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Photograph: Birch bark by Doug Mercer (Canada)

Bark Ecology

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available. It therefore presents state-of-the-art information on the topic it covers.

Plants provide humans with a diverse, unique, and irreplaceable set of goods and services. From food and fodder, to raw materials for shelter, medicine and rituals, plants have been and continue to be critical keystones in cultural development.

Among the many parts of plants that we use, bark products have always played exceptionally large roles. Traditionally, bark products have been particularly prominent as sources of medicines and raw materials, and more recently, for other industries (Figure 1, Figure 2, Figure 3, Figure 4, Figure 5).

One of the bark products that comes to mind when considering plant substances that have shaped the geo-political relationships between nations is rubber, a latex derived from Hevea brasiliensis, a species native to the Amazon Basin but now cultivated all over the tropics. Bark from trees in the Andean genus Cinchona has also shaped our destinies. The power of bark extracts from these trees as a treatment for malaria is well known, as is the monopoly held on “fever tree” bark for decades by the Jesuits (Honigsbaum 2001).

Bark management is important not only for securing the supply of diverse raw materials bark provides (e.g., fibers, dyes, and medicines), but also for protecting the trees supplying these resources. Horticulturalists and gardeners have long known about the effects of bark wounding. For example, bark-girdling is a standard practice used throughout the world to stimulate short-term fruit production. Stem-girdling improves fruit yield and quality because, by severing the conducting tissues in the phloem, the products of photosynthesis are trapped above the girdle, at least until the wound heals. In response to increased availability of carbohydrates in the crown, trees with damaged trunks often produce more and sweeter fruits (Layne and Flore 1991).

Traditional and modern forest managers girdle unwanted trees during stand-tending operations, a practice that is less costly and less damaging than felling. The practice also has the added advantage of gradually rendering resources (water, nutrients, and light) more available to neighboring trees, thus avoiding the shock of sudden access to resources that might compromise their development.

Rather than cataloguing the many uses of bark, the purpose of this review is to provide a better understanding of the nature of this interesting and complicated suite of tissues. I start by addressing the seemingly simple issue of what is bark. I then describe some of the prominent developmental, structural, and physiological properties of bark. Given the ecological and evolutionary importance of bark, as well as the multitude of ways humans use it, clarity about its structure and function seems critical.

I. What is bark?

Bark is the term loosely applied to the outermost covering of tree stems (Figure 6). A major source of complexity is that bark, more specifically defined as the complex of tissues located outside the vascular cambium, generally includes both live and dead cells produced by two cambia. Thus, treating bark as a single entity can obscure important aspects of its biochemistry, physiology, ecology, and evolutionary biology.

Among its many ecological roles, bark covers and protects the vascular cambium of most trees. Inner bark is produced by the vascular cambium, but the protective function of bark as a whole is augmented by cell divisions in the cork cambium (hereafter the phellogen). The vascular cambium, besides providing for girth increments in woody plants, also contributes to increased plant longevity by replacing conductive tissues lost by death or cavitation. Vascular and supportive tissues have apparently evolved repeatedly and represent major advances in the evolution of terrestrial plants (Cichan 1990)

a. Inner bark

Inner bark is produced by and adjacent to the vascular cambium. It is composed of living secondary phloem, the tissue responsible for most translocation of photosynthates and other metabolites around trees. Progressing towards the outside of the stem, inner bark is typically terminated by the innermost and most recent periderm, which is produced by the phellogen. The phellogen develops from parenchyma in the older phloem tissues or, in young stems, just beneath the epidermis. In some species, the phellogen produces phelloderm towards the inside of the stem and phellem towards the outside, but in many species only phellem is formed. Phelloderm is composed of living parenchyma cells whereas the phellem is made up of dead cells with chiefly suberized walls.

