“Basically, it’s suds,” said Alison Pearce, deputy director for programs at Nature Forward, formerly the Audubon Naturalist Society in Chevy Chase, Md.She explained that dust, pollutants and other particulates floating around in the air settle on a tree. The tree also exudes its own chemicals. When it rains, all that material is mixed together and flows down the tree, from the leaves, along the branches and down the trunk.“It happens more often with trees that have deep channels in the bark,” Pearce said. These include oak, tulip poplar and hickory...It’s the result of what naturalists call stemflow mixing: the aerating rush of water down the tree.
Most "foam" I see in nature comes from spittle bugs, though I wouldn't expect to see them on a tree trunk. This phenomenon was new to me. You learn something every day.
When I lived in San Francisco—where it doesn't rain for six to eight months of the year—this was a common sight on eucalyptus trees when the rains would come and rinse a season's accumulation of exuded oils and the dust trapped in it. It would accumulate at the base of the trunk, too, in a fragrant pool of suds. (I suddenly miss walking through Holly Park in the rainy season.)
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