Researchers discuss how dry pine forests, dry and wet mixed-conifer forests, and dry and moist mixed-conifer forests in the US Pacific Northwest (PNW) area were influenced by a continuum of 19th–21st-century sociopolitical activities and management techniques in this study. The research was published in Frontiers in Ecology and the Environment.

Image Credit: Grisha Bruev/

To address extensively changed forest successional circumstances, 20th- and 21st-century climatic shifts, and quickly shifting wildfire regimes in the present age, scientists suggest landscape-level adaptation.

Wildfires were viewed as entirely detrimental to western landscapes after the “Big Burn,” and laws were devised to ensure that subsequent flames were rapidly suppressed. Following that, the federal government prioritized fire control to safeguard people, infrastructure, and forests. With the implementation of the “10-am rule” in 1935, high suppression efficacy and reduced burnt area became obvious (that is, fires extinguished by 10 am after identified).

Old woods were iconic to many people who prized them for their inherent recreational, aesthetic, spiritual, and ecological significance. Old woods were prized by Native American tribal people for supplying food and medicine, preserving spiritual and cultural rituals, and fostering intergenerational linkages to ancient customs.

One proposed policy in inland PNW woods would prevent the cutting of any tree greater than the 53 cm diameter at breast height (dbh), which was seen to be a reasonable lower cutoff for ancient trees and forests at the time. The implementation of the policy increased the openness of forest management as it related to large trees, as well as alleviating public worries about giant tree removal; nonetheless, it ignored tree age and species, as well as rural community requirements.

As a result, enforcing the giant tree strategy will increase the density and layering of shade-tolerant but fire-intolerant trees, raising the risk of crown fire initiation and spread in forests where it had previously been rare. As forest fuels accumulated and drought conditions deteriorated, aggressive fire suppression—performed to support community, resource, and habitat preservation—became increasingly expensive and ineffectual.

Many fire-intolerant trees have developed into bigger size classes after more than 170 years of fire exclusion, seen in Figure 1.

Figure 1. Species abundance has changed in mixed-conifer forests in recent decades. Estimates of (a) basal area per hectare (BAH) and (b) trees per hectare (TPH) on public lands in eastern Oregon and Washington were compiled from national and regional forest inventories. For each diameter class, bars from left to right represent estimates for midpoint inventory years 1995 (1990–1999), 2004 (2000–2007), and 2014 (2010–2017). Species include the fire-tolerant and shade-intolerant ponderosa pine (Pinus ponderosa, PIPO) and western larch (Larix occidentalis, LAOC); the shade-tolerant white fir (Abies concolor) or grand fir (Abies grandis) (together, ABCOGR) and Douglas fir (Pseudotsuga menziesii, PSME); and all other tree species combined (Other). Species proportions in 2004 and 2014 are relative to the BAH (above) and TPH totals (below) in 1995, for each diameter class. Diameter class midpoints are (left to right) 10 cm (range 2.5–20 cm), 30 cm (range 20–40 cm), 50 cm (range 40–60 cm), and 90 cm (range 60–120 cm). Results show overall increases in BAH and TPH for tree diameters >40 cm, with ABCOGR and PSME increasing more than PIPO, and LAOC generally declining. Error bars represent the standard error of the mean estimate. (a) The proportions of total BAH in the 10-cm, 30-cm, 50-cm, and 90-cm midpoint classes in 2014 were 16.9%, 35.1%, 27.5%, and 20.6%, respectively. (b) The proportions of total TPH in the 10-cm, 30-cm, 50-cm, and 90-cm midpoint classes in 2014 were 79.9%, 14.5%, 4.2%, and 1.4%, respectively. Note that the largest increases in BAH and TPH occurred in the 50-cm class. Image Credit: Hessburg, et al., 2022

Their findings showed that removing substantial intended burning enhanced the consequences of fire exclusion, as well as the potential advantages of intentional burning in lowering uncertainty, increasing the chance of less severe fires, and reducing the burnt area.

As a result, damaged forests are frequently uniform, thick, and stratified, as seen in Figure 2, with fuel ladders extending from near ground level to the crowns of surviving huge, ancient, fire-tolerant trees in Figure 3.

