Are Bubbles Trouble for a Tree?

One of the students in a class I was teaching about rocks at Herrontown Woods noticed something decidedly un-rocklike on a black oak we were passing by.

Foam was collecting at the base of the tree. Might it indicate some malady like decay or disease?

The bubbles, clustering like the frog eggs now being laid in nearby vernal pools, reached at least ten feet up the trunk. There was, however, no obvious wound in the bark that would suggest sap was emerging and interacting with the water flowing down the trunk from the slow morning rain. 

Not surprisingly, what seems like a very curious and rare phenomenon turns out to have been written about many times over on the web. Soap is made of salts and acids, and in this case the salts in dust, accumulating on the bark during a dry spell, combine with the acidic sheddings of the tree itself. Rain generates "stemflow" on the trunk, and as the water drips down over rough bark, absorbing these salts and acids, bubbles are formed. The tree is perfectly fine.

If we had had time, we could have examined other trees to see which ones and which kinds were collecting foam at the base. Smooth-barked trees like beeches likely would not generate sufficiently turbulent stemflow to create bubbles. Perhaps tilted trees, on which the stemflow concentrates on the lower side as it flows downward, would have more bubbles. The pace of rain may also be a factor. That day, the rain was steady but gentle. 

One reason this bubbling seems so rare is that we don't usually pick rainy days to walk in the woods. It's our presence, not the bubbles, that are rare.

While the bubbles were heading down the tree, a couple earthworms were heading up, apparently to escape the too soggy soil.

Speaking of bubbles, here is a post with photos of bubble patterns in the ice of Lake Carnegie during the winter of 2015, back before our winters turned liquid.