Question
Why is there foam on my backyard tree after it rains?

Answer
With heavy rain, water works its way through the bark, especially towards the tops of the trees, to the wood underneath. As it runs down towards the base, between the bark and the trunk, it collects some of the compounds that the tree uses in its growth. Then, when it gets lower down, the build-up then forces the now somewhat soapy water out through the bark to the surface, where it tends to foam a bit.