Unsupported inferences of high‐severity fire in historical dry forests of the western United States: response to Williams and Baker

Abstract

Reconstructions of dry western US forests in the late 19th century in Arizona, Colorado and Oregon based on General Land Office records were used by Williams & Baker (2012; Global Ecology and Biogeography, 21, 1042-1052; hereafter W&B) to infer past fire regimes with substantial moderate and high-severity burning. The authors concluded that present-day large, high-severity fires are not distinguishable from historical patterns. We present evidence of important errors in their study. First, the use of tree size distributions to reconstruct past fire severity and extent is not supported by empirical age-size relationships nor by studies that directly quantified disturbance history in these forests. Second, the fire severity classification of W&B is qualitatively different from most modern classification schemes, and is based on different types of data, leading to an inappropriate comparison. Third, we note that while W&B asserted 'surprising' heterogeneity in their reconstructions of stand density and species composition, their data are not substantially different from many previous studies which reached very different conclusions about subsequent forest and fire behaviour changes. Contrary to the conclusions of W&B, the preponderance of scientific evidence indicates that conservation of dry forest ecosystems in the western United States and their ecological, social and economic value is not consistent with a present-day disturbance regime of large, high-severity fires, especially under changing climate. KeywordsFire regime, fire severity, General Land Office survey, historical range of variability, ponderosa pine, wildfire. INTRODUCTIONA recent study in Global Ecology and Biogeography (Williams & Baker, 2012, hereafter W&B) applied historical data from General Land Office (GLO) surveys c. 1880 to reconstruct historical dry forests on four large landscapes in Arizona, Colorado and Oregon (USA). W&B described forest composition as characterized by 'abundant ponderosa pine (Pinus ponderosa C. Lawson), with lesser amounts of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), grand fir (Abies grandis (Douglas ex D. Don) Lindl.), juniper (Juniperus L.), western larch (Larix occidentalis Nutt.) and lodgepole pine (Pinus contorta Douglas ex Louden)' . W&B infer that the presence of a certain proportion of trees below a site-specific diameter threshold is evidence of 'higher'-severity fire, a term they use to indicate a combination of 'mixed-plus high-severity fire' . They then compare the proportion of plots with evidence of 'higher'-severity fire to the distribution of fire severities estimated from satellite imagery of recent large wildfires in the western United States, concluding that modern fires are within the historical range of variability (HRV) of fire severity. They also found it 'surprising' that dry forests 'commonly thought to have been open and parklike' were heterogeneous and relatively dense. W&B conclude that current management pr...

Tamm Review: Management of mixed-severity fire regime forests in Oregon, Washington, and Northern California

“…Their inferences are based on conclusions drawn from vegetation reconstructions using General Land Office (GLO) or federal Forest Inventory and Analysis (FIA) data. While these data are useful for general descriptions and tabulations of historical vegetation conditions, they are unsuited to making spatially accurate inferences as to local historical vegetation conditions, or for inferring disturbance regimes from size distributions of trees (Fulé et al, 2014;Stevens et al, in press). While needs vary both regionally and locally, we strongly disagree with the contention that ecological restoration is unnecessary in MSForests of the Inland Pacific West, as do a host of authors throughout the Inland West: Barth et al (2015), Collins et al (2011aCollins et al ( , 2015, Gaines et al (2010aGaines et al ( , 2010b, Spies et al (2010), Hessburg and Agee (2003), Hessburg et al (1999aHessburg et al ( , 1999bHessburg et al ( , 2000aHessburg et al ( , 2005Hessburg et al ( , 2013Hessburg et al ( , 2015, Taylor (2004), Stephens et al (2009Stephens et al ( , 2010Stephens et al ( , 2015, Moghaddas et al (2010), Scholl and Taylor (2010), Hagmann et al (2013Hagmann et al ( , 2014, Merschel et al (2014), Perry et al (2011), Harris and Taylor (2015), and Franklin and Johnson (2012).…”

Forest tree neighborhoods are structured more by negative conspecific density dependence than by interactions among closely related species

“…Large trees shade their smaller neighbors, dominate the rooting zone, and harbor large populations of pathogens, herbivores and mutualists (e.g., mycorrhizae) (Laliberte et al 2015). Individual trees often live for centuries, and although age-size relationships can be highly variable (Youngblood et al 2004, Fulé et al 2014, larger individuals usually tend to be older than smaller conspecifics in forests that are in stable conditions (Leak 1985). Thus, we used the size of focal trees (diameter at 1.3 m above ground, DBH) as an integrator of the strength and duration of interactions with neighbors and asked whether the phylogenetic composition of neighbors surrounding focal trees shifted with focal tree size.…”

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

“…Fulé et al (2014) suggested some of our GLO methods were invalid. In response, we explained that our methods were extensively tested, validated, and shown to be accurate, and we added new corroboration .…”

“…Some are concerned that recent high-severity patch sizes are uncharacteristically large and damaging in dry western forests (Stephens et al 2013, Fulé et al 2014). However, these articles surprisingly presented no patch-size data (Williams and Baker 2014).…”