Mechanical thinning and prescribed fire benefit bumble bees and butterflies in a northern California conifer forest

Land managers across western North America are increasingly treating forests with mechanical thinning and prescribed fire to reduce stand density, with the aims of reducing wildfire risks and generating conditions that better align with historic forest structure. The effects of these treatments on invertebrates in forests is poorly understood, especially over longer time scales. We tested for effects of three forest treatments on butterfly and bumble bee abundance and species richness in a ponderosa pine and white fir forest in northern California, USA, that was treated systematically two decades prior to our surveys. The forest treatments included: mechanical thinning that selected for large diameter trees, thinning for a more pine dominated stand, thinning for pine dominance combined with prescribed fire, and control areas without any treatments. The most substantial difference in vegetation structure stemming from the past treatments was reduced forest overstory and greater flowering plant richness in all treatments relative to control. Butterfly and bumble bee richness was greater in all treatment types relative to control, though not always significantly so. Butterfly abundance was significantly greater in all treatments relative to control, and bumble bee abundance was significantly greater in pine treatments relative to control. Bumble bee and butterfly richness and abundance were most strongly positively related to greater plant species richness. Overall, it appears that bumble bees and butterflies in our study area generally benefited from the changes in forest structure that were induced by each of the three treatments, and those benefits persisted for decades. • The effect of forest thinning on bumblebees and butterflies is little studied. • On average more butterfly and bumblebee species were in forests that were treated. • Butterfly abundance was greater in thinned or thinned and burned stands. • Bumblebee abundance was greater in stands thinned to select for pine tree species. • Forest treatments benefit pollinators likely due to changes in vegetation.