Quantifying the effects of wildfire reintroduction on native pollinators in ponderosa pine forests of the Pacific Northwest

Abstract

Pollinators are an essential component of ecosystem function, and declining bee populations are a global conservation concern. Despite this importance, there is a lack of understanding regarding the distribution and demography of native bee species across western North American landscapes. Furthermore, the effects of disturbance on these essential fauna are understudied. Fire is a major driver of biodiversity and structural integrity in fire prone ecosystems. Over the last century, historical wildfire regimes have shifted due to climate change, exclusion of Indigenous fire stewards, and land management philosophies. As a result of these pressures, forests have experienced increases in fuel buildup that threatens dry forest resilience across western North America. In these post-fire landscapes, pollination is critical for vegetation recovery and ecosystem health. Pollinators are critical for successful angiosperm reproduction, food security, and ecosystem resilience. Because pollination services can vary by species and across spatial and temporal scales, land managers and ecologists seek to understand the spatial and temporal effects of fire reintroduction on native bee communities. An increased understanding of the effects of fire on pollinators can inform conservation management and ecosystem restoration. In this dissertation, I used field based and molecular methods to evaluate the effects of fire reintroduction on bee community composition across a temporal range of 1-9 years post-fire, and in an unburned control, in the Okanogan-Wenatchee National Forest in Washington, USA. In Chapter 1, I quantified native bee richness at nine forested plots using blue vane and pan traps, and determined that native bee genus richness was driven by the interaction between burn severity and burn age. Study areas that had been recently burned at higher severities had higher bee genus richness values. Furthermore, the proportion of above ground nesting bees in landscapes 1-8 years post-fire were greatest at sites that burned with moderate fire severity and had more available nesting habitat. In Chapter 2, I highlighted shifts in foraging resources for pollinator communities in post-fire landscapes based on DNA metabarcoding of pollen collected in blue vane and pan traps. The findings suggest that herbaceous perennials such as Achillea millefolium, Lupinus spp., and Phacelia hastata are important floral resources for post-fire pollinator communities. Burn age and burn severity were important drivers of plant-pollinator network richness, and plant species richness increased with burn age and burn severity. This work is the first to use pollen suspended in trap solutions to quantify floral resources for insect pollinators and highlights the utility of trap byproducts to provide insightful ecological information. In Chapter 3, I evaluated changes in pollinator demography to provide insights for bee community health following disturbance by wildfire. By deploying nesting materials across three burned landscapes and one unburned control, I quantified native wood-cavity-nesting bee reproductive success for two exemplar pollinator species: Osmia lignaria propinqua and Osmia kincaidii. The results suggest that burn age is the main driver of wood-cavity-nesting bee oviposition. Furthermore, DNA metabarcoding of pollen from nest boxes revealed herbaceous perennials such as Phacelia hastata and Epilobium brachycarpum to be important floral resources for wood-cavity-nesting pollinators. Finally, in Chapter 4, I documented new records of Melissodes nigracauda LaBerge, Dufourea dilatipes Bohart, Atoposmia abjecta abjecta Cresson, Coelioxys funerarius Smith, Dianthidium cressonii Dalla Torre, Dianthidium singulare Cresson, Osmia cyaneonitens Cockerell, and Stelis heronae Sheffield. These eight new records supplement the ~565 bee species previously documented in Washington state. Collectively, I assessed the effects of wildfire in ponderosa pine forests on native pollinator community composition, wood-cavity-nesting bee reproductive success, and plant-pollinator networks. These findings highlight the importance of fire reintroduction for native pollinator conservation in dry forest landscapes.