Native bee richness increases with wildfire burn severity in ponderosa pine forests.

Abstract

Wildfires are increasing in frequency and severity in dry forests across western North America and have direct effects on forest structure and ecosystem services. One important service to monitor is pollination, which promotes plant-pollinator biodiversity and is critical for post-fire vegetative recovery. Because pollination services can vary by species and across spatial and temporal scales, understanding the effect of fire on pollinator populations informs conservation management and ecosystem restoration. Due to the known positive effects of fire on bees, including increased floral resources, nesting habitat, and light availability, we hypothesized that pollinator richness would increase with burn severity (measured with the Composite Burn Index) and decrease with burn age. We quantified native bee richness in the Okanogan-Wenatchee National Forest in Washington, USA at nine forested plots that burned in 2021, 2018, and 2015, or were unburned since 1968. Data were collected in 2021, 2022, and 2023 biweekly from April to August using blue vane and pan traps. Our findings suggest that native bee genus richness was driven by the interaction between burn severity and burn age. Study areas that had recently burned at higher severities had greater bee genus richness. Furthermore, the proportion of above ground nesting bees in landscapes 1-8 years post-fire was greatest at sites that burned with moderate fire severity and had more available nesting habitat. Our findings suggest that mixed-severity fire in ponderosa pine landscapes promotes native bee biodiversity.