What's with the Foam I Saw on the Trees Yesterday?

A park visitor emailed us this question today:

Hi, I was walking in the woods behind my house in Fairfield, Vermont today and I noticed white foam on a number of our trees especially most of the yellow birch and also some hemlock. On some of the trees there was quite a bit of this foam. Is this something that we should be concerned about? I don't remember ever noticing it before. Thank you for your help.

Rebecca Phelps, our Conservation Coordinator, replies:

You are very observant, and what you were observing was Chemistry in action! What you saw on the trunks of these trees was the formation of a crude soap. It has been really dry for awhile in Vermont, and when it is dry particles from the air deposit themselves on the surface of tree bark (like dust). This dust includes bits of salts, acids and other particles in the air. When it rains, these particles dissolve into a liquid solution. Soap is essentially a chemical mixture of salts and acids formed into a molecule, and that is what happened when the water ran down the trunks of the trees. The primitive soap then foams as it moves down the trunk and hits a rough patch. You can also see this phenomenon on paved roads after a long dry spell followed by rain, where the tires on cars are what stirs up the soap bubbles.

This phenomenon was probably more pronounced than ever yesterday because of the amount of rain we had after being quite dry for some time.

Thanks again for sharing your discovery, we love exploring the mysteries of the woods in Vermont!