a closer look at tree bark, with michael wojtech

Copyright Michael WojtechIT’S ALWAYS RIGHT in front of us, 365 days a year on every tree. Perhaps that explains why we take bark for granted. We don’t look closely, or recognize more than a few obvious examples like a paper birch—let alone ask what bark is all about: what functions it serves, or why it looks so different from species to species.

Michael Wojtech of Know Your Trees dot com and author of “Bark: A Field Guide to Trees of the Northeast” (Amazon affiliate link) can answer those questions, plus this one: Can you actually learn to identify trees by their bark (an especially useful skill now through spring, when many are leafless)? Hint: The answer is yes.

Michael left a 15-year business career to pursue his love of natural history and writing, and earned his Master’s in Conservation Biology from Antioch University New England. His thesis, on tree bark, became the basis for the field guide. Though the book’s plant ID section covers trees of the Northeast, much of the material inside speaks to the characteristics and function of bark anywhere–so it’s fascinating wherever you live.

michael wojtech and bark book

Michael joined me from his home deep in the woods of Massachusetts—occasional crackly static of the phone line we reached him at attests to that, sorry!—to talk about bark on my radio show and podcast on November 9, 2014 show; the transcript follows.

my bark q&a with michael wojtech

Q. I loved reading about how this project began, Michael, with your master’s thesis project–and then grew, grew, grew. Tell us a little about its genesis.

A. I’d like to say it was an epiphany, and came all at once, but it did come quite gradually. I’m a transplant to New England; I had moved from the Pine Barrens of New Jersey to study at Antioch, and I was immersed right away in this brand-new landscape.

I was out in the field trying to do an inventory of trees growing in a local conservation area, and really just befuddled. It was about this time of year—late fall—and a stormy day, and misting, and all the leaves were just raining down upon me, and I was panicked and sprinting through the woods because the leaves were really my only connection to identifying the trees.

Q. Uh-oh! [Laughter.]

A. I was so immersed in learning the local flora and fauna. It was the way I was becoming connected with this landscape, and it was very profound. But I really felt there must be a better way, because it was really challenging.

Later when I needed to come up with a thesis topic, it was my adviser, Tom Wessels, who came up with the topic of bark. I was thinking about the writing, and creating a field guide, and I knew nothing about bark; I was a complete novice. I just took his word and dove in.

Q. In the book, I also learned delicious little details. Like the fact that the red-eyed vireo (a bird I hear singing all the time here in my upper field, which is backed by a grove of old birches) uses strips of paper-birch bark in its nests!

A. Trees are very foundational to a lot that goes on in the ecology of any place. I sometimes think of them as my charismatic megaflora. The more you look, there is a whole host of other questions you can ask, about why and how does this fit in with this bigger place?

Q. Let’s talk about bark structure. Like our human skin, with its epidermis, dermis and subcutaneous layer or hypodermis, bark is not a single layer like a piece of Saran wrap, is it?

A. It’s not, but it’s not something that’s obvious, because all we’re really seeing is the outer layer. That in itself is quite diverse depending on the species, and the age of the tree as well.

There are a whole number of things happening just outside the inner wood of the tree—what would be known as the inner bark—which is living tissue that provides a great deal of transportation function in the tree, but we never see.

Q. Is this where the vascular system of the tree is located?

A. Yes, it’s the phloem tissue, responsible for transporting everything that happens through photosynthesis. All that energy production gets transferred around the tree, horizontally and vertically, through this tissue.

Q. And this is the innermost bark layer?

A. Yes, and outside of that are a host of different layers. But what we’re looking at on the outside is actually dead tissue. So take a tree that has smooth bark on it, be it a young tree or a species that maintains that smooth bark as it gets older. What we’re looking at are cork cells, that die soon after they mature, and that outer layer is the protective layer protecting the tree from from drying out (from desiccation); from insect infestation; from pathogens, fungi and other things that want to gain access to the living tissues.

Q. It’s the armor.

A. It is—that’s a great word, because there are many species that really look armored, and with good reason.

Q. Again using the human skin analogy: A young tree’s bark and its bark at midlife and late life can be quite different, yes?  It expands as the tree grows, and undergoes other changes?

