Thin, smooth bark makes Madrone tree trunks seem cool to the touch
I may need to start petting trees more often. I’ve long known of trees that had particular colors and smells in their leaves and trunks, but I only learned in the last week that some trees hold surprises for your finger tips to discover. The tree in question was a Pacific Madrone (Arbutus menziesii), and was actually hard to miss thanks to its striking red bark peeling off the trunk. The surprise was that the tree was cool to the touch, which is why it’s sometimes called the “refrigerator tree.”
For something cool to the touch, Madrone trees need lots of sunshine to thrive. If conditions are right, they can grow to be nearly 100 feet tall, but at smaller sizes Madrone trees can be mistaken for some of their red-barked relatives, like the Manzanita (Arctostaphylos). Both plants’ eye catching bark grows thin and smooth, but this trait is especially striking in mid-summer when Madrone tree bark starts to peel off the trunk. At that point, a quick touch makes it hard to ignore how much cooler these trees are than the surrounding environment.
Cold or just conductive?
Except that they’re not really cooler. The trees’ temperature is likely the same as any of the other similarly-sized plants that grow near them, just like a paper book is the same temperature as a metal keys sitting in the same room. With sufficient time, the temperatures equalize, but when we touch the metal, or the Madrone trunk, it feels colder. This is because heat is more easily transferred to certain materials than others, and when heat from our hand is conducted away we perceive it as colder. Now, a Madrone tree obviously isn’t metal, but that thin, smooth bark isn’t as good an insulator as the rough, corky bark that you find on most trees. Your hand is able to come into more contact with the smooth surface, and the sap and fluids flowing inside the trunk can then wick your body heat away.
Even if refrigerator trees aren’t actually colder, their unusual bark obviously still stands out from that of their neighbors in the forest. The thin, peeling bark that exposes the trunk may have originally evolved as a form of defense. By shedding the outer layer of bark, the tree can dump any fungi, mosses, lichens or other parasites that tried taking up residence on the red wood. The red itself is likely another form of defense, as the tannins that make up that coloration would be bitter and possibly toxic to animals that might want to munch on the tree, not unlike the colorful bark found on rainbow eucalyptus. It’s good that the peeling is helpful to these plants, because now that they know about these chilled trees, it’s going to be hard to keep my kids’ hands off them.
Source: The Refrigerator Tree by Steve, Nature Outside