Bark Protective Survival Mechanisms Foil Deprivation, Injury, Invasion

Summary: Jeffrey Dawson of the University of Illinois at Urbana-Champaign says that bark protective survival mechanisms get trees past deprivation, injury and invasion.

In-bark performance of photosynthesis accounts for green colored bark of foothill Palo Verde tree (Parkinsonia microphylla), Springs Preserve garden, Las Vegas, Nevada: Stan Shebs, CC BY SA 3.0, via Wikimedia Commons

Bark protective survival mechanisms arise from the tissue's complex structure and straightforward function, according to Bark Traits are Important to Tree Health and Survival for the February 2016 issue of Arborist News.

Jeffrey Dawson of the University of Illinois at Urbana-Champaign begins with the outer bark's fatty, internal moisture-conserving, water-impervious, waxy suberin barrier to gas and nutrient exchanges. Dead-celled outer bark circles two single-celled layers of simple cambium cells whose differentiation into specialized cells coordinates with wood cells that carry water up from roots. Large and small-diameter sapwood xylem cells in "seasonally cold" or "seasonally dry" region-generated annual tree rings respectively describe earlier, faster and later, slower growth and uptakes.

Xylem enables transport of dissolved hormones and nutrients while hormone and sugar distribution entails sieve tubes in the living phloem's inner bark-forming cork cambium called phellogen.

Inner, living bark called phelloderm fits inside phloem to further exchange of gases, nutrients and water and storage of nitrogen and phosphorus compounds and of starches.

Starches get remobilized for "early growth and metabolism," despite bark-stripping, foraging, nutrient-seeking, winter-famished wildlife, since bark protective survival mechanisms generate bitter-tasting, toxic substance-filled, thick-walled, tough-fibered cells. Inner bark has nutrients while corky, thick outer bark has air-filled cells to hinder cold and hot transfers from temperature extremes and high heat from fires.

Forming, reforming bark invents arcs for scaly-barked maple and oak, cylinders for smooth-barked beech and birch, splits for fissured ash and alternating thicknesses for peeling birch. It juggles many advantages, such as discouraging climbing insects and tree-dwelling algae, lichens, mosses and orchids and supporting interbark-barrier gas exchange by lenticels in smooth-barked trees.

Stomata likewise keep carbon dioxide for photosynthesis and oxygen for respiration as gas exchange regulators on leaves and, until true bark forms, pre-green bark cells chlorophyll-filled.

Thin, translucent cork-layered trees lack the Palo Verde tree's green bark cell stomata and let surface-layered photosynthetic tissue recycle carbon dioxide from respiration back into carbohydrates. Bark musters not only photosynthates but also antimalarial quinine, antimicrobial tannins, cancer-fighting taxol and insecticidal juglone and resins from Cinchona, hemlock, yew, walnut and coniferous trees. Bark protective survival mechanisms also necessitate fire-resistant, thick-barked black and bur oaks, coast redwood, giant sequoia and pitch pine and, outside fire-prone habitats, flood-resistant thin-barked sycamores. They offer similar thick-barked protection from freeze-thaw cracks on cooling bark contracting from direct winter sunlight overhead southeast-facing trunks and killing cambium conifers and London planes.

The adhesion of bark to wood prevents environmental, human and mechanical damage least before the growing season's "new fibers fully differentiate" and most during dormant seasons.

Fiber-rich bark qualifies shagbark hickory and white ash, not fiber-poor, dormant-weak beech, red alder, sycamore and white birch, for greatest "dormant-season inner-bark strength, toughness, and adhesion." Bark protective survival mechanisms reinforce bark's "stiffening" young stems and results in high density and specific gravity from bundles of fibers with "thick, lignified cell walls." Bark shows lower adhesion, density, specific gravity, strength and toughness with fiber-poor, short sclereids clustered into groups of bone-like, branched, column-like, same-length same-width or star-like shapes.

Protective roots-to-shoots, species-specific bark thwarts deprivation, injury and invasion through fibrous cells, fire-resistant thickness, flood-resistant thinness, lenticels, phenols, photosynthetic tissue, sieve tubes, stomata, suberin and terpins.

Distinctive bark of shagbark hickory (Carya ovata) is fiber rich, a bark protective survival mechanism that accounts for inner-bark success during dormant season: Steven Katovich/USDA Forest Service/Bugwood.org, CC, via Forestry Images

Acknowledgment

My special thanks to:
talented artists and photographers/concerned organizations who make their fine images available on the internet;
University of Illinois at Urbana-Champaign for superior on-campus and on-line resources.

Image credits:

In-bark performance of photosynthesis accounts for green colored bark of foothill Palo Verde tree (Parkinsonia microphylla), Springs Preserve garden, Las Vegas, Nevada: Stan Shebs, CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Parkinsonia_microphylla_trunk.jpg

Distinctive bark of shagbark hickory (Carya ovata) is fiber rich, a bark protective survival mechanism that accounts for inner-bark success during dormant season: Steven Katovich/USDA Forest Service/Bugwood.org, CC, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=5538516

For further information:

Dawson, Jeffrey. February 2016. "Bark Traits Are Important to Tree Health and Survival." Arborist News 25(1): 12-17.

Gilman, Ed. 2011. An Illustrated Guide to Pruning. Third Edition. Boston MA: Cengage.

Hayes, Ed. 2001. Evaluating Tree Defects. Revised, Special Edition. Rochester MN: Safe Trees.

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