Carmella,
Interesting question. I have my favorite teas, so there are a lot of things I've never tried, but that seems like a good research
project to try making tea from all the different tree and shrub leaves in your area (after a bit of research).
I would approach the issue by families. For example:
Pine family (pines, spruce, fir, etc.): Use the needles of any species for a vitamin-C rich tea. Enjoy it thoroughly once in a while, but don't overdose on pine resins. Compare the Pine family with the Cedar and Yew families in Botany in a Day, so you get the right trees.
Willow family (willows, cottonwoods, aspens, poplars): Okay for tea. May have some super mild analgesic properties. Don't over do it (true for anything).
Rose family trees (cherry, pear,
apple): The leaves may contain some cyanide, like cherry pits or apple seeds. A little bit won't hurt you, and cooking it (making tea) destroys the cyanide, as does drying. Use in moderation.
Walnut family: I would try that.
Olive family (olive, lilacs,
ash): I would try tea of lilacs and ash and research other species a bit more.
I hope this helps!
Sincerely,
Thomas J. Elpel
http://www.GreenUniversity.com