Saturday, February 13, 2010

Nature Notes: Tree Foam

Tree foam looks like bubble bath.
There are many reasons to go hiking during the rain: the solitude, wildlife, scents, and vivid colors to name a few.  But at the top of my list has got to be tree foam.  Also referred to as "stemflow", when rain water drips down the trunks of trees, it picks up impurities and forms bubble-bath looking suds at the base of trees.  Sometimes, the pattern is localized, so that one one or two trees in the forest will have the bubbles.  We've also seen tree foam at the base of nearly every tree in the forest.  Tree foam forms in any season, but seems to be most dependent on the length, duration, and intensity of rainfall.  I find tree foam absolutely delightful and one of the special treats of hiking in the rain.

My Maryland hiking partner and I noticed this tree foam when we were hiking several years ago, and were unable to find an explanation.   For a long time, I would ask everyone I hike with if they'd ever seen tree foam.  I got a lot of blank stares.  One hiking friend actually had seen it, and she calls it "tree spit"!
I finally found some literature about "stemflow" which provides a good scientific description of the process:
As I understand it, rain water dissolves stuff from the treebark as it flows down the stem of the tree.  This changes the surface tension of the water, so that when it drips down towards the base of the tree,  air is introduced due to the turbulence of the water, and foam is formed because of the altered surface tension.

Foam isn't just restricted to trees either.  There is also "rock foam" and one time we even saw "trail foam" as the water was flowing downhill on a leaf-covered trail.
Closeup of the bubbles
On every tree


  1. Fascinating! I always thought bugs did that!

  2. Hope you researched by now that this is a bacterial infection called slime flux