The Protective Layer – Peace and Healing A Perspective of Traditional and Non-Traditional Methods of Healing The Protective Layer November 12, 2011 by Dan Williams, Psy.D.,P.A.-C. | 0 comments Our skin, the largest organ of our body. It protects us from the elements as well as helping with our body temperature. If one has ever cut down a tree you have noticed the protective layer of the bark. Also an insulator as well as a protector of nutrients that flow up the trunk. Damage the bark significantly and you see a scar for the rest of the tree’s life. Even our brain has a protective outer layer called dura mater, “tough mother.” The protection of many animals, trees, plants and even insects have protective outer layers to escape predators as well as protection from the elements. What about our emotions, our soul, our spirit? What is the protective layer of our emotions? Is it any wonder that paramedics, police officers, and those in active combat become calloused or to the contrary do not, and suffer PTSD, post traumatic stress disorder. It IS our very perception of self, combined with experience and our success, as well as failure that protects our soul our living spirit.

When met with repetitious life trauma, emotional devastation repeatedly we may weaken, however this is also the process that develops character. The outer layer that protects our spirit, our soul, our emotional demeanor is our perception of life and how we chose to handle it. Nothing more, nothing less. Why the surge on self help books, the quest for the “right” religion, the need to question our belief versus listening to a loved one try and convince us to accept a certain religious paradigm. When pushed to the limit of stress. When we feel we are ready to break. When the emotion upheaval is so heavy upon our ability to cope, we may rely on others to converse. When we hang up the phone, leave the therapy session or begin to walk our walk of life….in the end it is always “YOU” that goes to bed at night when the covers are tucked up under your chin. Recognize the protective layer is your perception, your attitude, your belief. It is the make or break in terms of happiness and success when dealing with stress and life trauma. If one has a shattered or weak perception of self, it takes minimal understanding to see one shaken, one rattled and disoriented on which path to take.

We sometimes over compensate and build a very calloused layer to protect the wounded soul. The emotional bark becomes thicker and at times impenetrable. It is up to us to soften the outer layer with time, personal growth and trusting others. Yes, it takes time. Sometimes, like the tree hit by lightning, the scar remains and rears it’s ugly head in the form of anger, tears, or condescending behavior. Sarcasm, the wonderful defense to softness. It is humorous, but when used frequently is a defense mechanism to avoid. The avoidance of closeness. I know few people that would want to hug a hawthorn tree with it’s wooden barbs emitting out from the trunk as if it were a Medieval weapon. Humans can grow these thorns as well as remove them. We have the opportunity to become guarded and protective. We also have the ability to let the guard down and become soft and giving. The balance. Ah, the balance. Humans must learn and understand their own protective layer. The skin of their soul. The bark around their spirituality. It is important to protect ourselves, however too much protection and we live emotionally lonely lives, where we can rationalize our very existence.

We can convince ourselves we are happy, when we are not. Understand the importance of your protective layer and how it changes with certain individuals. Decide how you want to live. Maybe you need to be more protective of your soul and just maybe you need to open up and become more vulnerable. Either way, recognize the protective layer and your responsibility in owning it. Anatomy of a tree at arborday.org Anatomy of a Tree The Inside Story The outer bark is the tree’s protection from the outside world. Continually renewed from within, it helps keep out moisture in the rain, and prevents the tree from losing moisture when the air is dry. It insulates against cold and heat and wards off insect enemies. The inner bark, or “phloem”, is pipeline through which food is passed to the rest of the tree. It lives for only a short time, then dies and turns to cork to become part of the protective outer bark. The cambium cell layer is the growing part of the trunk. It annually produces new bark and new wood in response to hormones that pass down through the phloem with food from the leaves. These hormones, called “auxins”, stimulate growth in cells.

Auxins are produced by leaf buds at the ends of branches as soon as they start growing in spring. Sapwood is the tree’s pipeline for water moving up to the leaves. Sapwood is new wood. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood. Heartwood is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact. A composite of hollow, needlelike cellulose fibers bound together by a chemical glue called lignin, it is in many ways as strong as steel. A piece 12" long and 1" by 2" in cross section set vertically can support a weight of twenty tons! Leaves Make Food For the Tree And this tells us much about their shapes. For example, the narrow needles of a Douglasfir can expose as much as three acres of chlorophyll surface to the sun. The lobes, leaflets and jagged edges of many broad leaves have their uses, too. They help evaporate the water used in food-building, reduce wind resistance—even provide “drip tips” to shed rain that, left standing, could decay the leaf.

