Wandering among the trees and bushes in the Bosque each day has peeked my curiosity about bark and it’s less obvious functions.

Some trees like aspen and birch have smooth skin all their lives, others like the Cottonwoods end up with heavily wrinkled bark that sometimes turns reddish with age in the Southwestern sun.

The term “bark” is often used to describe only the corky, visible outer surface of trunks and branches. In botanical terms, though, bark includes the entire, multi-layered shell of a tree that can be detached from the wood -that is, everything outside a thin ring of tissue called the vascular cambium. Cells divide and grow in the cambium layer, producing a ring of wood in the inside and a layer of bark tissue, called the active phloem, on the outside. The phloem transports sugars and nutrients throughout the tree, and is typically hidden from view, beneath the outer bark.

Outside the phloem, trees have three additional bark layers, collectively called the periderm. The first two layers are virtually invisible; the inner layer – the cork skin – usually contains chlorophyll and does some photosynthesizing. The middle, cork cambium layer facilitates cell growth. The third, outer layer is made up of cork cells that die soon after they mature. This cork layer protects the tree from infection, infestation, and drying out. The smooth, unbroken outer bark that all trees start out with is this cork layer.

As a tree grows, its wood thickens and pushes out against the bark that surrounds it. The different ways in which the outer bark adapts to this pressure is what gives each species its distinctive appearance. Some species maintain their original outer layers for their entire lives. In such cases, the outer bark expands to match the growth of the wood beneath it, and remains unbroken.

However, in other trees pressure from the faster-growing wood soon causes the initial periderm to split as a new layer forms in the inside, usually in overlapping sections that vary in shape, size, and thickness according to species. This process can repeat itself as a tree grows. Alternating layers of old periderm and dead phloem form the thick, craggy, wrinkly outer bark that is found on most mature trees.

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In each tree species, the bark’s appearance is determined by the shape of the overlapping sections of periderm, the type of connective tissue, and the rate at which layers of bark break apart.

Thick outer bark is generally a good investment, since it better protects a tree from wounds and provides more thermal protection. The outer bark’s air-filled cells function much like in home insulation, keeping the inside warmer or cooler than the outside. Ridges, scales, and vertical strips can dramatically increase the outer surface area and help maintain a more even temperature. Contoured barks also hold moisture, which slows the transfer of heat through the outer bark. I think these characteristics are really easy to see on the trunks of adult cottonwood trees.

Thick bark is especially important for fire protection. Redwoods, for example, have bark that is almost a foot thick making the tree impervious to all but the hottest fires.

Rapid temperature changes can also damage or kill sections of bark. In winter, for example, direct sunlight can warm bark to temperatures much higher than the surrounding air. When temperatures plummet rapidly, cooling bark can crack as it contracts. Extreme temperature changes create havoc.


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