How many bryophytes will you find? Perhaps 16 or so mosses with 10 common ones and about 5 liverworts, 4 of which will be fairly common.
The BIG 3 bryophytes of tree trunks in our area are:
Ulota crispa, Platygyrium repens and Frullania eboracensis. These three are so common, that if you learn them, you will know what grows on tree trunks 75% of the time!
There are several different growth forms that you will notice:
Cushion formers -these are many small upright plants crowded together to form either dense or loose cushions. Examples of these include Ulota crispa, Dicranum montanum and D. viride and the Orthotrichums.
Mat formers - these are those that form low, dense mats tight to the trunk and include such bryophytes as Platygyrium repens, Hypnum pallescens, Frullania and Radula. In moister areas or on large, old trees (often maple) you get a looser, deeper cushion such as Anomodon attenuatus (although this moss is more often found at the base of the tree, sometimes it can creep up quite high).
Finally you have the Shelf-like form, seen in such bryophytes as Neckera pennata, Leucodon andrewsianus and the liverwort Porella platyphylla, where the plant grows in overlapping shelves and sticks out away from the trunk of the tree.
There is also a difference between which species you might find on a tree depending on the type of forest you are in and what kind of tree you’re looking at. Species on apple trees in an orchard will be different from those found in rich beech/maple woods or drier oak/hickory woods or an old willow next to a stream. Trees found in old growth woods will also have a different community of bryophytes compared to those in second growth.