The Tall Timbers Fire Ecology Lab has been collaborating with entomologists Dr. Michael Ulyshen and Scott Horn of the U.S. Forest Service Southern Research Station to study the effects of season of fire on bee-flower interactions.
With our help identifying plant species, Ulyshen and Horn used the Tall Timbers Season of Fire Plots established by the Fire Ecology Lab to collect bees from February to October of 2022. The goal was to test the effects of fire in winter, spring, summer, and fall on which bees appear and which plant species they visit.
Over the course of the study Ulyshen and Horn collected 92 bee species visiting 79 flower species with 446 unique combinations of bee-plant species interactions.
This research has been recorded in two recent publications in Frontiers in Ecology and Evolution and Ecology and Evolution.
Overall the effects of fires were ephemeral, temporarily reducing the numbers of flowers and the bees that visit them. Because most plants are perennial and resprout after being topkilled by fire, they recover within a few to several weeks.
However, fires in winter (January) and fall (October) were the most disruptive, because it takes longer for the plants to recover after burning in those seasons.
Fall burns result in a particularly long period before plants recover, as most plants do not resprout or flower until the following growing season. Also, fall fires occur during the period when the largest number of plant species are flowering, so they have the largest impacts on pollinators.
In contrast, flowering plants recover quickly following fires the spring (March-April) and summer (June) growing season.
The effects of fires were ephemeral, temporarily reducing the numbers of flowers and the bees that visit them. Plants recover quicker following spring and summer burns, the study found.
In this light, historic lightning-initiated fires that occurred primarily in the late spring and early summer would have had particularly short effects on flowers and their pollinators. In fact, certain species, like goat’s rue (Tephrosia virginiana), increased in flowering after spring burns and attracted more bees than in other plots. Bee-flower species combinations in the spring and summer burn plots were distinct from those in the fall burn plots and to a lesser degree from winter burn plots.
This project also revealed that a larger than expected number of bee species are “oligolectic” or specialized to visit plants in only one or two families, composing about half of the species found. The greatest proportion of such bees were found in the fall and depend on plants in Asteraceae (sunflower family), such that fall burns have a larger impact on the most specialized bees.
Ulyshen and Horn also discovered that a surprisingly large percentage of bees found in this study (18-25%) depend on trees, including pine trees, for their survival.
Many require dead wood for nesting, as well as resin for nest construction. So, whereas it might seem that only the herbaceous plant community is important to pollinators, the trees are also essential, including dead standing trees and woody debris. Retaining some upland fire-tolerant hardwood species, such as southern red oaks (Quercus falcata) and mockernut hickories (Carya tomentosa), is also predicted to increase bee pollinator diversity.
Taken together, this work demonstrates that effects of individual fires on plants and plant pollinators are fairly short-lived regardless of season of burning. However, it also suggests that repeated fires in a given season applied over larger scales might shape patterns of flowering, pollinator communities, and abundance of plant species. Our research on effects of season of fire is just getting started, and there is more to come.
