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.