# Why is tree bark brown? **URL:** \ **Category:** Factual Questions\ **Created:** [January 3, 2001, 1:35pm UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227 "2001-01-03T13:35:16Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Phobos](https://avatars.discourse-cdn.com/v4/letter/p/3d9bf3/32.png) [@Phobos](https://boards.straightdope.com/u/Phobos)\ **Post date:** [January 3, 2001, 1:35pm UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/1 "2001-01-03T13:35:16Z")
ok. My 11-year old neice started asking me questions the other day. “Why is the sky blue?” HA! I knew that one! I’m not sure she understood the scattering of blue light, but at least it was the Straight Dope. Then she asked, “why is tree bark brown?”. Ug. I wasn’t prepared for that one so I made some lame joke about how trees grow in dirt and made a lame observation that not all tree bark is brown (like white birch, etc.). Anyway, now that I’m not on-the-spot, I’ve been thinking about it some more. My SWAG is that tree bark does not NEED to be any particular color so the pigments that make up its color are just a general mish-mash. Kind of like the brown you get when mixing paints of many different colors. So, can I go back to her with this answer or is there a plant biologist among our ranks that can give me a more complete answer? ---
**Author:** ![WIGGUM](https://avatars.discourse-cdn.com/v4/letter/w/ce7236/32.png) [@WIGGUM](https://boards.straightdope.com/u/WIGGUM)\ **Post date:** [January 3, 2001, 3:22pm UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/2 "2001-01-03T15:22:10Z")
I believe the outer layer of bark that we see is called “rhytidome”. As a tree ages, it adds newer and newer layers of bark to protect the tree from diseases among other things. The new layer, however, is added under the older layer of bark which eventually dies and falls off. My WAG is that bark is brown because as the chemicals within the living bark degrade, they begin to take on a brownish (though not in all cases) color. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [January 3, 2001, 7:06pm UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/3 "2001-01-03T19:06:08Z")
If you mean “What makes the brown color?” take WIGGUM’s answer and add bug poop. If you mean, “What is the purpose of the brown color?” I reckon it’s so trees can hide among camo-clad hunters. ---
**Author:** ![manhattan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manhattan/32/7_2.png) [@manhattan](https://boards.straightdope.com/u/manhattan)\ **Post date:** [January 4, 2001, 12:00am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/4 "2001-01-04T00:00:14Z")
So with the orange camo now all the rage among deer hunters, we can expect evolution to take a symbotic turn? Echhh. ---
**Author:** ![Ad\_Noctum](https://avatars.discourse-cdn.com/v4/letter/a/b19c9b/32.png) [@Ad\_Noctum](https://boards.straightdope.com/u/Ad_Noctum)\ **Post date:** [January 4, 2001, 12:20am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/5 "2001-01-04T00:20:18Z")
isn’t bug poop green? I thought only worms pooped brown… oh well, and anyhow-- I wanted to say that Wiggum’s response was ABSOLUTELY correct. as far the extent of my knowledge spreads. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [January 4, 2001, 4:22pm UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/6 "2001-01-04T16:22:39Z")
Bee poop is brown. Aphid poop is clear, but quickly turns black. It’s that sticky stuff that falls on your car if you park under a tree. You probably thought it was sap. Fly poop is black. That’s the end of my bug poop knowledge. Sometimes I’m astonished at the plethora of useless knowledge cluttering up my skull. ---
**Author:** ![casdave](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@casdave](https://boards.straightdope.com/u/casdave)\ **Post date:** [January 4, 2001, 6:18pm UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/7 "2001-01-04T18:18:26Z")
There are trees whose bark is not brown, Beech, Mountain Ash, Sycamore, many types of willow, London Plane are some that spring to mind. ---
**Author:** ![Doobieous](https://avatars.discourse-cdn.com/v4/letter/d/da6949/32.png) [@Doobieous](https://boards.straightdope.com/u/Doobieous)\ **Post date:** [January 4, 2001, 7:50pm UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/8 "2001-01-04T19:50:05Z")
I cant say why most tree bark is brown, but IIRC, Redwood and Sequoia bark is reddish because of high amounts of tannic acid (which will make rivers reddish in color). But, some trees have greenish bark (palo verde), and some have multi colored bark, such as Eucalyptus delgupta (Mindanao Gum, a jungle tree from the Philippines). This tree has flaking bark in blue, green, yellow, red and purple. So, at least with that tree the color is due to pigments. ---
