DNA metabarcoding reveals key floral resources for native pollinators in post-fire landscapes

Wildfires are a natural disturbance in dry ponderosa pine forest ecosystems, and are increasing in frequency and severity across western North America. The effects of wildfire on biotic and abiotic conditions have important implications for higher trophic levels, including native bees. We sought to quantify the responses of flowering plants to fire age and severity and determine the effects of these shifts on pollinator richness through time. We sequenced pollen from pan and blue vane traps deployed every 2 weeks from April to August in 2022 and 2023 to quantify shifts in foraging resources for pollinator communities in landscapes aged 1-8 years post-fire, and in an unburned area. DNA metabarcoding pollen suspended in blue vane and pan traps revealed herbaceous perennials such as Achillea millefolium, Lupinus spp., and Phacelia hastata to be key floral resources for post-fire pollinator communities. Burn age and burn severity were the main drivers of plant-pollinator richness, and plant species richness increased with burn age and burn severity. Bee species richness varied among burn severities, supporting management for patch mosaics and landscape heterogeneity as a fundamental principle of fire reintroduction. Our findings underscore the importance of bioinformatics threshold selection in plant list generation and highlight the role of burn age, burn severity, and plant richness in effectively conserving pollinator species across fire-effected landscapes. This study also used the novel approach of collecting pollen suspended in trap solutions to quantify floral resources for insect pollinators, highlighting the utility of trap byproducts to provide insightful ecological information. Understanding post-fire plant-pollinator relationships is critical for successful conservation of dry forest angiosperm flora and bee faunas.