Advanced Search—The Tree Guide at Arborday.org Anatomy of a Tree Leaves Leaves carry out photosynthesis, making food for the tree and releasing oxygen into the air. And this tells us much about their shapes. For example, the narrow needles of a Douglasfir can expose as much as three acres of chlorophyll surface to the sun. The lobes, leaflets and jagged edges of many broad leaves have their uses, too. They help evaporate the water used in food-building, reduce wind resistance and even provide “drip tips” to shed rain that, left standing, could decay the leaf. Branches and Twigs Branches and twigs grow out of the tree trunk and serve as support structures for leaves, flowers and fruit. They also transport materials between the truck and the leaves. Trunk The trunk of a tree is made up of five different layers. The outer bark is the tree's protection from the outside world. Continually renewed from within, it helps keep out moisture in the rain and prevents the tree from losing moisture when the air is dry. It insulates against cold and heat and wards off insect enemies. The inner bark, or “phloem,” is the pipeline through which food is passed to the rest of the tree.

It lives for only a short time then dies and turns to cork to become part of the protective outer bark. The cambium cell layer is the growing part of the trunk. It annually produces new bark and new wood in response to hormones that pass down through the phloem with food from the leaves. These hormones, called “auxins,” stimulate growth in cells. Auxins are produced by leaf buds at the ends of branches as soon as they start growing in the spring. Sapwood is the tree's pipeline for water moving up to the leaves. Sapwood is new wood. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood. Heartwood is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact. A composite of hollow, needlelike cellulose fibers bound together by a chemical glue called lignin, it is in many ways as strong as steel. Set vertically, a 1" x 2" cross section that is 12" long can support twenty tons! Roots Contrary to popular belief, tree roots are typically found in the top three feet of the soil.

They also expand well beyond the dripline, often occupying an area two to four times the size of the tree crown. A tree’s root system works to absorb water and minerals from the soil, anchor the tree to the ground, and store food reserves for the winter. It is made up of two kinds of roots: large perennial roots and smaller, short-lived feeder roots. We inspire people to plant, nurture and celebrate trees. The Arbor Day Foundation is a 501(c)(3) nonprofit conservation and education organization. A million members, donors, and partners support our programs to make our world greener and healthier. Biology Plant Tissues - Shmoop Biology No, we’re not talking about Kleenex or Puffs or other handy products during cold season, but the tissues that make up plant bodies. A tissue is a group of cells with a common function, structure or both. Some of the key tissues in plants are: vascular tissues dermal tissues ground tissues In animals, vascular tissue transports blood throughout the body, delivering oxygen, removing waste, creating Olympic athletes, and so on. In plants the tissues that transport things around the plant are called xylem and phloem. Want to hear a funny plant joke? Of course you do.

If a plant became a musician, he might play the xyle-phloem. OK, maybe "funny" was a bit of an overstatement. Non-vascular plants such as mosses don’t have vascular tissue, but in all other plants xylem transports water around the plant. Xylem cells are like zombies in that they are dead when functional. They don’t, however, eat brains. At least, not that we know of. Dead cells, which don’t have organelles filling them up, have a greater capacity for transporting water. There are two types of cells that make up the xylem: tracheids and vessel elements. Both of these cell types are dead when they are used in the xylem. Using dead cells, which don’t have organelles filling them up, allows more capacity for transporting water. Tracheids are long, narrow cells whose ends overlap. They have small holes between their ends, which allow water to move vertically between the cells. They also have small holes called pits on their sides, which allow water to move laterally. Tracheids look like flutes standing on one end. Vessel elements are shorter, wider cells.

If a tracheid is a footlong hot dog, vessel elements would be more like little breakfast sausages; shorter and wider. When they are stacked on top of each other, the walls between them dissolve and water can move through them. Only woody angiosperms (flowering plants) have both vessel elements and tracheids; other vascular plants have only tracheids. Phloem transports nutrients, most of which are products of photosynthesis. The main type of phloem cell is called a sieve element. Sieve elements are different than tracheids and vessel elements because they are alive when functional. But how could they possibly have room to transport all those nutrients if they are living cells filled with organelles? Unlike Dr. Who’s TARDIS , these cells aren’t magically bigger on the inside. Sieve cells rely on companion cells . Sieve elements are just cells, and if they are going to transport anything then they have to work together. The ends of the sieve element cells have little holes in them; where two sieve elements meet is called a sieve plate. Liquid can pass from one sieve element to the next through the sieve plate.

