Abstract
The effectiveness of low and high intensity prescribed fires in restoring the composition and spatial 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 to restore 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.
Highlights
Before Euro-American settlement, frequent fires influenced the composition, structure, and dynamics of mixed-conifer forests in the Sierra Nevada, California
Tree mortality was much lower and reduced total stem density by an additional 2% of the trees in the low intensity burn; second-year mortality in the high intensity burn was 13% of the trees that were still alive after the first post-fire year
The change in total basal area was relatively small, with a reduction due to fireinduced mortality of 3% in the low intensity burn (Table 2) and 20% in the high intensity burn (Table 3)
Summary
Before Euro-American settlement, frequent fires influenced the composition, structure, and dynamics of mixed-conifer forests in the Sierra Nevada, California. Due to the frequency of fire occurrence and subsequent fire-caused mortality, primarily in the smallest tree cohort, most areas of the Sierra Nevada mixed conifer forest type (approximately 5900 km2, Davis and Stoms 1996) probably had low densities of understory trees and shrubs. Under the pre-settlement fire regime, fires most often burned under low-moderate intensity with patches of high intensity. In these patches, direct fire-induced mortality and subsequent bark beetle (Dendroctonous spp.) attack on the damaged trees lead to scattered openings within a matrix of surviving trees (Stephenson et al 1991, Stephens et al 1999). Studies that have reconstructed stand composition and structure of these stands report much lower stem densities than today, larger trees, and a strong clumping of overstory trees (Harrod et al 1999) (Figure 1)
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