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Red Bark Phenomenon

In the past five years an odd phenomenon has been observed on the bark of over twenty species of trees in New England — an intense reddish-orange coloration.  It’s been determined that this is due to the presence of a microscopic green algae (Chlorophyta), tentatively identified as belonging to the genus Trentepohlia.  A branching mat of thick-walled algal cells containing a bright orange-red pigment alters the color of the bark.

Red Bark Phenomenon is especially prevalent on White Pine, Eastern Hemlock, Red Oak and American Beech trees.  Affected trees appear to be of varying ages and are often, but not exclusively,observed near bodies of water, such as swamps and rivers. Frequently (as pictured) only one side of a tree is affected.

The exact conditions that promote this growth of algae are not known, but theories include climate change in the Northeast, in particular warming seasonal temperatures, increased precipitation punctuated by droughts, and more turbulent weather.  (Photo by Adeline Casali)

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22 responses

  1. Kathy Rizzo

    Is the algae harmful to the trees?

    February 7, 2020 at 8:45 am

    • I haven’t found any information on this, Kathy. Could or could not be…

      February 10, 2020 at 4:46 pm

  2. Lynne Woodard

    Glad to have some information on this! I had observed it near Curtis Pond in Calais and wondered about it. Thank you.

    February 7, 2020 at 8:46 am

  3. Meghan Young

    Hi Mary,

    Thank you so much for this post! I have been noticing this coloration and have been curious about why and what and how!

    Take care, Meghan Young

    On Fri, Feb 7, 2020 at 8:42 AM Naturally Curious with Mary Holland wrote:

    > Mary Holland posted: “In the past five years an odd phenomenon has been > observed on the bark of over twenty species of trees in New England — an > intense reddish-orange coloration. It’s been determined that this is due > to the presence of a microscopic green algae (Chlorophyta” >

    February 7, 2020 at 8:50 am

  4. Alice

    I’m not aware of that. Is there a particular side of the tree that this happens on? Like moss growing on the north side?

    February 7, 2020 at 9:06 am

  5. Deb Fountain

    Thank you for this–I have seen this also and could not find any information on it. Is it known yet whether this will effect the health of the trees?

    February 7, 2020 at 9:07 am

  6. Evergreen Erb

    This is very interesting! I’m wondering if it does any damage to the trees. I hope not, as they have so much to contend with anyway.

    February 7, 2020 at 9:23 am

    • Hi Evergreen,
      I haven’t read that it’s harmful, but it could be — just can’t find any information on that topic!

      February 10, 2020 at 4:47 pm

  7. susantcloutier

    I have noticed that along Rt 202 in New Salem, MA, on mature white pines. It appears on the sunny side of the trunk. Thanks for the ID of the algae that cause the color.

    February 7, 2020 at 9:30 am

  8. Ron Willoughby

    This looks like a Red Pine tree.

    February 7, 2020 at 9:31 am

  9. Thanks Mary for this information. I have noticed the redness. Do “we” know if it is harming trees?

    February 7, 2020 at 10:05 am

    • Hi Barbara,
      I’m afraid I don’t know!

      February 10, 2020 at 4:48 pm

  10. Libby

    Is it too soon to know if there are any effects on the health of these trees? I’ve begun to notice this, too….

    February 7, 2020 at 10:47 am

  11. Jocelyn Kleinman

    I wonder why a green algae makes the bark red? I would have thought a red algae or even a fungus.

    Jocelyn Kleinman

    February 7, 2020 at 10:57 am

  12. mmwm

    I also wonder if it harms the trees. It’s quite prevalent at Clark Pond trails in New London NH on the beeches. Thanks for the info!

    February 7, 2020 at 11:05 am

    • Hi Libby, I have not come across any literature that addresses whether or not this harms a tree, so I have no real answer for you, I’m afraid! So sorry.

      February 7, 2020 at 4:11 pm

      • Libby

        Ah, then – time will tell! So Amy changes to our environment and so much not yet understood. Glad for the posting.

        February 7, 2020 at 5:13 pm

    • I don’t know the answer to that, mmwm!

      February 10, 2020 at 4:48 pm

  13. Troy Mullens

    Interesting. We are living in a time of many changes.

    February 7, 2020 at 2:00 pm

  14. bill on the hill

    Clearly it is the POTUS fault for backing out of the Paris Accord. Climate change, it’s all his fault & I sense once again, Impeachment Inquiry in the winds… LOL!

    February 10, 2020 at 12:35 pm

  15. KenC

    I have about 20 hemlocks on my lake property, in mid-central Maine, and all are affected to varying degrees. I first noticed it, as orange needles, several Winters ago. Since then, I’ve had to take down about 4 trees, and I fear all my hemlocks will eventually succumb. I don’t think this applies to all trees that get RBP, but the combination of stressors in my location have probably led to their demise.

    March 10, 2020 at 1:37 pm

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