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The Garden Scoop

Tree Bark

Winter time is somewhat of a downtime for deciduous trees.  After a growing season’s worth of beautiful foliage, culminating in the wonderful fall display of color, woody plants enter the long, cold season of dormancy that is part of their annual life.  For many of us, this is not a time of arboreal beauty or great ornamental value, but rather a time of dull, bare tree canopies cast against our wintery skies.  However, I find winter as a time to reflect on one of the most often overlooked aspects of tree beauty, which lies in the character of their bark. 

Each tree species has its own distinct pattern of bark arrangement.  While some are more difficult to differentiate, many have a distinctly unique pattern that can be quite showy and adds to their ornamental value across all four seasons. 

From a physiological standpoint, bark is an amazing plant structure.  It is composed of inner living tissue along with outer bark tissues that becomes dead or non-living at maturity, being pushed outward as tree trunks expand radially with growth.  Since outer bark tissue, commonly referred to as cork, is non-living it cracks and furrows as the tree trunk expands, creating the species-specific pattern we see on the outside of tree trunks.

The inner, living bark is comprised of vascular tissue, called phloem, which serves as a conduit for the transport of sugars produced by photosynthesis in leaves to the rest of the tree.  These sugars are the primary energy source for all physiological processes throughout the tree, so it is vitally important that the phloem tissue does its job.  Since the phloem is right beneath the bark, those accidental bumps with a mowing deck can be detrimental to young trees, which have thinner bark, if the phloem is gouged.

The outer, nonliving bark, or cork, is a commonly used material for many everyday products including everything from wine bottle stoppers to flooring.  It is actually a very sustainably harvested product, typically harvested once every decade from native cork oaks (Quercus suber) in the Mediterranean region.  Following harvest, trees are left to re-grow their cork for the next harvest. 

In recent years, plastic and other materials have been used for wine stoppers creating competition and driving cork prices down.  Conservationists fear this trend could have a very negative effect on Mediterranean ecosystems since the cork industry hosts a thriving oak community that may give way to other land uses if cork production becomes unprofitable.   

Mediterranean cork famers carefully harvest their crop, leaving a finite amount on the tree since the outer bark serves as a protective layer from environmental extremes.  It keeps moisture in and pathogens out, with cell walls infused with a substance called suberin that aids in these processes.

The characteristics of outer bark are often a result of the environment in which a tree species evolved, with very specific functions.  Trees that evolved in fire-driven ecosystems, such as the cork oak or our native oak species, have especially thick outer bark to save valuable phloem tissue from fire damage.   Oaks also have a large amount of compounds called tannins in their bark and other tissues.  Tannins are toxic to much of the insect world and much of the other fauna that may predate tree tissues, providing defense from herbivory. 

Bark character can be a highly ornamental trait of many trees we use in landscaping.  Among our native trees, I find that many of the thinner bark species have some of the most splendid cork adorning their outer boles.  American beech (Fagus grandifolia) and musclewood (Carpinus caroliniana) trees have an interesting, very smooth bark with a quite unique appearance.  In the case of musclewood, the growth form of the trunk produces shapes similar to muscles under the skin when covered with the thin, gray bark, which is an especially uncommon and attractive bark character.

Compared to their other thin baked compadres, there are several native trees that boast flamboyant, exfoliating bark, which is much less subtle.  Sycamores (Platanus occidentalis) have a dark-colored and flaky outer bark that sloughs off as the tree trunk expands to reveal a beautiful creamy-white inner bark, creating an excellent contrast between the lighter and darker tones of bark. 

Perhaps one of my favorites is the river birch (Betula nigra), with an exfoliating outer bark that beautifully accents the trunk with creamy shades of white as well as orange and brown.  Unlike sycamore which sheds it bark plates, the very thin, curly exfoliating bark of river birch tends to hang onto the trunk longer giving it an almost furry appearance.

This winter, take some time to appreciate tree bark patterns while we wait out the gardening off-season.  Some basic knowledge of bark can add to your winter tree id skills as well as better inform landscape design decisions with respect to winter interest.