Birch tends to be brittle once thoroughly dead. Any reaching the surface should crack under the impact of a hammer or a shovel blade. They're also going to rot and compost. Any that can be broken up a bit will speed the process.
Junipers (dunno if it's too wet there for it) are pretty tough and come in a wide variety...from creeping type ground cover to arching shrubs to rounded short trees to tall slender cones. Of course, getting it out later( if you decide it's not right) is a bigger problem. Arborvitae (spelling?) Is similar to juniper as an evergreen, and also comes in an array of shapes and sizes.
Some firs can arc over creating overhead cover and shade while maintaining space underneath for ladies to relax in, hide and climb.
What about some sort of nut tree? Hazelnuts/filberts are fairly small, I think...
Beech (not the same as birch) get tall but produce edible seeds (human edible, also). Chestnut could be fun too.
Some unusual ideas