A sieve plate is very similar to the colander you might use to drain the water from pasta after it is done cooking—except that a colander is round and a sieve plate isn’t. Xylem and phloem make up the vascular tissue. The vascular tissue is arranged in bundles, and is easy to see in a cross-section of a stem: Another important tissue is the dermal tissue, which is the outer protective layer. Plants don’t have skin exactly, but they do have an epidermis. (Did anyone else just picture a plant covered in human skin? No? Just us? Right then, moving on.) The epidermis is the outermost layer of tissue, the tissue that has to meet the cruel world out there. The epidermis is the tissue that encounters the rain, the heat, the pollution, and all the other elements. In non-woody plants, the epidermis is the main security guard against the big scary world, but leaves and stems also get protection from a waxy coating on top of the epidermis called the cuticle. In woody plants, the epidermis gets replaced by a protective tissue called the periderm as stems and roots get older.

Because the dermal tissue is the outermost layer of cells, it is the plant’s first layer of defense and is important for protecting the plant. One way the epidermis protects the plant is by growing little hairs, or trichomes. Before you start imagining plants that look like they’re related to Cousin Itt , keep in mind that these hairs are super-fine. Not superfreaky. You may have never noticed the hairs before, but take a look at the next plant you pass. The little stem that connects the leaf to the rest of the stem or branch often has tiny hairs on it. Trichomes have a few important functions, but one thing they do especially well is prevent insects from eating plants. Just as some people prefer nectarines to peaches (no fuzz), most insects would prefer trichome-less plants. No one wants a mouthful of hair when they’re eating. Trichomes can also help reflect sunlight, acting as a natural sunscreen for the plant. As it does with people, too much sun can harm plants because it damages cells and increases water loss. Roots have epidermal hairs too. Underground, the hairs do not provide as much protection from insects as they would aboveground.

Instead, the root hairs increase the surface area of the root and help take up more water. Plants are in a constant struggle to keep themselves hydrated, like a marathoner racing on a hot day. Bring on the Gatorade. Tissues that make up part of the plant but are not vascular tissue or epidermis are called ground tissue. Ground tissue can be found inside and outside the ring of vascular tissue in the plant stem. Tissue that is on the inside of the vascular tissue ring is called pith, and tissue on the outside of the vascular tissue is called cortex. Ground tissue is made up of three main cell types: parenchyma collenchyma sclerenchyma Parenchyma cells are the most common cells you encounter inside a plant. They have thin walls, are alive when functional, and are all around pretty awesome. (But beware! You might run into this term again, used differently, in animal biology. Don’t say we didn’t warn you.) These cells are involved in some A-list activities in the plant, including being the site of all that photosynthesis plants are doing.

But wait, you say, you’ve already read the section on photosynthesis , and you know that it happens in chloroplasts? That’s correct, but chloroplasts are organelles, and have to live inside cells. Those cells are parenchyma. Though photosynthesis usually happens in the leaf, parenchyma cells can be found all over the plant body. They often function as storage cells, serving as both attic and garage for extra starch. Parenchyma stores starch in roots, stems and even fruit. One other cool thing about parenchyma cells is their ability to divide and differentiate into different cell types, given the right conditions. Because of this property, parenchyma cells get involved in wound repair. We wonder when they had time for a first-aid class, with all the other things they’re doing. Collenchyma cells have thicker walls than parenchyma and are often involved in support. Since plants don’t have bones, they have to figure out some other way of standing up straight. Collenchyma cells like hanging out with each other so much that they often form strands, like the strings on a stalk of celery. Although they’re strong, collenchyma cells are flexible too.

Some cells have lignin in them, which is a polymer that makes the cells sturdier. Collenchyma cells do not have lignin in their cell walls, so they can be nice and bendy. A sclerenchyma cell, in addition to having a long name, has a tough job to do. It has to give its life to a greater cause: the support and protection of the plant body. Sclerenchyma cells get their name from "scleros," which is Greek for hard. These cells are hard because they have lignin in their walls (unlike Mr. Collenchyma up there) and are usually dead when functional. Sclerenchyma cells come in two types: fibers sclerids Fibers are important for structure and support, acting like a skeleton for a plant body. These cells are usually grouped in threads and are long cells. Some fibers don’t have lignin in their cell walls, which makes them soft and commercially attractive. Hemp and flax fibers are both used commercially. Sclerids are usually used for protective purposes in addition to being support cells. They have thick walls and are perfect for making up seed coats and nutshells.