**Author:** ![Pyrhan](https://avatars.discourse-cdn.com/v4/letter/p/2acd7d/32.png) [@Pyrhan](https://boards.straightdope.com/u/Pyrhan)\ **Post date:** [December 10, 2013, 12:23am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/9 "2013-12-10T00:23:38Z")
Hi! I just registered because I was asking myself the same question, and after giving it some thought, I’d say it is probably due to some conjugated aromatic compounds coming from lignin degradation, such as fulvic and humic acids. And maybe anthocyanins and carotenoids also play a part. There’s an answer on [Ask.com](http://Ask.com) saying it is a matter of carbon content in the tree, but it is clearly wrong and denotes no understanding of chemistry from the one who wrote it. (since, obviously, many carbon-rich compounds are perfectly clear, from, say, glucose and most organic compounds to diamond) ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [December 10, 2013, 1:04am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/10 "2013-12-10T01:04:22Z")
Bears. Y’know. ---
**Author:** ![kaltkalt](https://avatars.discourse-cdn.com/v4/letter/k/bb73d2/32.png) [@kaltkalt](https://boards.straightdope.com/u/kaltkalt)\ **Post date:** [December 10, 2013, 4:39am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/11 "2013-12-10T04:39:19Z")
A related question i’ve had is why tree trunks/bark is not green with chlorophyll … more surface area to collect sunlight and make energy. some stems on smaller plants are green. some green surfaces, like cacti, are very hard/strong. Why would trees evolve to have a large portion of their surface area not produce energy? I realize not as much light gets to the trunk as gets to the leaves but some certainly does, maybe even a lot depending on the density of the trees around it. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [December 10, 2013, 5:28am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/12 "2013-12-10T05:28:09Z")
> [@kaltkalt](#): > > A related question i’ve had is why tree trunks/bark is not green with chlorophyll … more surface area to collect sunlight and make energy. some stems on smaller plants are green. some green surfaces, like cacti, are very hard/strong. Why would trees evolve to have a large portion of their surface area not produce energy? I realize not as much light gets to the trunk as gets to the leaves but some certainly does, maybe even a lot depending on the density of the trees around it. Because for most trees protecting the vital cambium and phloem layers beneath the bark is more important than adding a little more photosynthetic capacity. Bark thick enough to be protective against insects or being gnawed on by mammals will be opaque and block photosynthesis. For bark or the outer surface of the plant to be photosynthetic, it has to be too thin to offer very good protection. Cacti, of course, have lost their leaves in order to avoid excessive water loss, so they need to have trunks and branches that can photosynthesize. But they protect these surfaces with spines and thorns rather than bark. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 10, 2013, 6:05am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/13 "2013-12-10T06:05:58Z")
Most tree bark isn’t brown. Looking out the window here, i can see at least 6 species of tree, and not a trace of brown bark. There are various shades of grey, black, silver, white, yellow and red, but no brown. As for why bark is (usually) darker than the underlying wood, it’s because the tree pumps it full of dark coloured chemicals as it’s being produced. It’s not an accidental process. Tree bark protects the tree’s vascular system from predators and pathogens and fire, amongst other things. It has to be tough. The first step in that is usually to pump it full of tannins. Tannins are great at crosslinking organic fibres, making them much harder to tear. That not only makes the fibre physically tougher, it makes it difficult to digest making it a much less tempting meal.That’s why we tan leather. It’s also no coincidence that tannins for tanning are traditional extracted from tree bark. The other thing that tannins do is make the bark bitter, making it less tempting for animals to chew on. Tannins aren’t the only chemicals that trees pump into the forming bark. There’s a whole raft of other compounds, but they all serve the same general purpose: make the bark hard, physically resistant, chemically indigestible and unpalatable. One of the side effects of these chemicals is to make the bark dark. Instead of having the neatly aligned fibres that you see in sapwood that allow light in and scatter it to produce pale wood, you get crosslinked fibres that absorb light at all sorts of random wavelengths. IOW, the colour of bark by itself probably isn’t of any survival advantage, but the process of toughening the dark requires crosslinking, which literally tans the bark and makes it dark. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 10, 2013, 6:18am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/14 "2013-12-10T06:18:49Z")
> [@WIGGUM](#): > > As a tree ages, it adds newer and newer layers of bark to protect the tree from diseases among other things. The tree adds new layers of bark because that’s the only way it can grow, and it needs to grow to stay alive. If a tree could not add new layers of bark and not die as a direct result, then it wouldn’t \*need \*to do so in order to be protected form disease. > [@](#): > > The new layer, however, is added under the older layer of bark which eventually dies and falls off. The old bark is dead by the time the new bark has been laid down. And on many trees the bark never falls off, it just becomes progressively thicker and more fissured. > [@Doobieous](#): > > … some have multi colored bark, such as Eucalyptus delgupta (Mindanao Gum, a jungle tree from the Philippines). This tree has flaking bark in blue, green, yellow, red and purple. So, at least with that tree the color is due to pigments. Most of the bark colours in that family of eucs is due to diffraction, not pigments. IOW it’s a physical property of the way the fibres in the bark are arranged. You couldn’t grind it up and extract a blue pigment anymore than you could grind up a starling feather and extract a blue dye. ---
