December 28, 2017

By Maggie Abe

 Textured Trunks: Exploring Exfoliating Bark

Today’s WildAware article is submitted by Acton resident, Maggie Abe. Maggie is a local artist who is a member of the Board of Directors of The Friends of the Acton Arboretum, Inc.

As winter approaches, the landscape grows starker. The green of summer dulls, then goes dormant. All too soon, fall’s conflagration flares and fizzles, the leaves becoming brittle and brown. One might think the Arboretum has little to offer in these bare months, but this is a wonderful time of year to examine features often overlooked in the surfeit of spring and summer. Exfoliating bark is a fascinating adaptation of a number of species showcased in the Arboretum and provides a welcome pop of texture during the leaner months.

Exfoliating or not, bark is vital to a tree’s health. A tree’s outermost bark is dead and shields the inner, living bark. The inner bark, phloem, carries the products of photosynthesis throughout the plant, becoming the outer bark as the tree grows. In the cambium layer, new phloem forms just under the bark; on its other side, the cambium also produces new sapwood, or xylem, which makes up the bulk of the tree and transports water. We see this growth in cross-section as annual rings, with the youngest rings being those farthest from the central heartwood.

Bark is referred to as “exfoliating” when it exhibits a peeling or flaking behavior. It is a natural process and allows the tree to quickly recover from injury—common in temperate regions with snowy winters—as well as rid itself of disease and pests. Some theorize that in birches (Betula spp.), exfoliating bark facilitates the removal of light-blocking lichens, enabling sunlight to reach the inner bark for photosynthesis. Shedding old bark could also provide a way for trees to undergo rapid growth in diameter without their bark fissuring, which weakens it. There are likely other benefits of exfoliation still unknown to us!

Detailed below are some trees with exfoliating bark living in the Arboretum. Appreciate their unique beauty year-round!

Dawn redwood (Metasequoia glyptostroboides)

The dawn redwood is an ancient tree originating in China and thought to be extinct until the 1940s. Harvard’s Arnold Arboretum is responsible for disseminating its seeds throughout the U.S.; today, it can be found in parks around the world. It has reddish bark, a sinewy trunk, and green needles that turn orange in the fall. There are four dawn redwoods in the Acton Arboretum: one across from the rain garden, another between the grape arbor and the hosta garden, and two in the rhododendron garden.

Japanese stewartia (Stewartia pseudocamellia)

Peer through the branches and you will see this tree’s beautiful trunk, silvery and mottled like the neck of a giraffe. The bark exfoliates in smooth patches. In the summer, it grows round white flowers with yellow centers. The Arboretum’s stewartia is located on the main lawn where the path first branches, visible from the parking lot.

Paperbark maple (Acer griseum)

This tree’s copper color stands out against the white of winter. Its bark peels off in luminescent, peach curls. You can find this tree by the Arboretum’s stone reading circle.

Bald cypress (Taxodium distichum)

This tree is reminiscent of something you might see in a primordial mangrove. Like the dawn redwood, it is a deciduous conifer, with soft green needles that turn russet in the fall. Curved protrusions called “knees” shoot off its roots and break the soil around the tree; they brace the tree and provide extra support in sodden soil and inclement weather. The tree’s flaky bark is especially evident on the knees. Head over to the rhododendron garden to find the Arboretum’s specimen.

River birch (Betula nigra)

While all birches have exfoliating bark, the sort that comes away in papery sheets and delicate curlicues, the river birch is “adaptable to both flood and drought and is more disease-resistant and heat-tolerant than any other birch” (Carlsmith, 1983). It has orange-tinged bark and maroon twigs. The Arboretum’s river birch grows in the understory study.

Bibliography

• “Anatomy of a tree.” Trees, U.S. Forest Service, 2017. <fs.fed.us/learn/trees/anatomy-of-tree>
• Carlsmith, Anne. “The River Birch.” Arnoldia, vol. 44, no. 1, 1983, pp. 29-31. <jstor.org/stable/42954180?seq=1#page_scan_tab_contents>
• Kozlowski, Gregor. “Why do some barks peel?” Online posting. ResearchGate, 15 Oct. 2016. <researchgate.net/post/Why_do_some_barks_peel_What_are_the_advantages_of_exfoliating_for_a_tree
• “Oh! That Exfoliating Bark!” Life on the Greenway, Rose Kennedy Greenway Conservancy, 31 Jan. 2013. <blog.rosekennedygreenway.org/2013/01/31/oh-that-exfoliating-bark/>
• Rhoades, Heather. “Trees With Interesting Bark—Using Exfoliating Bark On Trees For Seasonal Interest.” Gardening Know How, Gardening Know How, 25 Mar. 2015. <gardeningknowhow.com/ornamental/trees/tgen/exfoliating-bark-trees.htm>
• “Stewartia pseudocamellia.” Plant Profiles, Holden Arboretum, 2017. <holdenarb.org/horticulture/plant-profiles/stewartia-pseudocamellia/>
• “The Dawn Redwood [Metasequoia].” About, The Bailey Arboretum, 2017. <baileyarboretum.org/the-dawn-redwood/>

 

 

Paula Goodwin is a member of the Acton Conservation Commission who introduced WildAware with Acton Natural Resource Assistant Bettina Abe. WildAware is a program sponsored by the Town of Acton Natural Resources Department that began in September, 2015. The purpose of WildAware is to educate the community about the existence and habits of wild creatures, and the goal is increased community awareness of shared habitats. For information, call 978-929-6634 or send email to nr@acton-ma.gov.