Tradeoffs in growth and fuel reduction when using prescribed fire in young mixed conifer stands

Abstract Background Following high-severity wildfires in conifer forests that rely on wind dispersal for regeneration, reforestation practices are used to hasten the development of large, fire-resistant trees that are better able to persist through the next fire. Planted forests, however, are particularly prone to high-severity wildfire effects for the initial decades following their establishment. This vulnerability suggests the need for surface fuel reduction treatments in the early stages of stand development. Such treatments would be aligned with the disturbance regime of frequent-fire forests and could also hasten the development of multi-aged structures dominated by fireresistant trees. Conventional treatments early in stand development include shrub competition reduction and precommercial thinning, but prescribed fire is typically avoided. To assess the potential for inclusion of prescribed fire in young stand management, we investigated how four different treatments – mastication, mastication plus herbicide, two prescribed burns, and mastication plus two burns – affected individual and stand-level growth versus fuel loads in young mixed-conifer plantations in the north-central Sierra Nevada, California, USA. Results Total surface fuel load more than doubled over five years in the mastication only and mastication plus herbicide treatments. Fuel accumulation was avoided in the two treatments that included prescribed fire. Stand growth was similar across the mastication only, mastication plus herbicide, and mastication plus burn treatments. Stand growth was lowest in the burn only treatment. The mastication plus herbicide treatment maximized individual tree growth, especially for white fir and incense-cedar. Individual tree growth was similar among the burn only, mastication only, and mastication plus burn treatments. Conclusions Mastication followed by repeated prescribed burning could be a viable management strategy to reduce wildfire hazard without sacrificing growth in young mixed-conifer stands that are entering a vulnerable stage of fire risk. Mastication in combination with herbicide may facilitate the growth of large, fire-resistant trees, but does not address surface fuel buildup. The use of fire alone can effectively reduce fuels, but stand growth may be somewhat low relative to mastication and herbicide. Incorporating prescribed fire is a promising approach for protecting planted stands from high-severity fire while transitioning them into multi-aged structures. Gapbased silvicultural systems that facilitate the development of multi-cohort stands can also use prescribed fire broadly in order to restore heterogeneity and low surface fuel loads.