UC Davis

Fire is the dominant source of disturbance that impacts the distribution of species, but in many regions of the world, fire regimes are changing in response to global change, with cascading impacts on ecosystem diversity and function. In the coniferous forests of the western United States, incidences of extremely large and high-severity fires (i.e., megafires) are increasing due to a century of fire suppression combined with a warming, drying climate. Loss or degradation of ecosystem processes, such as pollination, may be especially detrimental after large-scale, high-severity fire removes biomass and fragments habitat. I studied the response of pollinator communities and floral resources to the King Fire, a large-scale, high-severity fire which burned in 2014 in the mid-elevation coniferous forest and meadows of the Sierra Nevada, California.In chapter 1, I investigated how burn severity in upland and meadow habitat interacted with floral phenology to impact the pollinator resources available after fire, using flowering plant survey data. I modeled floral abundance and floral richness to evaluate the influence of burn severity, habitat, and time since snowmelt on pollinator resources. I further compared alpha and beta diversity and community composition among burn-habitat classes at three points in time during the flowering season. Meadows tended to have more diverse and abundant flowering resources throughout the season; fire increased floral abundance and diversity in upland habitat. All burn-habitat classes had similar amounts of β diversity, but fire increased habitat heterogeneity temporally, with upland habitats peaking in diversity at different points of the season. Forests that have experienced some fire may be more resilient to climate-induced shifts in flowering if the overall community compensates through this increased temporal heterogeneity. Chapter 2 investigated the impacts of burn severity, habitat, and floral resources on pollinator abundance, by modeling their effects on count data of floral visitors (bees, wasps, flies, beetles, and true bugs), butterflies, and hummingbirds. Although many relationships were not significant, I found highly consistent positive responses to floral richness, negative responses to burn severity in meadows, and mixed responses to burn severity in uplands for floral visitors. Hummingbirds and two families of butterflies responded positively to burn severity in uplands. Hummingbird abundance was also significantly higher in high-severity meadows. Whereas high-severity fire, particularly in meadows, has the potential to affect most pollinators negatively, moderate-severity fire can improve the quality of previously fire-suppressed upland habitat. Given that much of the Sierra Nevada is predicted to burn at high severity, reducing risk of high-severity fire around meadows may protect sensitive pollinator communities. In chapter 3, I focused on how pollinator diversity was impacted by burn severity in upland and meadow habitat. I compared alpha and beta diversity and community composition among burn-habitat classes and modeled species richness and beta diversity as functions of burn severity, habitat, and floral resources. Alpha and beta diversity were similar among all burn-habitat classes at the broader scale, but when plot-level metrics were modelled, burn severity had significantly negative impacts on richness in meadow habitat. Community composition differed among green and high-severity meadow, and green and high-severity upland. As expected, floral richness was positively associated with pollinator species richness. Pollinator diversity is more sensitive to the negative impacts of high-severity fire in meadows, and as such this habitat should be the focus of conservation measures given the high diversity and abundance of species it supports.