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

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 early‐successional 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.