Merging prescribed fires and timber harvests in the Sierra Nevada: Burn season and pruning influences in young mixed conifer stands

Abstract

In dry, productive forests where historically infrequent high-severity fires are now common, new silvicultural systems will be needed to better align management activity with the ecosystem's dependent disturbance regime of frequent low and moderate-severity fires. Merging timber harvests with prescribed fire programs can be advantageous because each disturbance provides benefits that the other cannot provide alone. We conducted a study aimed at providing information to managers interested in merging gap-based silviculture with frequent prescribed fire. We studied the influence of burn season (spring versus fall) on canopy mortality and damage by conducting prescribed burns in 13-14 year old stands that had been regenerated with gap-based silviculture. We also pruned sugar pine (Pinus lambertiana) and incense-cedar (Calocedrus decurrens) prior to burns to evaluate the influence of pruning on fire related mortality and damage. Fall (21%) and spring (19%) burns resulted in similar amounts of mortality two years following burns, but the fall burns consumed considerably more fuel compared to spring burns. Percent volume crown scorch was greater in spring burns and greater when crown bases were low to the ground. Fall burns were generally favorable assuming a management context where fuel consumption and survival is desirable. However, either burn season may be acceptable using fire as a thinning mechanism is desired to encourage the development of low-density, mature stands. As a pre-fire treatment, pruning did not clearly reduce fire-related mortality or crown damage. Considering that pruning itself is a form of crown damage, it could be considered counterproductive as a pre-fire treatment because of increased heat entering pruned crowns as a result of increased surface fuel and the loss of heat-buffering lower branches. Merging gap-based silviculture with prescribed fires in perpetuity may be initially complex operationally in the Sierra Nevada, but it offers managers a disturbance regime-guided method for sustaining heterogeneity at fine and coarse scales while maintaining low surface fuels. The timing of introducing fires into young stands as well as traditional timber management tools such as rotation ages and harvest intervals can be altered depending on exact objectives.