A good illustration of the importance of bark is given by a
growing tree that has a completely hollow trunk. The inner trunk may have been
burned out, rotted, or decayed by insects such as termites. Just below and
protected by the bark are three layers, the phloem, the cambium and the xylem,
that are like pipelines carrying supplies to enable such a tree to keep
growing.
Rotten to the core but still growing strong |
The cambium, the second and thinnest layer, is the most
important as it provides all the cells that form the trunk and branches.
Just outside the cambium, immediately under the bark, is the
phloem. Made up fibrous material, the phloem transports sugars from the leaves
down to the root system.
The xylem starts just inside the cambium and consists of the
sapwood and heartwood. The sapwood carries water from the roots up to the
foliage. The heartwood consists of layers of old sapwood.
(The transportation of water and nutrients up and down a
tree is what enables a tree to grow and is a topic for another day)
The outer layer of bark is much like our skin. It protects
the inner layers from damage and insect attack and trees with thick bark, like
ironbark species, are even insulated from attack by fire.
Eucalypt bark protects the outer layer of sapwood that
contains epicormic buds. When a eucalypt loses its crown in a fire, these buds
will survive and produce green foliage that enables the tree to continue to
live. (Eucalypts and fire is also a topic for another day)
Epicormic shoots on eucalypts appear after a fire |
The bark of many trees contains resins that exude out when the
tree is damaged, sealing off the area from further harm such as invasion by
insects and fungi. It is now understood that the inner layers of bark also
sequester carbon.
The bark on the trunk and branches of a tree also provides
habitat for a host of organisms, giving rise to entire micro-ecosystems that
are vital for the biodiversity of a region. Many insects and spiders, reptiles
and mammals make their homes on and inside the bark of a tree. Moss, lichen and
fungi species have adapted to growing on bark.
This well camouflaged Two-tailed Spider is a typical bark-dweller |
Many birds are
especially adapted to extract a diet just from the animals that live there.
The Varied Sittella has long toes and a thin bill to help capture invertebrates in the bark of trees |
White-throated Treecreeper - another bark specialist |
The humble layer of bark on a tree plays a vital role in the
survival of the tree itself and the health of the ecosystem in which it exists.
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