Brain Snack Don’t be freaked out by all these cells that are dead but still working in the plant. Humans use dead cells too—our hair and nails are made up of dead cells, and many people spend more time prettying up their dead cells than their living ones with gel, mousse, hair dye and nail polish. The Language of Bark - American Forests American Forests Home /The Language of Bark The Language of Bark The search for a better all-season field guide inspires a closer look at the unexpected beauty of bark. Organizing photos of various bark types Story and photos By Michael Wojtech I spot the neon-orange flag, unmistakable among the more earthly hues of October leaves, and crash through the underbrush to reach it. I quickly begin cataloguing the surrounding trees. On a low-hanging branch, I find familiar looking needles and record eastern hemlock on my data sheet. I peer up through binoculars to recognize the leaves of a broad beauty — an old red maple. The next tree, a tall one, looks like a hickory. Pignut? Bitternut? Mockernut? Its leaves, buds and twigs are too high to see.

I write down bitternut, based on the moist, lowland terrain I find myself in, but wish I could be more certain. The flag marks one of 30 plots scattered throughout this western Massachusetts forest in which I have been inventorying trees over the last several weeks. Although it is still morning, a deepening dusk descends along with a steady drizzle; a downpour seems imminent. It is the wind, though, that spurs me on. With each gust, more of the season’s last leaves rain down upon me — leaves that provide critical clues to species identification. Today is my last chance. The coming storm will leave behind a canopy of bare branches. By late afternoon, I reach the last plot and document the last tree. I leave the forest drenched, but relieved. Inner bark is a primary food source for beavers, who have stripped it from the base of this American beech. In the following months, as I work toward my master’s degree in conservation biology, I yearn for an easier, all-season tree identification guide — one that would have helped me on that October day. Ecologist Tom Wessels, my thesis advisor at Antioch University New England, suggests a solution: bark.

I recognize the peeling, gleaming white trunks of paper birch and the smooth, light-gray bark of American beech, but most often, I see bark as a blur of browns and grays. Bark, however, can easily be inspected in any season, and I decide it is the perfect subject for a regional field guide — one I can create for my thesis. What I learn as I begin field work is how little I know. Bark’s diversity, especially within the same species, is far greater than I imagined. It will take me three years to complete my thesis and another four and a half years for my field guide to be published. A NEW LANGUAGE A friend traces the deep furrows of bark with his finger. “That’s an eastern white pine,” I tell him. When he asks, “How can you tell?” I am speechless. Over a year into my project, I recognize most tree species by their bark, but still struggle to describe them for others. Hundreds of bark photographs are spread across my office floor. Although I have been sorting through them for months, it has been difficult to explain why some photos are similar and what makes others look so different.

Cross section of paper birch trunk showing the orange-brown inner bark, or phloem, surrounded by the white, protective cork layer of the periderm I need a new language, which I find by discovering how bark’s multiple layers affect its surface appearance. The growth and function of these layers help provide criteria for separating the photos into categories, which I call bark types — the first level of identification in my bark key. Differentiating the species within each bark type will require a second level of identification, but with this start, I begin to see the forest — and the photos on my floor — through new eyes. I gather together photos of trees with smooth, unbroken bark, which become my first bark type. These are mostly young trees, with bark that will change in appearance as they grow older. Now, to develop my next bark type category, I need to sketch out the multi-layered structure of bark. The inner bark layer, the phloem, transports sugars produced by photosynthesis throughout the tree. This living layer is the food of beavers, porcupines and other mammals, including humans — the phloem of eastern white pine and other species can be dried and ground into flour.

The green cork skin found in the interior layer of bark can produce supplementary energy through photosynthesis. The layers that are part of the outer bark are collectively referred to as the periderm. The outmost periderm layer — made up of cork cells that die soon after their protective qualities have developed — is the visible, touchable surface of smooth-barked trees. Cork protects the tree by helping prevent desiccation and keeping insects and pathogens from penetrating to the living tissues beneath it. The dead, mostly air-filled cells also help insulate the tree and account for the light weight of the bark. Beneath the cork in the periderm are the cork cambium, an area where cell division occurs, and the thin, green cork skin, found by gently scraping away the outer bark on a young twig. The cork skin is capable, to a lesser degree, of the same energy-producing photosynthesis as leaves. Supplemental energy from bark photosynthesis helps trees stay healthy and can boost recovery from defoliation due to insect infestation, disease, storms or severe drought. Distinctive pores, called lenticels, facilitate the controlled gas exchange — necessary for bark photosynthesis — of carbon dioxide and oxygen through the protective outer bark.

Lenticels are readily visible on many species, especially on smooth bark, where you can also feel how they protrude from the surface. They become criteria for another pile of photos and are the distinguishing characteristic of my second bark type. The variety of shapes, sizes and colors of lenticels — from the dark, horizontal lines of yellow birch to the diamond-shaped structures of young bigtooth aspen — will later help me identify the species within this bark type. TEASING APART THE BROWNS AND GRAYS A few species, such as American beech, maintain smooth, unbroken bark for their entire lifespan, as their initial periderm continues to grow around the increasing circumference of the trunk and branches, but many of the uncategorized photos still spread across my floor show bark that has broken apart and grown thicker in a multitude of ways. These trees have all reached a point where their wood is growing faster than — and pushing outward against — the bark that surrounds it. The bark of each species responds differently to this pressure, resulting in specific bark characteristics that provide clues for species identification.

