What is forest resilience?

Excerpts from a UC Berkeley Forest Research and Outreach blog post, with info aimed at landowners. It reminds us that thinning and fuels reduction isn’t only about reducing fire intensity/severity: it’s also about forest health.

What is forest resilience? Forest resilience is a measure of adaptability. It focuses on retaining a forest’s essential structure and composition to a range of stresses or complex disturbances. In other words, a resilient forest may lose some trees to drought, fire or insect attack, but the mortality rate will not overtake the forest’s ability to continue growing trees and provide habitat. Some will die, but many will live.

Today’s [Sierra] forests have huge increases in basal area and tree density compared to the historical record. Historically, forests were generally low in density yet highly variable in their structure, with open patches and clumps of trees. Twenty-two trees per acre was not uncommon in the Sierras, but those 22 trees were huge! The older and bigger trees get, the more adaptations they have, like thicker bark, a high canopy and higher levels of resilience to disturbances. We have normalized high density, homogeneous stand structure and high competition forests that would not have occurred historically. Today’s forests are more vulnerable to fire and drought related mortality due to a legacy of timber harvesting in early 1900’s that focused on large tree removal; a century of fire suppression policy and action, and climate change effects such as less humidity recovery in the night.

Timber harvesting or commercial thinning:

  • Sometimes you have to commercially thin in order to restore lower density forest conditions;
  • You can leverage saw logs/forest products to pay for other management activities; and
  • It is effective in reducing tree density in the canopy and ladder fuels and reduces competition for the remaining trees.

Commercial thinning plus prescribed fire: Very effective in reducing tree density, ladder fuels and surface fuels.  Combining meaningful thinning and prescribed fire can mitigate fire hazard and improved the growth and vigor of your trees to maximize resistance and resilience to wildfire and drought-related tree mortality!

15 thoughts on “What is forest resilience?”

  1. The second bullet is laughable: “You can leverage saw logs/forest products to pay for other management activities”. That is true if, and only if, industry has managed to hang on through the timber wars of late.

    Regions 8 and 9 are doing swell; stumpage values pay for all cultural work associated with timber sales. The Ouachita, when I was there in 2006-mid 2010 had in excess of 40 million in KV! When I got to Region 3, we were paying, yes paying, $500/acre to log. These trees being taken out were sawlogs. They were chipped and fed into a biomas (28 MW) power plant.

    Region 2, except for the Black Hills, has only a marginal market for competition for sawlogs, or other biomass material. Region 6 has a bit of a market, but nothing like they once had, and certainly not enough to generate much value above base. I don’t know about 4 and 5; Region 1 and 2 are about in the same boat.

    Private landowners can thank the FS for running off forest industry (excluding 8 & 9), and no one wants to invest 100 million dollars into anything resembling a modern mill, tied to the FS, without a 20 year guarantee! And that 20 year guarantee is the tipping point for investments.

    Most operators don’t want to “fiddle” with Stewardship due to these other tasks the FS is so fond of hanging onto sale areas. As one operator told me, “my equipment and employees are suited to logging timber, not plowing fire lines, building wildlife ponds or pre commercial thinning!”


  2. What is the scientific evidence for the following statements?
    “Historically, forests were generally low in density yet highly variable in their structure, with open patches and clumps of trees. Twenty-two trees per acre was not uncommon in the Sierras, but those 22 trees were huge!”

  3. Matching stand densities to the ‘new normal’ precipitation levels is something preservationists don’t want to talk about. It’s an important concept, folks. It is easier to change stocking levels than to change precipitation levels.

  4. There are two words to define in “forest resilience”. The article, as presented, focuses almost entirely on the trees. In today’s world, it is important to remind the reader the the forest is more than just the trees. A Forest is a biotic community of many, many species of plants and animals, interacting with a diverse and dynamic physical environment. — At it’s basis, resilience depends upon diversity. Resiliency of the forest community depends on species diversity that, if sufficient, fulfills every vacant niche that occurs following disturbance. Then there is species-population diversity, that depends upon the genetic diversity of the population. — Resilience of a forest is much more that “getting back” to a wood-producing condition. JAB

    • That’s a good point.. I think people might define it differently. Some people might say “treed vegetation cover” and others might want also want different other kinds of diversity.. say prions or mosquitoes, or carnivores or whatever. A diversity of species, then diversity of genetics within the species. I can safely say that for most species we have no clue how much genetic diversity is good; in fact if we wanted to go back to pre European times we mostly don’t have an idea of what is was then. That’s one of the reasons many geneticists are skeptical of the “return to the past” idea.

      • Re. “how much genetic diversity is good”. Good for what has to be answered before there can be an answer to the question. I believe we have a responsibility to leave almost as much genetic diversity as we can, as options that future generations deserve. Exceptions, arguably, are human disease organisms, for one. (We have committed our species to rely on medical science to replace costly “herd immunity”. But it’s a race we sometimes barely win, or somewhat lose.) Re. we don’t know how much genetic diversity existed in pre-Columbian times, or whenever, there is clear evidence that much more diversity existed for many species, especially in increasingly fragmented species. — Because we don’t know everything, and never will, is not an excuse to do nothing. JAB

  5. This is an incorrect statement that is stated over and over again as though it is fact, and a big reason why people don’t understand the issues facing the Sierra and old forest species: “Today’s [Sierra] forests have huge increases in basal area and tree density compared to the historical record.”

