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Living Hollow Trees

12-31-14 hollow yellow birch 027Occasionally one comes across a living tree with a portion of its trunk, or its entire trunk, hollow. How is it possible for a tree to thrive even when its center, or heart, has completely decayed? It comes down to the different kinds of wood that are produced by a tree: sapwood and heartwood.

Sapwood (often light-colored) is the younger, living, outermost portion of a woody branch or tree trunk (just beneath the bark), while heartwood (often dark-colored) is the dead, inner wood. All wood in a tree is first formed as sapwood. Sapwood’s principal functions are to conduct water from the roots to the leaves (via xylem tissue) and to disperse nutrients made by the leaves to the rest of the tree (via phloem tissue). Heartwood (so called because of its central position, not because it is essential to the health of the tree) is basically non-functioning xylem tissue that has become blocked with resins, tannins, and oils. Although the dead heartwood can lend stability to a tree, it is no longer part of the transport system, and therefore, not vital to the tree.

Cavities and hollows typically result from an injury to a tree (usually caused by fire, storms, lightning, insects or birds) that exposes the heartwood. Bacteria and fungi lose no time moving in and beginning the decaying process, which can result in a hollow tree. Because the sapwood, and therefore the transport system, is still intact, the tree lives, despite the loss of its inner heartwood.

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12 responses

  1. On my frequent walks on back country roads in Central Vermont (and elsewhere in the world), I’ve come to admire the old roadside trees. They are usually not perfect specimen trees but malformed, poorly cared for old things that are the home to many other living creatures mainly because of their cavities. Trees in the woods are similar—few are what we’d like to call “perfect.” They may not live to their optimum age but they live a good life that is part of a larger ecosystem. I think they are lovely.

    December 31, 2014 at 1:42 pm

  2. And because most of the rigidity of a cylindrical shape is in the outer portion, hollow tubes remain quite strong: think bamboo.

    December 31, 2014 at 2:07 pm

  3. I completely agree, John. And the shelter they provide wildlife could provide me with posts for the next year!

    December 31, 2014 at 2:10 pm

  4. Agree with all. Cavity trees are an extremely important habitat element and should be a priority consideration in any forest management plan. The short list for recent sightings of cavity use here includes raccoons, fishers, porcupines, gray squirrels, screech owls and flying squirrels. The compartmentalization of decay in trees (CODIT; original work by Shigo) is a marvelous adaptation for survival in the world of trees.

    December 31, 2014 at 3:19 pm

  5. Peter Hollinger

    Just some nit-picking: Active sapwood, where the xylem cells have formed hollow tubes, is no longer alive; only the young cells forming inside the cambium are still living. Also, phloem forms on the outside of the cambium (inner bark) and is not part of the sapwood. Sap from the roots is pulled up through the xylem passively, while sugars from the leaves are pumped actively by the phloem.

    December 31, 2014 at 4:39 pm

    • Thanks for the excellent details, Peter. Much appreciated!

      January 1, 2015 at 11:12 am

  6. etymologist

    RE: Xylem and Phloem. Some sixty years ago, my high school biology teacher permanently imprinted the function of xylem and phloem: “Water zips through the xylem. Food flows through the phloem.”

    Speaking of Xylem: I recently read of a bacterial infection (Xyelella fastidiosa) that threatens to decimate olive trees throughout groves in Calabria. It infests the trees via insect vectors, and preferentially proliferates in the tree’s xylem to a degree that it completely blocks movement of water, causing death by dessication. There is no treatment or cure for this infection as yet. The infected trees must be burned.

    December 31, 2014 at 9:24 pm

    • Fascinating information about the bacteria and olive trees!

      January 1, 2015 at 11:03 am

  7. judilindsey@comcast.net

    Wow! Crazy cool info! Happy New Year, Mary! Judi 🙂

    January 1, 2015 at 2:35 pm

  8. Peter Kallin

    Mary-

    I enjoyed your post about hollow trees. Reminded me of a presentation I gave a few years ago to our local tree commission, who asked me to talk about the ecological benefits of trees. Turns out that a dead tree is actually about 9 times more alive than a live tree on a living cell per volume basis. You may enjoy the attached.

    Regards,

    Peter

    January 6, 2015 at 1:12 am

    • Hi Peter, Unfortunately WordPress doesn’t let me see any attachments. If you felt like sending it to me at my email address (mholland@vermontel.net) that would be great! Many thanks!

      January 6, 2015 at 4:20 am

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