Managing Fuels, Tree, and Stand Growth Using Prescribed Fire and Alternatives in Young Mixed Conifer Stands

Background In the wake of increasingly frequent and severe wildfires in California, artificial regeneration and density management facilitate prompt reforestation and the rapid growth of large, fire-resistant trees. Young plantations are particularly prone to high-severity wildfire effects, suggesting the implementation of fuel reduction treatments in the early stages of stand development. The extent to which density management (i.e., thinning) and fuels management (i.e., prescribed fire) can work together is uncertain given their potentially conflicting effects on tree and stand level growth. 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 mixed-conifer plantations during young stand development in the north-central Sierra Nevada, California, USA. Results The mastication plus herbicide treatment maximized individual tree growth, especially for white fir and incense-cedar, but fuel loads doubled after five years without the use of fire. The mastication only treatment resulted in a 151% increase in fuel loads over the same period, and individual tree growth was comparable to the burn only and mastication plus burn treatments. The burn only treatment greatly decreased fuel loads but also resulted in low relative stand growth. The mastication plus burn treatment prevented fuel accumulation and generally did not slow down individual tree growth. In addition, stand growth occurred at a rate similar to that of the mastication plus herbicide treatment. 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 grow trees to a large, fire-resistant size more quickly, but does not address fuel buildup. The use of fire alone can effectively reduce fuels while not substantially impacting individual tree growth, but stand growth may decline relative to mastication and herbicide.