Countering Omitted Evidence of Variable Historical Forests and Fire Regime in Western USA Dry Forests: The Low-Severity-Fire Model Rejected

Abstract

The structure and fire regime of pre-industrial (historical) dry forests over ~26 million ha of the western USA is of growing importance because wildfires are increasing and spilling over into communities. Management is guided by current conditions relative to the historical range of variability (HRV). Two models of HRV, with different implications, have been debated since the 1990s in a complex series of papers, replies, and rebuttals. The “low-severity” model is that dry forests were relatively uniform, low in tree density, and dominated by low- to moderate-severity fires; the “mixed-severity” model is that dry forests were heterogeneous, with both low and high tree densities and a mixture of fire severities. Here, we simply rebut evidence in the low-severity model’s latest review, including its 37 critiques of the mixed-severity model. A central finding of high-severity fire recently exceeding its historical rates was not supported by evidence in the review itself. A large body of published evidence supporting the mixed-severity model was omitted. These included numerous direct observations by early scientists, early forest atlases, early newspaper accounts, early oblique and aerial photographs, seven paleo-charcoal reconstructions, ≥18 tree-ring reconstructions, 15 land survey reconstructions, and analysis of forest inventory data. Our rebuttal shows that evidence omitted in the review left a falsification of the scientific record, with significant land management implications. The low-severity model is rejected and mixed-severity model is supported by the corrected body of scientific evidence.

Contemporary Wildfires Not More Severe Than Historically: More Fire of All Severities Needed to Sustain and Adapt Western US Dry Forests as Climate Changes

“…Baker [3] showed this for nearly all dry forests, except for parts of California. Also, the low-severity fire model for historical dry forests was rejected by Baker et al [11] after correcting extensive false and omitted evidence in Hagmann et al [7], and an alternative mixed-severity fire model was accepted. Under this supported alternative mixed-severity fire model, primarily low-to moderate-severity fire was excluded in the modern era, and high-severity fires are not burning recently at exceptional rates [11,12].…”

“…Also, the low-severity fire model for historical dry forests was rejected by Baker et al [11] after correcting extensive false and omitted evidence in Hagmann et al [7], and an alternative mixed-severity fire model was accepted. Under this supported alternative mixed-severity fire model, primarily low-to moderate-severity fire was excluded in the modern era, and high-severity fires are not burning recently at exceptional rates [11,12]. Under the mixed-severity fire model, the primary ecological fire need is to just restore the low-to moderate-severity fires that were excluded; reducing high-severity fire under this mixed-severity model is fire suppression, well known to be ecologically damaging [13].…”

“…These CFI count estimates have been shown to be incorrect and substantially too short [15], because they do not measure the areas of fires, but just count fires equally and measure intervals between them in lists, although most are small. Newer landscape-scale methods are available that explicitly measure fire areas and avoid this and the many other limitations of these older small-plot CFI or MFRI estimates [11]. Accurate regression methods are also now available [15] to correct old CFI estimates to FRs, as is done here with the uncorrected Landfire estimates.…”

Surveyor and analyst biases in forest density estimation from United States Public Land Surveys

“…Although density estimates are derived through a seemingly esoteric exploration of historic surveying methodology and the statistics of point pattern analysis, accurate density estimates have potentially profound influence and broad applications. The use of historic land surveys has been invaluable to current research ranging from the influence of Indigenous populations and the prevalence of open ecosystems (e.g., Abrams et al, 2022; Hanberry, Bragg, et al, 2020; Whitney & Steiger, 1985), to the role of fire and the restoration and management of forests (e.g., Baker et al, 2023; Knight et al, 2020; Meunier & Shea, 2020), to the determination of aboveground biomass of undisturbed forests (e.g., Hanberry & He, 2015; Rhemtulla et al, 2009), or to the parameterization and validation of theoretical vegetation models (e.g., Blankenship et al, 2021; Raiho et al, 2022). Unfortunately most previous density estimates lack any evaluation of bias or methodology for assuring the accuracy of historic survey data.…”

“…For over a century, myriad studies have reconstructed early species composition from these records. Recently measurements of the distance to witness trees have been used to infer historical forest structure (e.g., Baker et al, 2023; Bourdo, 1956; Cottam, 1949; Goring et al, 2016; Knight et al, 2020; Kronenfeld & Wang, 2007; Manies et al, 2001; Paciorek et al, 2021; Rhemtulla et al, 2009).…”