On some species, such as paper birch, this pressure causes thin layers of the outer cork to separate and peel away from the trunk and branches in horizontal, curly strips — creating my third distinctive bark type and a new stack of photos. More often, pressure exerted by the wood splits the entire periderm, including its protective cork layer. The first visible sign of this process is often vertical cracks in otherwise smooth bark, a feature I noticed long ago on northern red oak. I designate this as my fourth bark type. Deep furrows separate thick, corky ridges on mature eastern white pine. Because the periderm protects the tree from outside elements before it breaks apart, a new, active periderm forms beneath the old one, within the active phloem tissue. The cells outside of this new periderm then become isolated and die. This process, which can occur once or multiple times depending on a tree’s age and species, results in alternating, non-living layers of old periderm and old phloem tissue called the rhytidome — the Greek word for wrinkle.

My remaining, uncategorized photos all seem, at first, to display the same rough, thick, multilayered rhytidome — the iconic representation of bark that I held in my mind’s eye before starting this project. Slowly, though, as I closely observe my photos and get my hands on living specimens, differences begin to emerge. On some bark, gaps appear between the outer rhytidome layers, where one could easily catch an edge and pry pieces away from the trunk. Bark with this characteristic can further be divided into three new bark types: scales of bark like on a black cherry; thick, irregular plates like on a black birch; and vertical strips like on a red maple. Other trees have rhytidomes with more tightly adhered layers, creating an additional three bark types: intersecting ridges like on a white ash; ridges broken horizontally like on white oak; and uninterrupted ridges like on northern red oak. For the first time in months, I now have a clear walking path through my office thanks to 10 neat piles of photos representing each of my 10 bark types, which become the foundation for an identification key.

I enjoy months in the woods searching for my next set of descriptive clues to differentiate the species within each bark type. At a particular stage in the growth of white pine, for example, I discover fine, horizontal cracks that are evenly spaced, like writing paper. I spend weeks looking at nothing but sugar maple trees before noticing that the surface of their bark is crackled, like old china. BARK ECOLOGY After almost three years, I complete the bark key and hand in my thesis. But my field guide is still missing an important piece: I have yet to address the environmental influences behind bark’s grand diversity. Drought, fire, temperature extremes, limited growing seasons and interactions with other organisms all have influenced the evolution of different bark characteristics. I begin to research this missing piece by investigating the functions of paper birch’s bark features, which I have been wondering about for some time. The habitat and range of the thin-barked paper birch includes high altitudes and the far northern regions of North America — places where temperate fluctuations are most extreme.

Trees can be damaged by sunscald and frost cracks, where abrupt transitions from sun-soaked warmth to cold, or the reverse, can crack or kill sections of bark and open pathways for insects, fungi and other harmful invaders. Yet, this species only maintains a thin outer bark as it matures, even though thick, multi-layered rhytidomes generally provide the best thermal and structural protection. The white trunk, peeling bark and prominent lenticels make paper birch easy to identify. The white coloring of paper birch bark helps make up for its lack of insulating thickness by reflecting sunlight and reducing the potential for damage. Its peeling, curly strips keep the outer bark thin, allowing more sunlight to reach the photosynthetic cork skin — even on the trunk. The peeling mechanism also strips mosses, algae and lichens from the surface of paper birch’s trunk and branches where, if left to accumulate, they could block sunlight for bark photosynthesis, prevent gas exchange by clogging lenticels and, if dark-colored, contribute to damage from overheating.

Damage to a tree — such as wounds in a tree’s outer bark from fire, sunscald, tunneling bark beetles, gnawing rodents or broken branches — are an inevitable part of its lifecycle. Each species has developed a specific set of chemical and structural mechanisms in their bark that help heal wounds and protect the tree, and some of these defenses offer clues for species identification. For instance, betulin, the compound that whitens paper birch bark, also deters against desiccation and bacteria, fungi, insects and gnawing animals, and the curious, concentric cracks often found on red maple bark result from repeated cycles of fungal infections and attempts by the bark to wall off, or compartmentalize, the intruders. I also discover olfactory clues to bark identification that result from a bark’s defensive mechanisms. A protective chemical in black cherry bark that deters browsing animals yields a bitter, almond scent when the outer bark is scraped away from a twig. But this smell of the chemical doesn’t deter humans, as it is used to make cough medicines, expectorants and throat lozenges.

