The first time I noticed tree foam I was hiking in the mountains of Virginia. It was Fall and delightfully cool, and to make it better it was raining. A hike in the rain can be terrific. You can give yourself over to the rain. The rainfall creates a smaller world with you at the center. All around you is the random fall of the drops. They drum on the leaf canopy above and then drip down onto the understory below – including you. The rain and the trees and the understory have created a universe that is bounded by a curtain of the rhythm of the rain. As the drops beat on the leaves they create a barrier against the outside world of noise from cars and planes, and from the general hub-bub of humanity. It becomes just you in the much smaller, much cleaner universe that is centered on your hearing. You can turn and look around and look up and see the dimensions of your new world. And for a while you can enjoy the true uniqueness and quietude of a world that is all your own.
As you look around, you may see what looks like sea foam building up at the base of a tree. It’s a small but growing mass of white bubbles right at the roots. I mainly notice it on the pine trees. And the foam is not just at the base. The foam collects at the base, but it can be seen coming down the tree as pale, flowing streaks of rain water. The mass of foam at the base of the tree billows and grows at more than one spot. As I look further, I realize there is foam at the base of several of the trees. What is it? Is it a disease? Is it a fungus? Not necessarily. It’s a common occurrence brought on by the chemistry of the tree, the roughness of the bark of the tree, and the surface tension of the water that usually holds the shape of the drops as they roll down the tree.
When it rains in the woods few if any of the drops reach the ground directly from the sky. The rain that falls directly to the forest floor by-passing the canopy of the trees and the growth of the understory is considered “through-fall”. Some of the rain falling into the canopy is captured and remains on the leaves and branches. Other droplets flow down the tree’s exterior to reach the ground. The drops that do not flow all the way to the ground are given up to the atmosphere though evaporation. The water droplets that roll down twig and branch and then flow down the trunk in numerous little rivulets is “stem flow”.
As the water’s stem flow passes over the tree’s bark it picks up tiny bits of organic material and the chemical residue from the surface of the tree. These bits create a chemical change in the water’s molecular bonds which reduces the surface tension of the water droplets. The reduced surface tension allows more air to become entrained in the water. The droplets gather into larger rivulets and flow over and around the bark of the tree. This acts like waves in the ocean or rivulets in a stream and exposes more and more of the surface of the water droplets to the air. This stirring action creates the foam that can be seen flowing down the trunk and which accumulates as the mass of bubbles at the base of the tree.
The rain water that reaches the base of the tree may run off on the surface of the forest floor to be absorbed into the ground where it may be taken up by the tree’s root structure. Other run-off that is absorbed by the ground will infiltrate further downwards to mix with the water table and perhaps enter a stream that flows down towards the ocean. Eventually, through root uptake and transpiration by the tree, or through evaporation from a stream or the ocean, the water is taken back up into the atmosphere and from there to fall again as rain on the joyful hiker.
Information on tree foam may be found in a NOAA site, www.oceanservice.noaa.gov/facts/seafoam.html.
A similar article can also be found at a terrific hiking blog, http://ramblinghemlock.blogspot.com/.