Restoring Northern Sierra Nevada Mixed Conifer Forest Composition and Structure with Prescribed Fires of Varying Intensities

Abstract

The effectiveness of low and high intensity prescribed fires in restoring the composition andspatial structure in a mixed conifer forest in the Northern Sierra Nevada is examined. The overstocked pre-fire stand had 480 trees ha-1, a basal area of 39.5 m² ha-1, and an inverse J shaped diameter distribution with an average dbh of 23 cm. Prescribed fires produced tree mortality in the lower and intermediate dbh-classes and affected trees up to 40 cm dbh. In the low intensity prescribed fire, total tree density was reduced by 33% and basal area by 3% three years after fire. In the high intensity prescribed fire, total tree density and basal area was reduced by 73% and 20%, respectively, two years after the fire. The high intensity prescribed fire changed the dbh distribution from inverse-J to bell-shaped. The spatial structure of the stand burned under low intensity, assessed with the Winkelmass-method, was not altered. In the area burned with high intensity prescribed fire, post-fire tree mortality created gaps in the overstory and led to a higher degree of spatial clumping, attributes that are similar to some old growth stands. The low-intensity prescribed fire was not intense enough to change forest structure significantly. Prescribed fires of at least moderate intensity may be needed to begin torestore current mixed conifer stands to pre-settlement conditions. Burning the accumulated surface fuels created from fire suppression and past harvesting with a low intensity fire may be useful in reducing fire hazards but this may not produce other restoration goals of lower tree densities and canopy gaps.