It seemed a fair assumption that darker bark would have a higher temperature than lighter bark, and this certainly seemed to hold true. All of the darker barks had temperatures of 133ºF or higher. The lowest of the dark barked trees, pin cherry, was one of the darkest barks but it compensated for this by having glossier, which reflected more light. The lightest bark, paper birch which is effectively white, has an extremely high albedo and was “only” 27ºF above air temperature.
Ridged bark also seemed to have a cooling effect on the trunk. And interestingly, ridged barks were also the most variable within a log, with temperatures varying by as much as 8ºF within a ¼” patch, likely due to shading caused by ridges. I imagine there are three reasons that ridged bark is on average cooler: microclimates caused by the shadows in the “valleys”, the surface of the ridges are at an angle to the sun rather than perpendicular to the sun (the bark doesn’t receive directly sunlight and the incoming sunlight (insolation) is dispersed over a larger surface area), and, much like a radiator sheds heat, so too would the ridges on bark help shuttle heat away in the breeze (I did try to shelter the bark from wind to minimize this factor).
I was also surprised at how quickly the surface of the trunk could cool down when shaded, even for a few moments or when a gentle breeze blew by. I took the hottest bark, staghorn sumac at 155ºF, and cast a shadow on it. I measured the temperature continuously over 30 seconds and the bark dropped a staggering 36ºF to just 119ºF!
Next week I’ll dig into the adaptive significance behind the different bark colors.
Figure 2: Surface temperature of bark of different trees, but this one coded by bark texture; ambient temperature was 91ºF). Yellow dots represent smooth bark, purple is rough, and blue furrowed.