What makes birch bark white

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Several species of the tree genres Betula have a white bark,especially when they are young. This is quite peculiar to trees - at least in central Europe; so I'm wondering if there is a ecological reason behind it.

The first idea I have is that they protection against the sun. But normally protection of nature against the sun is to make the opposite: put more pigments in the skin, so that the sun does not damage the underlying DNA.

If this is the explanation, how do other tree species protect against this? It is protection against rapid warming of the cambium layer. Although some species of birch are similar in appearance to aspen, aspen bark does not peel. Birch leaves are oval in shape and dark green and smooth on the upper surface.

Each leaf is two to three inches long and about an inch wide. The birch flowers, arranged in long clusters known as catkins, appear from April through June with the fruit maturing in the fall. The fruit is made up of several tiny winged seeds that will drop throughout the fall and winter. Birch trees will start producing seeds at 15 years of age. Compared to other deciduous trees, birches have a relatively short lifespan of 80 to years.

The river birch Betula nigra can be found in the floodplains and swampy bottomlands of the eastern U. This birch species have copper-like two-toned bark and gold-colored leaves in the fall. The paper birch Betula papyrifera is often called the canoe or white birch. 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. In order to better understand this possible defense strategy, I studied several paper birch trees and other nearby trees for two months in midwinter in Lyme Hill Conservation Area, owned and managed by the Upper Valley Land Trust.

Specifically, I tried to answer the question: do paper birch trees have lower surface temperatures than darker colored trees? If lighter-colored bark protects paper birch trees from heat-related injuries, then those trees should exhibit lower surface temperatures than neighboring darker trees. After several weeks of study, I came away with a much more complicated answer than I expected. While paper birches tend to exhibit lower surface temperatures than darker trees, there is a lot a variation in this trend.

Many factors appear to influence tree surface temperatures, including air temperature, sun exposure, cloud cover, and tree trunk diameter. One of my most interesting findings was that temperatures can vary substantially across the surface of a single tree, whether light or dark, with higher sections being up to 6. Though this trend was not consistent among trees, it certainly raises more questions about the distribution of heat across their surfaces.

Surface temperatures of birch and dark trees. Although not statistically significant, the difference in surface temperatures between birch and dark trees suggests that light colored bark allows birch trees to maintain lower surface temperatures than darker trees.

Each dot represents the mean of each group.