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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).

IV. Damage to tree stems

There are many potential causes of damage to tree stems. I will discuss the particularities of the most common types and will reflect on the ecological aspects of each kind of damage to tree stems.

a. Mechanical Damage

Winds and storms can snap stems and falling trees often scrape against their neighbors. In temperate forests, large masses of ice freed during spring thaws can cause serious damage to tree stems in riparian forests (Filip 1989) and, as mentioned earlier, frost cracking damages many trees in temperate latitudes. Animals also wound tree stems when they feed on inner bark or cambial tissues and when they rub their bodies or antlers against tree trunks. Finally, bole damage during logging, road widening and, increasingly, by use of lawn care machines is exceedingly common.

The extent of damage to tree stems and more specifically to bark should be evaluated on the basis of which tissues are affected. Damage to the inner bark interrupts the transport of photosynthates whereas damage to the phellogen reduces the ability of this cambium to produce storage cells and tissues important for stem protection. More fundamentally, what is described as “bark damage” often includes the exposure and death of the vascular cambium. In contrast, damage can be superficial and only affect the phellem, perhaps only rendering the tree more susceptible to further mechanical or heat damage.

b. Herbivores and Pathogens

Herbivore damage to tree stems is particularly common where other browse and water sources are scarce. A wide range of animal species feed on inner bark and bark exudates including orangutans, rhinos, beavers, squirrels, porcupines, and spiny rats. Herbivores are selective in their choices of tree species for foraging and can have profound effects on plant community structure and species composition (Figure 12). Elephants and kudus, for example, actively select barks with high water or nutrient contents (e.g., baobab, Adansonia digitata; Swanepoel 1993; Figure 13; McNaughton 1988 ).

Many mammals, including several species of primates (e.g., baboons and bushbabies), feed on resins and gums produced by bark. These substances are exuded in response to wounding, presumably to protect the stem from further damage, but contain carbohydrates, minerals, and proteins that are important dietary components for some animals (Harcourt 1986).

Perhaps the best-studied bark-animal interactions involve the effects of plant pathogens transported by bark “feeding” animals. Pathogens that affect bark have been responsible for local and regional extirpation of numerous tree species. Among the most famous cases are the American chestnut (Castanea dentata), which was decimated in the early 1900s in eastern American forests by an Asian fungal pathogen (Endotia parasitica), and the Nectria-scale insect problem currently affecting populations of beech (Fagus grandifora) in the same region (Latty et al. 2003).

c. Fire

Heat from outside of stems can be transferred into the cambium and wood by conduction, radiation, and convection, depending on the steepness of temperature gradients and bark properties. Although bark thickness has repeatedly been shown to be the primary correlate with avoidance of cambial damage during surface fires (Pinard and Huffman 1997, Dickinson 2002), interspecific differences in bark tissue properties (e.g., specific gravity, density, and moisture content) are also important. A particular example of the interplay of these properties is the bark of the cork oak, which is both very thick and has very low heat conductivity (Figure 14, Figure 15; Cork Oak Review). Additionally, the presence of volatile compounds (e.g., benzene-soluble extractives), proteins, and lignin all affect the heat of combustion, which tends to be higher in softwoods than hardwoods (Chang 1955).

The vascular cambium of some tree species is apparently protected from heat damage by bark that readily ignites. Bark flammability is a function of its chemistry and aeration. Barks that retain thin strips of partially sloughed periderm, for instance, tend to ignite easily. Flammable shedding barks serve as fire ladders that carry flames from the understory up into the canopy where they can turn into crown fires. Species with barks that readily carry fires into the canopy typically have good post-fire regeneration capacities, of which a good example is the stringy-bark group of Eucalyptus (Burrows 2001).

The ability of trees to replace crowns lost in fires by production of epicormic branches is influenced by bark and bud properties. Buds buried below thick bark are often mechanically prevented from emerging. Several species of fire-adapted Australian Myrtaceae, among other genera, avoid this dilemma by producing epicormic bud strands that are meristematic from the cambium to the outer bark. In these species, even if the outer portions of the elongated buds are destroyed, resprouting is still possible from portions deeper in the bark (Burrows 2002).

