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Bark (botany)
Bark (botany)
Bark (botany)
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The bark of Pinus thunbergii is made up of countless shiny layers.

Bark is the outermost layer of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term.[1] It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost layer of the periderm. The outer bark on older stems includes the dead tissue on the surface of the stems, along with parts of the outermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the living periderm is also called the rhytidome.[2][3][4][5][6][7][8][9][10][11][excessive citations]

Products derived from bark include bark shingle siding and wall coverings, spices, and other flavorings, tanbark for tannin, resin, latex, medicines, poisons, various hallucinogenic chemicals, and cork. Bark has been used to make cloth, canoes, and ropes and used as a surface for paintings and map making.[12] A number of plants are also grown for their attractive or interesting bark colorations and surface textures or their bark is used as landscape mulch.[13][14]

The process of removing bark is decortication and a log or trunk from which bark has been removed is said to be decorticated.[15][16][17][18][19]

Botanical description

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Bark is present only on woody plants - herbaceous plants and stems of young plants lack bark.

Tree cross section diagram

From the outside to the inside of a mature woody stem, the layers include the following:[20]

  1. Bark
    1. Periderm
      1. Cork (phellem or suber), includes the rhytidome
      2. Cork cambium (phellogen)
      3. Phelloderm
    2. Cortex
    3. Phloem
  2. Vascular cambium
  3. Wood (xylem)
    1. Sapwood (alburnum)
    2. Heartwood (duramen)
  4. Pith (medulla)

In young stems, which lack what is commonly called bark, the tissues are, from the outside to the inside:

  1. Epidermis, which may be replaced by periderm
  2. Cortex
  3. Primary and secondary phloem
  4. Vascular cambium
  5. Secondary and primary xylem.

Cork cell walls contain suberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, and prevents infections by bacteria and fungal spores.[21] The cambium tissues, i.e., the cork cambium and the vascular cambium, are the only parts of a woody stem where cell division occurs; undifferentiated cells in the vascular cambium divide rapidly to produce secondary xylem to the inside and secondary phloem to the outside. Phloem is a nutrient-conducting tissue composed of sieve tubes or sieve cells mixed with parenchyma and fibers. The cortex is the primary tissue of stems and roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the pericycle.[22][23][24][25][26][27][28][29][3][excessive citations]

As the stem ages and grows, changes occur that transform the surface of the stem into the bark. The epidermis is a layer of cells that cover the plant body, including the stems, leaves, flowers and fruits, that protects the plant from the outside world. In old stems the epidermal layer, cortex, and primary phloem become separated from the inner tissues by thicker formations of cork. Due to the thickening cork layer these cells die because they do not receive water and nutrients. This dead layer is the rough corky bark that forms around tree trunks and other stems.

Cork, sometimes confused with bark in colloquial speech, is the outermost layer of a woody stem, derived from the cork cambium. It serves as protection against damage from parasites, herbivorous animals and diseases, as well as dehydration and fire.

Periderm

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Damaged bark of a cherry tree

Often a secondary covering called the periderm forms on small woody stems and many non-woody plants, which is composed of cork (phellem), the cork cambium (phellogen), and the phelloderm. The periderm forms from the phellogen which serves as a lateral meristem. The periderm replaces the epidermis, and acts as a protective covering like the epidermis. Mature phellem cells have suberin in their walls to protect the stem from desiccation and pathogen attack. Older phellem cells are dead, as is the case with woody stems. The skin on the potato tuber (which is an underground stem) constitutes the cork of the periderm.[30][31]

In woody plants, the epidermis of newly grown stems is replaced by the periderm later in the year. As the stems grow a layer of cells form under the epidermis, called the cork cambium, these cells produce cork cells that turn into cork. A limited number of cell layers may form interior to the cork cambium, called the phelloderm. As the stem grows, the cork cambium produces new layers of cork which are impermeable to gases and water and the cells outside the periderm, namely the epidermis, cortex and older secondary phloem die.[32]

Within the periderm are lenticels, which form during the production of the first periderm layer. Since there are living cells within the cambium layers that need to exchange gases during metabolism, these lenticels, because they have numerous intercellular spaces, allow gaseous exchange with the outside atmosphere. As the bark develops, new lenticels are formed within the cracks of the cork layers.

Rhytidome

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The rhytidome is the most familiar part of bark, being the outer layer that covers the trunks of trees. It is composed mostly of dead cells and is produced by the formation of multiple layers of suberized periderm, cortical and phloem tissue.[33] The rhytidome is especially well developed in older stems and roots of trees. In shrubs, older bark is quickly exfoliated and thick rhytidome accumulates.[34] It is generally thickest and most distinctive at the trunk or bole (the area from the ground to where the main branching starts) of the tree.

