Bark color as adaptation (pt 2)

This is part of a larger series on how the color of bark may be an adaptation for different ecological conditions. You can find the whole series here. For more natural history delivered right to your inbox, sign up for our (mostly) weekly newsletter.

In my study (results), paper birch had the lowest temperature when exposed to the sun. It also has bright white bark and one of the farthest north distributions of any hardwood tree (see map below). Other white barked trees (e.g. quaking aspen, balsam poplar, and gray birch) also extend well into the boreal forest (see aspen’s range map below) indicating that having white bark is a specialized adaptation for coping with extreme cold.

ABOVE: The striking white bark of paper birch (My Neighbors Woods, Richmond)
BELOW: Range map for paper birch (taken from iNaturalist data)

ABOVE: Young growth on paper birch showing reddish stems but a mostly white trunk (Victory Mountain, Victory)
BELOW: Range map for quaking aspen (taken from iNaturalist data)

While it might seem counterintuitive for more northern trees to have bark that keeps the trunk cooler, this is exactly the case, but only for hardwoods. Let’s consider a deciduous tree with dark bark on a cold, sunny day in January. The dark bark absorbs sunlight and rapidly heats up (like your cat’s fur when it lays on the ledge next to the window). In just a few minutes the staghorn sumac in my setup went from ambient temperature (91ºF) up to 155ºF (I’ll be reconducting this study during the winter to see how cold weather impacts the results). Objects tend to expand as they heat up and contract as they cool (a tree is no exception), so the warm, outer part of the tree slowly expands.

As the bark continues to absorb sunlight throughout the day, it conducts heat inward, and the core of the tree slowly warms and expands as well. As soon as the sun sets (or is blocked by another tree) the outer part of the trunk cools rapidly and therefore contracts. When I shaded the sumac it dropped 36ºF to just 119ºF in just 30 seconds! If a tree cools too fast it can crack (and even explode).

Factors associated with frost cracking:

1. Cold days with full sun and no wind: wind would disrupt the boundary layer and cool the trunk down
2. Growing in the open or on an edge: no other adjacent trees to shade the trunk
3. Larger diameter tree: stores more heat, can expand more
4. Dark bark: dark colors absorb more heat
5. Smooth bark: rough bark, like a radiator, dissipates heat
6. Thin bark: Less insulation to protect wood from thermal expansion
7. Defects in the wood: doesn’t handle contraction nearly as well as healthy wood

Both aspens and birches have thin bark which allows for photosynthesis during the late fall and early spring (about 15% of an aspens energy budget can come from bark photosynthesis). While the absence of leaves exposes the trunks of hardwoods to sunlight for photosynthesis, it also makes them far more susceptible to frost cracking than evergreens whose needles shade the trunk. Trees with smooth, shiny, white bark have higher albedos (a measure of how much light is reflected), which minimizes the risk of frost cracking on cold and sunny winter days by reflecting rather than absorbing sunlight.

When things do go wrong, however, frost cracks can appear on the trunk of a tree. These vertical cracks tend to form where prior damage has already weakened part of the trunk (that is, frost and/or freezing is not the primary cause of damage to the tree). Damage occurs in the winter and callus tissue forms in the summer as the tree attempts to seal up the wound.

Repeated frost cracking can lead to frost ribs, a pattern particularly noticeable on oaks (as shown above). On some species (e.g. American elm, hemlock), the callus tissue – whether from mechanical damage, a hungry beaver gnawing the bark, or frost damage – forms clear annual growth lines that you can use to get a rough estimate of when the damage first occurred.

At the start of the series, if I asked you to draw a tree with crayons, you would likely choose some shade of brown to shade in the bark, maybe sprucing it up with some vertical lines. But you could just as well choose green, red, gray, pink (white ash in spring), yellow (aspens in spring), or even leave the trunk white. Those vertical brown lines could be swapped for horizontal peeling sheets of paper or diamond shaped furrows. You could finish the drawing with a nice glossy polish or leave it a dull matte. And we haven’t even been asked to draw the tree’s bark as it changes over different stages of its life.

The idea is here is that the combination of texture and color can produce thousands of possible expressions of “bark,” which we can conveniently use to help us identify say a pin from a black cherry. But more importantly, these variations are clues to the underlying ecological adaptations behind bark color. Temperature regulation is just one of the more specialized functions of (the core functions of outer bark are: protection from herbivores, parasites, and pathogens, protection from the environment (heat, cold, fire, ice damage, etc.), and gas exchange (oxygen, water, carbon dioxide)).

We can think about how color might play a role in these functions, looking for patterns across different species and habitats. When we start to think of bark as an adaptation for confronting the immediate challenges of that tree’s environment it starts to become clearer why bark can look so different. Like our skin, bark is a tree’s first line of defense in maintain an internal balance, protecting it’s inner system for piping nutrients, sugar, and water throughout its body.