Why Paleo Climate/Fire And Other Study Methods Provide Better Historical Fire Records

Severe deforestation on the Wallowa Whiteman National Forest, Oregon justified by fire scar reconstructions. Photo George Wuerthner 

One of the biggest problems, and also a source of disagreements in wildfire discussions, stems from the use of different temporal and spatial scales. What may seem like excessive wildfire under one set of temporal and spatial assumptions may lead to different conclusions under another study design.

For instance, if you were to view the 1988 Yellowstone Fires, which charred approximately 1.2 million acres of the Yellowstone Park and adjacent area with a hundred-year time scale, these blazes can be considered excessively large. On the other hand, if you were to look back at fire occurrence over a thousand-year period, you might discover that large landscape blazes as occurred in Yellowstone in 1988 are quite the norm for the area.

  A chart showing episodic burned acreage in Yellowstone from 1300 to 1900s. 

Numerous paleo climate-wildfire studies normalize large blazes that are almost always correlated with periods of drought and higher temperatures.

For instance, a wildfire study of Battle Ground Lake in Washington using charcoal in a lake noted a number of long-term changes in vegetation and wildfire, primarily due to climate influences. The study concludes: “From the middle to late Holocene (ca. 5200 cal yr BP to present), forest dominated by Pseudotsuga, Thuja-type, and Tsuga heterophylla supported less frequent, but mostly large or high-severity fire episodes. Fire episodes were least frequent, but were largest or most severe, after ca. 2500 cal yr BP. The fire history at Battle Ground Lake was apparently driven by climate, directly through the length and severity of the fire season, and indirectly through climate-driven vegetation shifts, which affected available fuel biomass.”

While native people living in Yosemite Valley may have increased fire occurence, the overriding influence on fire occurrence at the regional scale was climate. Photo George Wuerthner 

Vachula et al. (2019) reviewed the fire history of what is now Yosemite National Park where, historically, large Indigenous communities resided. Their research found a direct correlation between climate and the amount of burning on the landscape.

“We analyzed charcoal preserved in lake sediments from Yosemite National Park and spanning the last 1400 years to reconstruct local and regional area burned. Warm and dry climates promoted burning at both local and regional scales…

Regional area burned peaked during the Medieval Climate Anomaly and declined during the last millennium, as climate became cooler and wetter and Native American burning declined.

Our record indicates that (1) climate changes influenced burning at all spatial scales, (2) Native American influences appear to have been limited to local scales, but (3) high Miwok populations resulted in fire even during periods of climate conditions unfavorable to fires. However, at the regional scale (< 150 km from the lake), fire was generally controlled by the top-down influence of climate.” (Vachula et al. 2019) Geographer, Thomas Vale came to similar conclusions (Vale, 2002).”

The method used to reconstruct fire history also influences the conclusions. For instance, most fire history studies use fire scars recorded in the tree rings to create an average fire interval. Many problems with using tree ring fire studies are typically not disclosed.

Due to several issues, including the use of “composite” fire scars, some of these studies tend to overestimate the occurrence of low severity blazes and discount or ignore high severity fire. The shorter fire rotations resulting from fire scars are used to justify more human inventions in forest management, including “fuel reductions” by logging or prescribed fire under the assumption that high severity blazes result from abnormal fuel build-up.

Fire scar seen at the base of old growth ponderosa pine in the Blue Mountains of Oregon. Photo George Wuerthner 

Among them is the selective bias of researchers focusing on trees with fire scars. This is not a random process and could bias conclusions.

A second problem with fire scar studies is that their accuracy (to the degree they are accurate) declines the further you go because you have fewer and fewer trees to sample. In other words, finding a tree over 2-300 years old is difficult because fewer trees that live long are available.

The third problem is that many tree ring fire scar studies depend on very few trees. Baker looked at fire scar studies for ponderosa pine and found that more than half utilized only a few trees to draw conclusions about the fire history of entire regions.

The use of “composite” fire scar studies also creates a bias in historical fire reconstructions. Most fire studies add up all the fire scars recorded into a “composite” timeline. As one fire scientist suggests, all this does is count fires.

The problem with this technique is that the more scars you find and count over bigger and bigger areas, the shorter the fire interval becomes and the riskier your assumption that any fire recorded by one tree burned throughout the entire study area, even though some trees didn’t scar in the fire event.

To give one hypothetical example of how this counting fire might skew the results, let’s pretend we have a 1000-acre study area. In that study area, we find evidence for at least one fire every year between 1900 and 2000 or a fire interval of 1 year. However, if each of those fires only burned 1 acre then in a hundred years at most only 10% of the 1000 acres would have been burned. At that rate, the fire rotation would be 1000 years.

A study of forests in the Sierra Nevada concluded that high severity blazes affected 31-39% of the land acre. Photo George Wuerthner 

Other methods for determining past wildfire history often differ from those produced by fire scars. For example, Baker, using General Accounting Office records for California’s Sierra Nevada forests which are commonly described as historically influenced by low severity blazes, came to different conclusions. He found: “that low-severity fires 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.”

