Twenty-five years of tree demography in a frequently burned oak woodland

Due to decades of fire suppression, much of the Upper Midwest savanna habitat has converted to oak woodland. In efforts to restore oak savanna habitat, fire has been re-introduced in many of these woodlands. A primary purpose of these burns is to kill the fire-sensitive mesophytic tree species, which had established themselves during the decades of fire suppression, reduce the number of understory trees, and preserve the larger more widely spaced oaks. It is clear from ongoing efforts that restoring oak savannas will require frequent fires over decades. But frequent fires over the long term can also threaten the desirable oaks. Long-term demographic studies at savanna restoration sites experiencing frequent fires are necessary to determine the extent to the frequent burns are supporting and/or confounding restoration goals. Results presented here are from a twenty-five-year demographic study of an Upper Midwest bur oak (Quercus macrocarpa) savanna/woodland experiencing frequent fire, during which both the survival and growth of more than 9,000 trees were documented. Survival was assessed annually and growth every five years. In the face of frequent fires, stem survival was found to be strongly associated with tree species, stem size, and stem growth. In turn, stem growth was found to be related to tree species and stem size. Decades of frequent burning in this oak woodland have substantially reduced the abundance of unwanted trees, specifically mesophytic species and Q. ellipsoidalis, the latter which outcompetes Q. macrocarpa in the absence of fire. While Q. macrocarpa mid-sized (10-25 cm dbh) and large (> 25 cm dbh) trees are quite resistant to fire and now dominate the savanna landscape, they are not immune from fire-induced mortality. It is recommended that the number and density of these trees should be re-evaluated every few years to ensure that desirable numbers remain. If necessary, fires should be suspended for a period of time. This will give smaller Q. macrocarpa trees time to grow larger and become more fire-resistant, thereby ensuring successive generations of Q. macrocarpa.