Chemical composition

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Bark tissues make up by weight between 10 and 20% of woody vascular plants and consists of various biopolymers, tannins, lignin, suberin and polysaccharides.[35] Up to 40% of the bark tissue is made of lignin, which forms an important part of a plant, providing structural support by crosslinking between different polysaccharides, such as cellulose.[35]

Condensed tannin, which is in fairly high concentration in bark tissue, is thought to inhibit decomposition.[35] It could be due to this factor that the degradation of lignin is far less pronounced in bark tissue than it is in wood. It has been proposed that, in the cork layer (the phellogen), suberin acts as a barrier to microbial degradation and so protects the internal structure of the plant.[35][36]

Analysis of the lignin in the bark wall during decay by the white-rot fungi Lentinula edodes (Shiitake mushroom) using 13C NMR revealed that the lignin polymers contained more Guaiacyl lignin units than Syringyl units compared to the interior of the plant.[35] Guaiacyl units are less susceptible to degradation as, compared to syringyl, they contain fewer aryl-aryl bonds, can form a condensed lignin structure, and have a lower redox potential.[37] This could mean that the concentration and type of lignin units could provide additional resistance to fungal decay for plants protected by bark.[35]

Damage and repair

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Bark can sustain damage from environmental factors, such as frost crack and sun scald, as well as biological factors, such as woodpecker and boring beetle attacks. Male deer and other male members of the Cervidae (deer family) can cause extensive bark damage during the rutting season by rubbing their antlers against the tree to remove their velvet.

Living tree bark enveloping barbed wire

The bark is often damaged by being bound to stakes or wrapped with wires. In the past, this damage was called bark-galling and was treated by applying clay laid on the galled place and binding it up with hay.[38] In modern usage, "galling" most typically refers to a type of abnormal growth on a plant caused by insects or pathogens.

Bark damage can have several detrimental effects on the plant. Bark serves as a physical barrier to disease pressure, especially from fungi, so its removal makes the plant more susceptible to disease. Damage or destruction of the phloem impedes the transport of photosynthetic products throughout the plant; in extreme cases, when a band of phloem all the way around the stem is removed, the plant will usually quickly die. Bark damage in horticultural applications, as in gardening and public landscaping, results in often unwanted aesthetic damage.

The degree to which woody plants are able to repair gross physical damage to their bark is quite variable across species and type of damage. Some are able to produce a callus growth which heals over the wound rapidly, but leaves a clear scar, whilst others such as oaks do not produce an extensive callus repair. Sap is sometimes produced to seal the damaged area against disease and insect intrusion.[citation needed]

A number of living organisms live in or on bark, including insects,[39] fungi and other plants like mosses, algae and other vascular plants. Many of these organisms are pathogens or parasites but some also have symbiotic relationships.

Bark of mature mango (Mangifera indica) showing lichen growth
  • The patterns left in the bark of a Chinese Evergreen Elm after repeated visits by a Yellow-Bellied Sapsucker (woodpecker) in early 2012.
    The patterns left in the bark of a Chinese Evergreen Elm after repeated visits by a Yellow-Bellied Sapsucker (woodpecker) in early 2012.
  • The self-repair of the Chinese Evergreen Elm showing new bark growth, lenticels, and other self-repair of the holes made by a Yellow-Bellied Sapsucker (woodpecker) about two years earlier.
    The self-repair of the Chinese Evergreen Elm showing new bark growth, lenticels, and other self-repair of the holes made by a Yellow-Bellied Sapsucker (woodpecker) about two years earlier.
  • Alder bark (Alnus glutinosa) with characteristic lenticels and abnormal lenticels on callused areas.
    Alder bark (Alnus glutinosa) with characteristic lenticels and abnormal lenticels on callused areas.
  • Sun scald damage on Sitka spruce
    Sun scald damage on Sitka spruce

Uses

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The inner bark (phloem) of some trees is edible. In hunter-gatherer societies and in times of famine, it is harvested and used as a food source. In Scandinavia, bark bread is made from rye to which the toasted and ground innermost layer of bark of scots pine or birch is added. The Sami people of far northern Europe use large sheets of Pinus sylvestris bark that are removed in the spring, prepared and stored for use as a staple food resource. The inner bark is eaten fresh, dried or roasted.[40]

Bark of pine was used as emergency food in Finland during famine, last time during and after civil war in 1918.

Bark can be used as a construction material, and was used widely in pre-industrial societies. Some barks, particularly birch bark, can be removed in long sheets and other mechanically cohesive structures, allowing the bark to be used in the construction of canoes, as the drainage layer in roofs, for shoes, backpacks, and other useful items.[41] Bark was also used as a construction material in settler colonial societies, particularly Australia, both as exterior wall cladding and as a roofing material.[42][43]

Backpack made of birch bark. Museum by Lake Baikal, Russia

In the cork oak (Quercus suber) the bark is thick enough to be harvested as a cork product without killing the tree;[44] in this species the bark may get very thick (e.g. more than 20 cm has been reported[45]).

