Woody fuel structure and fire in subalpine fir forests, Olympic National Park, Washington

The fuel structure and flammability of subalpine fir (Abieslasiocarpa (Hook.) Nutt.) stands were studied to determine the relationship between these forests and fire. It has long been known that subalpine fir forests burn catastrophically, but the contributions of fuel structure and fuel moisture to this pattern of burning have been relatively unstudied. This investigation discovered two relationships. First, over twice as much fuel in subalpine fir forests accumulated around the bases of the fir trees than in the forest as a whole, and the many dead branches on the lower trunks may allow fire to travel up into the canopy. Second, the fuels in subalpine fir forests were more flammable at the end of the summer than at the beginning, and maximum flammability was achieved in early August when the fuel moisture was between 16 and 22%. We also found that the fuel structure of subalpine fir was different from that of fire-stable ponderosa pine (Pinusponderosa Laws.) forests. The fuel around the bases of the trees in ponderosa pine forests was not significantly different from that in the entire forest, and there were few branches on the lower trunks.