How are epiphytes symbiotic? I understand epiphytes are not parasitic. But how exactly are they helping the tree on which they grow?

It's important to note that not all symbiotic relationships must be expressed in terms of a positive-negative (parasitism) or a positive-positive (mutualism) framework. I would guess that most tree epiphytes exist in a commensal framework with their host trees. That is, the epiphyte has a positive interaction (it gets somewhere to grow), and the host species has neither positive nor negative effects.