The proportions of other cell types in inner bark (e.g., parenchyma, fibers, and sclereids) vary among species. Thick-walled cells (e.g., fibers and sclereids) often result from cell wall thickening and lignification of parenchyma; these cells constitute the mechanical support tissue of the inner bark. Inner bark may also contain oil glands, laticifers, and resin canals, but such structures may also occur in the cortex, pith, leaves, and xylem. Types of sieve elements, their sizes, and arrangements in the phloem vary among species. There is also a great deal of species-specific variation in the size and arrangement of bark parenchyma cells. Likewise, the arrangement of fibers and sclereids is diverse, often forming patterns of taxonomic importance.

b. Outer bark

Outer bark (= rhytidome) includes all of the tissues from the innermost (i.e., most recent) periderm to the outside of the stem. This complex of tissues includes the products of the phellogen or phellogens, as well as any epidermis, cortex, and primary and secondary phloem that are exterior to the youngest phellogen and its products.

Subsequent phellogens (and therefore the most recent periderm) typically originate in the outermost living layer of secondary phloem, often leading to the isolation of all tissues more peripheral on the stem. Older outer bark can be sloughed or retained as trees grow. Outer bark structure depends on the relative abundances of its constituent tissues but generally contains collapsed and otherwise non-conducting secondary phloem that has become isolated by cell divisions in the phellogen. Although some species have only a single periderm (e.g., Bursera, Fagus, Pachicormus), most species develop several over the course of time. These sequential periderms vary in the degree to which they are continuous around the stem, concentric, and parallel to each other. Discontinuous periderms result from variation in meristematic activity in the phellogen around the bole in apparent response to the mechanical stresses imposed by radial growth. New periderms often arise in contact with older ones, forming a ramifying system that influences features of bark external texture such as the size of exfoliating bark flakes.

The surface of the outer bark of many species is perforated by corky lenticels that allow increased rates of gas exchange by living tissues inside the stem. Lenticels commonly arise beneath stomata in the epidermis just prior to the formation of the first periderm. As lenticels develop, the epidermis is ruptured and thereafter the inner lenticel boundaries are generally continuous with the phellogen (Dickison 2000).

Phellem is typically the thickest of the three dominant tissues of the outer bark even when it is reduced by sloughing. It mostly consists of densely packed parenchymatous cells with walls that become suberized or lignified (or both) before the cells die and become air-filled. Phellem is sometimes referred to as “cork,” but since its cells are not always suberized, this term is best reserved for the phellem of the cork oak, Quercus suber (Trockenbrodt 1990). Stem aeration and other physical properties depend in part on the patterns of layering of different cell types in the outer bark, especially the compactness of the phellem.

Outer bark appearance is influenced by the locations of the sequent phellogens and by how their derivatives develop. These sorts of anatomical characteristics determine bark texture (Figure 7, Figure 8, Figure 9, Figure 10, Figure 11). Smooth textured barks, for instance, develop from sequent periderms formed just beneath the trunk surface, which causes bark scales to be thin. Alternatively, smooth bark can derive from the formation of a single periderm and continuous shedding of phellem. Bark develops prominent scales, in contrast, when a series of initially parallel and ring-like periderms are formed that later vary in their growth trajectories due to the presence of fiber bundles. Bark fissuring can be induced by dilating phloem rays that form zones of fracture in the periderm. When fissures are present, thick bark scales can be rapidly sloughed or retained for many years.

Structural characteristics of bark are influenced by the proportional representation and distribution of different tissues. For example, granular inner bark usually indicates the presence of sclereids. Bark can also be brittle and crumbly due to the inclusion of calcium oxalate crystals or stone cells. A more fibrous texture is found in barks with abundant fibers and with periderms that separate easily, but the shape and size of conducting elements also influence whether bark is fibrous.

More detailed discussions of bark structure and growth can be found in Esau (1967) and Roth (1981). A comprehensive attempt to capture the diversity of bark uses can be found in Saunders and colleagues (1994). Bark terminology is explained by Trockenbrodt (1990)and Junikka (1994).