Some stem barks have significantly different phytochemical content from other parts. Some of these phytochemicals have pesticidal, culinary, or medicinally and culturally important ethnopharmacological properties.[46]

Bark contains strong fibres known as bast, and there is a long tradition in northern Europe of using bark from coppiced young branches of the small-leaved lime (Tilia cordata) to produce cordage and rope, used for example in the rigging of Viking Age longships.[47]

Among the commercial products made from bark are cork, cinnamon, quinine[48] (from the bark of Cinchona)[49] and aspirin (from the bark of willow trees). The bark of some trees, notably oak (Quercus robur) is a source of tannic acid, which is used in tanning. Bark chips generated as a by-product of lumber production are often used in bark mulch. Bark is important to the horticultural industry since in shredded form it is used for plants that do not thrive in ordinary soil, such as epiphytes.[50]

Bark chips

Wood bark contains lignin which when pyrolyzed yields a liquid bio-oil product rich in natural phenol derivatives. These are used as a replacement for fossil-based phenols in phenol-formaldehyde (PF) resins used in oriented strand board (OSB) and plywood.[51]

[edit]
  • Eucalypt bark
    Eucalypt bark
  • Close-up of living bark on a tree in England
    Close-up of living bark on a tree in England
  • Acer capillipes (Red Snakebark Maple)
    Acer capillipes (Red Snakebark Maple)
  • Monterey Pine bark
    Monterey Pine bark
  • A rare Black Poplar tree, showing the bark and burls.
    A rare Black Poplar tree, showing the bark and burls.
  • The typical appearance of Sycamore bark from an old tree.
    The typical appearance of Sycamore bark from an old tree.
  • Quercus robur bark with a large burl and lichen.
    Quercus robur bark with a large burl and lichen.
  • Kauri bark
    Kauri bark
  • Beech bark with callus growth following fire (heat) damage
    Beech bark with callus growth following fire (heat) damage
  • Damaged bark
    Damaged bark
  • Common oak bark
    Common oak bark
  • "Rainbow" Eucalyptus bark on the Hawaiian island of Maui
    "Rainbow" Eucalyptus bark on the Hawaiian island of Maui
  • The old bark has a metallic luster. In 2016, the height of the tree was 15 meters and the bust circumference was 5.7 meters. Samanea saman
    The old bark has a metallic luster. In 2016, the height of the tree was 15 meters and the bust circumference was 5.7 meters. Samanea saman

See also

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Wikimedia Commons has media related to Bark.

References

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Other references

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Bark (botany)

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In botany, bark refers to all the tissues in the stems and roots of woody that lie external to the , forming a protective and functional covering that constitutes 5–28% of the tree's total mass depending on , age, and environmental factors. It develops during , where the produces new outward and inward, while a separate (phellogen) generates the periderm to replace the fragile primary as the thickens. This structure divides into inner bark (living secondary ) and outer bark (dead tissues including rhytidome), with the boundary marked by the current-season's . The inner bark primarily consists of sieve tubes, companion cells, parenchyma, and fibers, enabling the downward transport of photosynthates (sugars and nutrients) from leaves to other parts of the via a process stimulated by growth hormones like auxins in spring. The outer bark, formed from successive layers of periderm, includes phellem (cork cells that are suberized and impermeable), phellogen (the meristematic layer producing new cells), and phelloderm (living inward), often accumulating secondary metabolites such as for defense. In many , the cortex—a layer between the and —persists in younger bark but splits and contributes to the scaly or furrowed appearance as the tree ages. Bark serves multiple critical functions, including mechanical protection against physical injury, pathogens, and herbivores; regulation of water loss and through its impermeable yet lenticel-punctured surface; and insulation from temperature extremes, fire, and frost. The phloem's conductive role ensures nutrient distribution, while the outer layers act as a dynamic barrier that renews annually, shedding old rhytidome in species like birches or remaining intact in others like oaks. Bark morphology varies widely—smooth and thin in young trees, thick and ridged in mature ones—reflecting adaptations to specific habitats, and it often stores reserves or indicates environmental stress through cracking or discoloration.

Overview and Formation

Definition

In botany, bark is defined as the collective term for all tissues located exterior to the in the stems and roots of woody plants, encompassing the secondary and periderm. The serves as the dividing layer between the inner wood (secondary ) and the outer bark. This structure arises during , distinguishing bark from primary tissues like the . Unlike woody plants, herbaceous plants lack true bark because they do not undergo and instead retain a simple for protection and . In these non-woody , such as many annuals and perennials, the outer covering remains thin and living throughout the plant's lifecycle, without the development of lignified or corky layers characteristic of bark. The term "bark" derives from bōrc, meaning the rind or covering of a , rooted in a Proto-Indo-European base related to and hiding. Its early botanical usage appears in 17th-century texts, such as John Evelyn's Sylva, or A Discourse of Forest-Trees (), where it refers to the external layer of trees, including processes like (stripping the bark). This work marked a foundational application of the term in systematic .

