The Characteristics and Interactions of Mixed Pyrophyte and Mesophyte Litter Fuel Beds

Abstract

Oak trees are of great ecological and economic importance in the southeastern United States. These trees are being replaced through the mesophication process, presenting a major risk to regional ecosystems. This study applies an established method to measure the burning characteristics of longleaf pine ($Pinus$ $palustris$), southern red oak ($Quercus$ $falcata$) and sweetgum ($Liquidambar$ $styraciflua$ L.). The combustion fraction, the mass loss rate, the temperature at multiple heights and the propagation rate were measured for each individual species and mixtures with 30\% oak mass and variable pine and sweetgum mass. Pine was found to be the most consistently flammable species with the highest median measurements for all measured burning characteristics. Conversely, sweetgum had the lowest median values for all burning characteristics. The results for oak were bimodal. The low flammability mode had results similar to those of sweetgum, whereas the high flammability mode had lower temperatures than that of pine but higher mass loss and propagation rates. Burning characteristics generally decreased as the sweetgum content increased; however, using principal component and cluster analysis, five clusters were identified. In the first cluster, the test had higher mass loss and propagation rates than any of its constituent species. In the second cluster, the propagation rate and flame height decreased, but the temperature along the fuel bed was greater than that of its constituent species. In the third cluster, the temperature decreased to expected values. The fourth cluster contained tests with combustion fractions lower than that found for any of the individual species. However, in the fifth cluster the combustion fraction approached the values found for sweetgum. It was concluded that there exist distinct pine-oak and oak-sweetgum interactions, which lead to competing changes in the overall fuel-bed burning characteristics. These results contribute to the current discussion on how the progression of mesophication impacts the forest floor flammability; and can be used to improve prescribed burning to suppress mesophication.