II. Bark chemistry

Bark differs from all other plant parts in its development, anatomy, and chemistry. Chemical compounds found in low concentrations in other plant parts are in some cases highly concentrated in bark (Young 1971), but some barks contain novel compounds as well. Unfortunately, most investigators of bark chemistry do not specify the bark tissues from which the compounds were derived. As a result, accurate discrimination of the sites of production and storage of chemical compounds can usually not be made even to the level of inner or outer bark.

Discussions of bark exudates suffer from the same terminological confusion that plagues bark in general. Adding to the problem of inconsistent and overlapping uses of terms such as “sap” and “gum” is the fact that, within a tree, the same exudates can be produced in bark, wood, and pith. To foster communication I offer the following definitions, based mainly on the work of Langenheim (2003).

Mineral compounds are up to ten times more abundant in bark than in wood, with bark ash contents up to 60%, predominantly calcium, silica, and phosphorus (Jensen 1963). Bark extractives can represent 20-40% of bark mass with most of the remaining components being suberin, lignin, lignans, and phenolic acids, along with some non-structural carbohydrates. Phenolic acids in bark are mostly found in outer bark cells. Cellulose is the principal carbohydrate in both bark and wood but the latter also often contains substantial amounts of glucose and sucrose. Other sugars present in low concentrations in bark include galactose, mannose, and starch, the latter only in inner bark.

In addition to phenolics and carbohydrates, low molecular weight extractives are often very abundant in bark (low molecular weight extractives include fatty acids, alcohols, resin acids, alkaloids, pigments, phlobaphenes, glycosides, tannins, carbohydrates, fats, waxes, terpenes, amino acids, vitamins and steroids). Sometimes, there are also small amounts of essential oils (volatile low molecular-weight terpenes) that give characteristic aromas to the barks of some trees (e.g., incense cedar, ponderosa pine, and eucalypts, among many others). Finally, there are pigments in the bark such as flavonoids, some of which have economic importance (e.g., quercetin, taxifolin), as well as anthocyanins and leucoanthocyanins.

Proteins, and less frequently single amino acids, are stored in the inner bark of some species in concentrations that typically fluctuate seasonally. These substances are typically translocated from leaves to bark immediately before leaf senescence (e.g., Pinus; Pomeroy et al., 1970). Bark proteins reportedly increase frost resistance of some temperate tree species (Siminovitch and Briggs 1949).

A number of other compounds found elsewhere in the plant are concentrated in the bark of some species. For instance, alkaloids form salts with a range of acids in bark (e.g., citric, tannic, among others). Vitamins (e.g., C, B1, among others) are also found in the barks of some species in concentrations that fluctuate seasonally.

The chemical composition of bark tissues varies as a function of ontogeny, history of disturbance (e.g., herbivory, fire, disease), environmental conditions, and even the height on the tree where the sample was taken. For example, the concentration of magnolol in Magnolia, a chemical used in the pharmaceutical industry, varies with bark thickness (Zhao 1999). In some conifers, resin pressure, crystallization rate, viscosity, and chemical composition vary with stress levels and other factors, and there can be chemical differences in the resin produced before and after wounding even within a single tree (Berryman 1972; Klepzig et al. 1996).

Bark chemistry also determines its decomposition rate, digestibility, and flammability, as discussed below. The high concentration of poisonous, digestibility reducing, and otherwise defensive compounds in bark is expected, given its location on the outside of tree stems and its covering of the vascular cambium.

III. Bark Functions

Bark tissues have a wide range of functions that are critical for tree survival and growth. The primary functions of the inner bark include transport and storage of photosynthates, but in some cases inner bark is active in carbon fixation. Major functions of outer bark include reduction in water loss from stems and roots, prevention of pathogen entry, avoidance of mechanic injury to underlying tissues, and general insulation of the stem against environmentally adverse conditions (e.g., extreme cold and heat).