Development Process

Bark development in woody plants occurs primarily through , a process driven by lateral meristems that increase stem girth after primary growth establishes length. The , a cylindrical layer of meristematic cells located between the primary and , plays a central role by producing secondary phloem cells outward, forming the inner bark, while simultaneously generating secondary (wood) inward. This secondary phloem consists of sieve tubes, companion cells, fibers, and , facilitating transport before much of it becomes non-functional and is sloughed off over time. The outer bark arises from the cork cambium, or phellogen, which differentiates from the pericycle or cortex in young stems and produces the periderm—a protective barrier comprising phellem (cork) outward and phelloderm inward. In the initial stages of development, typically in young stems of dicotyledonous plants, secondary growth begins shortly after the vascular cambium forms a continuous ring, leading to the replacement of the epidermis with periderm as the stem expands. Annual increments contribute to multilayered bark formation, where seasonal activity of the vascular and cork cambia adds new layers of phloem and periderm each year; older periderm layers often split and are shed, resulting in the textured, ridged appearance of mature bark. This incremental process ensures continuous protection and adaptation as the plant ages. Notable differences exist between dicots and monocots in bark development. Dicotyledons, such as oaks and maples, undergo extensive secondary growth via the , producing true bark with distinct inner and outer components. In contrast, monocotyledons like palms lack a and thus do not form true bark; instead, they develop fibrous sheaths from that provide structural support but no secondary thickening. Environmental factors significantly influence bark development, particularly thickness, which varies with to enhance . In arid regions with frequent s, such as savannas or Mediterranean ecosystems, trees evolve thicker bark to insulate the from lethal heat, with studies showing bark thickness correlating positively with and rainfall variability. For instance, in fire-prone environments allocate more resources to periderm production, resulting in bark that can be several centimeters thick, compared to thinner bark in mesic forests. and gradients further drive these adaptations, with warmer, drier conditions promoting greater bark investment for protection and water regulation.

Anatomy

Inner Bark

The inner bark, also known as the secondary , is the living portion of the bark that develops from the and serves as the primary conduit for the downward transport of sugars and other organic compounds in woody plants. It consists mainly of tubes, companion cells, phloem fibers, and cells, which collectively enable efficient nutrient translocation while providing structural support. These components remain metabolically active until they are gradually compressed and displaced by the expanding outer bark layers. Microscopically, the inner bark features sieve plates—specialized perforations in the end walls of sieve tube elements—that facilitate the pressure-flow mechanism of transport, allowing to move rapidly through the . In many temperate , the secondary phloem exhibits annual rings analogous to those in the , formed by seasonal variations in cell production from the , with wider bands of in spring and narrower, fiber-rich zones in summer. These rings reflect the dynamic growth patterns and can be observed in cross-sections under light microscopy. Over time, the functional transition in inner bark occurs as older tissues collapse under pressure from new growth, leading to the obliteration of sieve tubes and the lignification of fibers and sclereids, which renders them non-conductive and integrates them into the supportive framework. This process ensures continuous renewal of transport tissues adjacent to the , with non-functional persisting as part of the inner bark until fully incorporated into the outer layers. In trees such as Quercus () species, the cambium-adjacent remains highly active, demonstrated by robust flow rates during the , which can exceed 1 liter per hour in mature stems under optimal conditions.

Outer Bark

The outer bark, also known as the rhytidome in mature trees, forms the outermost protective layer of the stem and , consisting primarily of dead tissues derived from successive periderms and sloughed secondary . It develops as the initial is replaced by the periderm during , providing a durable barrier against environmental stresses. The periderm is a lateral meristematic tissue system comprising three main layers: the phellogen (), which divides to produce phellem (cork) cells outward and phelloderm cells inward. The phellogen originates from the cortex or and remains meristematic, generating rectangular phellem cells that mature into dead, suberized tissue forming the bulk of the outer bark. The phelloderm, a living layer of parenchyma-like cells, faces inward toward the secondary and supports metabolic functions, though it is often thin or absent in some . As the stem expands, new phellogen layers form inward, causing older periderms to accumulate and eventually slough off, contributing to the rhytidome's scaly or furrowed appearance in mature trees. The rhytidome represents the buildup of multiple old periderms interspersed with dead and bark scales, which detach irregularly through sloughing to accommodate growth and shed damaged tissue. This process is particularly evident in like oaks and pines, where the outer layers crack and peel, exposing fresher periderm beneath. Physically, the outer bark exhibits high impermeability to and gases due to the suberized walls of phellem cells, which form a hydrophobic barrier preventing and entry. Thickness varies widely by and age, ranging from a few millimeters in young stems to up to 10 cm in mature baobab trees (), enhancing mechanical protection against fire and physical injury. To facilitate for underlying living tissues, the periderm develops lenticels—porous regions of loosely packed, complementary cells that penetrate the phellem layer. These structures form above stomata during early periderm development and persist as raised or sunken spots, allowing oxygen while maintaining overall impermeability.

