Natural range of variation for yellow pine and mixed-conifer forests in the Sierra Nevada, southern Cascades, and Modoc and Inyo National Forests, California, USA

Synergistic effects of long‐term herbivory and previous fire on fine‐scale heterogeneity of prescribed grassland burns

“…Herbivores are known to increase the heterogeneity of vegetation at both larger (between‐patch) and smaller (within‐patch) spatial scales, often via the creation of bare patches (Taylor et al 1994, Porensky et al 2013). Different fire frequencies and severities can also promote heterogeneity of vegetation, as has been observed in systems ranging from tallgrass prairie to mixed‐conifer forest (Gibson et al 1990, Safford and Stevens 2017).…”

Evidence for widespread changes in the structure, composition, and fire regimes of western North American forests

“…Paired tree-ring and sedimentary charcoal-based fire histories from the same locations show 20th-century decreases in fire occurrence that are unprecedented in recent millennia (Allen et al 2008, Beaty and Taylor 2009, Swetnam et al 2009). Frequent fire reduces the intensities and severities of subsequent fires by maintaining tree densities and live and dead fuel loads at levels below those that local site productivity could readily support (Reynolds et al 2013, Stine et al 2014a, Safford and Stevens 2017, Addington et al 2018, Accepted Article Battaglia et al 2018). Overlapping fires limited the spread of crown fire and other contagious processes (e.g., insect outbreaks and disease epidemics, Hessburg et al 1994Hessburg et al , 1999b by reinforcing discontinuities in canopy cover, species composition, tree size and age classes, and surface fuel abundance (Roccaforte et al 2008, Collins et al 2009, Fulé et al 2012a, van Wagtendonk 2018.…”

Biogeography of fire regimes in western U.S. conifer forests: A trait‐based approach

“…Some functional traits of trees (e.g., thick bark) promote survival during fire through protective structures that reduce fire exposure, whereas others (e.g., flashy litter) can alter the fire spread and intensity by influencing the fuel environment (Hood, Varner, van Mantgem, & Cansler, 2018; Keeley, Pausas, Rundel, Bond, & Bradstock, 2011). It is common practice to rank species along a continuum from “fire tolerant” to “fire intolerant” (e.g., Brown & Smith, 2000; Safford & Stevens, 2017), but species rankings are often based on a qualitative understanding of the natural history of species rather than a quantitative assessment of traits associated with surviving fire.…”

“…We distinguish this fire adaptation strategy from other fire‐adapted life histories, such as “fire‐embracing” strategies (Keeley, 2012), which involve loss of aboveground biomass and post‐fire regeneration via resprouting or serotiny and may be adaptive under less frequent, higher‐intensity fire regimes (Pausas, Keeley, & Schwilk, 2017; Schwilk & Ackerly, 2001), and fire‐avoiding strategies, which involve ecosystems that burn infrequently and do not select for fire‐adaptive traits. We chose to focus on fire resistance rather than fire‐embracing traits in our analysis because the degree of fire resistance of different species is hypothesized to be strongly associated with the frequency and spatial extent of surface fire in forests of the western USA (Safford & Stevens, 2017; Steel, Safford, & Viers, 2015), and there is strong morphological variation among widespread species. Furthermore, the question of post‐fire recovery, which is influenced by fire‐embracing traits, dispersal traits and seedling niche requirements, is distinct from the question of which species are best adapted to survive frequent fires, which is the dimension of fire regimes we are considering here.…”