a. Transport

Photosynthates as well as some proteins and RNA are principally transported around trees from sources to sinks through sieve elements in the phloem. Conducting tissues can comprise more than 60%of the inner bark (Jensen 1963). These conducting tissues are called sieve tubes in angiosperms and sieve cells in other vascular species. Sieve tubes and sieve cells both lose their nuclei when they become functional as conduits for photosynthates. These cells are always adjacent to living cells that are called companion cells in angiosperms and albuminous cells in other species. These associated cells aid in the loading and unloading of sieve elements with photosynthates.

b. Growth

The girth increments in tree stems that make height growth biomechanically and hydraulically possible are mostly due to expansion of cells derived from the vascular cambium, supplemented by cell production in the phellogen(s). To an even lesser extent, stem growth is augmented by the meristematic tissues responsible for ray dilation as well as by the expansion of parenchymatous cells remote from meristems.

c. Biomechanical support

Bark is subjected to huge mechanical forces, but because the proportion of the cross section occupied by bark decreases with stem girth, these contributions are likely to decrease with growth. Bark is generally flexible (i.e., low modulus of elasticity), not stiff, but exceedingly tough (i.e., high work to fracture in tension, compression, and torsion). Bark toughness might help trees avoid frost cracking, a prevalent problem in temperate forests that might also occur near timberline in the tropics.

As important as rigidity is to upright tree stems, flexibility is also critical. Trees that can bend or twist without damage can develop streamlined forms in response to flowing water or air and thus avoid further mechanical loading. By having a covering of flexible bark over their comparatively rigid column of xylem, trees may avoid serious tissue damage where the strains associated with bending and twisting are the most severe — near the surface. The system of interlocking fibers should also reduce the risk of cracks propagating from the periphery of the stem inwards to the core, thereby helping to avoid mechanical failure.

d. Defense

Inner bark is alive and therefore can defend itself either by possessing standing traits (i.e., constitutive defenses such as spines, thorns, toxins, and digestibility reducing compounds) or by synthesizing defensive compounds upon damage (inducible defenses such as many different phenolic compounds). Outer bark, in contrast, depends almost solely on constitutive defense provided by its thick-walled dead cells, as well as the presence of extractives and other stored secondary compounds.

e. Storage

Particularly in environments where resource availability and plant demands vary seasonally or diurnally, the barks of many species provide important storage sites for a range of materials including non-structural carbohydrates, nitrogen, and water (Pomeroy et al. 1970). Stem water contents in some tree species can be substantial, up to 64% by volume in baobab (Fenner 1980), and much of this storage is in bark. Marked seasonal fluctuations in proteins stored in the vacuoles of phloem parenchyma have also been reported in some tropical tree species (Schmidt and Stewart 1998) as well as in several temperate hardwood and conifer species (Wetzel 1989, Wetzel et al. 1991). These cells, as well as phelloderm parenchyma, can also store carbohydrates, fats, oils, latex, and resins.

f. Carbon Fixation

Many tree species have photosynthetic cortical, epidermal, or inner bark tissues on their twigs and young shoots. Less common is photosynthetic bark on larger stems. When present, the specific bark tissues that are photosynthetic in adult trees vary among taxa. In some species the photosynthetic tissue is a persistent epidermis with abundant lenticels (e.g., Betula) whereas in others photosynthesis occurs in cortical tissues rejuvenated by continued cell division (e.g. Populus).

In yet other species, particularly in arid environments, the phellogen arises immediately inside the epidermis or in the most external layer of the cortex, and produces a very thin layer of phellem cells that cover the chloroplast-containing parenchyma cells in the secondary phloem but allow for light penetration (e.g., Bursera).

Finally, photosynthetic tissues in tree stems can be present in wood ray parenchyma and even the pith (Wiebe 1975) where they may help prevent stem anaerobiosis (Pfanz et al. 2002). Although bark photosynthesis is often discounted, the contribution of stem photosynthesis to whole plant carbon gain can be as high as 40%-50% during periods of water stress (DePuit 1975, Nilsen, 1992).

Finish reading this review on page 2.

The photograph at the top of the page shows the outer bark of birch (Betula) and was taken by Doug Mercer (Canada).

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