Bark Variations

Bark structure exhibits significant variation between gymnosperms and angiosperms, reflecting differences in their vascular tissues and protective strategies. In gymnosperms, particularly such as pines (Pinus spp.), the bark is typically thicker and more fibrous, featuring prominent resin canals that traverse the outer bark and provide chemical defense against pathogens and herbivores. These resin canals, lined with secretory cells, are a defining feature of conifer bark , contributing to its durability and resinous quality. In contrast, angiosperm bark, especially in hardwoods like oaks (Quercus spp.) and (Acer spp.), tends to be thinner and smoother, with less pronounced fibrous layers and fewer specialized secretory structures, allowing for greater flexibility in stem expansion. Environmental conditions further drive bark diversity, with adaptations tailored to local climates and ecological pressures. Tropical trees, such as certain eucalypts (Eucalyptus spp.), often develop smooth, exfoliating bark that periodically sheds in large sheets, facilitating the removal of epiphytes, lichens, and fungal growth that could otherwise accumulate in humid environments. This shedding process exposes a fresh, clean surface beneath, reducing biotic burdens in consistently wet conditions. In temperate regions, bark is commonly furrowed and ridged, as seen in species like redwoods (Sequoia sempervirens), where the thick, fibrous outer layer provides thermal insulation against seasonal temperature fluctuations and frost damage. These furrowed patterns enhance the bark's insulating properties by trapping air within the ridges, moderating heat exchange with the environment. Certain plants deviate from typical bark formation, exhibiting anomalies in secondary growth that alter or eliminate traditional periderm development. Cacti (Cactaceae family), for instance, lack conventional bark; instead, they retain a persistent, succulent epidermis throughout their lifespan, which thickens with a waxy cuticle to serve protective functions without forming a corky periderm. This epidermal layer, reinforced by underlying chlorenchyma, maintains photosynthetic activity and water storage in arid habitats, bypassing the need for bark replacement. In climbing vines, such as those in the genus Bauhinia, anomalous secondary growth produces irregular vascular patterns, including successive cambial rings or included phloem, resulting in lobed or contorted stems with uneven, patchy bark formation. These variants allow vines to achieve rapid, flexible thickening without the uniform radial expansion seen in self-supporting trees. The evolution of bark traces back to early vascular plants, transitioning from rudimentary epidermal coverings to sophisticated periderm systems. In primitive vascular plants of the period (approximately 400 million years ago), protection relied on a simple that shielded against and mechanical injury, but lacked secondary thickening. Over time, the emergence of vascular cambia enabled , leading to the development of periderm in woody species, which replaced the epidermis with layered cork tissues for enhanced durability and renewability. This evolutionary shift, coinciding with the rise of forests, allowed modern woody to achieve greater stature and longevity through repeated periderm production.

Functions

Protection

Bark serves as the primary protective layer for woody , forming a robust barrier that shields the underlying vascular tissues from a range of external threats. This outer covering, composed of and structural materials like and , prevents penetration by physical forces, biological invaders, and environmental stressors, thereby ensuring the survival and longevity of the . The effectiveness of this protection varies with bark thickness, composition, and structure, adapting to specific ecological niches. In terms of physical protection, bark acts as a mechanical against from , falling , and animal activity. Its lignified tissues provide tensile strength and elasticity, absorbing impacts that could otherwise damage the and . For instance, the accumulation of thick outer bark in many species serves to cushion against such mechanical damage, reducing the risk of or tissue rupture. Additionally, tough, fibrous bark deters herbivory by making it difficult for browsers like deer or to access inner tissues; examples include oaks with rough, tannin-rich bark that discourages chewing. Bark also confers resistance in fire-prone habitats, where thick layers insulate the from lethal . In giant sequoias (), the bark can reach 60 cm in thickness and features a loose, air-filled structure with low density, enabling it to withstand external temperatures while keeping the inner below the 60°C threshold that causes . This insulation is enhanced by the fibrous composition, which dissipates thermal energy effectively. Biologically, bark functions as a multifaceted defense against pathogens and , combining physical barriers with chemical deterrents. The periderm layer, rich in , forms an impervious seal that blocks fungal hyphae and bacterial entry, while constitutive compounds like phenolics, terpenoids, and acids in polyphenolic cells inhibit microbial growth and activity in invaders. For example, in , bark contains juvabione-like sesquiterpenes that disrupt reproduction and feeding. In response to breaches, bark facilitates wound sealing through the rapid formation of traumatic ducts and suberized tissues, compartmentalizing damage via lignified cell walls and necrotic zones to prevent spread. This process isolates infested areas, as observed in species where ray cells swell and produce defensive phenolics, limiting nutrient availability to pests like the balsam woolly adelgid. Environmentally, bark provides insulation against temperature extremes, buffering the cambium from and . Its low thermal conductivity, due to air pockets and fibrous matrices, minimizes conductive loss or gain; in temperate , this reduces damage by maintaining inner tissues above freezing during cold snaps. For instance, bark in trees like maples acts as a thermal barrier, preventing rapid freezing that could rupture cells. In drought conditions, bark regulates water loss through low vapor conductance (g_bark), conserving stem hydration when is limited— with thicker, less permeable bark exhibit greater tolerance by limiting from stems. This hydraulic control helps maintain turgor and prevents in vessels. Bark also facilitates through lenticels, small pores that allow oxygen and while minimizing water loss, supporting respiration and in underlying tissues. Ecological adaptations further illustrate bark's protective role, such as periodic shedding in certain species to maintain barrier integrity. In birches (Betula spp.), the peeling of thin, papery bark in horizontal strips removes accumulated lichens and mosses that could block light or harbor pathogens, preventing overgrowth and ensuring efficient through the chlorenchymatous inner bark. This shedding, driven by internal pressures and seasonal changes, renews the surface and enhances overall resilience in moist, shaded environments.

