Spatial, seasonal, and diel forest floor moisture dynamics in Jeffrey pine-white fir forests of the Lake Tahoe Basin, USA

Abstract

Variation in consumption of forest floor fuels in temperate coniferous forests have been linked to variation in fire effects, including mineral soil heating, tree mortality, and erosion. Moisture content of forest floor fuels influences consumption and therefore is a primary predictor of fire behavior and effects. To better understand spatial and temporal variability in forest floor moisture, we collected fuels from each forest floor horizon, cones, and woody fuels in four long-unburned Jeffrey pine (Pinus jeffreyi) – white fir (Abies concolor) stands within the Lake Tahoe Basin, USA. To isolate the effects of spatial position within a stand, fuel moisture was measured at the base of trees, beneath the crown drip line, and beyond the crown in open gaps across a fire season. We compared spatial moisture dynamics at the m2, ha, and Basin scales and temporal dynamics at the day (one 24-h sampling period), month, and year scales (one site was measured in 2009 and 2010). Duff moisture varied spatially within stands, whereas moisture in litter, cones, and woody fuels did not. Forest floor moisture 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-h woody fuel moisture. A better understanding of the complexity of forest fuels will help land managers manage fire in Tahoe Basin forests and inform the understanding of fuels dynamics in other temperate coniferous forests.