When a Tree Burns, Does It Fall?

All trees fall down. Some fall when they are alive. Some fall after they die. Some fall when they are young. Some fall when they are old. Wind knocks trees over. Trees fall over from root decay. Trees also fall after being burned in forest fires.

Wait a second . . . go back and click on that last link. The one about trees falling over after forest fires. Most of those lodgepole pine trees are still standing. And they’ve been standing long enough for the aspen in the understory to sprout and grow.

So how long does a tree stand after being killed by fire? It’s an area of research almost wholly neglected by so-called tree hazard evaluations (“There are no scientific publications, however, that evaluate, test or compare the [tree hazard] procedures or methods”). Ecologists, however, have studied snag persistence to assess wildlife habitat.

In a western Idaho study, for example, 95% of Douglas-fir snags were still standing four years after fire. Over 80% remained up-right 11 years later.

If the Forest Service thinks wilderness should remain closed until the fire-killed trees have fallen over of their own accord, they’ll have to lock the public out for years. The Forest Service could treat wilderness trails as it does roads and cut the potentially hazardous trees, if doing so “preserves its wilderness character.” Or the Forest Service could do as it has in Idaho — open the wilderness and warn visitors to “abandon hope all ye who enter here.”

10 Comments

  1. Thanks for the info Andy. Yes, abandon all hope!
    But wait, what if the agency put a box of hardhats at each trailhead with instructions to “please wear one while hiking and to please return it to the box when you leave?”

  2. Is it possible that the USFS is closing burned areas until trees have fallen in order to stoke more pro-salvage-logging sentiment? I.e., “You could go hike and hunt and enjoy your own land, but the logging companies just need to clear out the dead wood first. If it wasn’t for those pesky environmentalists you could be there now.”

    • It might be possible, but it doesn’t seem likely since the FS is closing areas in the Angeles (CA) and the Pike (CO) where there is no salvage logging. Plus of course there is none in wilderness, except perhaps for hazard trees.

    • Sometimes it sure seems like that! And all of the fire-fighter comments this past summer about “if it weren’t for all of those dangerous snags…this fire would be much smaller because we could fight fire closer to the fire…” And while it may not be for that reason, these types of closures/statements can certainly give that impression.

  3. Andy, I think that there are a lot of scientific publications on trees falling, especially lodgepole after MPB attacks. Remember those back in the day in Central Oregon? It depends on a lot of things as articulated here.
    “Factors affecting Tree Decay and Rate of Fall:
    • Time since death: Exact number of years since a tree was attacked and died may be
    difficult to determine just by looking at it but you can get a rough idea by its outside
    appearance.
    o Trees under first year of attack will still be green to yellow.
    o First two years after attack trees canopies will be red and will still be holding
    most of their needles.
    o Three years after the attack and beyond, trees will appear grey and will lose
    more and more of their canopies from this point on. Observing a tree in the grey
    phase should be a clue that it may be decaying and unstable. Many studies have
    shown that the half-life (time it takes for half of the snags in a stand to fall) of
    beetle-killed trees lies somewhere between 6 and 9 years (Lewis and Hartley;
    Farris and Zach).
    • Climate is the other primary factor, along with time, that determines the speed and
    extent of fungal decay spread through the structural fibers of the tree. Decay spreads
    more easily in warmer, moister conditions. Decay growth may be slowed or stopped
    completely if conditions become too cold or dry. The microclimate that the affected
    trees are in will also have a major effect on the rate of decay. Trees on high elevation
    exposed ridges with dry soils will decay slower than trees in the bottom of a wet draw
    on the same slope at a lower elevation (Mitchell and Preisler).
    • Stand structure has also been shown to have a significant bearing on how quickly snags
    will begin to fall. One study showed that trees in thinned stands began falling 3 years
    after attack and trees in unthinned stands began falling 5 years after attack. It was
    thought that this was due either to the thinned stands being more exposed to wind, or
    the additional light transmission in the thinned stands warming the soil at the root collar
    which sped up the growth of root decay fungi, or both (Mitchell and Preisler).
    • Wind is probably the biggest mechanism actually causing the fall of snags. A study done
    on stands of beetle-killed Ponderosa snags in Colorado showed that 90% of the snags or
    greater fell in the same direction, in line with prevailing winds (Schmit, Mata, and
    McCambridge). This suggests that most of the trees came down when winds were
    present.
    • Tree diameter also has a bearing on how long a snag will stand. Larger diameter pines
    have been shown to stand longer than smaller diameter pines. This is due in part to
    having more wood to decay, and to larger trees having more decay resistant heartwood
    (Cluck and Smith).
    • Bark thickness helps control how quickly the tree loses moisture. The thick bark of
    ponderosa pine retains more moisture than the thin bark of lodgepole pine. This allows
    decay fungi to propagate much further up the bowl on ponderosa (Bull, E.). This means
    that while a ponderosa snag may have extensive rot 2/3 of the way up the bole, under
    the same environmental conditions, the rot in a lodgepole snag may be confined to
    within a few feet of the ground. Because of this, lodgepole pine almost always breaks at
    ground level while Ponderosa have more of a tendency to break off incrementally down
    the bole (Everett et.al.).
    • The Sapwood/Heartwood ratio of trees also effects how quickly they decay.
    Heartwood is much more resinous than sapwood and is filled with extractives(waste
    products of photosynthesis). There are many species of decay fungi that can
    decompose sapwood, but very few of these can decompose heartwood, and even for
    the ones that can it takes much longer (Lowell et.al.). ”

    There are other citations as well.. https://www.nwcg.gov/sites/default/files/htsc-mountain-pine-beetle-stands-risk-levels.pdf
    My point is that there is scientific research as well as practitioner knowledge on the subject and it shows that falling depends on a lot of things.

    • In my experience, large p. pines (and sugars) usually fall as one unit when the roots rot. Older pines often already have existing ‘cat-faces’, with some associated rot. Additionally, contact with the ground has an accelerating effect, as well. When a tall pine falls downhill (towards a road) as a unit, the impact is quite spectacular and ‘messy’ with lots of wooden shrapnel.

  4. I say – let ’em fall if they aren’t salvaged and let the user take the risk signing a note absolving Uncle Sam of all responsibility since the user is already taking numerous risks even when not around dead trees. If you don’t know what you are doing, you don’t need to be in the woods.

  5. Interestingly on many USFS experimental forests, in control, harvesting and fire study plots, the scientists track each tree over 4 in dbh over its life – or until it becomes a snag, whereby it is tallied as dead and then on plot re-entry in subsequent years never looked at again. There are decades of missed data that could have informed us about snag longevity, snag use, interactions with fire (as the mortality agent or as something that might hasten snag fall). Yet on national forests information about value and “management” of snags is needed from New Hampshire to California. Missed opportunity by a deputy area of the USFS that is in a terrible need of a top to bottom evaluation and redirection.

    • Forestgreen.. I too am a big fan of experimental forests, and I don’t disagree with you about FSR. I am sympathetic to them because they must work in the social context of the Science Establishment and yet try to do work that is useful to people. Not an easy task.

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