Transport and Support

The inner bark houses the secondary , a critical that enables the downward transport of photosynthates, such as , and signaling molecules from photosynthetic source organs like leaves to sink tissues including and growing shoots. This transport occurs via pressure-driven bulk flow through longitudinally aligned sieve elements, enucleate cells specialized for conduction with parietal cytoplasm and sieve plates facilitating connectivity. In trees, phloem sap concentrations of range from 65 mM to 1 M, supporting efficient mass flow while also carrying signaling entities like the protein FLOWERING LOCUS T and various mRNAs for inter-organ communication and developmental regulation. Beyond conduction, the cells in the inner bark serve as key sites for storage of and other nutrient reserves, which accumulate during periods of surplus and are mobilized seasonally to fuel growth and maintenance. In many woody , such as poplars, bark storage proteins—comprising up to 62% of soluble proteins under short-day conditions—build up in vacuoles of inner bark in autumn and winter, providing a reservoir that declines in spring to support new shoot elongation. Similarly, in sugar maples, reserves in trunk tissues, including bark, peak in late summer and are hydrolyzed during winter dormancy, contributing to the spring flow rich in that sustains early-season . compounds, stored as phospholipids and glucosamine-6-phosphate in bark, undergo analogous seasonal cycling, with mobilization in spring via transport to developing buds on nutrient-poor soils. Mechanically, bark contributes to stem support through specialized sclerenchyma elements, including fibers and sclereids, which enhance tensile strength and overall rigidity against environmental stresses like wind or tilting. Phloem fibers, often arranged in tangential bands 5 cells wide and composed largely of high-cellulose gelatinous types, provide flexible reinforcement to protect conducting tissues, while clustered sclereids offer compressive resistance and prevent structural collapse. In Amazonian trees like Pachira, trellis-organized phloem fibers generate consistent tensile stress, aiding postural reorientation during ontogeny and maintaining stem stability as bark thickness scales with diameter (allometric exponent ~0.66–0.78). However, transport efficiency in the diminishes over time due to compression and obliteration of older sieve elements as expands the inward, leading to gradual loss of functional conducting area in the inner bark. This age-related sclerosis, exacerbated by mechanical pressure from accumulating wood layers, shifts reliance to newer phloem layers while older ones become non-conductive, potentially limiting resource allocation in mature stems.

Chemical Composition

Structural Components

Bark's structural integrity relies on a matrix of carbohydrates, including , , and , which form the primary components and provide mechanical support. In bark, typically constitutes 18-38% of the dry mass, 15-33%, and 30-50%, with being notably higher than in to enhance rigidity. For example, in pine bark, these values are approximately 27% , 23% , and 50% on an ash- and extractives-free basis. contributes to flexibility by binding cellulose microfibrils, while lignin's cross-linking reinforces the structure against compression and tension. In cork layers of the periderm, —a complex —replaces much of the content, comprising up to 58% of the dry mass and forming a barrier that imparts and gas impermeability through its aliphatic and aromatic domains. This deposition occurs in , reducing and enhancing long-term durability without relying on living metabolic processes. Water content differs markedly across bark layers, reflecting their physiological roles; living inner bark often exceeds 50% (on a wet basis), facilitating transport, while dry outer bark falls below 20% due to its protective, non-living nature. Overall, inner bark is 7-10 times higher than outer bark, with fresh bark averages ranging from 58-150% on a dry basis depending on , such as 120% in Scotch . Minerals, primarily in the form of (1.5-10% of dry ), include silica and calcium deposits that bolster mechanical by reinforcing cell walls. Calcium predominates (often over 70% of ), forming crystals, while silica contributes to , as seen in grass epidermises where opal phytoliths enhance resistance to abrasion—structures sometimes analogous to bark in non-woody . Bark's physical metrics underscore its protective role, with varying from 0.2-1.0 g/cm³ across and conditions; air-dried bark typically measures 0.37-0.59 g/cm³, though cork typically ranges from 0.12-0.24 g/cm³. Tensile strength is generally lower in bark than in comparable , with bark-fiber composites showing reduced modulus compared to wood-flour equivalents, though bark can match wood in specific gravitropic stress .