Safrole in the bark of sassafras deters insects and other pests, but I find its sweet, licorice-like odor quite pleasing. Efforts by red maple bark to combat a fungal infection result in concentric cracks called target canker, which can be used to identify this species. Then, there are the beneficial associations between bark and other organisms. Helpful fungi often congregate around lenticels and branch junctions — where invasive organisms are most apt to gain entry — and deter or feed upon other harmful fungi. Slugs often leave trails across the bark of American beech as they glean algae that could otherwise block sunlight for bark photosynthesis. I’ve discovered that the environmental influences on bark are even more varied and numerous than the many different bark characteristics that can be used for species identification. Even after finishing a chapter on bark ecology and seeing my field guide published, I know I have only touched the surface of this seemingly infinite web of interactions. Now, when I walk through a forest, I look at each tree and imagine what is going on beneath the visible surface of its bark — as if, in a way, I have x-ray vision.

I have learned to use visual and tactile clues to see and understand what used to leave me feeling lost in a sea of complexity. Instead of a blur of brown and gray trunks, I now see individual trees with species names and a host of ecological relationships. Beyond the practical reasons for learning to identify bark, I realize that I have been learning and teaching the art of perception. Bark may not seem exotic. It may not, at least initially, leave you in awe like the panoramic view from a hilltop or a glimpse of a crimson morning sky. But learning to see formerly mundane or hidden layers of beauty and function opens up a world of detail and nuance that allow what is local to become spectacular, bringing us closer to home. Michael Wojtech, a freelance writer, educator, photographer and illustrator, adapted this piece from his book, Bark: A Field Guide to Trees of the Northeast. Learn more at . Getting to Know Bark | Winter 2011 Getting to Know Bark 1 Comments As the vibrant colors of fall give way to the more subtle hues of winter, tree bark becomes a focal point in the forested landscape.

The splendid contrasts of bark are highlighted by the low-riding winter sun – the white, curly strips of paper birch juxtapose with the dark, burnt cornflake scales on black cherry; the furrowed ski track patterns on the red oaks complement the smooth, bear-scarred bark of American beech. In the Northeast, one can encounter 67 native and naturalized tree species, each with its own distinctive bark that often changes as trees mature. Spend a contemplative afternoon in the woods, and you might begin to wonder where all this diversity comes from. Why do species such as American beech maintain smooth, unbroken bark for their entire lifespans (if they avoid the ravages of beech bark disease), while the bark of northern red oak, black cherry, and most other species in this region cracks and thickens over time? The gleaming white trunks of paper birch are easy to recognize – but why are they so different from their gray and brown neighbors? These questions are a great entry point into the world of bark ecology. Drought, temperature, fire, growing seasons, and interactions with other organisms all influence bark characteristics.

But there are more subtle factors at play here as well, including physiological adaptations that go back millions of years. Bark Structure To understand why different trees display different styles of bark, we must first understand the inner workings of a tree. The term “bark” is often used to describe only the corky, visible outer surface of trunks and branches. In botanical terms, though, bark includes the entire, multi-layered shell of a tree that can be detached from the wood. That is, everything outside a thin ring of tissue called the vascular cambium. Cells divide and grow in the vascular cambium layer, producing a ring of wood to the inside and a layer of bark tissue, called the active phloem, to the outside. The active phloem transports sugars and nutrients throughout the tree, and is typically hidden from view, beneath the outer bark. Outside the active phloem, trees have three additional bark layers, collectively called the periderm. You may need a scalpel and a microscope to find the first two layers, which are quite thin and also hidden from view. The inner layer – the cork skin – usually contains chlorophyll and does some photosynthesizing. The middle, cork cambium layer facilitates cell growth.

The third, outer layer is made up of cork cells that die soon after they mature. This cork layer protects the tree from infection, infestation, and drying out. The smooth, unbroken outer bark that all trees start out with is this cork layer of the periderm. As a tree grows, its wood thickens and pushes out against the bark that surrounds it. The different ways in which the outer bark adapts to this pressure is what gives each species its distinctive appearance. Some species maintain their original outer layers (the initial periderm) for their entire lives. In such cases, the outer bark expands to match the growth of the wood beneath it, and remains unbroken. On the smooth bark of beech trees, for instance, you can still decipher decades-old graffiti. In most northeastern species, however, pressure from the faster-growing wood soon causes the initial periderm to split like a tight pair of jeans. When the initial periderm breaks, a new layer of active periderm forms to the inside. This new periderm usually forms in overlapping sections that vary in shape, size, and thickness according to species. Any cells trapped outside the active periderm become isolated and die.