    According to Safford and Stevens (2017, https://www.fs.usda.gov/treesearch/pubs/55393), a USFS comprehensive review of available data and studies on the Sierra Nevada historical vs. current conditions: “Among the variables assessed, only basal area, overall plant species richness, and percentage cover of grass/forbs and shrubs appear to be within or near the NRV.”


    If one logs to TPA, then basal area falls well below historical metrics due to the missing large tree component. For every doubling of the diameter, the area of a circle increases by 4 times (there’s 4 times as much pizza in a 16 inch pie than an 8 inch pie). For species associated with higher canopy cover, there’s more canopy packed into each acre, because each tree has more canopy spread out vertically, increasing the volume of habitat on each acre.

    The large trees were liquidated up until the spotted owl protections resulted in this practice ending in the early 1990s. Thinning to a historical TPA doesn’t magically grow back the large trees (or habitat volume). That takes time, centuries in fact. Yes, logging can increase growth rates and residual tree health, but there must be a balance, ensuring that all of the species are carried forward and integrity is maintained, while also ensuring that fire and climate resilience are provided. This is no small task, one that gets more difficult every year.

    • “Fact: On average, CURRENT BASAL AREAS ARE WITHIN THE HISTORICAL RANGE in the Sierra!!!!!”

      Thanks, Anonymous, for reminding us that some readers have a technical background in forestry.

      It is axiomatic that a site’s inherent productivity is generally fully occupied. There are two standard measures of “occupancy” — basal area and leaf area. Since basal area is easier to measure, that’s the metric generally used by foresters.

      The empirical observation is that across a wide range of tree densities (i.e., tree stems/acre), basal area remains constant. Thus it should come as no surprise that whereas today’s forests might have ten or a hundred-fold the number of trees per acre compared to the 19th century, the basal areas are the same.

    • Unlike historic forests, much more of today’s basal area, in Sierra Nevada forests, are in flammable species like incense cedar and true firs. Changing the species mix to a more drought-tolerant combination is more ‘natural’ than not doing it.

      Some people continue to want to have a pre-human landscape, in a world dominated by billions of people.

    • One question. How do you, we, define (biotic?) integrity? It’s getting to be another buzzword used frequently. (Also, “integrity” is found in the Wildlife Refuge System Improvement Act of 1997, and probably elsewhere in government policies.) I like to think of “integrity” as related to “integrated”, meaning that, at least to the extent practicable, the biotic diversity in integrated, that is functioning naturally (not controlled by us). For many species, integrity requires a large available and diverse landscape. JAB

      • For national forest lands there is a book answer (36 CFR 219.19):
        “Ecological integrity. The quality or
        condition of an ecosystem when its
        dominant ecological characteristics (for
        example, composition, structure,
        function, connectivity, and species
        composition and diversity) occur within
        the natural range of variation and can
        withstand and recover from most
        perturbations imposed by natural
        environmental dynamics or human
        This begs the question of what the “natural range of variation” is. The Planning Handbook (§23.11a) attempts to explain it, but does so poorly; however, it is safe to say that it is not necessarily the same as the “historic range of variation.”

  6. Forest Health and Forest Resiliency are vague and unscientific terms.

    Montana Public Radio reported on this in 2017:

    One of the lines Senator Daines often uses when talking about public lands management is, “a managed forest is a healthy forest.”
    “One of the problems is, ‘healthy’ doesn’t have a scientific definition,” Larson said, “so, when we come at it from a technical perspective, it can mean whatever we want it to mean. Some of the most intensively managed forests in the world are in Northern Europe, and they are in a biodiversity crisis, because they have mismanaged their dead wood. They never let their trees get old, they never let ’em die. They cut ’em down and take them to the mill, and there is a horrible deficit of dead wood in those forests. And as a consequence, they’re compromised, they’re not functioning, they’re not providing the habitat for all the native biodiversity, the native wildlife species.

    ERIC WHITNEY 2017. Forest Ecologist Comments On Senator Daines’ Fire Call. Montana Public Radio. Sept 14, 2017. http://mtpr.org/post/forest-ecologist-comments-senator-daines-fire-call

    … as ecologist Robert Lackey describes, there is no universal definition of ecosystem health, yet many environmental policy issues are based on the idea of restoring or improving the health of ecosystems. Lackey calls ecosystem health a “value-based ecological concept” based on subjective assumptions that “masquerade as science.” Ecosystems have no preferences; people do.

    Regan, S. 2016. . ENVIRONMENTALISM WITHOUT ROMANCE – Science alone cannot resolve most environmental issues. PERC Report: Volume 35, No.1, Summer 2016 http://www.perc.org/articles/environmentalism-without-romance.

    • There are a great many ways to ‘manage’ forests, to ‘balance the harms’. Extremists just don’t want to consider anything closer to the middle. Forests in many mountainous areas have been ‘managed’ in some sort since the last glaciers receded.


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