Bioactive Compounds

Bark contains a diverse array of bioactive compounds, primarily secondary metabolites that contribute to defense, stress response, and ecological interactions. These include phenolics, terpenoids, and occasionally alkaloids, which are synthesized in response to environmental pressures and stored predominantly in the inner bark for rapid deployment. Phenolics represent one of the most abundant classes of bioactive compounds in bark, encompassing and that provide chemical protection against herbivores, pathogens, and abiotic stressors. , polyphenolic compounds classified as hydrolyzable or condensed, can constitute 8-20% of dry (Quercus spp.) bark weight, imparting astringency by binding to proteins and reducing tissue palatability to deter feeding. This defensive role is evident in their ability to inhibit microbial growth and enzymatic digestion in potential attackers. , another phenolic subgroup, accumulate in bark to absorb (UV) radiation, shielding underlying tissues from photooxidative damage; their biosynthesis is upregulated following UV exposure, enhancing epidermal protection across various species. Terpenoids, including resins and oleoresins, are prominent in conifer barks and serve as key agents. In pines (Pinus spp.), —a of monoterpenes, sesquiterpenes, and diterpene acids—exhibits broad-spectrum activity, with monoterpenes primarily inhibiting fungal pathogens and diterpenes targeting , thereby preventing microbial invasion at wound sites. These compounds flow through bark canals to seal injuries, reinforcing physical barriers with chemical deterrence. Alkaloids, though less common in bark compared to other plant tissues, occur in select species and contribute to pharmacological defense. In cinchona (Cinchona spp.) bark, quinine and related quinoline alkaloids comprise 1.6-1.9% of dry weight, with total alkaloids reaching 4.8-5.2%; these nitrogenous compounds disrupt metabolism, offering protection against insects and microbes. The concentration of bioactive compounds in bark exhibits significant variability, influenced by environmental stresses such as herbivory, which induces elevated levels as a targeted response. For instance, signaling post-herbivory can increase accumulation in bark within days, enhancing resistance without excessive energy cost. Extraction methods like for phenolics or for alkaloids are used in analysis to quantify these fluctuations, revealing species-specific and seasonal patterns.

Damage and Response

Types of Damage

Bark damage in trees can arise from abiotic factors, which are non-living environmental stresses, or biotic agents, such as living organisms that invade and degrade bark tissue. These injuries compromise the bark's primary role in protecting underlying vascular tissues, potentially leading to decline or if extensive. Abiotic damage includes several distinct forms. Frost cracking occurs when rapid freezing and thawing cause the outer bark to split longitudinally, often on the southwest side of trunks due to fluctuations and sun exposure, particularly in young or thin-barked trees. Sunscald results from intense solar heating the bark on exposed surfaces, leading to cellular and cracking, especially in with light-colored bark like maples. strikes can instantly girdle trunks by exploding bark and scorching wood, creating entry points for secondary infections. Mechanical wounds from abiotic sources, such as high winds snapping branches or fracturing bark, further expose inner tissues to or pathogens. Biotic damage is inflicted by pathogens and pests that target bark directly. Insect borers, like the emerald ash borer (Agrilus planipennis), tunnel through the phloem layer, creating galleries that disrupt nutrient flow and cause bark sloughing in ash trees across North America. Fungal pathogens induce canker diseases, where sunken, discolored lesions form on bark as hyphae invade and kill cambial tissue, exemplified by Nectria species affecting hardwoods. Bacterial infections, such as those from Erwinia species, produce wetwood or slime flux, leading to bark blistering and cracking through internal pressure buildup. Girdling represents a severe form of damage where bark and are removed in a complete ring around the trunk or , severing the and preventing downward transport of photosynthates, which starves the roots and leads to canopy dieback. In urban settings, this often results from repeated mechanical injury by lawn mowers or string trimmers scraping the base of young trees, a common issue in landscaped areas. Such damages have significant global prevalence, as seen with (Ophiostoma novo-ulmi), a fungal vectored by bark beetles that has devastated populations worldwide since its emergence in in the 1910s and spread to by the 1930s, causing bark necrosis and vascular blockage.

Repair Mechanisms

When bark is damaged, trees initiate a response primarily through the formation of tissue derived from undifferentiated cells in the and adjacent ray initials. This proliferates from the edges, initially consisting of cells that divide via and to cover the exposed surface. Concurrently, suberization occurs, where ligno-suberized layers develop rapidly around the to seal exposed tissues, preventing water loss and invasion by depositing and in cell walls. A key aspect of bark repair in trees is the compartmentalization of decay, described by the CODIT model developed by plant pathologist Alex Shigo. This model outlines four barriers that isolate damaged areas: Wall 1 forms immediately via plugged vessels to limit vertical spread; Wall 2 uses existing growth ring boundaries for longitudinal containment; Wall 3 relies on for radial isolation; and Wall 4, the strongest, arises from new cambial activity post-wounding to chemically and physically separate healthy from affected wood. These barriers confine decay to the wood present at the time of injury, protecting subsequent growth, though their effectiveness varies by species and wound age. Regrowth involves the differentiation of into a new periderm and , restoring bark functionality around the . The surface develops a periderm layer, while underlying cells form a regenerated that produces new and bark tissues, often completing closure in fast-growing species within one month for strip wounds. Success rates are high for small wounds; for example, lesions under 500 cm² achieve approximately 90% closure, enabling full compartmentalization and minimal decay risk. Repair is limited if the is extensively destroyed, such as in injuries exceeding 25% of the trunk circumference, as this prevents callus initiation and new tissue formation, leading to branch dieback and potential tree decline. In such cases, compartmentalization may still occur in remaining healthy sectors, but overall regeneration fails without intact cambial tissue.

