Saturday, August 12, 2017

Bark - Essential for Survival

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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