Fire refuges

Concerns are often raised about the detrimental effects of prescribed fire on animals, especially relatively nonmotile invertebrates. Since (at least some) invertebrates survive prescribed burns, escape or avoidance mechanisms must come into play. During my recent surveys of the results of our two large burns, I frequently observed patches, some tiny, others reasonably large, that had not burned. These unburned areas are generally called refuges ("refugia" in the scientific literature).

The heterogeneity of savanna and prairie landscapes is self evident. I have spent quite a lot of time watching fire spread across the landscape. Although many areas show virtually 100% burn, in other areas, small or large unburned patches occur. These tend to be more common in savannas than prairies, but even prairies show small unburned sites.

The photos here provide a few examples, and illustrate the mechanisms by which fire refuges are created.
The base of a tree is often a fire-avoidance zone. Partly this is because the tree itself creates a reduced-fuel environment. Prairie grasses, the most flammable fuels, require full sunlight, and because of the leaf canopy the base of a tree is fairly shady. Also, fire tends to move in complicated patterns around trees, and some parts remain unburned. The above photo shows the unburned zone around the base of a large white oak.
The above photo shows the effect that a large horizontal log had on fire behavior. This black oak had been cut as part of savanna clearing, and was overlooked when the area was canvassed for fire wood. This log provided a good barrier and fire simply passed around this small site. Neither the grasses nor oak leaves, both good fuels, got burned.
The above photo shows an area that did not burn when we did the big south-slope fire. This is a fairly small area, but there were areas as large as 50 feet in diameter that did not burn. The photo below provides a closer look. Since most invertebrates are small, such a site could provide a reasonable hiding place.