Fuels treatment longevity of mechanical mastication and growth response of ponderosa pine (Pinus ponderosa) in northern California

Many fire-prone forests in western North America suffer from hazardous fuel conditions. Mechanical mastication is an increasingly common method of fuels treatment, but little is known regarding long-term effectiveness. A randomized block design at two sites (northern Sierras and southern Cascades) compared ladder fuels and overstory growth among treatments including mastication alone and mastication followed with prescribed fire or herbicide 10 or 11 years post-treatment. Subsequent herbicide application reduced ladder fuels in comparison to mastication alone or with prescribed fire. Prescribed fire further reduced ladder fuels at the southern Cascades site, however, in the northern Sierras post-fire ladder fuels were positively related to overstory absence. Mastication alone effectively released ponderosa pine at the southern Cascades site, whereas neither herbicide nor prescribed fire affected pine radial growth. This study demonstrates the feasibility of prescribed fire and herbicide for increasing treatment longevity of mastication, but also highlights potential limitations and important considerations.