Print PageSoutheastern pine savannas experience frequent fires. Longleaf pines (Pinus palustris) produce needles that, when dry, are highly flammable and facilitate rapid spread of fire through savanna groundcover. Mockernut hickory, Carya alba, is one hardwood that persists through fires via resprouts from underground root crowns. A single understory hickory stem produces approximately 50g (dry mass) 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 hickory might suppress fire. We manipulated hickory and pine litter beneath understory hickory stems prior to prescribed fires at a second-growth pine savanna, Camp Whispering Pines in eastern Louisiana in 2010. We investigated effects of litter on temperature increase and residence time of fire, and survival of hickory stems. For each treatment, all litter was initially removed and replaced with 0 or 50g of pine litter, in combination with 0, 50, or 100g of hickory litter, four months before fires. We anticipated that addition of hickory litter, especially with no pine litter, should generate the lowest temperatures and shortest residence times, and increase survival of hickory stems, if hickory leaves suppressed fire.
Results/Conclusions
Pine and hickory fuels produced different effects on fire characteristics and understory hickories. Lowest maximum fire temperatures occurred when only hickory litter was present; any amount of pine litter present resulted in elevated maximum fire temperatures. Analysis of temperature increases indicated significant effects of pine litter (F1,18.6=4.87, p=0.040). Analysis of fire residence time also revealed significant effects of pine litter (F1,19.3=20.76, p<0.001). Temperatures remained elevated the shortest period of time when hickory leaves were the only fuels, and the presence of any amount of pine litter increased fire residence times. All hickories survived all combinations of fuels. Nonetheless, most hickories resprouted from buds located aboveground along the original stem when only hickory litter was present. In contrast, when any pine litter was present, resprouting occurred only from the underground root crown. Height of surviving stem was significantly related to temperature increase (F3,24=10.07, p<0.001, R2=0.546). Understory hickory stems can survive fires in the absence of pine fuels, but when longleaf pine fuels are present, hickories are top-killed and survive by resprouting from the root crown after fire. We conclude that characteristics and effects of fires are reduced by hickory fuels, but only when no longleaf pine fuels are present.
