Shun the Sun

Mosses will grow on the north, south, west and east of trees (and, indeed, buildings), wherever there is enough moisture to sustain them - and that depends on various factors, such as prevailing winds, and not exclusively on shade. Besides, if the south side of the tree you're looking at happens to be partly shaded - perhaps by another tree, this being, after all, a wood - moss will colonise it readily.