What is the symbiotic relationship between epiphytes and trees?
It depends, of course, on the particular epiphyte and the particular tree. While there is not necessarily any symbiotic relationship between epiphyte and host tree, such relationships are known to exist. For example, many of the species of moss that so often grow thickly on the bark of bigleaf maple (Acer macrophyllum) trees in the Pacific Northwest can fix atmospheric nitrogen; the trees grow aerial roots into the mats of moss that grow upon them and absorb the nitrogen fixed by the moss. See http://academic.evergreen.edu/projects/ican/education/TRFwebsite/PDF/TRFcurriculumPart2.pdf .
