Influence of topography and fire history on Sierra Nevada mixed-conifer stand conditions under an active-fire regime

Background/Question/Methods

Following 90 years of fire suppression and fuels accumulation, Sierra Nevada forest management is often focused on reducing the risk of high-severity fire without compromising habitat for sensitive species. One method proposed for balancing these objectives is to manage forest landscapes for the heterogeneity of forest conditions once produced by an active-fire regime. Reconstructions can provide some insight into historic structure and composition of forests but their methods do not provide reliable estimates of the density and species composition of small trees, and they may not represent how current forests would respond to frequent fire under modern climate conditions. This study measured structure and composition of unmanaged mixed-conifer forests with the best available approximation of an active-fire regime (> 2 low intensity fires within the last 60 years) under recent climate conditions. Objectives of the study were to quantify the stand structure, composition, diameter distribution, and fuel loads of active-fire forests, and to determine how these forest conditions varied with fire history and topographic position (i.e., slope position and aspect).

Results/Conclusions

Regeneration, shrub cover, and small-tree density were predominantly associated with fire history metrics such as intensity and time since last burn, while large tree density was more dependant on topographic location. Fuel loads were influenced by a combination of fire history and site moisture. Trees <40 cm DBH accounted for 46% of live stems, and 13% of basal area. Structural differences between topographic positions were subtle, excepting ridge top plots. Together, lower, middle and upper slopes had an average basal area of 60 m²/ha, of which 52% were shade-intolerant species. In these plots, average stem density was 238 trees/ha, canopy cover was 48%, and the downed woody fuel density was 44 Mg/ha. On ridges basal area was 21 m²/ha, 74% were shade-intolerant species, stem density was 111 trees/ha, canopy cover was 19%, and woody fuel density was 17 Mg/ha. Our research suggests greater small tree densities than most reconstruction studies and may provide preliminary guidelines for the number and size of smaller leave-trees in fuels treatments. In general, near surface forest conditions (i.e., fuels, shrubs, regeneration, and small tree densities) were strongly associated with fire frequency and intensity, while large tree density was more associated with topographic differences in productivity.