Trees in Polychrome

From the nudibranch’s stunning neon markings …

to the geometric mosaics of the turtle’s shell;

the diatom’s stained-glass motifs …

image by Dr. Stephen S. Nagy

to the slender elegance of the calla lily,

nature is brimming with beautiful designs.

Indeed, every now and then you come across a life form that makes you stop and wonder, “… How can this even be real?”

One of my favorite examples is Eucalyptus deglupta, the only eucalyptus species found naturally in the Northern Hemisphere. Its most striking feature is its multi-colored bands of bark, which give the evergreen its common name – the rainbow eucalyptus.

The rainbow eucalyptus’ streaks of radiant color are produced by the patchy exfoliation of the tree’s bark. Each time a tree sheds a strip of bark, the trunk’s bright green inner layer becomes exposed. Then, as the young bark ripens and matures, it progresses through a series of different sunset hues. First, a careless blue, followed by fervent purple; on to dynamic orange, and finally a rich maroon, before the exfoliation process repeats itself all over again. With this irregular shedding sequence, each individual tree cycles through a constant, lava-lamp flow of colors and patterns. At any given time, each color pattern is unique, and no two trees look alike.

Many familiar trees, such as oaks, have a thick, uneven layer of bark that, for the most part, permanently encrusts the surface of their trunks. In contrast, the bark of the rainbow eucalyptus is thin, smooth, and constantly replacing itself. When a patch of bark has expired, it peels away at thin-walled junctions within the bark’s tissue called the phloem parenchyma.

There are several possible reasons for why certain trees have exfoliating bark, according to Dr. Marc Abrams, Professor of Forest Ecology at Penn State University. For fast-growing species, such as sycamores, exfoliating bark can accommodate for rapid rates of trunk expansion. In floodplain environments, where there is no use for the water conservation provided by thick bark, thin bark allows trees to lose water through transpiration more rapidly. Trees that more efficiently cycle water through their system then grow faster and thereby benefit from exfoliation. Furthermore, peeling bark may allow for greater gas exchange between a tree’s trunk and the atmosphere, which can prove beneficial in waterlogged, anaerobic soils.

However, there are trees with peeling bark that are not exceptionally fast-growing, such as the shagbark hickory, as well as many non-shedding species that inhabit floodplains, such as green ash and pin oak. Another explanation is that exfoliation provides trees that don’t have the protection of thick bark with an alternative defense against herbivory. Frequent peeling can ward off fungi, parasites, and epiphytes that grow on bark.

These explanations shed insight into why E. deglupta might bear peeling bark. Rainbow eucalyptus trees grow rapidly, attaining heights of 75 meters and diameters of 24 meters. They also prefer humid, lowland rainforest habitats, and form dense stands along river banks, where water conservation is not a priority. Moreover, fungi and epiphytes, such as mosses and lichens, flourish in these moist environments.

Indigenous to the island of Mindanao in the Philippines (the tree is also sometimes called the Mindanao gum), the rainbow eucalyptus occurs natively in Indonesia, Papua New Guinea, and the Philippines. Now, the tree has spread throughout the world’s tropics, and is commonly found as an exotic species throughout the South Pacific, Southeast Asia, and South America.

Humans cultivate and harvest rainbow eucalyptus trees, mostly for pulpwood and ornamental purposes. Paradoxically enough, its vibrant wood is used primarily in the manufacture of white paper. Because of its rapid growth, people also frequently plant E. deglupta as part of reforestation efforts.

More recently, the rainbow eucalyptus has been shown to benefit coffee cultivation by providing shade for coffee plants. E. deglupta is an ideal coffee shade species because it grows so rapidly. Furthermore, research has shown that, when rainbow eucalyptus is planted between rows of coffee, the two plants display complementary use of soil resources. Though the eucalyptus trees may grow faster above ground, the coffee plants grow competitive root systems that restrict the shallower roots of the eucalyptus below ground. In the end, the two species co-exist without either out-competing the other.

Ultimately, the rainbow eucalyptus exemplifies this wonderful quality about nature: just by existing and doing what they do, nature’s various organisms bring diversity and function to the world. In this vein, the rainbow eucalyptus provides an arresting and remarkable example of beauty, simply through its natural process of growth and re-growth.

I’ll leave you with this lovely haiku, written by Shelley Krause over at But Wait, There’s More! You can read more of Shelley’s haikus here.

by Shelley Krause

References:

Schaller, Michaela. “Species and site characteristics that permit the association of fast-growing trees with crops: the case of Eucalyptus deglupta as coffee shade in Costa Rica .” Forest Ecology and Management. 173.1-3 (2003): 205-215. Web. 28 Dec. 2011. <http://www.sciencedirect.com/science/article/pii/S0378112702000798&gt;.

Still, Douglas, and Fiona Watt. New York City. Parks & Recreation. Daily Plant. New York: , 2004. Print. <http://www.nycgovparks.org/news/daily-plant?id=19242&gt;.

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10 thoughts on “Trees in Polychrome

  1. Steph, amazing work! I have quickly jumped on the fan bandwagon of the rainbow eucalyptus – its generous aid of coffee growth just put me over the top :). I read your color adjectives a few times through too because I fell in love with your use of language! It reminded me of an assignment I was assigned in creative nonfiction – write about a color. No rules, just write. Not sure why, but I found it to be pretty challenging. Not too many people can combine creative writing and science (in an interesting, informative, and unique way) like you can. Have you looked into any science writing masters programs?

    • Thanks so much mick! I’m so glad you’re a fellow rainbow eucalyptus groupie. I am indeed looking into science writing masters programs (I know there’s one a Johns Hopkins :) ). I’m actually taking next semester off to do thesis research, but an added bonus is that I’ll be able to write a lot more and hopefully submit my writing to a few places. Hopefully that’ll help me next fall when I’m applying to those science writing programs

  2. Nice post and very well presented. It makes me wonder about the function of the colours; why would the tree bother to be so colourful? What advantage does it have?

    As someone else commented earlier, anything which benefits coffee growth is a friend of mine.

    • Thank you! Also, excellent question. There is actually a surprising scarcity of information regarding the specifics of why the rainbow eucalyptus presents the colors it does. I am definitely no expert on trees, but the way I understand it, the color of bark mostly has to do with its chemical and structural composition.

      Chemically, different aromatic compounds and pigments produce different colors; carotenoids make orange-reds, anthocyanins make purples, tannins make darker colors.

      Structurally, wax layers or air spaces within the bark tissue can perform high backscattering of light, which produces lighter colors. The exposed bark of the rainbow eucalyptus is a vibrant green because it contains photosynthetic tissue.

      Ultimately, each color is product of the chemical and structural compounds within the bark tissue. Of course, these different pigments and structural components serve important functions. Then, when you put it all together, you get the colorful paint-stroke effect of the rainbow eucalyptus.

      Hope that helps! Let me know if you find any other information pertaining to why the rainbow eucalyptus boasts such a magnificent spectrum of colors.

  3. Pingback: WTF?? Hippie trees look like tie dye, but they’re real! « hippielessons

  4. Pingback: Rainbow tree | Ray Cannon's nature notes

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