A. Take a common species such as American beech, which maintains this smooth, light gray bark throughout the life of the tree, if it’s not diseased or has some other compromise happening. That is one of the reasons that beeches around the world have long been famous for carving initials and words in—because the bark doesn’t break apart, so the words can last.

Q. And the letters get bigger in “point size,” don’t they?

A. They actually grow more prominent as time goes on. So this bark is actually adding material to confirm to the expanding circumference of the tree.

For most species in the Northeast, and a lot of other species as well—at some point the outer layer begins to break apart. I was actually surprised doing the research on how bark grows and how it functions to learn that this breaking apart is actually in response to the growth of the wood beneath it.

So for each species that has these multiple layers of bark, the outer layer reaches the  point where it can no longer grow as quickly as the wood growing beneath it. These are literally stress and pressure cracks.

Q. Like stress fractures!

A. It is. Remember, this is the protective outer layer of the tree, so if nothing else happened [when the cracks occurred], this tree would soon succumb to a whole host of invasions, and become very diseased and probably eventually die.

But invisible to us, a new protective layer is forming beneath this outer layer, and then that becomes part of this outer bark–becomes part of the protective layer made up mostly of dead cells. This process can happen once or twice or many multiple times in the life of the tree, and that’s how this armored bark forms over time.

This process and the way that it happens really dictates all of the different forms of bark that we see in all the species. It’s really a product of its evolutionary heritage, and the landscape it evolved in.

copyright Michael WojtechQ. Speaking of bark’s appearance, you paint a wonderful, overwhelmed picture in  the book about standing in your office for some time with the floor literally covered in photos of different kinds of bark, trying to figure out the code–the keys to what would become your basic bark types that organize the plant-by-plant ID section.  Talk about that process and the “aha” that eventually came.

A. These photos remained spread out on the floor of my office for many, many months. What I was trying to do is come up with a language for explaining the differences. Even once I started to notice the first line of observation—“Oh yes, this tree is different from the one next to it”—I still couldn’t really explain why in words.

What I started to do was take photos of bark that looked similar, and place them in piles. So then I had all these piles, and started to try to explain what makes everything in this pile the same, and makes one stack different from the next one.

I’d spread them all out, and try again, and as I started to really look at the structure, or bark physiology that we spoke about, it really started to provide the language, and an understanding of what was happening differently in the growth cycles of each species. And that really gave me the language and understanding of how to explain why one was different from the other.

Q. In this new language, how many bark types are there? At the end of the process, how many piles were there?

A. In the unique subset of species I was working with—64 species in New England and New York—I ended up with 10 different bark types. I had seen other people work on this kind of bark categorization, but because I was working on this subset, I created my own. That could be different in a different region—and any tree could be more than one of those types. There is overlap.

wojtech_bark_10_paper-birch-trunk_webQ. What are some of the types—what was the first you named?

A. I started easy, with “smooth and unbroken,” like the beech. Another that seemed to have some clarity was “peeling horizontally in curly strips.” Like a paper birch [above], or river birch.

Q. One of the most interesting sections–probably because like you, I am someone who’s always asking “why, why why?”–is the portion of the book called “Bark Ecology.” What is bark ecology?

A. Bark ecology is getting away from the individual tree and looking at the environment.  And this doesn’t just mean the environment you find the tree in, because this evolution of what the bark looks like happened over many, many years.

Beech, for example, has smooth bark. Many researchers think that is a lineage that dates back to a time when beech was a tropical species. Looking in the pollen fossil records, they have traced the northern migration of beech trees over many years after the last glaciers melted.

Smooth bark is not only an advantage, but also really a necessity for any tree in the tropics. Any kind of rough bark maintains a lot of moisture and a lot of surface area that allows epiphytes and other organisms to grow on the bark. Because of the tropical climate and the moisture levels, these organisms can amass in such great number they can weigh down the tree and break off a branch or bring down a whole tree.

