Ponderosa pine is one of the most common eastside Oregon ecosystem types. It occurs along the east slopes of the Cascades, extends into the hill country of Klamath and Lake counties and the North Warner Mountains, and is common in the Ochoco and Blue Mountains of central and northeastern Oregon (Figure 1). Ponderosa pine often occurs in relatively pure (single species) stands, but it is also a significant component of dry mixed conifer stands. If you own or manage forestland in dry side Oregon, you almost certainly interact with this important tree species.
Ponderosa pine and fire have had a close connection for millennia. Lining up a map of Oregon’s fire regimes with Figure 1 shows that the ponderosa forest type neatly corresponds to the frequent, low severity fire regime. Historically, fires burned through the understories of ponderosa stands as frequently as every couple of years. The range is typically five to 20 years, although stands with very little understory growth may have experienced longer or more variable fire return intervals, simply because surface fuels accumulated so slowly.
These frequent fires consumed needles falling from the trees, along with upper portions of native bunchgrasses. They also killed seedlings and kept shrubs at bay. The result was open stands of ponderosa pine with little understory development. Note that tree seedlings in these frequent-fire stands seldom developed beyond very small size — they were killed by fire. Only rarely could seedlings make it through that early stage and reach fire-tolerant size.
Ponderosa pine has some unique characteristics that allow it to so effectively coexist with fire:
- The base of the living crown (living branches) becomes relatively high by early maturity. This is partly due to its intolerance of shade — even its own shading effect can cause its lower branches to die, separating its living branches from fires burning in surface fuels. This separation, or crown lift, is enhanced by the action of fire itself — heat from surface fires scorches and kills foliage and buds on lower limbs. While this may seem detrimental, the effect is to increase the distance between the tree’s crown and surface fire — a positive factor for trees living with frequent fire.
- Its flaky outer bark curls when heated, encouraging the flakes to fall from the tree taking fire with them. This allows ponderosa pine to literally shed fire.
- Its foliage is clustered near the ends of its branches, so if any fire does happen to climb the stem it generally fails to reach foliage. This gives the tree resistance to torching, which is fire that engulfs a single tree.
- The open structure of the crown allows heat and embers from fire to pass through relatively unimpeded, making it less likely that the foliage will catch on fire. If a tree’s crown does ignite, the limited amount of foliage means less heat is generated, making it less likely its neighbors will also be ignited. This gives ponderosa pine resistance to crown fire, which is fire that advances through tree crowns.
- To maintain this open crown, ponderosa pine drops older needles each fall. Even these dead needles add to the tree’s fire resilience. The mat of accumulating needles serves as a mulch that keeps other plants from growing under the tree, minimizing surface fuel depth. If shrubs or seedlings become established under the tree, dead ponderosa pine needles, which occur in clusters, hang in them — a condition known as needle drape. When fire burns through the needle litter it does so in a slow-moving, low intensity manner, punctuated with higher intensity fire in shrubs and seedlings with needle drape. This is the primary mechanism by which ponderosa pine stands maintain their open nature.
- Frequent, low severity fire stimulates resin duct response in ponderosa pine, making it more resistant to bark beetle attacks. Resin is the primary defense mechanism for trees to fend off pests. When a resin duct is severed, resin flow can trap and flush out the boring invader.
Why we're talking restoration
Those who live among or manage forests in the areas Figure 1 shows as ponderosa pine forest will likely see a lot of other tree species in those forests. They may even doubt the map and think that those areas would be better characterized as mixed conifer. Researchers are increasingly recognizing that many ponderosa pine stands have shifted away from their historic conditions, and that the shift has significant potential to be detrimental in the long run.
Oregon’s dry ponderosa pine forests used to consist primarily of large, old, relatively widely scattered ponderosa pine trees, with some western larch and a smattering of other species. It’s now much more common to find few remaining large, old trees, and that the space between these trees has become densely infilled with other tree species. Foresters and ecologists refer to this as altered stand structure, composition, and density. The changes can be attributed to three things:
- Past harvesting practices that removed large old trees, either because they were considered more valuable or because they were assumed to have slowed their growth rates. By removing them, younger, more “thrifty” trees could better utilize the site. This had the effect of shifting the site from ponderosa pine, and similarly fire-resistant western larch, to less fire-resistant but more shade-tolerant tree species such as grand/white fir and Douglas-fir.
- Past grazing practices that allowed cattle and other livestock to consume plants that would have been fuel for surface fires, and to disturb the pine needle litter. This changed fire effects in pine-dominated areas, allowing more seedlings and shrubs to accumulate. Grazing also contributed to spread of nonnative grasses.
- Very effective suppression of wildfire and the near complete elimination of Indigenous use of fire as an ecosystem management tool over the past 100 to 150 years. By excluding fire from these stands, tree and other plant species that are intolerant of fire have become far more common than they were historically. This has resulted in much higher forest densities — more trees packed in the same area — and a shift in composition to grand/white fir, Douglas-fir, western juniper, etc. Concurrently, understories have become increasingly dominated by shrubs and small trees, at the expense of native grasses and forbs.
The most readily observed outcome of these altered forest conditions is that fires that escape suppression have the potential to become very large and have severe impacts on plants and soil and the myriad organisms and natural processes that rely upon them. We also observe that forests are under stress from over-competition, all too often promoting bark beetle outbreaks. We likely observe significant mortality in grand/white fir as well. While very capable of expanding into fire-suppressed ponderosa pine forests, grand/white fir can’t overcome the fact that they are poorly suited to those sites. We may fail to recognize that the changes in forest structure, composition, and density have also changed habitats in ways that favor some species over others. This means that understory plants, soil microbiomes, and insect and wildlife populations have also changed.
