Effects of a Fall Wildfire on Herbaceous Vegetation on Xeric Sites in the Selway-Bitterroot Wilderness, Idaho

Abstract

Trends in productivity and mineral content of herbaceous vegetation in Ponderosa pine and montane grassland over a 4-year period are reported. Dry matter production on burned areas was 1.4, 1.3, 2.2 and 1.6 times that on unburned sites in the four successive years following the fire. Annual forbs and annual grasses contributed 56% of total dry matter the first year following fire. Perennial forbs contributed 40, 75, 75, and 77% of dry matter on the burned sites in successive years following the fire, compared with 66% on the unburned site. Herbage mineral concentrations did not change appreciably following fire, although low values for nitrogen and potassium were apparent in the first year.Responses of herbaceous understories in Ponderosa pine (Pinus ponderosa) communities to fire in the northern Rocky Mountains have not been extensively evaluated. Although investigations in Arizona by Pearson et al. (1972), Campbell et al. (1977, and in central Washington (Weaver 1951) have been made, Mueggler (1976) concluded that generalizations are difficult because each combination of climate, vegetation, and soil must be considered separately. Further, none of these investigations evaluated trends in productivity, vegetative composition, and nutrient content simultaneously.A lightning-caused fire in Ponderosa pine and adjacent montane grassland on August 10, 1973, in the Upper Selway River (White Cap Creek), Idaho, provided opportunities to assess understory responses. It burned for 43 days over an 1,100-ha area, when it was extinguished by rain (Mutch 1974). The area is a portion of a fire management zone in the Selway-Bitterroot Wilderness where fires are allowed to burn under most natural conditions.Objectives of this study were to follow changes in vegetative production, composition, and mineral content on the burned area and compare with adjacent unburned sites. Study AreaElevations on the burn ranged from 935-1,830 m, with all study plots on south-facing slopes at 1,000-1,300 m above sea level. Soils were derived from Idaho batholith parent material consisting of decomposed granite, gneiss, and rhyolite (Greenwood and Morrison 1967). Habeck (1972) characterized these soils as low in fertility, water holding capacity and of weak structure. Average annual precipitation was 76 cm with peaks occurring November-January and April-June (Finklin 1977 The fire occurred under extremely dry conditions, with relative humidity as low as 11%, maximum temperatures to 32° C, and winds gusting to 45 km/ hr. Ground cover on south-exposed slopes was sparse and unevenly distributed. Burned study sites showed evidence of burned stubble but no litter. The fire burned rapidly over these sites and left little ash. Adjacent unburned slopes of similar topographic characteristics were used for comparisons.Elk (Cervus elaphus) and deer (Odocoileus hemionus, 0. virginianus) used these areas as winter-spring range. McCulloch (1955) reported heavy utilization of browse by elk on adjacent shrub winter ranges in the early 1950's, and pr...

Shrub Responses after Fire in an Idaho Ponderosa Pine Community

“…Nevertheless, overall response from burning was lower than found on more mesic sites further north in Idaho (Leege and Hickey 1971). The observed response of shrubs to this burn was similar to the effect on herbaceous species on adjacent xeric sites (Merrill et al 1980).…”

Effects of fire exclusion on forest structure and composition in unlogged ponderosa pine/Douglas-fir forests

“…Fire exclusion is thought to increase stand densities, increase shade-tolerant species in the overstory, increase overall plant cover, decrease grasses, slow nutrient cycling, and decrease overall species diversity (Cooper, 1960;Merrill et al, 1980;Covington and Moore, 1994a,b;Arno et al, 1995b;Fulé et al, 1997;Keane et al, 2002;MacKenzie et al, 2004;DeLuca and Sala, 2006). However, it is increasingly recognized that fire effects in ponderosa pine forests can vary considerably in space and time (Brown et al, 1999;Schoennagel et al, 2004) and www.elsevier.com/locate/foreco Forest Ecology and Management 237 (2006) [418][419][420][421][422][423][424][425][426][427][428] vegetation responses may reflect this variability.…”

Principal short-term findings of the National Fire and Fire Surrogate study

“…Log mass and cover were reduced with burning, especially with fall burns compared to spring burns, while the density of logs was unchanged at the Southern Sierra FFS (Knapp et al 2005 (Laughlin et al 2004, Merrill et al 1980, and combined treatments (Fulé et al 2005 Charring temperature is a critical variable for production of charcoal that may enhance plant growth, suggesting that fi re intensity is important for predicting the extent to which site productivity is enhanced. Spatial heterogeneity in productivity was associated with the creation of charcoal at different temperatures, corresponding to spatial heterogeneity in fuels, fi re intensity, and fi re severity.…”