Effects of tree cutting and fire on understory vegetation in mixed conifer forests

[Display omitted] •41 studies examined effects of tree cutting and fire on mixed conifer forest understories.•Understories often decreased within four years after treatment, then increased.•Some native species were most common after cutting+fire than cutting alone.•Amount of overstory canopy reduction, slash treatment, and herbivory affected response.•Reducing overstory canopy cover well below 40–50% elicits the strongest response. Mixed conifer forests of western North America are challenging for fire management, as historical fire regimes were highly variable in severity, timing, and spatial extent. Complex fire histories combined with site factors and other disturbances, such insect outbreaks, led to great variation in understory plant communities, and management activities influence future dynamics of both overstory and understory communities. This variation needs to be considered as part of ecosystem-scale efforts to influence future fires and restore the composition and structure of mixed conifer forests. We undertook a systematic review of published studies evaluating effects of tree cutting and fire on understory vegetation in western North American mixed conifer forests. Forty-one studies, published in 50 articles, met inclusion criteria and encompassed projects in seven states in the USA and British Columbia in Canada. Total understory plant abundance (cover, biomass, or density) commonly declined in the short term within 4years after treatment. This may result from damage to plants during tree cutting operations or fire, heavy loadings of slash, little change or even expansion of tree canopies after low-intensity treatments, herbivory, or drought. In contrast, all 7 studies measuring understories longer than 5years since treatment reported increases in understory metrics. Treatments in these long-term studies also persistently decreased tree canopy cover. Most or all native species endured (even if reduced in abundance) through cutting operations or fire. A model of understory response has emerged that treatments generally do not eliminate species, and often benefit species absent or uncommon in untreated forest. Groups of native species (e.g., Epilobium spp.) appear fire-dependent, because they are uncommon or absent in unburned mixed conifer forests and after tree cutting alone. Cutting and prescribed fire applied together resulted in the greatest invasion of non-native plants, but non-native cover was minimal compared to native cover. Few stu