Structure and composition of forest floor fuels in long-unburned Jeffrey pine-white fir forests of the Lake Tahoe Basin, USA

In spite of the mechanistic links between forest floor fuels and fire behaviour and effects, little information is available on their composition and structure. We collected fuels from well-developed forest floors in four long-unburned Jeffrey pine (Pinus jeffreyi)-white fir (Abies concolor) forests in the Lake Tahoe Basin in California and Nevada. We measured forest floor (litter, fermentation and humus, where present) load, depth, bulk density and mineral ash content at the base of each tree, near the crown drip line and beyond the crown in open gaps (n=40 Jeffrey pine, 40 white fir). We found substantial variability in composition between the two conifers' forest floor fuels and across species. Forest floor was mounded near tree stems, with the majority of the depth (and mass) composed of fermentation fuels. Humus was present across spatial locations, but was patchier with increasing distance from trees. Ash content varied between trees (fir>pine) and with depth (humus>fermentation>litter). Results highlight the high spatial variation in forest floor fuels and underscore the need for building a greater understanding of forest floor fuels in long-unburned sites.