground-nesting bees at 18 sites located in 2 different burn areas (Buffalo Creek and High Meadow). Within each of these 2 fires, we studied 3 burn severities (low burn, high burn, and unburned) with 3 replicate sites for a total of 18 sites.

We recorded groundcover using Fire Effects Monitoring and Inventory System groundcover codes. We also recorded the diameter at breast height of snags (standing dead trees) in every site and percent canopy cover surrounding focal trees. We found fire severity significantly affected percent canopy cover and the frequency of groundcover types. Fire severity did not significantly affect mean snag basal area (a measurement of forest density), but higher severity fire sites did have a higher snag basal area. Our research aids in understanding the long-term effects of fire on native bees and ecosystem function.