The Ecology of Bark

Shark sliver clouds raced over the horizon. Temperatures were in the forties when my dogs and I took an unfamiliar trail that had been packed down by machines. It is mid – February – I note that the buds on trees and bushes are already swollen and have been for about 10 days. Not a good thing.

Wandering among some larger trees, I was paying close attention to the bark on young trees, too. Although I can identify trees when they are adults, when they are leafless, I can be tricked by saplings. Smooth bark obscures identification. When I start questioning who’s who I turn it into a game. I check the ground for clues, and the neighbors too. Look to the sky to determine the tree’s general shape. Today one stately tree with fluted upturned bark gave the tree’s name away immediately. An adult shagbark hickory is easy to identify. Very satisfying.

I am delighted to meet this tree knowing that it can live for 350 years, and feeds many woodland animals. Less known is that the shagbark hickory is a host plant for butterflies and moths, including the Luna moth. I am reasonably certain that I identified a smooth-skinned youngster not far from the adult – but uncertainty still lurks. Rubbing my hands over the adult’s rough edges, my curiosity about bark took over; this was a healthy tree, and I was reminded how important bark is to a tree…

Some trees, like aspen and birch, have smooth skin all their lives; others, like white pine, end up with heavily wrinkled bark that sometimes turns slightly reddish with age. A mature Red pine always looks to me as if it has thick overlapping scales; this bark is definitely red.

The term “bark” is often used to describe 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 on 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, the 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 on 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.

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. These characteristics are easy to see on the trunks of adult hemlock, pine, or hickory 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.

With all the protective advantages of thick bark, why does the bark of some trees remain thin? Smooth bark is easier for the tree to grow, but a major advantage of thin bark is its increased ability to photosynthesize. Scraping away the outer bark on a twig or young branch reveals the thin skin that photosynthesizes, in some cases, almost as efficiently as the leaves of trees can.

Since thick bark blocks most or all sunlight from reaching the cork skin, photosynthesis levels are usually much higher in species that maintain thin bark on their trunk and branches. Energy produced by bark photosynthesis is thought to support regular cell maintenance in the trunk and branches and can help trees recover from defoliation due to insects, storms, or severe drought.

Bark photosynthesis works best when leaves do not shade the bark. Even in a seemingly dormant forest if the sun warms the bark it can photosynthesize even when air temperatures are below freezing.

Thin bark also helps thwart mosses, lichens, and algae. Epiphytes can block sunlight, preventing efficient photosynthesis. They can also absorb heat, which increases a tree’s risk of damage from temperature changes.

Some trees, like paper birch, feature strips of bark that peel away from the trunk and take with them any light-blocking epiphytes that may have become established there. An amazing adaption, that! Just last week I picked up some gracefully curled white birch bark that was laying on the ground while walking around at our local land trust (MLT).

Outer bark also protects a tree against intruders. In general, thin-barked species like American beech are easier to penetrate than species with thick bark. But bark of any thickness has weak spots at pores, cracks, and furrows, and at branch junctions where wrinkles bring the inner bark closer to the surface.

Wounds in the outer bark open pathways for fungi, bacteria, and insects, but in healthy trees, the bark’s chemical and structural defenses can often overcome infections and infestations.

For example, resins in conifers and the gum in black cherry bark repel insects and infectious agents, seal small wounds to prevent infection, and trap insects. Betulin, a substance that whitens the bark of paper birch, deters gnawing animals, fungi, insects, and other invaders. The inner bark of aspen and other members of the willow family contains salicin, which deters bacteria, fungi, and insects.

Structural mechanisms also defend a tree against infection and infestation. Fungi that breach the outer bark, for example, can be walled off or compartmentalized. This action contains the infection, but it also kills sections of bark by blocking the incoming flow of water and nutrients. The resulting small areas of discolored, sunken, or cracked bark are called cankers.

Even when infection or infestation is controlled, a tree must contend with the breach that has occurred in the protective outer bark. The inner bark generates cork to surround a wound and can close small openings and narrow or close large gaps over time.

In Maine, Eastern hemlock is the only species that produces wound cork in annual increments that you can count – like rings of wood – to determine a wound’s age. Unfortunately, despite its multiple chemical and structural defenses, bark can’t protect trees from all attackers, especially introduced organisms for which a species has no evolved resistance. In many cases, introduced pests successfully kill a tree.

However, some bark-inhabiting fungi and bacteria do no harm. Other fungi and bacteria defend their hosts by out-competing or preying upon canker-causing fungi. These beneficial organisms are often found near weak areas of bark where pathogens might gain entry.

When tiny insects, such as springtails and bark-lice, inhabit bark and feed on mosses, lichens, and fungi growing there, they can benefit their host tree by attracting spiders, ants, and other predators that can help control populations of defoliating insects.

At any given moment, there are thousands of interactions between the bark and its environment that most of us take for granted. Especially during the winter months, if you pay attention to bark, you may, like me, develop a deep respect for the unparalleled beauty and for the protective skin of every tree you meet.

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