Brenna L. Decker and Alexandra N. Harmon‑Threatt, 2019




Seasonal timing of prescribed burns had differential effects on vegetation and bees in prairie fragments.

Neither dormant nor growing season burns negatively affected native bee communities in the prairie fragments.

Landscape characteristics surrounding prairie fragments may influence recolonization by bee communities following prescribed burns.

In this study, the authors investigated the effects of prescribed burn seasonality on vegetation and native bee communities in south-central Illinois tallgrass prairie fragments. They compared bee nesting and foraging resources, native bee species richness, and the abundances of above- and below-ground nesting bee species following dormant and growing season burns, and in nearby unburned prairie. The study was designed to help guide pollinator-friendly prairie restoration and management activities.

Contemporary prescribed fire management in midwestern US tallgrass prairies favors winter burns, due in part to a perceived lower effectiveness of growing season fires. Winter burns typically have higher fire intensities, consume more vegetation, and promote late-season grass and forbs; late summer and fall burns promote early flowering forbs and suppress grass dominance. However, empirical data are lacking for how fire season affects pollinators in these habitats. For example, some bee species nest above ground in stems, rotting wood, or grass duff, while others nest below ground in existing burrows or by excavating holes in bare soil; these nesting resources may be differentially affected by burn seasonality.

To investigate the effects of prescribed fire seasonality on native bees, the authors surveyed paired prairie fragments (≤ 1 km apart) in blocks with similar vegetation and soils over a two-year period. Pairs included one with a dormant season burn and another with a growing season burn. Fragments left unburned during the previous three years were included in each block during the first year of surveys. Investigators averaged observed values from within sampling units to estimate percent cover of bare soil, dead vegetation, grass, forbs, and moss. The mean number of flower heads from the quadrats estimated available flower resources. Bee communities were sampled passively with colored pan traps and blue vane traps and actively hand-netted in the morning and afternoon. Vegetation and bee sampling were conducted every 4 weeks between May and August each year.

Field site in May, 2016, showing the setup of one station which includes a blue vane trap and pan traps at 1 meter and at ground level (below vegetation). The five trap stations at each site were left for an 8-hour period.
(All photos: Brenna Decker)

The proportion of bare ground differed among all three treatments, with growing season burn treatments having the largest proportion, followed by dormant season burn treatments. Unburned fragments had lower overall bee abundance and lower below-ground nesting bee abundance compared to growing season burns, but a higher proportion of above-ground nesting bees compared to the two burned treatments (see figure below).

Comparison results between burn treatments (x-axes) for proportion of bare ground, bee abundances, proportion of above-ground nesting bee abundances, and below-ground nesting bee abundances. Means with standard error are presented, with letters indicating significance between treatments.

Bee species richness and flower abundance did not differ among treatments, but plant species richness was marginally greater in the growing season burn treatment compared to the unburned treatment; the authors posit that a longer application of the burn treatments may be necessary to observe floral community shifts. Additionally, adjacent semi-natural lands and unburned portions of the prairie likely maintained bee communities via recolonization of the burned prairie fragments.

Common long-horned bee (Melissodes sp.) on a black-eyed Susan (Rudbeckia hirta).

The authors conclude that growing season burns did not negatively impact these prairie fragments, that their benefits may be similar to those from dormant season burns, and that their use provides a longer timeframe throughout the year for prescribed burning without negatively affecting native bees or the plant community.