OUTER BARK THICKNESS DECREASES MORE WITH HEIGHT ON STEMS OF FIRE-RESISTANT THAN FIRE-SENSITIVE FLORIDIAN OAKS (QUERCUS SPP.; FAGACEAE)

Premise of the study: In ecosystems maintained by low-intensity surface fires, tree bark thickness is a determinant of fire-survival because it protects underlying tissues from heat damage. However, it has been unclear whether relatively thick bark is maintained at all heights or only near the ground where damage is most likely. Methods: We studied six Quercus species from the red and white clades, with three species characteristic of fire-maintained savannas and three species characteristic of forests with infrequent fire. Inner and outer bark (secondary phloem and rhytidome, respectively) thicknesses were measured at intervals from 10 to 300 cm above the ground. We used linear mixed-effects models to test for relationships among height, habitat, and clade on relative thickness (stem proportion) of total, inner, and outer bark. Bark moisture and tissue density were measured for each species at 10 cm. Key results: Absolute and relative total bark thickness declined with height, with no difference in height-related changes between habitat groups. Relative outer bark thickness showed a height-by-habitat interaction. There was a clade effect on relative thickness, but no interaction with height. Moisture contents were higher in inner than outer bark, and red oaks had denser bark than white oaks, but neither trait differed by habitat. Conclusions: Quercus species characteristic of fire-prone habitats invest more in outer bark near the ground where heat damage to outer tissues is most likely. Future investigations of bark should consider the height at which measurements are made and distinguish between inner and outer bark.