A comparative study of tree-ring fire histories with charcoal sediment recorded in lakes of the Klamath Siskiyou region of California-Oregon border revealed: “The comparison indicates that the tree-ring records detected individual fires not evident in the lake-sediment profiles, whereas the charcoal data disclosed variations in fuel loading and general levels of burning at broader spatial scales.”

Proponents of Native America burning making historical fire reconstruction from fire scars often exaggerate the influence of anthropogenic fire by extrapolating the fire frequency from locations used frequently by tribal people to large landscapes.

The Willamette Valley of Oregon may have sustained some of the highest density of Native Americans on the Continent yet studies demonstrated that human ignitions usually were localized and did not influence the larger landscape. Photo George Wuerthner 

For example, Whitlock and Knox looked at wildfire occurrences in Oregon’s Willamette Valley. Due to abundant food resources and mild climate, the Willamette Valley historically had one of the highest population densities of Native Americans on the continent. However, in a study using lake bottom charcoal and pollen, Whitlock notes: “The idea that Native Americans burned from one end of the valley to the other is not supported by our data … Most fires seem to have been fairly localized, and broad changes in fire activity seem to track large-scale variations in climate.”

Noss, et al. 2014 assert: “Despite ample evidence that lightning fire was a primary ecological driver in the NACP [North American Coastal Plain], the myth persists that most fires before the arrival of Europeans were set by Native Americans. For example, Mann (2005; 361) provides a map that shows essentially the entire pre-Columbian NACP, including the lightning-riddled Gulf Coast and Florida peninsula, as ‘dominated by anthropogenic fire’ or with ‘widespread forest clearing for agriculture’. No evidence is offered to support these claims.”

Contrary to the “fire suppression” “fuel Buildup” frequently given for large fires, new research demonstrates a conclusive relationship between climate and wildfire severity and size.

A meta-analysis of wildfire concluded that “global fire activity is low when conditions are cool and high when conditions are warm” demonstrating the overriding influence of climate conditions on wildfire size and frequency.

An NOAA study confirms that climate is the overriding influence on wildfires, found that “climate change enhanced the drying of organic matter and doubled the number of large fires between 1984 and 2015offsite link in the western United States. A 2021 study supported by NOAA concluded that climate change has been the main driver of the increase in fire weather link in the western United States.”

An area that was thinned prior to the climate-weather driven Dixie Fire, one of the largest in California history. Photo George Wuerthner

The NOAA Research shows that changes in climate create warmer, drier conditions, leading to longer and more active fire seasons. Increases in temperatures and the thirst of the atmosphere due to human–caused climate change have increased aridity of forest fuels during the fire season. These drivers were found to be responsible for over half the observed decrease in the moisture content of fuels in western U.S. forests from 1979 to 2015, and the doubling of forest fire burned area over the period 1984–2015.

Thus, we see strong differences between interpretations that utilize fire scars record and other methods that tend to provide a more generalized perspective on landscape scale blazes methods like charcoal, pollen studies, air photo studies, and General Land Office records often come to different conclusions about the occurrence and influence of Native American burning.

Another study of the Klamath Siskiyou region, another region that some researchers who used fire scars characterized as low-to mixed-severity fire regime, found that: “The record reveals a highly episodic pattern of fire…The 11 largest charcoal peaks are significantly related to decadal-scale drought periods.”

They go on to conclude: “This history is best described as alternations between centennial-scale periods of little to no fire with intervals of frequent fires, probably of small size and low severity, punctuated by larger and/or more severe fires that are associated with distinct erosional signature.”

Contrary to some fire scar studies which created open, park-like forest conditions, other methodologies concluded that the Blue Mountains of Oregon had dense vegetation in places and periodic high severity blazes. Photo George Wuerthner

Contrary to assertions by fire scar advocates the Blue Mountains of Oregon were dominated by open forests characterized by low severity blazes,  review by Williams and Baker titled: “Spatially extensive reconstructions show variable-severity fire and heterogeneous structure in historical western United States dry forests which included the Blue Mountains found: “Park-like stands of large trees maintained by low-severity fire predominated only in parts of the study landscapes. Only 3, 12, 40 and 62% of the four landscapes fit a low-severity fire model; 38–97% had evidence of higher-severity (mixed- and high-severity) fire.”

CONCLUSIONS:

Climate-weather conditions, not fuels, drive most large wildfires. The research methods used to create historical fire occurrence, size, and severity can lead to contrary conclusions. In general, fire scar studies tend to shorten fire intervals and record low-severity blazes that may only burn a few acres. In contrast, General Accounting Office records, air photos, charcoal, and pollen studies, among other methods, tend to record the larger landscape scale blazes that may occur at long intervals of centuries and are almost always a consequence of climate and weather conditions. Awareness of these different methods and how they influence temporal and spatial scales must be considered when discussing forest management. Studies that util;ize methods that record longer fire history records are more likely to be sutiable for creating fire managmeent policies.