, 2000) or with managed active fire programs (e.g., Sequoia/Kings Canyon

National Parks; Webster and Halpern, 2010), the key evolutionary process of low- and mixed-severity fire has been excluded after settlement (Heinlein et al., 2005, Baker et al., 2007 and Falk et al., 2011). Fuel loads accrued during the 1900s support severe, stand-replacing fire regimes in many areas (Freeman et al., 2007, Crotteau et al., 2013 and Fornwalt and Kaufmann, 2014). Tree density and basal area have increased on average by orders of magnitude, now often exceeding 1000 trees ha−1 and 30–80 m2 ha−1 basal area (Cocke et al., 2005, North et al., 2007 and Fulé et al., 2009). Tree composition has generally shifted toward an increased proportion of species with low fire tolerance and higher shade tolerance, at the expense of fire-tolerant species such as Pinus ponderosa Gemcitabine datasheet (ponderosa pine; Barbour et al., 2002, Vankat, 2011 and Abella et al., 2012). Concomitant with increased tree

density, light reaching the forest floor has decreased, while O horizons have thickened ( Bigelow and North, 2012 and Lydersen et al., 2013). Stocking levels of livestock (primarily cattle and sheep) peaked in the mid-1800s or early 1900s among regions, with likely profound but poorly understood CH5424802 impacts ( Riggs et al., 2000). A suite of non-native species, ranging from tree pests to plants, can dramatically influence mixed conifer forests at local to regional scales ( Hessburg and Agee, 2003). Associated with these land use and forest structural changes,

examples of repeat-photography studies and historical records have frequently revealed dramatic changes in understory vegetation since the early Euro-American settlement period. In early settlement photos of Rocky Mountain mixed conifer forests in Idaho and Montana, Gruell (1983) showed examples ∼50–100 years later of before herbaceous understories of Lupinus spp. or Pseudoroegneria spicata (bluebunch wheatgrass) largely disappearing under expanded tree canopy; reduced shrub understories such as of Shepherdia canadensis (buffaloberry); and shifts in shrub dominance such as to Cercocarpus ledifolius (mountain mahogany). Striking aspects of the geographically extensive photos included abundant evidence of disturbance (predominately fire, timber cutting, and livestock) in the late 1800s/early 1900s which related to mosaics of different understories, and numerous pathways of vegetation change in the 1900s, but generally significant expansion of conifer trees and reduced herbaceous plants and shrubs ( Gruell, 1983). In analyzing historical inventories from 1897–1902 in California mixed conifer forest reserves, McKelvey and Johnston (1992) concluded that understories were sparse at that time (even though overstories remained open and dominated by large, old trees) owing to drought in the late 1800s, intensive livestock grazing, and severe burning by sheepherders.