Reintroducing fire in regenerated dry forests following stand-replacing wildfire.

Abstract

Prescribed fire use may be effective for increasing fire resilience in young coniferous forests by reducing surface fuels, modifying overstory stand structure, and promoting development of large trees of fire resistant species. Questions remain, however, about when and how to reintroduce fire in regenerated forests, and to what end. We studied the effects of spring prescribed fires on stand structure and canopy fuel properties in 25- to 34-year old ponderosa pine forest that was planted following the Entiat wildfire in 1970. Six adjacent units were ignited over the course of four days within a 256-acre, south-facing management unit in the Preston Creek drainage, near Entiat, Washington. Fire effects were assessed on a grid of 264 small (0.014 acre) plots, of which 219 (83%) contained at least one tree. Fires reduced mean tree density from 426 to 280 trees per acre and reduced mean stand basal area from 47 to 38 ft²/acre. Fires also modified canopy fuels, raising mean canopy base heights from 1.0 to 6.3 feet and reducing canopy bulk density from 0.0064 to 0.0061 lbs/ft³. Fire behavior and fire effects were heterogeneous within treatment units, however, and local fire severity was positively correlated with local stand basal area. Tree mortality probabilities declined with increasing tree diameter for all species. For any given diameter, however, mortality probabilities increased with local stand basal area, probably due to higher fuels and local fire intensity. At the median basal area (37 ft²/acre), fires killed mostly small trees (dbh < 2 inches). In more dense stands, fires also killed larger trees (dbh up to 5-6 inches). Mortality rates varied little among species except for larger trees in patches with high basal area, where survival rates were higher for ponderosa pines than for Douglas-firs and other conifer species. Overall, prescribed fires were effective for thinning stands from below, raising canopy base heights, and, to some extent, favoring ponderosa pine over Douglas-fir and lodgepole pine. Additional fires (and possibly some mechanical thinning) may be needed, however, to maintain low surface fuel loads, further modify canopy fuels, and further increase forest resilience to future wildfires.

Parent Publication

• Restoring fire-adapted ecosystems: proceedings of the 2005 national silviculture workshop

Citation

Peterson, David W.; Hessburg, Paul F.; Salter, Brion; James, Kevin M.; Dahlgreen, Matthew C.; Barnes, John A. 2007. Reintroducing fire in regenerated dry forests following stand-replacing wildfire. In: Powers, Robert F., tech. editor. Restoring fire-adapted ecosystems: proceedings of the 2005 national silviculture workshop. Gen. Tech. Rep. PSW-GTR-203, Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture: p. 79-86