The bubbles you see is are the formation of a crude soap on the bark of a White Pine. During dry periods salts, acids and other particles from the air coat the surface of the bark. When it rains, these mix with the water and form a solution. The foam is from the agitation of the mixture when it encounters a barrier (bark) during its flow toward the ground.
As to why this occurs primarily on pines, botanist Ken Sytsma states, “Pines produce a whole array of natural hydrocarbons for herbivory defense. One set of these is “pine tars” that have been used in the past to make soap. As precipitation works its way down trunks of pines, they accumulate these compounds. What you may be seeing is natural pine soap in the making.” (Thanks to Brenda & Steve Hillier for Mystery Photo idea.)
There are roughly 90 species of oak trees in North America, several of which can be found in New England. (Eastern White, Northern Red, Eastern Black, Burr, Common Chinkapin, Swamp White, Pin, Chestnut, Bear, Scarlet and Common Post). When identifying oaks, several characteristics, such as buds, bark, branching pattern and leaves, can be used. Most Northeastern species of oak have lobed leaves, with the lobes deep or shallow, pointed or rounded.
One thing all oak leaves have in common is their variability. Even on a single tree, you can find leaves of widely differing shapes. One reason for this is that the amount of sunlight that reaches them affects their shape. Leaves that are shaded are not only often larger than those that are bathed in sunshine, but their lobes are far more shallow. Both of these traits maximize the intake of sunlight. Canopies of oaks have a larger proportion of small, deeply-lobed leaves than lower down on the trees, where you can often find relatively large leaves that appear to lack lobes completely. The two pictured leaves come from the same Northern Red Oak. Can you tell where on the tree they probably grew? (Thanks to Penny March for post idea and leaves.)