Why are Paper Birches so White?

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Certainly there’s much to admire about paper birch trees, also known as white birch (Betula papyrifera). Ecologists marvel about how adaptable they are, growing natively over a transcontinental range and doing so in all types of soil and topographic situations. Generations of happy homeowners rave about paper birch’s value as beautiful veneer and as solid-wood furniture and cabinetry. Even Robert Frost famously extolled their flexibility and capacity to provide gymnastic thrills to young boys and other “swingers of birches” in his poem, Birches.

But really, which among its many traits and features can rival paper birch’s bark, with its great, peeling, papery sheets of white? Paper birch trunks stand out in such bright contrast to their darker neighbors – be they maple and cherry or spruce and fir – that one has to wonder: why so white? And does it do the tree any good?

White is what we see when an object absorbs no visible light but instead reflects back to our eyes all colors in equal proportion. Paper birch trees appear white to us because they reflect most of the sun’s rays. In contrast, dark-barked trees – which is to say, pretty much all other trees – reflect very little but instead absorb nearly all colors. This is key: dark trees absorb light, white trees reflect it.

It turns out that the high reflectivity of paper birch bark may be related to the transcontinental distribution of the species that so excites ecologists. Consider that with light from the sun also comes heat. Absorption of sunlight by a tree’s bark causes heat gain. Oddly enough, in northern climates, mid-winter warming of a tree’s bark is not a good thing. Such warming may cause rapid fluctuations in the temperature of the cambium, that all-important, thin veil of regenerative cells between the bark and the wood. Extreme fluctuation in the temperature of these cells, especially from warm back to really cold, commonly results in cell death and severe injury to the stem and bark. Sunscald, frost cracks, loss of sap conduction, and sometimes even the death of the tree are all possible outcomes. Could it be that being white helps paper birch trees avoid such dangers? Is whiteness an adaptation for living in cold environments and avoiding midwinter heat gain and its potential harm?

It sure would seem so. Paper birch is one of just four deciduous tree species that reach the northern limits of tree growth in North America. The others are Alaska white birch, trembling aspen, and balsam poplar. What else do these species have in common, besides being the northernmost survivors? You got it: they all have thin, light-colored bark. Researchers have hypothesized that light-colored bark reduces the risk of winter injury and may explain why the northernmost deciduous trees are those with highly reflective bark.

In one study, researchers painted paper birch trees brown and noted that mid-winter cambium temperatures were higher in the artificially darkened stems. The brown-painted trees also cooled more rapidly than their lighter-colored counterparts. This same study found that after two years, brown-painted aspen stems in the Yukon had higher incidence of wounding than did whitepainted and natural trees. The researchers suggest a real and meaningful connection between bark color and tree survival in northern environments.

Still, what makes paper birch bark white? What makes it so highly reflective and conceivably more suitable for northern climates? Evidently, birch bark is white because it is rich in betulin (the name having been derived from Betula, the birch genus.) A triterpene to your chemist friends, betulin occurs as crystalline deposits in cells in the outer layers of the bark. These betulin crystals are physically arranged in such a way (not unlike snow) as to appear white. This admittedly dry fact comes to life when we consider that betulin has great pharmacological and therapeutic promise for humans as well. It is said to be bactericidal, fungicidal, antiviral, analgetic, spermicidal, anti-allergenic: it may even prevent cancer. How fitting, then, that the very whiteness that evidently helps paper birch trees to live at the northern limit of trees just may also help us live to enjoy them.

Michael Snyder is the Chittenden (Vermont) County Forester.

 
Discussion
  1. Sue Pike → in York ME
    Jan 07, 2012

    Hi, I write a local nature column and had fixated this week on finding out why paper birch bark is so white—what makes it white versus why the light color is a good adaptation to northern latitudes.  You talk about this at the end of this article.  I had thought that yellow birch bark is higher in betulin than paper birch bark (am probably wrong), but, if this is true, is it the arrangement of the crystals in the outer bark that make paper birch more reflective? Or conversely, something else in the yellow birch bark that make it more reflective.  Your’s is the first article I’ve found about what makes paper birch white—do you have any suggestions for more background on this?

    If I do write this up, can I quote/reference your Northern Woodlands article?

    Thanks
    Sue

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