This process can repeat itself as a tree grows. Alternating layers of old periderm and dead phloem form the thick, craggy, flakey, outer bark that is found on most mature trees. Botanists call this protective outer bark the rhytidome, from the Greek rhytidoma, for wrinkle. In each tree species, the bark’s appearance is determined by the shape of the overlapping sections of periderm, the type of connective tissue, and the rate at which layers of bark break apart. In black cherry, for example, the scales correspond to the size and shape of the periderm sections, which break apart from each other. In species such as northern red oak, sections of outer bark stay connected to form thick, vertical ridges. Thick or Thin? It’s impossible not to notice how some mature tree species, like black oak, have remarkably thick bark, while others, like beech, do not. Thick outer bark is generally a good investment, since it better protects a tree from wounds and provides more thermal protection. The outer bark’s air-filled cells function much like in home insulation, keeping the inside warmer or cooler than the outside.

Ridges, scales, and vertical strips can function as radiator fins, dramatically increasing the outer surface area and helping to maintain a more even temperature. Contoured barks also hold moisture, which slows the transfer of heat through the outer bark. Thick bark is especially important for fire protection. For example, black oak, which typically grows in drier, more fireprone habitat than any other species in the red oak group, develops thick bark earlier than any of these relatives. The outer bark of pitch pine, the most fire-adapted species in this region, thickens while the trees are still young and develops into deep, corky blocks that allow it to grow in dry habitats on rocky summits and sand plain forests that are susceptible to fire. Fire is not the only thermal threat to trees; rapid temperature changes can also damage or kill sections of bark. In winter, for example, direct sunlight can warm bark to temperatures much higher than the surrounding air. When the sun sets and temperatures plummet, the rapidly cooling bark can crack as it contracts. With all the protective advantages of thick bark, why does the bark of some species in our region remain thin?

As well as requiring less effort to produce, a major advantage of thin bark is the increased ability to photosynthesize. Scraping away the outer bark on a twig or young branch reveals the thin, surprisingly green cork skin. This layer can perform, to a lesser degree, the same photosynthesis as leaves. Levels of energy production are highest on newer growth, where up to half of available sunlight can penetrate the outer bark and reach this green layer. Since thick bark blocks most or all sunlight from reaching the cork skin, photosynthesis levels are usually much higher on species that maintain thin bark on their trunk and branches. Energy produced by bark photosynthesis is thought to support regular cell maintenance in the trunk and branches and can help trees recover from defoliation due to insects, storms, or severe drought. Bark photosynthesis has its heyday in early spring and late fall, when leaves do not shade the bark. But energy can also be produced in a seemingly dormant winter forest. Sunlight warms bark on the south- and southwest-facing sides of trunks and branches, making it possible for bark to photosynthesize even when air temperatures are below freezing.

Thin bark also helps thwart the mosses, lichens, and algae (collectively called epiphytes) that grow on trees and sometimes become a nuisance. Epiphytes can block sunlight, thus preventing efficient photosynthesis. They can also absorb heat, which increases a tree’s risk of damage from temperature changes. Beech developed smooth bark, which is difficult for epiphytes to gain a foothold on, because it evolved in tropical climates where epiphytes can amass in such large numbers that they break branches or pull down trunks; it maintained this trait as it migrated to cooler climates. Some trees, like paper birch, feature strips of bark that peel away from the trunk and take with them any light-blocking epiphytes that may have become established there. This peeling process also keeps lenticels, which are vital to bark photosynthesis, from becoming blocked. Bark Defenses Like the exterior walls of a house, outer bark not only regulates temperature, it also protects against intruders. In general, thin-barked species like American beech are easier to penetrate than species with thick bark, like northern red oak.

But bark of any thickness has weak spots at lenticels, at cracks and furrows, and at branch junctions, where wrinkles bring the inner bark closer to the surface. Wounds in the outer bark open pathways for invaders such as fungi, bacteria, and insects, but in most cases the bark’s chemical and structural defenses can overcome infections and infestations. Resins in conifers (which give species like balsam fir their pleasing smell) and the gums in black cherry bark repel insects and infectious agents, seal small wounds to prevent desiccation and infection, and trap insects. Black cherry bark also contains hydrocyanic acid, which gives off a sour, bitter-almond scent that wards off browsers, and has long been used by humans in cough medicines, as an expectorant, and to sooth sore throats. Betulin, which whitens the bark of paper birch, deters gnawing animals, fungi, insects, and other invaders, helping to make up for the easy penetrability of this species’ thin outer bark. A host of medicinal properties are under investigation for betulin, including its potential as an anti-cancer drug.

