How can many faces be mapped to a texture? A tree model with leaves looks like (created by sapling addon):



Using unwrap and reset shows orientation issues like a leaves stem pointing to the outside instead of the branch.



Before UV reset the UV-map shows this pattern, some leave pairs are upright and some are rotated by 90°.



How should all the UV islands be rotated to match the images orientation? Is there a method that is less time consuming than manual select and transform?

I tried almost all options from Mesh / Select / Similar to unwrap at least the upright and rotated pairs of faces separately.
My advice is to switch your leave settings from Hexoganol to Rectangular. This will make single planes without cuts (extra edges) with 1 origin and it will unwrap as a single plane (like resetting the uv map). It is then easier to make adjustments for the direction of the leaves and will effect all the leaves in the same way (from the root). I don't know of any other workflow's for this situation.