Overall conclusions from these, and other studies in mixed conifer forests (e.g., Kaufmann et al., 1998, Gruell, 2001 and Taylor, 2004), indicate that: (i) understory plant cover has generally decreased during the past ∼100 years, likely linked with increased tree density and supported by negative relationships

between tree abundance and understory vegetation ( Larson and Wolters, 1983); (ii) grazing, fire exclusion, different climatic conditions (from the 1800s to today), and other factors (e.g., air pollution) have likely interacted to change composition or abundance of understory plants in ways not well understood; and (iii) it is difficult to ascertain specific past reference conditions for these understories, suggesting SCH900776 opportunity for developing find more reference information based on how contemporary vegetation responds to disturbance to help guide future forest management efforts. The primary question of our systematic review was: How do tree cutting and fire influence understory vegetation in western North American mixed conifer forests? We had five specific questions, each with anticipated outcomes: (1) Do tree cutting, managed fire (prescribed or wildland fire use), tree cutting + managed fire, and wildfire

have different effects on total understory plant abundance (cover, biomass, density, or other reported measures) and species richness? We anticipated that relative treatment effects would increase in the order: managed fire < cutting < cutting + managed fire < wildfire. Our rationale was that, owing to negative relationships between overstory tree abundance and understory vegetation in undisturbed mixed conifer forests (Larson and Wolters, 1983), treatments should represent a gradient of decreasing overstory

correlated with increasing understory vegetation. Note that we purposely refer to wildfire as a ‘treatment’ in this paper, because wildfire often is an eventual ‘de facto’ treatment implemented via passive management in these forests (Stark et al., 2006, Knapp et al., 2012 and Crotteau et al., 2013). Mixed conifer forests frequently occupy elevations between those supporting lower-elevation P. Galactosylceramidase ponderosa or Pinus jeffreyi (Jeffrey pine) forests and higher-elevation forests such as pure Abies concolor (white fir) or subalpine including Picea-Abies (spruce-fir) among different regions supporting mixed conifer forests ( Battaglia and Shepperd, 2007). Minimum elevation required to support mixed conifer forests generally decreases from southern to northern latitudes ( Klenner et al., 2008). Major physiographic regions occupied by mixed conifer forests include the inland Pacific Coast (e.g., Klamath and Sierra Nevada Mountains), Intermountain Region, and Rocky Mountains ( Fig. 1).