The inner bark of aspen and other members of the willow family contains salicin, a compound that deters bacteria, fungi, and insects. Salicin has long been used by humans as a pain reliever, and is the precursor of synthetically produced aspirin. Structural mechanisms also defend a tree against infection and infestation, and can alter the appearance of bark in the process. Fungi that breach the outer bark, for example, can be walled off, or compartmentalized. This action contains the infection, but it also kills sections of bark by blocking the incoming flow of water and nutrients. The resulting small areas of discolored, sunken, or cracked bark are called annual cankers. Perennial cankers form when fungi that were initially compartmentalized overcome this defense by continuing to grow while the tree is dormant, or by colonizing the wood beneath the bark and bypassing the blockade. This cycle of incursion and walling off can repeat year after year, causing sections of bark to crack, break apart, and eventually separate from the trunk. Some structural responses cause visible changes in the outer bark that help with tree identification.

In this region, the perennial target canker – a series of concentric cracks in the outer bark – is found exclusively on red maple, though not on every tree. The circular cracks mark the cycles of the disease’s progress and the tree’s subsequent compartmentalization. Neither the tree nor the fungus seems to gain the upper hand, and this competition can continue year after year. If an infection spreads around the circumference of the trunk, the resulting cankers can girdle and kill the tree. In the early 1900s, infections of this magnitude, caused by a nonnative fungus, nearly eliminated American chestnut. Even when an infection or infestation is controlled, a tree must contend with a breach in the protective outer bark – like a broken window in a house – that other invaders could exploit. The inner bark generates cork to surround a wound, and can close small openings and narrow or close large gaps over time. Eastern hemlock is the only species in this region that produces wound cork in annual increments that you can count – like rings of wood – to determine a wound’s age. Despite its multiple chemical and structural defenses, bark can’t protect trees from all attackers, especially introduced organisms for which a species has no evolved resistance.

Beech bark disease illustrates the domino effect that can be initiated by a simple wound. The thin bark of American beech provides many advantages, but this species has no defense against the beech scale, a tiny insect smaller than the head of a pin, accidentally introduced from Europe in the 1800s. When these insects penetrate the bark to feed on sap, they chemically alter the bark in a way that allows a fungus to invade at a later time. A tree’s efforts to compartmentalize these infections typically fail. The resulting perennial cankers, which render the trunk and branches almost unrecognizable as beech, facilitate secondary infections and insect infestations that most often lead to the death of the tree. Not all associations between bark and other organisms are parasitic. Some bark-inhabiting fungi and bacteria are thought to survive solely on exudates from their host, and do no harm. Other fungi and bacteria defend their hosts by out-competing or preying upon canker-causing fungi. These beneficial organisms are often found near lenticels or other weak areas of bark where pathogens might gain entry.

When tiny insects, such as springtails and bark-lice, inhabit bark and feed on mosses, lichens, and fungi growing there, they can benefit their host tree by attracting spiders, ants, and other predators that can help control populations of defoliating insects. At any given moment in your local forests, there are thousands of interactions between bark and the environment. We can’t easily see most of them, but visible features of bark hint at this invisible interplay. On a winter hike you might spy the luminous, whitebarked branches of a sycamore tree and imagine bark photosynthesis cranking out a little supplemental energy. Or conversely, you might feel kinship with an eastern hemlock – its multi-layered bark providing the same hedge against the winter chill as your multi-layered clothing. Take some time to look at bark up close. Notice the resin blisters on balsam fir, the patchwork of thick plates on mature black birch, the way poplar bark changes so dramatically as the trees age.

Perhaps you will be willing to strip off your gloves in the cold and get your hands on some bark – you might compare the smooth surface of a beech trunk with the rough ridges of a red oak, or notice the multiple layers of scales on a black cherry. At the very least, you will find that the world of bark is more than just a monotonous blur of browns and grays. To a tree, it’s a complex world of function; to an observer, it’s a source of beauty that shows itself best against the contrast of snowy winter woods. Michael Wojtech is the author of Bark: A Field Guide to Trees of the Northeast. He is a freelance writer and teaches workshops on trees throughout the region. Photo Gallery © 2011 by the author; this article may not be copied or reproduced without the author's consent. Tags: bark, beech, conservation, forest management, forestry, new hampshire, northern woodlands, sustainability, trees, wildlife, willow. Y does water leak from from this particular tree? Join the discussion To ensure a respectful dialogue, please refrain from posting content that is unlawful, harassing, discriminatory, libelous, obscene, or inflammatory.

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Question: What is the name of the protective outer layer of trees?
Answer: Bark