Lowering stand density reduces mortality of ponderosa pine stands

Received this press release today from the Pacific Southwest Research Station. This is research that confirms what foresters have long known. I’ll bet that some groups will call this “best available science,” but others will contest or ignore it.


Lowering stand density reduces mortality of ponderosa pine stands

REDDING, Calif.—As trees grow larger in even-aged stands, competition develops among them. Competition weakens trees, as they contend for soil moisture, nutrients, and sunlight. Competition also increases trees’ risk to bark beetles and diseases, and subsequently leads to a buildup of dead fuels. A recent study, led by Dr. Jianwei Zhang, research forester at the U.S. Forest Service’s Pacific Southwest Research Station, considered if the onset of this risk could be determined. The study, which appears in the Canadian Journal of Forest Research, also considered if the relationship between density and mortality varies with site quality as ponderosa pine stands developed.

Based on the analysis of 109 long-term research plots established on even-aged natural stands and plantations from 1944 to 1988, and 59 additional ponderosa pine plots measured by the Forest Service’s Forest Inventory and Analysis group, these researchers found that site quality affected the relationship between density and mortality.

“Any silvicultural treatments that enhances growth will reduce mortality rate for a given stand density.” Dr. Zhang said. “By establishing the self-thinning boundary lines from the size-density trajectories, the onset of mortality risk can be determined for ponderosa pine stands.”

The research also confirmed the added value of such long-term study sites which allow new questions to be addressed that were not included in the original studies. Other recently published research from this group of scientists demonstrated thinning forest stands to a lower density reduces fuel buildup significantly, and enhances its economic value by increasing growth of residual trees.  Specifically, stand basal area, which is the cross sectional area of all trees in a stand measured at breast height, is not affected by thinning ponderosa pine stands to half the normal basal area of a specific site quality. If the stand has experienced high mortality caused by bark beetles, it can be thinned more heavily without sacrificing timber, biomass, or volume increment and plant diversity.

In addition, results from these long-term studies show that early shrub removal and tree density control are the most effective and efficient ways to reduce fuel buildup. Under Mediterranean climatic conditions, shrubs reduce overstory tree growth and keep tree crowns in contact with the shrub canopy.  In turn, this growing fuel ladder can carry a ground fire into the crowns of the overstory trees.  Although carbon stocks may be the same with or without understory vegetation, by controlling competing vegetation, carbon is reallocated into the trees instead of shrubs; and carbon loss to wildfire is reduced.

These findings provide useful information for managers in their stand treatment projects within National Forest and private forestlands.

To read the full article, go to http://treesearch.fs.fed.us/pubs/45108; or http://www.fs.fed.us/psw/programs/efh/staff/jzhang/ for other articles.

13 thoughts on “Lowering stand density reduces mortality of ponderosa pine stands”

  1. They did teach us these things in Forestry School years ago. They just needed to make it politically correct by framing it with the carbon storage story.

  2. Yup, we even knew it in prehistoric times when I was an undergraduate (class of ’63). And we even had zillions of studies to prove it for most commercially valuable species. This is a time proven underlying principle of forest science and plant physiology (access to sunlight and soil nutrients for survival). Unfortunately, it seems that passing on knowledge is not a strong human trait in that the young up and comers seem to discount the value of what their seniors have to say. I have also seen it in a fair number of SAF publications. There is also the possibility that part of the problem may arise from literature searches being mostly limited to the internet and thereby excluding many foundational studies.

    It sure would be cheaper to the taxpayers to have the pre-internet studies uploaded rather than paying for redundant research. In addition, forest science would be better served if the dollars were spent on establishing or debunking less established science or demonstrating the harmful impact of managing for aesthetics and or minor species at the expense of providing for a balanced succession of the keystone species.

  3. The press release mentions carbon but not the study itself.

    The authors made no effort to weigh the carbon “loss” from treatment versus the carbon “gain” from fire modification. The number in other studies do not turn out too well for using commercial logging to increase carbon storage. In the vast majority of cases, many more acres need to be logged in order to alter fire behavior on just a few acres, so there is much more carbon lost to logging than gained by modifying fire. The only exception is when the smallest fuels are removed form the forests with the shortest present-day (as opposed to historical) fire return interval. See Mitchell, Harmon, O’Connell. 2009. Forest fuel reduction alters fire severity and long-term carbon storage in three Pacific Northwest ecosystems. Ecological Applications. 19(3), 2009, pp. 643-655.

    • Short term carbon losses in pro-management proposals lead to longer term benefits in a wider array of issues. Eco’s often subscribe to a “do no harm” mindset, where it takes some impacts to achieve a more long term benefit. Flammable and overstocked forests tend to release most of its carbon in catastrophic wildfires, in today’s human-dominated world. Reality says we must treat these situations, or suffer very broad degradation on more and more acres, year after year. Pretending that wildfires are “natural and beneficial” is a poor excuse for forest “management”. I think it is better to manage for all the other benefits that managed forests can supply, including the slightly less short term carbon storage that preservationism MIGHT offer. Forests need to offer more than just short term carbon storage, at the expense of a broad forest benefits.

  4. Does anyone know what % of the forests in the western US are actually ponderosa pine stands? And of those ponderosa pine stands what % of them are currently unlogged “natural” stands?

  5. To add to the chorus. Why are we spending our research dollars on stuff that was well understood back in 1943. At Penn State we didn’t talk too much about ponderosa pine or carbon sequestration but our experimental forest included studies on thinning stands of mixed oak and scotch and white pine. Same results (minus carbon). Larry’s comment on our current “carbon obsession” to the exclusion of other multiple management considerations is spot on.

    • Mac, I received a press release a couple of years ago entitled “Effectiveness of fuel treatments in the west depends on thinning intensity.” I kept a copy in my “Duh” file. However, the paper it refers to is “Simulating fuel treatment effects in dry forests of the western United States: testing the principles of a fire-safe forest,” which seems like a reasonable line of inquiry.

  6. I guess one reason is to make sure that people know that through long-term studies these things are still true. Otherwise people could say “that’s not the latest science, that’s old.”
    There’s also the factor that only new things get published which leads people to believe old things (null hypotheses) may not be true
    for that reason it could be very important to keep stating in the publication record what continues to be true.
    The reason I brought up the fact that we knew it, was just to check my memory, not necessarily to critique them for publishing it.

  7. The authors ackowledge the long history of PP research. cites Here’s a paragraph from the introduction to the paper:

    Foresters often manipulate stand density and structure to enhance
    stand function for the goals of managed lands (Pretzsch
    2009). Ever since Reineke (1933) conceived stand density index
    (SDI) for characterizing self-thinning based on even-aged ponderosa
    pine (Pinus ponderosa Lawson & C. Lawson) stands in California,
    SDI has been used to determine how individual stands
    compare with a limit of stocking, beyond which the risk to mortality
    from natural causes is high. It has been extensively used in
    guiding forest management for ponderosa pine stand growth
    (Cochran and Barrett 1995; Oliver 1995, 1997; Long and Shaw
    2005), resilience to insects (Oliver 1997; Fettig et al. 2007), wildlife
    habitat development (Moore and Deiter 1992), and fuel treatment
    (Zhang et al. 2010). Not only has it been developed for other forest
    species (Reineke 1933; Weller 1987; VanderSchaaf and Burkhart
    2007), but it has been applied to uneven-aged or mixed-species
    stands (Long and Daniel 1990; Solomon and Zhang 2002; Woodall
    et al. 2003, 2005). As Zeide (2005) stated, SDI “may not be a good
    measure of density, but it is still the best we have” in forest management.”


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