Earth Notes: Tree Bark Thickness

Jun 21, 2017

Next time you pass a mature ponderosa pine, notice its broad plates of orange-red bark etched with black crevices. That thick, puzzle-shaped bark helps the tree survive moderate forest fires by protecting the inside of the trunk from overheating; severe fires though can kill even the thickest-barked trees.

Ponderosa pine bark
Credit Michael Collier

A recent study of hundreds of trees worldwide found that ponderosas are just one of many species that have evolved thicker bark in ecosystems with frequent natural fire.  

Beyond fire resistance, bark can benefit trees in other ways. In cottonwoods growing along streams and springs in the Southwest, bark thickness and roughness varies a lot - with rougher-barked trees supporting more abundant and diverse lichen communities.  

Research at Northern Arizona University indicates that genetic variations are responsible for around half the variations in cottonwood bark roughness. Some cottonwoods possess genes promoting a shift from smooth to rough bark earlier in their growth. This trait allows lichens to establish earlier on the trunks of those trees than on same-aged individuals with smoother bark.

There’s even evidence that rough-barked trees provide habitat for a wider variety of insect communities—in turn encouraging more birds and mammals to forage there.

If forest fires become more severe, trees will have to adapt, or suffer greater devastation.