Measuring surface fuel litterfall and decomposition in the Northern Rocky Mountains, USA
Fire exclusion policies and the successful fire suppression program across western United States and Canadian landscapes over the last 70 years have resulted in excessive accumulations of surface fuels that have increased the potential for severe and potentially dangerous wildland fires. Government land management agencies are advocating extensive fuel treatments and ecosystem restoration activities to reduce the possibility of severe and intense wildfire that could damage ecosystems, destroy property, and take human life. Knowledge of fuel litterfall and decomposition rates before and after fuel treatments could help managers prioritize, design, and implement more effective fuel treatment programs, but these rates remain relatively unknown for many ecosystems. In this study, the rates of deposition and decomposition were quantified for six surface fuel components across major forest types in the northern Rocky Mountains to estimate fuel dynamics parameters for use in complex landscape models of fire and vegetation dynamics. Fuel litterfall was measured for more than 10 years with semi-annual collections of fallen biomass sorted into six fuel components (fallen foliage, twigs, branches, large branches, logs, and all other material). This litterfall was collected using a network of seven to nine, 1 m2 litter traps installed at 28 plots established on seven sites with four plots per site. Decomposition was measured using litter bags installed in three sets of three bags each on five of the seven sites and the bags were monitored for biomass loss each year for 3 years. Deposition and decomposition rates are summarized by plot, cover type, and habitat type series. Foliage litterfall rates ranged from 0.057 kg m-2 on the dry Pinus ponderosa stands to 0.144 kg m-2 on mesic Thuja plicata stands, while foliage decomposition k values ranged from 0.085 to 0.283 along a moisture gradient. Fallen foliage and fine woody fuel (twigs, branches) tended to be more homogeneously distributed than large woody fuel (large branches, logs) across the traps and across each year of the 10+ year study. Spatial and temporal properties of both litterfall and decomposition are also evaluated and their implications to fuel modeling and mapping are discussed.