Forest floor characteristics and moisture dynamics in Jeffrey pine-white fir forests of the Lake Tahoe Basin, USA

In temperate coniferous forests, forest floor fuels have been linked to variation in important effects of fire, most notably mineral soil heating and post-fire tree mortality. To improve the understanding of fire effects, I collected forest floor fuels in long-unburned Jeffrey pine (Pinus jeffreyi) - white fir (Abies concolor) forests of the Lake Tahoe Basin, USA. Fuels from each forest floor horizon (litter, fermentation, and humus), as well as other important woody fuels (1-hour fuel, 10-hour fuel, and cones), were collected around the bases of large ( 50 cm diameter) Jeffrey pine and white fir. To isolate the effects of spatial position, I quantified fuel loading, depth, and bulk density at the base of each tree, beneath the crown drip line, and beyond the crown in open "gaps". Seasonal fuel moisture trends were measured during the 2009 and 2010 fire seasons. To understand the structure of the litter horizon, both spatially and between species, litter composition was examined, and mineral ash contents of each forest floor horizon were measured. Little variation was detected in forest floor bulk densities, depths, and moisture contents between the two conifers. There was also little variability in forest floor moisture across the Lake Tahoe Basin, but isolated rainfall events significantly altered forest floor moisture patterns. Duff (the fermentation and humus) moisture varied spatially throughout stands, whereas other important forest floor components did not. Results from the interannual moisture study revealed that moisture of the forest floor horizons differed between years while woody fuel moisture did not. The variability in field results underscore the importance of measuring duff moisture content, as well as 10-hour woody fuel moisture, prior to prescribed fire. Forest floor bulk density and depth did not differ between Jeffrey pine and white fir. However, Jeffrey pine forest floors were more flammable than white fir forest floors in the laboratory, where samples of forest floor litter were burned under controlled conditions. Forest floor fires beneath Jeffrey pines could potentially burn at greater intensities and smolder longer than white fir forest floors due to differences in burning characteristics. In the Tahoe Basin, a better understanding of the complexity of forest fuels will help land managers manage and restore these fire-prone forests. More broadly, these results help inform the understanding of fuels dynamics in other temperate coniferous forests.