Fire, climatic variability, and grazing by large herbivores have historically limited woody vegetation in the tallgrass prairie region of North America to gallery forests in fire-protected areas along rivers and streams. Consequently, we expect that dominant tree species in these forests have developed mechanisms for tolerating periodic surface fires. Susceptibility of trees to fire damage depends in part on key properties of bark which influence heat transfer to the vascular cambium, including thickness, density, and moisture content. An earlier survey of Konza Prairie Biological Station in northeast Kansas, USA indicated that gallery forests were co-dominated by Quercus macrocarpa and Q. muehlenbergii, while Celtis occidentalis occurred as an important sub-dominant species. Populus deltoides, Gleditsia triacanthos, and Juniperus virginiana were relatively uncommon. To test the hypothesis that dominant gallery forest tree species are more resistant to fire damage than uncommon species, fire was applied to the bark of 10 individuals of these six species under conditions mimicking grass fires (400 °C for 120 seconds). Maximum temperature at the vascular cambium, bark thickness, bark moisture content, and bark density were measured. Trees were considered fire-resistant if the vascular cambium temperature remained below the thermal cell death threshold, 60 °C, throughout the treatment.
Results/Conclusions
Bark thickness was found to be most highly correlated with fire resistance (rs= -0.871, P<0.0001). Across species, a minimum bark thickness of approximately 10 mm was necessary to maintain the vascular cambium temperature below 60 °C under fire conditions applied in this study. Trees that produce thick bark quickly in juvenile size classes (P. deltoides, Q. macrocarpa, and Q. muehlenbergii) experienced lower temperatures at the vascular cambium than those which do not develop thick bark with increasing diameter (C. occidentalis, G. triacanthos, and J. virginiana). While resprouting from protected belowground meristems following fire damage can also provide fire tolerance, protection from fire damage by rapid development of thick protective bark appears to be important in regulating forest community composition in this gallery forest system. As fire frequency and intensity decrease in remnant tallgrass prairie of North America as a result of habitat fragmentation, fire suppression, and changing land management, we can expect gallery forest composition to shift from dominance by fire-resistant tree species to dominance by fire-sensitive species.
