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
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
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.
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.
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.
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.
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).
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,
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.
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.
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;
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).
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
oak, which is both very thick and has very low heat conductivity
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).
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
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).
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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This review is also
available in the following
Portuguese (coming soon) Spanish (coming soon)
The author is: Dr.
Claudia Romero (PhD in Botany)
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).
for the figures were taken by J. Pausas, C. Romero and J. Romero.
The proper citation is:
Ecology. ECOLOGY.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:
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