**Author:** ![kaltkalt](https://avatars.discourse-cdn.com/v4/letter/k/bb73d2/32.png) [@kaltkalt](https://boards.straightdope.com/u/kaltkalt)\ **Post date:** [December 10, 2013, 6:38am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/15 "2013-12-10T06:38:21Z")
> [@Colibri](#): > > For bark or the outer surface of the plant to be photosynthetic, it has to be too thin to offer very good protection. Any chance you could elaborate on this a lil’ bit? Why would it have to be too thin? There’s no way nature could evolve thick, photosynthetic bark to get the photosynthesis-produced sugars into the tree? Is there a physical reason for this, something like how a tree supposedly can’t be taller than (i forget the number) feet because capillary action can’t overcome gravity past that height? ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [December 10, 2013, 6:54am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/16 "2013-12-10T06:54:38Z")
> [@kaltkalt](#): > > Any chance you could elaborate on this a lil’ bit? Why would it have to be too thin? There’s no way nature could evolve thick, photosynthetic bark to get the photosynthesis-produced sugars into the tree? Is there a physical reason for this, something like how a tree supposedly can’t be taller than (i forget the number) feet because capillary action can’t overcome gravity past that height? I am thinking the light would not get through to the chlorophyll. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [December 10, 2013, 7:04am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/17 "2013-12-10T07:04:09Z")
> [@casdave](#): > > There are trees whose bark is not brown, Beech, Mountain Ash, Sycamore, many types of willow, London Plane are some that spring to mind. Pshaw! Take a gander at the [Rainbow Tree](http://www.travelstar1.com/rainbow-eucalyptustree-stop-stare/), a type of eucalyptus. There’s a reason why they call it that. Scroll down for many more pictures. > [@](#): > > The secret behind the Rainbow Eucalyptus is actually pretty simple. The trees shed multiple patches of bark every year, but not at the same time. As the patches are gone, the green inner bark is exposed, and, as it matures, every new patch first turns bluish, then orange, purple and maroon. This creates the rainbow effect that makes these trees so nice to look at. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [December 10, 2013, 7:14am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/18 "2013-12-10T07:14:13Z")
(Missed edit window.) Aha, I see that **Doobieous** ninja’d me – 13-some years ago! (What color are zombies?) The _Eucalyptus delgupta_ of which he speaks is none other than the Rainbow Tree that I mentioned! ---
**Author:** ![Enola\_Gay](https://avatars.discourse-cdn.com/v4/letter/e/3ab097/32.png) [@Enola\_Gay](https://boards.straightdope.com/u/Enola_Gay)\ **Post date:** [December 10, 2013, 7:23am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/19 "2013-12-10T07:23:40Z")
I always presumed that tree bark was like skin. Melanin ( or the tree version) helps to protect our skin from the sun. Trees have varying shades of brown, just like human skin. Maybe that’s too simplistic an explanation, but that’s always how I’ve thought of it. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [December 10, 2013, 9:48am UTC](https://boards.straightdope.com/t/why-is-tree-bark-brown/48227/20 "2013-12-10T09:48:14Z")
> [@kaltkalt](#): > > Any chance you could elaborate on this a lil’ bit? Why would it have to be too thin? There’s no way nature could evolve thick, photosynthetic bark to get the photosynthesis-produced sugars into the tree? Is there a physical reason for this, something like how a tree supposedly can’t be taller than (i forget the number) feet because capillary action can’t overcome gravity past that height? In simple terms, to photosynthesis you need light and you need air. You also need access to the water, protein, minerals and sugar in the sap stream. So for the trunk to photosynthesise the bark needs to be thin enough to the \*outside \*enough to let in both light and air. It also needs to be thin enough to the \*inside \*to let in sap. The problem is that pathogens and predators also want access to the water, protein, minerals and sugar in the sap stream. So by making the bark just a thin layer to the outside, and just a think layer to the sap, you’ve effectively removed the only thing physically preventing every animal and microbe in the world from eating the tree alive. IOW bark needs to be thick because it needs to provide a substantial physical barrier between the outside world and the sap stream. Make it thin enough to allow air and light to pass through from the outside, and sap to pass through form the inside, and you effectively \*have \*no barrier.