Dry winds brought down the leaves. Rain is currently making them slick hazards as we walk under an increasingly bare canopy on Capitol Hill. We enter a time of year when you can look up and fully appreciate the magnificent spread of a massive oak and the bright papery bark of a silver birch. I particularly enjoy this opportunity to engage with the barks of plants in winter, both because they pose interesting points about survival, and because they are simply artistically captivating, (especially without those gaudy distractions, leaves).
While it’s likely we all know this, it’s worth saying: Bark exists to protect woody plants, which grow year after year, from harm. Trees, shrubs, and some vines grow bark as a shield from fire, insects, and fungi as well as from freezing temperatures or moisture loss. When that delivery truck mangles the bark of an ash tree on Broadway, it opens up the tree to potential pathogens, gives it less protection from freezing weather, and can even stop it from transferring nutrients if badly damaged. Trees, our main focus here, need bark like we need our skin.
I think it’s worth noting that the terms “tree,” “shrub,” and “vine” are just useful terms for general growth forms that have developed independently all over the plant world. Just because a plant is tall, has a thick woody trunk and we call it a tree, say a Douglas Fir and a Bigleaf Maple, doesn’t mean they are closely related. And of course not all bark is exactly the same either, instead the varied results of adaptations necessary for trees their particular ecological niches.
Trying to understanding bark apart from a plant is unhelpful. I find it’s better to consider a cross section of a tree trunk to demonstrate bark’s role. I like to start in the middle layer of this cross section, called the cambium, because it’s the center of a growing tree trunk. Each season the cambium adds a layer of xylem or sapwood (really, just wood), which is like a series of stacked straws, transporting water and providing strength. As the cambium adds more layers of sapwood in annual growth, the inner wood eventually becomes heartwood, dead material that is mostly structural at the center of the tree. Cambium also helps create more bark, the layer sitting above and protecting this fragile layer. Directly on top of the cambium is the phloem, or inner bark, the layer that carries food and hormonal signals throughout the tree. Above this is the cork layer, that tough external barrier which is what we actually see and generally call bark.
I want you to ponder why this is so fascinating: Every species we get the pleasure of enjoying as we stroll across the Hill, native or otherwise, has a different type of bark that provides different services. Native trees can have radically different bark for wildly different reasons but still grow side by side. This becomes far less obvious when we take a plant from a far off place and plop it down in our neighborhood, but it doesn’t make it less interesting or beautiful, just harder to understand outside of the context of place.
Turning to native trees offers easy ways to understand some of the variety of bark as they relate to our climate and ecosystems. Douglas Firs (Pseudotsuga menziesii) have heavy outer bark, a cork layer that grows several inches thick, shielding mature trees from moderate fires and burrowing beetles, part of the natural fire cycles that have swept our forests for millenia. Pacific Madrone (Arbutus menziesii), the only native, broadleaved evergreen tree in the region, has a beautifully varied bark, which is thin and sheds papery layers in summer to reveal an intense chartreuse alongside deep orange hues. The freshly revealed green patches are full of chloroplasts, cells that photosynthesize in all green plants, and in this case help maximise photosynthesis in dry conditions, when madrones also drops old leaves to reduce water loss and spur on new growth. Red Alders (Alnus rubra) often display deep vertical splits in the otherwise smooth outer bark of older trees, visible proof of early growth so fast the bark couldn’t keep up. Alders also give us clues about air quality. In cleaner air they host lichens that turn them a variety of blotchy grays, whites, and greens. In heavily polluted places they are a dark, nearly uniform gray and lack epiphytic diversity.
Appreciation of bark can take many forms. For some, it’s all about usefulness: cherry for coughs, alder for dyes, birch for basketry, cedar for all manner of things. For others it’s the challenge of identifying trees without their leaves, a further extension of knowing the world around us in all seasons, making dreary days more interesting. Simply enjoying the diversity of tree trunks for their inherent beauty is worth our time and will make our days brighter still. We have a huge diversity of species growing in our old neighborhood, all the more opportunities to get out and enjoy something free in spaces that seem to mostly revolve around commerce. Next time you’re on a walk, look at the differences: flakiness, grooves, colors of lichens that grow on the exterior, rough hard bark and others slippery smooth. May they bring you a bit of pleasure and questions to ponder.
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