Also, I just noticed that those trees which looked like birches are, according to the template name, supposed to be beeches. So maybe we could write up a possible art task on the forums or trac to create birch trees.

Call it fungi or mushrooms? I suspect the former would be more in line with the flora and fauna directory.

svn mv binaries/data/mods/public/art/actors/flora/mushrooms binaries/data/mods/public/art/actors/fungi

Yeah sure. will change it if you like. Not sure if it's worth it, nor if that's worth raising a concern either. Would it make more sense to move it to the prop folder ?
See below.

Also is it intended that flora only contains the trees subdirectory while the bushes like bush_desert_a.xml remain in art/actors/props/flora/?

Not really as both of those are by definition props. Would such a change be worth it. I can't tell.

Also, I just noticed that those trees which looked like birches are, according to the template name, supposed to be beeches. So maybe we could write up a possible art task on the >forums or trac to create birch trees.

The task already exists in the update post here : https://wildfiregames.com/forum/index.php?/topic/17730-art-dev-task-tracking/ in the flora subsection.

I've added them to Hellas, but I didn't notice them yet (can be a placement bug too).
Hopefully we can add them to Danubius for the release (maybe @smiley volunteers?)