Given the typical human lifespan, we’ve largely become accustomed to these altered conditions and have come to consider them normal. It takes a willingness to open our eyes and minds, and some retraining of our perceptions, to recognize that our many dry ponderosa pine forests have become significantly deviated from their historic conditions. In other words, they are outside of their historic range of variation (HRV).
HRV, also known as historic range of variability or historic range and variability — depending on who you talk to — is used by planners and practitioners to express past conditions thought to be in equilibrium with historic fire regimes and other disturbance mechanisms, and therefore desirable. This acknowledges that there was no one historic condition — there was always variation within and between locations. Current forest conditions, particularly those of dry eastside Oregon forests, are frequently well outside of their HRV.
What can be done to restore these ecosystems to more resilient conditions? We never know exactly what that former condition was for any location, and even if we did, we couldn’t possibly completely replicate it. Forested ecosystems are far too complex and interactive. And, if we could approximate a former condition, our climate has become hotter and often dryer so what was “ideal” 150 years ago may not be ideal today. A key principle of effective ecological restoration is to aim for restoration of resilience in the face of current and anticipated conditions and disturbances. In dry side Oregon, this often means restoration of an ecosystem’s resilience to fire, and a way of allowing fire to play its important regenerative role while mitigating the risk of large, high intensity fire events.
One school of thought is that we should simply stop suppressing fires whenever possible and allow fire to return ecosystems to their resilient states. But the potential for large, high intensity fire is always there. The fact that almost two million acres burned in the summer of 2024 in Oregon tells us the “just let it burn” approach is unlikely to be socially or ecologically acceptable. That said, there are instances where letting a fire burn may be a reasonable approach. A fire that ignites late in the fire season, in a remote area, burning upslope toward sparse fuels, might well be a candidate for the let-burn approach. An early season burn with the potential to move toward human populations or into valued timber or other resources would not a good let-burn candidate.
Furthermore, forests with significantly altered structure, composition, and density may not be in conditions where fire will accomplish restoration objectives. Fire in these forests may be too intense and result in high severity effects that make it difficult for the ecosystem to recover. In some cases, it may lead to long-term alteration of ecosystem composition and function. Intermediate actions that reduce density, remove ladder fuels, and/or reduce stress on leave trees may be needed to increase the odds that the ecosystem will benefit from fire. This gives us a general pathway for approaching restoration.
What restores forests to wildfire resiliency?
A recently released meta-analysis investigated the effects of thinning, prescribed fire, and wildfire on the severity of subsequent wildfire in conifer-dominated western U.S. forests. A meta-analysis pools results from published studies to better elicit results. This analysis sorted through many forest treatment studies and focused on those that assessed fire severity in both treated and untreated areas (i.e., they had controls), which allowed the size of the effect to be clearly identified.
The results are compelling. A dense stand with surface and ladder fuels that experiences a wildfire is highly likely to experience severe fire effects — all or most of the vegetation will be killed and soil will be altered. If we thin that stand to remove small trees and reduce overall density, but leave the slash in place, a subsequent wildfire is likely to also be quite severe. The wildfire may not be as severe as in the untreated stand, but a high level of mortality and soil impact should be expected.
If we had thinned the stand, then piled and burned the slash, a subsequent wildfire would have had less surface fuel to consume — soil impacts and mortality in the leave trees would be significantly reduced. Thinning and underburning in combination produces even lower mortality and further mitigates fire-induced soil impact. Underburning without thinning can produce results like thinning and underburning, but if a stand is overly dense, the underburn itself may yield unacceptable results. It’s also important to note that regardless of the treatment type or lack thereof, weather conditions prior to and during a wildfire will exert a strong influence on the fire intensity and resulting severity.
So, to answer our original question, we can enhance forest resiliency to wildfire by taking actions that reduce stand density and provide more space between tree crowns, but we need to couple this with management of surface fuels. How we accomplish this will depend on the condition of the stand. A dense stand likely requires both thinning and prescribed burning, while a relatively open stand may only require underburning.
Parting thoughts
Effective restoration requires careful evaluation of a stand’s current condition and anticipated environment, a tailored approach to treatment, and a commitment to ongoing treatment consistent with the location’s fire regime and specific management objectives. Consider the following:
- Restoration can be a very logical stewardship objective and motivation, but the key is to move your forest toward a resilient ecological condition rather than a specific structure.
- Knowing the HRV can be a helpful indicator, but it shouldn’t be used as a hard and fast target. Given our generally hotter climate, it may be prudent to maintain forests at densities toward the lower end of their HRV.
- The common practice of removing small trees — fuel reduction projects, for example — is a reasonable start in many instances, but if this is the only treatment forests can be left significantly overstocked. Removal of larger trees may be needed to bring densities to appropriate levels.
- Thinning alone is rarely effective at reducing the risk of high intensity fire. Thinning in combination with prescribed fire is a winning strategy, and a combination of pile burning and understory burning, in stages, may be advisable when treating a dense forest.
- Fresh pine slash is likely to attract pine engraver (Ips) beetles. Employ best practices for management of slash three to eight inches in diameter to avoid buildup of the pine engraver beetle population and subsequent mortality in small diameter pines or tops of larger pines.
- Ponderosa pine is, unfortunately, no longer a high value product in eastside log markets, so funding restoration work may be challenging. The Natural Resources Conservation Service (NRCS) has programs that help fund fuel reduction (small diameter) projects, and they may also have funding for ecological restoration that can include larger trees. The Oregon Department of Forestry (ODF) might have supplemental programs and your ODF Stewardship Forester is often an excellent first stop for questions about financial assistance.
- While many of these concepts could be applied to mixed conifer forests, those forests are different beasts and merit their own consideration.
