Historical forest structure and fire in Sierran mixed‐conifer forests reconstructed from General Land Office survey data

Abstract

Abstract. Dry forests of the western United States (ponderosa pine, dry mixed conifer) are often considered at risk of uncharacteristic severe fires, but recent research has found historically extensive severe fire. This has left divergent perspectives about how to restore dry forests, protect people and infrastructure from fire, and interpret the ecological effects of large fires, such as the 2013 Rim fire, a human-set 104,000 ha fire in the western Sierra Nevada Mountains. To help resolve this uncertainty, I used new methods to reconstruct historical forest structure and fire and test 11 hypotheses about them, using A.D. 1865-1885 General Land Office surveys, across 330,000 ha of Sierran mixed-conifer forests. The reconstructions show these historical forests were open and park-like in places, but generally dense, averaging 293 trees/ha; shade-tolerant trees and large trees were abundant, but smaller (,60 cm diameter) pines and oaks numerically dominated. These smaller trees, along with common understory seedlings and saplings and almost pervasive shrubs, created abundant ladder fuels. It is not surprising, given these conditions, that just 13-26% of historical Sierran mixed-conifer forests had only low-severity fire, with mixed-severity fire over 43-48%, and high-severity fire over 31-39% of the land area. The high-severity fire rotation was 281 years in the northern and 354 years in the southern Sierra, short enough to contribute to high levels of heterogeneity, including abundant areas and large patches (up to 9400 ha) of earlysuccessional forest and montane chaparral, but long enough to allow recovery of old-growth forest over large land areas. Proposals to reduce fuels and fire severity would actually reduce, not restore, historical forest heterogeneity important to wildlife and resiliency. Sierran mixed-conifer forests are inherently dangerous places to live, which cannot be changed without creating artificial forests over large land areas. However, people can adapt to fires by channeling development to safer areas and modifying ignition zones near houses and communities to survive fire.

Historical and current landscape‐scale ponderosa pine and mixed conifer forest structure in the Southern Sierra Nevada

“…Our estimate of past mixed conifer and ponderosa pine high severity fire (1-6%), which are similar to estimates from other work (Stephens et al 2007a, Mallek et al 2013, are much lower than those reported by Baker (2014) for the Sierra Nevada (31-39%). Perhaps information from contemporary restored fire regimes, for which fire severity patterns can be quantified more robustly, can provide insight.…”

“…Baker (2012Baker ( , 2014 used this system (GLO), which used eight trees per section (259 ha) that were marked to assist in the relocation of survey section corners, to reconstruct historical forest conditions in eastern Oregon and the Sierra Nevada. Four townships (or 144 sections) in eastern Oregon (Baker 2012) overlap an area that used similar historical transect data analyzed in this work (Hagmann et al 2013).…”

“…One challenge in the comparison of these results is possible differences in lower diameter limits in these studies. The transect studies (Collins et al 2011, Hagmann et al 2013, 2014, this work) all have explicit lower diameter limits whereas the GLO reconstructions do not include this information (Table 7). One study that also compared these two methods determined that the large differences in tree densities between the direct inventory and GLO could not be reconciled by possible differences in diameter limits of the two datasets (Hagmann et al 2013).…”

“…That stated, we have identified a historical dataset that covers a large geographic extent (.10,000 ha) and has unbiased sampling locations. This particular dataset consists of early timber inventories conducted by the US Forest Service (USFS) and includes robust tree measurements over large areas, which contrasts with the reconstruction of forest structure from very low density tree measurements from General Land Office records (e.g., Baker 2014). In California, these inventories were conducted ca.…”

Historical Fire Regimes in Ponderosa Pine and Mixed-Conifer Landscapes of the San Juan Mountains, Colorado, USA, from Multiple Sources

“…Ponderosa pine and dry mixed-conifer estimates, if pooled, would be roughly congruent with pooled estimates of 271 years in the Colorado Front Range [37] and 281-354 years in the western Sierra [38], but longer than the 217-year rotation on Black Mesa, northern Arizona [37], and the 175-year rotation on the Uncompahgre Plateau, which is just north of the San Juan Mountains [39].…”