Fire history and the structure and dynamics of a mixed conifer forest landscape in the northern Sierra Nevada, Lake Tahoe Basin, California, USA

Abstract

The goal of this study was to understand how fire regimes promote fine- and coarse-grain vegetation patterns in an old-growth mixed conifer forest dominated landscape in the General Creek watershed on the west shore of Lake Tahoe, California. We quantified the structure (e.g., composition, age, and size) of old-growth mixed conifer stands located across a range of environmental settings. Fire histories were reconstructed using fire-scar dendrochronology, and the influence of regional climatic variability on fire occurrence was assessed by relating the fire record to regional climate reconstructions. Fire regimes parameters varied across topographic gradients at landscape scales promoting fine grain forest structural patterns. The timing and extent of fires was related to inter-annual and inter-decadal variation in drought which was linked to the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. Coarse scale vegetation patterns where related to upper slope positions and relatively infrequent high severity fires. Fire regimes and forest structure have changed since EuroAmerican settlement with virtually no fires and structural shifts towards higher stand densities and a greater representation of fire intolerant species. At the landscape scale, fire regimes and forests patterns in mixed conifer forests are influenced by a variety of process operating at multiple spatial and temporal scales. Coarse scale heterogeneity related to topography and moderate to high severity fire is superimposed on fine scale variability related to topographic gradients and local variability in fuel and forest structural characteristics. Fire suppression has resulted in a more homogenous landscape particularly with regard to the loss of coarse scale heterogeneity.