V. Responses to stem wounding

In contrast to leaves that can be replaced after being damaged, tree stems are not ephemeral and thus need to avoid or recover from wounds. Although injuries that penetrate through the bark down into the wood are often noted, superficial damage that does not reach the vascular cambium is much more common. The ability to compartmentalize around bark wounds or repair them, to show compensatory re-growth in affected sections, and ultimately to replace damaged tissues should be of great selective value.

Although there has been comparatively little research on tree responses to bark damage (but see Dujesiefken and Liese, 1990) the responses of trees to wounds that penetrate into the wood have been fairly extensively described by Shigo (1984). His CODIT (Compartmentalization Of Decay In Trees) model describes the sequence of events after xylem is exposed. Some of the substances synthesized after damage neutralize the effects of pathogens or otherwise inhibit their development, thus confining their horizontal and vertical spread (Klepzig et al. 1996).

After inner bark is damaged, callose, a carbohydrate derived from parenchyma, is deposited to seal off damaged sieve tubes and otherwise cover the wound. In most tree stems, the vascular cambium adjacent to the wound lays down a single-cell layer called a barrier zone, which separates the tissues formed before and after wounding, isolates the damaged tissues, and blocks the spread of pathogenic and saprophytic organisms. Once the barrier zone is formed around wounded inner bark tissues, several other chemicals are produced and further anatomical boundaries are formed. Later, parenchyma cells adjacent to the wound become suberized, and thereby constitute a strong barrier against desiccation and further pathogen spread (Biggs 1984, 1986).

In many tree species a new phellogen then differentiates adjacent to the remnants of pre-existing phellogen as a new vascular cambium forms as an extension of the original cambium. In species with bark exudates, many insects and pathogens appear to be deterred as these substances dry out on exposed surfaces after damage. When the defense mechanisms that usually protect trees from fungal infections fail, trees often die due to the combined effect of fungal colonization of the cambium, phloem, and xylem parenchyma coupled with the effects of extensive necrosis and girdling (Shrimpton 1973, Cook et al. 1986).

Interestingly, there are some tree species that can survive complete girdling. For example, in the African baobab (Adansonia digitata), xylem parenchyma cells just below the exposed wood surfaces de-differentiate into cambial derivatives that then regenerate xylem and phloem tissues (Fisher 1981). Another species known to survive girdling is Broussonettia papyrifera, the bark of which is used to make traditional cloths and paper in Asia (Cui et al. 1989).

VI. Final remarks

Through the review of aspects of bark structure and function just presented, I have provided a baseline in terminology pertaining to anatomical, morphological, and physiological characteristics of tissues composing tree stems. I have also tried to introduce some of the evolutionary constraints and opportunities that have shaped bark into the wide structural and functional diversity that we see today. I hope that this effort will be useful for promoting the sustainability of harvesting practices in species with economically important barks. Likewise, I hope to foster more detailed and integrative research on bark ecology in general and bark ecophysiology in particular.


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Information about this Review

This review is also available in the following languages:  

Portuguese (coming soon)   Spanish (coming soon)

The author is:  Dr. Claudia Romero (PhD in Botany)

The photograph at the top of the page shows a family of elephants feeding on the bark of a baobab (Adansonia digitata).  The bark of this tree has a high water content and so is especially important for elephant survival in times when water is scarce.  Photo by Jen Gebbie (USA).

Photographs for the figures were taken by J. Pausas, C. Romero and J. Romero.

The proper citation is:

Romero C  2012    Bark EcologyECOLOGY.INFO #34

If you are aware of any important scientific publications about bark ecology that were omitted from this review, or have other suggestions for improving it, please contact the author at her e-mail address: 

cromero {at} botany.ufl.edu

© Copyright 2007-2012 Ecology Online Sweden.  All rights reserved.

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