In California and across the American West, fire suppression, lack of cultural burning, drought and rising temperatures have led to large and high-severity wildfires that can be detrimental to humans and wildlife alike. Land managers are increasingly using forestry treatments like mechanical thinning and prescribed fire to reduce wildfire risk in forests that are more primed to burn due to climate change and many decades of active fire exclusion. Researchers (including those at IBP) are studying the effects of these treatments on the wildlife associated with these forests, species like California Spotted Owls, American Goshawk, Great Gray Owl and Black-backed Woodpecker. But these treatments also affect species typically associated with more open habitats- insects like bumble bees and butterflies. As native pollinator populations are generally in decline, it is important to understand how these widely used forestry treatments impact them.

In a paper published this week in Forest Ecology and Management, researchers from The Institute for Bird Populations and the US Forest Service examined the effects of mechanical thinning and prescribed burn treatments on bumble bees and butterflies in the Goosenest Adaptive Management Area on the Klamath National Forest in northern California. They found that approximately 20 years after treatment, the forestry treatments still had substantial effects on the richness (number of species) and abundance (number of individuals) of both bumble bees and butterflies.

The Goosenest Adaptive Management Area is a long-term ecological research project initiated by the US Forest Service in the 1990s to evaluate the efficacy of mechanical thinning and prescribed fire treatments to create late-successional forest characteristics like the presence of larger trees and more open understories. The area consisted of a second-growth conifer forest, approximately 100 years old and dominated by fir rather than pine due to fire suppression. Treatments included removal of non-pine species to encourage pine dominance (Pine treatment), removal of non-pine species in addition to the use of prescribed fire (Pine with Fire treatment), mechanical treatment intended to shift the forest composition towards large-diameter trees regardless of species (Big Tree treatment), and no treatment, where the forest was left to continue on it established course (Control). Although these treatments were designed to create late-successional forest characteristics, they are very similar to treatments currently being used to reduce the risk of high-intensity, stand-replacing wildfires in forests across California.

The researchers chose to study the effects of these treatments on butterflies and bumble bees because they are native pollinators that are of conservation concern and are relatively easily identified in the field without the need for lethal sampling. Both groups forage on a variety of flowers, but butterflies can be considered specialists as many species select only a few or even a single host plant species for their caterpillars, while bumble bees are more of a classic generalist. The researchers sampled bumble bees and butterflies across all four treatments in the summer of 2021. Insect surveyors also surveyed flowering plants at sampling points and along transects. Fieldwork was cut short before all intended samples were collected due to the Antelope Fire which burned the study area in August 2021.

The researchers found that differences in the vegetation structure and plant communities between the treatment types corresponded to differences in the abundance and species richness of bumble bees and butterflies. Compared to control, the three other treatment types had substantially less overstory and more species of flowering plants in the understory. This was most pronounced in the Pine treatment, where non-pine tree species were removed. Species richness of both bumble bees and butterflies was greater in treated sites relative to control. The abundance of butterflies was significantly greater in all treatments relative to control. Bumble bees were also more abundant in treated sites, however this difference only reached statistical significance in the pine treatment. The researchers found that greater flowering plant richness was associated with greater abundance of both bumble bees and butterflies. Butterfly abundance was also associated with greater flowering shrub richness.

The study’s lead author, IBP’s Jerry Cole, was surprised by the number of bumble bee and butterfly species they encountered in the study. “I thought maybe there would be one or two, but we actually had quite a nice mix of species in the treated regions,” he says. “I usually imagine these forests as dense and dark, but in the treated plots there was a fair amount of floral diversity - something pollinators appreciate!” A similar positive relationship between a more open overstory with greater variety of flowering understory plants and greater abundance and variety of bumble bees and butterflies has been shown in other regions in other studies, but the persistence of these effects approximately two decades post treatment is notable in this study.

While the treatments in this study were not primarily designed to reduce fire risk when they were implemented in the 1990s, they are very similar to fire-risk reduction strategies being implemented today. In fact, a recent study in the journal Fire Ecology found that when the Goosenest Adaptive Management Area burned in the Antelope Fire of 2021, the treated areas burned less severely than control areas, with the Pine with Fire treatment showing the greatest reduction in fire severity.

This study demonstrates that selective cutting and/or prescribed burning to address forest changes that have resulted from many decades of active fire suppression can have multiple benefits. “We can create forests that are closer to historic conditions (healthier), reduce fuel loads (lower fire risk), and create openings in the understory that allow flowering plants to flourish which provide food resources for pollinators,” says Cole. “While the forest openings created aren't the pinnacle of habitat for pollinators, they are used by them, and every additional bit of habitat can help. If these treatments are applied across a large landscape, then that habitat can add up.”