Hardwood trees are a part of frequently burned longleaf pine woodlands that are important for wildlife, and create areas of lower fire intensity which may aid in longleaf pine regeneration. They are common and persistent in the understory, surviving fires by re-sprouting after aboveground stems are killed, and growing into the overstory only when aboveground stems grow to a fire resistant size. The potential to survive fire varies by fire intensity and by each hardwood tree’s specific resistance to heat damage. We investigated the effects of fire on hardwood stems by relating fire intensity to tissue damage in four hardwood groups common to longleaf pine woodlands: mesic oak, upland oak, sandhill oak, and non-oak. We measured maximum temperature and duration of heating during prescribed fires using thermocouples in longleaf pine woodlands, and measured, stem radius, bark thickness, and depth of heat damage by staining living tissue in hardwood stems that were harvested within one meter of thermocouples. We sought to answer 1) what is the heat threshold for survival versus death for stems of various types and sizes of hardwood trees, and 2) how do differences in stem anatomy in each group contribute to fire resistance?
Results/Conclusions
Bark thickness in both upland and sandhill oak increased with stem radius at rates that were approximately double the rates for mesic oak and non-oak species. We also found a threshold where hardwood stems were more likely to survive average maximum temperature of prescribed fire at a bark thickness of ca. 9.4 mm. This bark thickness corresponds to 50 mm radius stems for the upland and sandhill groups, and 100 mm stems for the mesic oak and non-oak groups. We conclude that upland and sandhill oaks can survive higher intensity fire than mesic oaks and other non-oak species of the same stem diameter. These results suggest that varying prescribed fire intensity may be required to recruit diverse hardwoods to the overstory while maintaining longleaf pine woodland structure.