Frequent fire in longleaf pine forest maintains an open canopy by preventing understory trees from growing into the overstory; however, variation in the type and amount of overstory litter in the fuelbed may influence duration of flaming and smoldering (residence time), which could affect understory hardwood control. Longleaf pine litter has been observed to increase fire behavior, but oaks and other hardwoods in longleaf pine forests have varied effects on fire behavior; it’s thought that some species impede fire while others facilitate fire. We investigated the effect of fire on understory hardwood survival by relating a spatially explicit analysis of the overstory distribution of twelve tree species common to longleaf pine forests in Southwestern Georgia, USA to fire residence time. Seventy-one fine-wire thermocouples were placed near oak trees in a longleaf pine forest matrix, trees within 20 m of the thermocouples were mapped, and temperatures were logged at two-second intervals during prescribed fires. We used an information theory approach to ask 1) whether the influence of individual trees on residence time differed among pines, fire facilitating oaks, and fire impeding oaks and other broadleaved trees, and 2) whether there was an antagonistic interaction between pines and fire-impeders.
Results/Conclusions
The positive effect of individual pines on residence time was readily detectable. We found a negative exponential relationship between distance from an individual tree and residence time for pines (predominantly longleaf); the effect pines on residence time was half of maximum at 6 m from the tree. Neither fire facilitating oaks nor fire impeding oaks and other hardwoods had any substantial effect on residence time, and there was no evidence for an antagonistic effect between litter of pines and of fire-impeding trees. We conclude that fire-facilitating oaks are not functionally equivalent to pines in their effect on fire behavior (and, by extension their ability to suppress understory hardwoods, because the shrub-killing effect of pines has mainly been linked to residence time). Our results do not support the notion that oaks and other hardwoods of southeastern forests have materially different effects on one important element of fire behavior (residence time). In contrast, they support the idea that pines, specifically longleaf, are keystone species for southeastern upland forests.