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.
Robert Keane, Deputy Program Manager, Fire, Fuel, and Smoke Science (FFS); Research Ecologist; Director, Fire Modeling Institute (FMI)
Staff: Violet Holley, Todd Carlson, Kirsten Schmidt, Wayne Lynholm, Courtney Couch, Laurie Dickinson, Myron Holland, Curtis Johnson, Micha Krebs, Eric Apland, Daniel Covington, Amy Rollins, and Ben McShan, Matt Rollins, Russell Parsons, Helen Smith, and Kathy Gray, Elizabeth Reinhardt
GOALS AND OBJECTIVES
To quantify the litterfall and decomposition rates of the major fuel components in common forests of the northern Rocky Mountains of the United States.
METHODS AND RESULTS
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). There were 28 stands on seven sites in the study (Figure 1) that represent the major forest types in the northern Rocky Mountains including ponderosa pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menzeisii), western red cedar (Thuja plicata), subalpine fir (Abies lasiocarpa), and whitebark pine (Pinus albicaulis), and lodgepole pine (Pinus contorta). 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 were 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 (Figure 2), while foliage decomposition k values ranged from 0.085 to 0.283 along a moisture gradient (Figure 3). 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 were also evaluated and we found that logs were underrepresented in the collection and it would have taken four times as many traps to detect log litterfall. These data can provide managers with valuable estimates of fuel deposition and decomposition rates that can be used to determine the longevity of fuel treatments and prioritize fuel treatment areas. This can be accomplished by using the fire behavior models to calculate how long it would take to accumulate enough surface fuels to ignite or support a crown fire or kill overstory trees using the fire behavior models. While the large woody fuel litterfall and decomposition estimates measured in this study may contain high error rates, the fine fuel dynamics, which is critically lacking in the literature, has been sufficiently estimated for use in models and management. Major findings include:
- Litterfall rates are highest on productive plots with shade-tolerant conifers and plots with high LAI. The most productive habitat type series have the highest litterfall rates across all fuel components.
- While foliage litterfall rates vary widely across forest cover types and habitat type series, rates of woody fuel components are about the same across all plots and types, especially the largest woody fuels (large branches, and logs).
- Decomposition appears to be positively correlated to a moisture gradient where the highest decomposition rates occur on the most productive plots.
- The temporal and spatial distribution of fine fuels (foliage and twigs) is more homogeneous than large woody fuels (branches and logs) because of the consistent timing and distribution of the litterfall for these fine fuel components.
- Many litter traps are needed across a large area to adequately sample log (1000 hr) deposition rates (>200) and this precludes efficient sampling for research and management. A better approach would be to quantify tree life tables to estimate eventual mortality and snag fall
Figure 1 The geographic locations of the seven litter collection sites in the northern Rocky Mountains, USA
Figure 2. Distribution of litterfall rates (kg m-2 yr-1) for all fuel collected in all litter traps by surface fuel components for each stand.
Figure 3. Distribution of a) decay rates (k value) and b) mass loss rates (percent loss) for all 28 plots in this study by four fuel components, and c) decay rates for all components arranged by plot from high to low foliage decomposition
This work was partially funded by the USGS National Biological Service and Glacier National Park's Global Change Research Program under Interagency Agreements 1430-1-9007 and 1430-3-9005 and the USGS CLIMET project.
Keane, R.E. 2008. Biophysical controls on surface fuel litterfall and decomposition in the northern Rocky Mountains, USA. Canadian Journal Forest Research 38:1431-1445
Keane, Robert E. 2008. Surface fuel litterfall and decomposition in the Northern Rocky Mountains, USA. USDA Forest Service Rocky Mountain Research Station Research Paper RMRS-RP-70. 22 pages
Data product containing litterfall, decomposition, and leaf area index data for "Surface Fuel Litterfall and Decomposition in the Northern Rocky Mountains, USA"