Authors: Eric Knapp; Jamie Lydersen; Malcolm North; Brandon CollinsHistorical forests shaped by fire were variable, resilient, and biodiverse, suggesting that managing for multiple resources may benefit from restoring forest hetergeneity. Units in second-growth mixed conifer forest in the central Sierra Nevada of California were either mechanically thinned using a prescription designed to create structural variability based on the spatial arrangement of trees in historical forests, thinned to an even crown spacing, or left unthinned. Half of the units were followed with a prescribed burn. Post-treatment vegetation data were collected one and three years after implementation. Mechanical thinning removed 77% of trees and 40% of the basal area, and by favoring pines over fir and cedar, produced a species composition similar to nearby historical reference plots. The prescribed fire only treatment killed 29% of trees but did not significantly reduce basal area. Variable thinning increased within-stand structural heterogeneity relative to even thinning or prescribed fire alone and did so at spatial scales similar to heterogeneity found in historical forests. Understory shrubs, forbs, and grasses responded positively to both thinning and prescribed fire treatments, with the combined treatments having the strongest effect. While results are still early, the high variability thinning/prescribed fire treatment produced the best outcome by restoring overstory structure and surface fuel loads, both of which should improve resilience while also maintaining or enhancing understory habitats.
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