Experimenting with trees
Last week I was out teaching the Science of Trees, and we had a great time tinkering with different ways of studying trees. Of the different tools I brought in to illustrate various adaptations of trees, my highlights was measuring the temperature of bark in trees using an infrared thermometer. While I should probably be writing this in December and not during the hottest point on our planet in the last 200 years (more on why next week), I thought I’d share the results anyways because it’s just so cool (pun intended)!
Setting up the study
I was building a bridge over a stream near our base camp and thought it would be a nice detail to make the railing out of logs from different species. While I wound up not going for a railing I did collect logs from over 30 species, all relatively straight and of similar diameter (about 3″). Turns out this also made for an excellent basis for a study on bark temperatures.
I lined the logs up against a garden bed, left them out for a couple of hours, and then used a simple infrared thermometer to measure both the surface and shaded temperature of each log (26 different species in total). I took the temperature at about 6″ off the ground to minimize cooling effect from both the grass and from the wind. To make sense of the variation in surface temperature, I also categorized each log based on its color, glossiness, and texture. You can see the full spreadsheet of data here.
Figure 1: Surface temperature of bark of different trees (ambient temperature was 91ºF). Green dots represent light colored bark, orange medium, and red dark.
What I learned
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.
