Southeastern pine savannas experience frequent fires. Longleaf pines (Pinus palustris) produce highly flammable needles that, when dry, facilitate rapid spread of fire through savanna groundcover. Black hickory, Carya texana, is one hardwood species that persists through fires and resprouts from underground root crowns. A single understory hickory stem produces approximately 50g (dried) of leaves in one season. These leaves, when shed during the dormant season, typically lie flat and form thick mats of litter at the bases of stems. We hypothesized that litter produced by black hickory might suppress fire. We manipulated hickory litter beneath understory hickory stems prior to 2008 and 2009 prescribed fires at a second-growth pine savanna, Camp Whispering Pines in eastern Louisiana, to investigate the effects of litter on maximum temperatures and residence times of fires. For three treatments, hickory litter was initially removed. For the removal treatment no litter was replaced; for the manipulated control 50g of hickory litter was replaced; and for the addition treatment 100g of hickory litter was replaced. A fourth treatment was an unaltered control that used all litter present naturally. We anticipated that the addition treatments should generate the lowest temperatures and shortest residence times if hickory leaves suppressed fire.
Results/Conclusions
Analysis indicated significant effects of litter treatments on maximum fire temperatures (F3,44=4.52, p=0.0076), but not residence time (F3,38.8=0.14, p=0.9335). For fire temperature, unaltered controls differed significantly from manipulated treatments (F1,44=11.11, p=0.0018). In contrast to expectations, however, all manipulations produced higher maximum temperatures than unaltered controls. Individual comparisons indicated that litter additions (p=0.0197) and manipulated control (p=0.0210) generated significantly greater temperatures than unaltered controls, while the removal treatment was not significantly different (p=0.3956). Although manipulation treatments affected maximum fire temperatures, different levels of litter addition did not differ significantly in their effects on maximum fire temperatures. All other comparisons were not significant. We hypothesize that manipulation of fuels increased aeration of fuels, resulting in more combustion and hence higher maximum fire temperatures. We project that hardwood fuels, such as produced by C. texana, might reduce damage to understory plants not by the production of non-flammable fuels, but by compression of the large, flat leaves that reduces air space around the root crown and thereby reduces combustion of fuels close to dormant buds, which are located just below the ground surface. In this way, shrubs might persist until rare longer fire-free intervals enable growth into reproductive tree stages of the life cycle.
