Foaming at the bark

A good while ago, I noticed two trees with foam on them. It was raining, and that explains the water, but what makes them foam?

Oak:

^{Olympus Stylus 1s, 42mm, ISO400, f4, 1/60s}

Birch:

^{Olympus Stylus 1s, 42mm, ISO400, f4, 1/50s}

I know some insects can generate foam to protect themselves, like the meadow froghopper aka meadow spittlebug (Philaenus spumarius), schuimcicade in Dutch, but they don't usually live on trees and the foam looks different (also note the stumped ants):

^{Olympus XZ-1, 28mm, ISO100, f2.8, 1/320s}

So, what makes a tree foam? Any ideas?