There are advantages and disadvantages, now that this smooth bark is growing in a different climate. Thin bark does not offer the same kind of thermal protection or mechanical protection as thicker bark—so it’s always a give and take, and that’s the way all the organisms have adapted over time, trying to find their niche.

stewartia-trunkQ. In the garden (as in nature) I think exfoliating bark is especially beautiful—and you mentioned the kind that peels in horizontal strips. Then there are ones that peel in camouflage-type patterns, such as on the American planetree or sycamore, or garden beauties like Stewartia (above) and Kousa dogwood or Pinus bungeana, the lacebark pine. What purpose does that peeling bark of either type serve; why does it happen?

A. It was really rewarding to find that something that made writing the key for the field guide easier—that was really an identifiable characteristic—had such a functionality to it as well.

We can pick out one familiar species, paper birch or white birch, which has this peeling bark. One of the things that happens functionally is that any lichens or mosses or algae that happen to be growing on the bark of that tree also come off with the outer layers when they come off.

Q. So it sort of cleanses itself.

A. One of the advantages this tree has is that it has white bark. Trees can be subject to a lot of damage from abrupt heating and cooling that happens in the wintertime. The sun shines on the trunk, and then the sun goes down and causes abrupt cooling and can cause breaking of the bark, when it dries out and contracts.

A white tree, such as paper birch, isn’t going to heat anywhere near as much as a dark trunk, so it’s quite functional, but if it’s covered by mosses and lichens, that mitigates that benefit.

wojtech_bark_7_cork-skin_webThe other aspect that was quite interesting is that there is a very thin layer in bark that contains chlorophyll, and can conduct photosynthesis. It’s typically supplemental energy, and nowhere near the energy produced by the leaves. But in a species like paper birch their strongest competitive niche is at high latitudes and high altitudes—they’re one of the most Northerly sited hardwood species in North American. In those places, that extra supplemental energy from bark photosynthesis can make a huge difference. [Above, the green cork skin found in the interior layer of bark.]

Q. Do you have a favorite bark of all?

A. I’ve always been partial to sycamore trees.

Q. People always ask me: What about lichen on bark? Is it bad?

A. It’s usually indifferent—unless in places like the tropics, as we mentioned, they accumulate in such mass. In the case of a tree that relies on bark photosynthesis, there are pores called lenticels—those dark horizontal or vertical lines, such as on cherry bark—that are absolutely necessary for photosynthesis, to provide gas exchange.

If they get blocked, it can cause an issue (which is why the peeling helps).

Pinus bungeana damage by sapsuckerQ. You mentioned frost cracks from temperature swings—but there are other enemies of tree bark, too, that you mention in the book: insects, deer in rut who scrape bark off, beavers, porcupines, yellow-bellied sapsuckers….[photo above of sapsucker damage to Pinus bungeana].

A. There are a number of things a tree needs to defend itself: They have structural defenses, and chemical defenses. Normally it takes something quite extreme or a cascade of events, such as a drought weakens the tree and it has one type of a problem, then something else takes advantage of that weakness and so on.

enter to win the bark field guide

Wojtech_Bark_coverI‘LL BUY BOUGHT A LUCKY READER a copy of, “Bark: A Field Guide to Trees of the Northeast” (affiliate link). All you have to do to enter is Answer this question in the comments box at the very bottom of the page, after the last comment (UPDATE: Giveaway is done):

What’s your favorite bark? Michael and I seem to agree on sycamore, with its camouflage pattern in multiple colors. How about you?

No answer, or feeling shy? Just say, “Count me in” or something to that effect, and I will. A random winner will be was chosen after entries closed at midnight Monday, November 17, 2014. U.S and Canada only. Good luck to all.

(All photos except Stewartia and Pinus bungeana copyright Michael Wojteck, used with permission.)

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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its seventh year in March 2016. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).

  1. I am interested in all the comments as I search for two trees to be close to large number of windows: dining room windows and master bedroom windows with much protection on two sides so full sun east and south. Windows are about 12 feet high to top of ceiling. Tree can be no bigger than 15 feet or so or it will block solar pannels!

    I need a tree with good winter bark interest as well as the ability to see through the tree to the rest of the yard. In both cases the tree will be about eight feet from the windows. One of the trees should be columnar probably due to porch and bulk head access. The other can be16-20 feet wide…..

    Must be a special tree hiding somewhere (we are on south coast of MA, zone 6 or maybe seven with such protection!)


    Maybe a Japanese Maple but I cant find which one!

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