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

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 4 years 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 5 years 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 studies examined influences of intensity of tree cutting or severity of prescribed fire, but overstory-understory relationships suggest that treatments must substantially reduce overstory density from maximum values (which can exceed 3000 stems ha-1 and 80 m2 ha-1 basal area) and tree canopy cover to <30-50% cover to elicit appreciable responses from the forest understory. Few studies examined understory dynamics after wildfire relative to unburned forest, and further work is warranted because wildfire is a likely eventual outcome of passive management in these forests. Across a broad region from the southwestern United States into Canada, prescribed fire and tree cutting consistently increased disturbance-promoted native species in the short term and total understory abundance in the long term. Active management using tree cutting and fire will likely benefit both biodiversity conservation and fire management in current mixed conifer forests.