Bark color as adaptation (pt 1)

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 looking at the results of my bark temperature study, it was clear that lighter colored bark had significantly cooler surface temperatures than dark colored bark. This was a fairly intuitive result as darker colors absorb more light and therefore heat up more while light colored objects (like snow) have a higher albedo and reflect most of the incoming light. At the extreme ends of the spectrum, when the air temperature was 91ºF, paper birch bark was just 118ºF while the much darker bark of staghorn sumac was 155ºF. So this begs the question of what advantage would this confer on a tree? There are three possible ways to think of this (with a big asterisk): 

1. Bark temperature is largely irrelevant
2. Having warmer bark is advantageous
3. Having cooler bark is advantageous

And then the big asterisk: bark color is not a one-size-fits-all situation. In some contexts it might be advantageous to heat up and in others to stay cool (or at least closer to the ambient temperature). It’s also important to note here that the main functions of bark include things like protection from parasites/predators, insulation from harsh weather, gas exchange, and photosynthesis, and not to either heat or cool down a trunk.

Let’s start with the assumption that bark temperature is unimportant factor to trees. I was measuring temperatures on logs that were exposed to direct sunlight, but there are plenty of situations where trunk aren’t exposed to direct sunlight either during the day, during the year, or during part of its life, meaning bark temperature could remain relatively stable through time (you might remember that the shaded side of the logs were all between 105 and 112ºF). Take conifers and other evergreen trees, which retain their leaves throughout the year. Since their trunks are mostly shaded, bark color has little bearing on bark temperature and we wouldn’t expect any specialized adaptations to deal with or take advantage of sunlight. This is also true for understory trees and shrubs that are shaded by the canopy throughout much of the growing season and during the winter when they might still be mostly shaded by the trunks of other trees in the forest.

One of the darkest and hottest trunks was striped maple (145ºF). To make sense of this, it’s helpful to place striped maple in its ecological context: striped maple is a shade tolerant, deciduous understory tree with photosynthetic bark. After striped maple has lost its leaves in the late fall, striped maple can continue to photosynthesize using its bark, effectively extending its growing season. It’s not exactly clear how late into the fall/winter it continues to photosynthesize (there isn’t any research that I could find about temperature conditions required for them to photosynthesize), but research on other trees with significant levels of bark photosynthesis (as with aspens) suggest that bark (or stem recycling) photosynthesis might account for as much as 15% of the energetic budget for the tree across the whole year! Other research on beech (source) has shown that increasing bark temperatures can exponentially increase bark photosynthetic rates outside the growing season. So we might expect bark that absorbs more light to be found in understory, shade tolerant trees (like beech, striped maple, etc) that have photosynthetic bark even at maturity.

We also see a similar pattern in trees like pin cherry (or young paper and gray birch, see image above) where the bark is dark, but more importantly it is a deep reddish purple. The color is the results of anthocyanins, an important class of pigment molecules that shield a plant from harsh UV radiation (same as the red colors found in fall foliage). As the bark thickens, anthocyanins are no longer required to protect the cortex tissues, and, at least on the birches, the bark matures into a lighter color.

I wonder also if dark bark might function similar to dark fur and feathers in some animals. On turkey vultures, the dark feathers help kill parasites by heating up in the sunlight. There’s a general trend in the patterns of animal colors where species in humid, tropical climates tend to be darker than in arid climates (this is called Gloger’s Rule). As with turkey vultures, the idea is that darker colors heat up more, which can help to kill parasites, which are a bigger problem in humid, tropical areas. This might also be true for trees with darker bark, where the heating affect kills off ectoparasites. I’ve reached out to the authors of this paper on Gloger’s Rule in plants (plants in hot, dry environments, like the desert, tend to be white and downy to help reflect sunlight), and will follow up if I get a response.

In the next post, we’ll look at cases in which having cooler bark is advantageous.