Human Uses

Traditional Applications

Throughout history, bark has played a vital role in across various cultures. Willow bark (Salix spp.), containing —a compound that serves as a precursor to aspirin—has been used for pain relief and treating inflammatory conditions since ancient times. Records from the , dating to approximately 1550 BCE in , document its application for ailments like fever and joint pain. Similarly, ancient Greek physicians such as prescribed willow bark preparations around 400 BCE for purposes, establishing its enduring therapeutic reputation. In crafts and construction, indigenous peoples of North America extensively utilized birch bark (Betula papyrifera) for its waterproof and flexible properties. Northeastern Native American communities, including the Ojibwe and Algonquian groups, crafted lightweight canoes from stitched sheets of birch bark, which were essential for travel, hunting, and trade across waterways; these vessels could span up to 20 feet and were renowned for their durability. The bark's papery outer layer also served as a writing surface for recording symbols and stories, while inner layers were woven into baskets and roofing for wigwams. In the Mediterranean, cork oak bark (Quercus suber) has been harvested since Roman times for practical applications, including as stoppers for wine amphorae sealed with pitch to prevent leakage during storage and transport. Ancient Greeks similarly employed it for sealing vessels containing olive oil and wine, highlighting its role in early preservation techniques. Bark contributed to food and ritual practices in diverse societies. Cinnamon bark from Cinnamomum verum, originating in , was traded as a prized spice along ancient routes as early as 2000 BCE, reaching where it was used in and culinary flavoring for elite dishes. Its aromatic qualities made it a symbol of luxury in ancient Near Eastern and Mediterranean cultures. In the Pacific Islands, bark cloth known as tapa—produced from the inner bark of (Broussonetia papyrifera)—held ceremonial importance; Polynesian and Melanesian communities created elaborately decorated sheets for weddings, funerals, and gift exchanges, often featuring motifs symbolizing ancestry and community bonds. Bark also carried deep cultural symbolism in and , often representing protection or exposure. The act of "stripping bark" appears as a for and violation in ancient Roman , as in 58, where the verb glubit (to strip bark or skin) evokes the raw exposure of inner layers, paralleling human fragility and power imbalances. This imagery underscores bark's dual role as a shield and a device for themes of and resilience in traditional storytelling.

Modern and Industrial Uses

In the , extracted from the bark of species remains a key antimalarial agent, with global production of quinine alkaloids estimated at 300-500 metric tons annually from 5,000-10,000 metric tons of harvested bark, primarily sourced from the Democratic Republic of Congo. bark extracts, rich in —a precursor to aspirin—are incorporated into modern herbal supplements and over-the-counter remedies for treating conditions like , , and , leveraging their and anti-inflammatory properties as natural alternatives to synthetic derivatives. Bark-derived materials play a significant role in industrial applications, particularly through extracts used in adhesives and dyes. Tannins from tree barks, such as those of and , serve as eco-friendly binders in wood adhesives for plywood and particleboard production, reducing reliance on petroleum-based alternatives. Similarly, polyphenolic extracts from these barks are employed as natural dyes in the , providing colorfast shades on fabrics like without metallic mordants, promoting sustainable processes. Cork, harvested from the bark of (cork oak), is widely used in wine stoppers, , and insulation; the global cork materials market was valued at approximately USD 5.3 billion in 2024, driven by demand for renewable, lightweight composites. Sustainable harvesting practices ensure bark resources are renewable, exemplified by cork oak management where the outer bark is stripped every 9-12 years without felling the tree, allowing full regeneration and supporting in Mediterranean ecosystems. Bark waste from and processing operations is increasingly converted into , such as through the production of high-calorie briquettes or pellets, which provide a source and help mitigate accumulation in forestry industries. Emerging research highlights bark's potential in advanced applications, including the development of nanoparticles for . Exosome-like nanoparticles derived from mulberry bark have shown efficacy in preventing gut by activating specific cellular pathways, offering a biocompatible carrier for therapeutic agents. Bioengineered silver nanoparticles from bark extract demonstrate promise as peptide inhibitors targeting cancer cells, enhancing drug specificity and reducing toxicity. Recent studies (as of 2025) have evaluated aqueous extracts from the bark of six European tree species for their antioxidant, antimicrobial, and wound-healing properties. Additionally, studies on bark's role in models emphasize its contribution to tree biomass, accounting for about 20% of above-ground carbon stocks, which informs strategies for mitigation.

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