Figure 2. A densely treed landscape emerges. Panoramic photographs – taken from Duncan Hill, Washington, looking southeast along the Entiat River drainage to the Columbia River – show the majority of the 238,000-ha Entiat drainage in (a) 1934 and (b) 2012. Fire exclusion and selection cutting broadly homogenized successionally diverse pine forests, and dry and moist mixed-conifer forests. In the absence of wildfires, bark beetles kill trees, increase fuels, and synchronize large areas for burning. (a) RR Sarlin; National Archives and Records Administration, Seattle. Image Credit: Hessburg, et al., 2022

Figure 3. View near Tronsen Ridge (in the background), 2013, Okanogan-Wenatchee National Forest, Washington. Only a handful of trees in this scene were present 125 to 150 years ago. The largest ponderosa pines are 300 to 400 years old, developing under a frequent fire regime. Most other trees are fire-intolerant grand fir and Douglas fir that established over the period of livestock grazing and fire exclusion. A few dwarf mistletoe-infested younger western larch are dead in this scene owing to extreme intertree competition for soil moisture and nutrients, and mistletoe infection severity. Image Credit: Hessburg, et al., 2022

Influence of Management Techniques on Forest Evolution

Wildfire and drought continue to threaten these forests and the biota that relies on them, prompting many fire and climate experts to wonder whether major changes to 21st-century landscape composition and structure are required to promote climate—and wildfire—adapted conditions and restore habitat variety.


Scientists also have a better understanding of the fact that, while many huge trees perform vital ecological tasks, not all large trees are biologically equal, and that tree size is an inconsistent measure of tree age, as some aged trees are small and other young trees have great diameters. Adaptability to wildfires and climate warming is also influenced by the tree species kept.

The remaining trees provide shade and have been fire-resistant for more than 170 years; examples include grand fir (Abies grandis), immature Douglas fir (Pseudotsuga menziesii), and white fir (Abies concolor). These three species are fire-resistant until they grow and have a thick bark. They were once small companions of the most often burnt inland Pacific Northwest woods.

Low-density fire-tolerant species are also better suited to shifting climate and wildfire regimes, planned burning and managed wildfires, and have lower insect susceptibility.

As the modern climate continues to modify biophysical ecosystems and wildfire regimes, landscape-level planning is required to identify how fire-tolerant and intolerant species, and also open and closed canopy forest successional conditions, should be conserved.

More reading: Drone Reforestation: The Future of Rebuilding Earth’s Forests with Scalable Technology

Research has also shown that regulations that apply the same rules to everyone are incompatible with managing resilient landscapes and their supporting systems. Researchers identify different environmental gradients throughout montane PNW landscapes that may sustain a wide range of forest structural and compositional patterns, as well as their associated biodiversity.

Management recommendations that allow for the creation of structural and compositional complexity that enables biodiversity retain wildfire regimes, and other forest attributes will be more likely to succeed. It is still necessary to strike a balance between many goals, such as preserving habitat for animal species that rely on shade-tolerant trees.


Changes in forest health and fire regimes, according to studies, amplify the consequences of global warming and continued active fire suppression. This more diverse patchwork would limit fire spread rates, fireline intensity, flame length, and crown fire potential, as well as foster more beta habitat variety (Figure 4).

Figure 4. Young shade-tolerant and fire-intolerant trees rapidly fill in the forest. Over 150 years, a dry and moist mixed-conifer forest landscape has become densely filled with Douglas fir and grand fir pole-, small-, medium-, and large-sized trees. View looking southwest into Stafford Creek, North Fork Teanaway River watershed, Cle Elum, Washington, in (a) 1934 and (b) 2013. (a) RL Cooper; National Archives and Records Administration, Seattle. Image Credit: Hessburg, et al., 2022

Federal managers have the potential to significantly boost native biodiversity and forest resilience to wildfires and future climate change by:

Harvesting small to big white fir while conserving huge and ancient pines has been agreed upon by several collaborative organizations. Furthermore, if collected trees are processed locally, there is more support for removal procedures, resulting in economic advantages for local communities.

It will be a social as well as an ecological concern whether to encourage climate and wildfire adaption in inland PNW forests. Those who simply oppose tree removal or wish to save shade-tolerant species may find the recommended climate and wildfire adaption strategies difficult to accept.


Federal land management agencies are now struggling to build the social capital required to adapt forests and boost resilience to wildfires, climate change, and other environmental changes. Agreements will be easier to reach if policies and planning standards explicitly define adaptation targets. To generate a common vision for management, effective public involvement, collaboration, and tribal consultation are essential.

Demonstration projects that create “zones of agreement” and acquiescence to adaptation and targets monitoring can help to speed things up. Success is rare if agencies rush to adjust policies without building trust and consensus.

Journal Reference

Hessburg, P.F., Charnley, S., Gray, A.N., Spies, T.A., Peterson, D.W., Flitcroft, R.L., Wendel, K.L., Halofsky, J.E., White, E.M., and Marshall, J. (2022). Climate and wildfire adaptation of inland Northwest US forests. Frontiers in Ecology and the Environment, 20(1), pp.40-48. Available Online:

References and Further Reading