I'd consider a red buckeye (Aesculus pavia) It's a very nice, small tree that isn't commonly grown because it browns out with normal moisture.

sorry i didn't see this til now. yes madgal is spot on; terrif suggestion and one of my top 3 'shrubs'.

as to styrax, styrax obassia is more hardy than japonica(we lost our styrax j. twice but probly too exposed; new one this yr is v sheltered). obassia is in its 2nd yr;growing in full sun til later in afternoon. growing like gangbusters.sorry i know not about styrax roots.

Halesia (my fav is UConn Wedding Bells) is also a wonderful lovely smaller and tiered, graceful flowering tree w/ coool seed pods after flowering.

Not small trees but just fyi, dogwood bushes really like moisture, as does cephalotaxus, button bush, and nyssa (fab BRIGHT red fall color but i have never grown it)and tiger eye sumac (cool yellow cut leaves and red stems.)

you know, you have a great resource in the Tower Hill Botanical garden-i would really call them and ask your styrax question- for the most accurate info.Also, Andy Brand at Broken Arrow Nursery in Hamden Ct. is a VERY helpful expert; he's the guy I always call w/ questions like yours.

If it turns out that you are told you would have success w/ both aeschulus and styrax in that spot, i would choose the aesch. pavia, hands down. because you get 2 seasons out of it w/ the flowers and the fruit(which people in fall- think are pears!)The drawbacks are: 1) the winter moths MURDER these guys. and 2) they lose their leaves very early.
best,
mindy
www.cottonarboretum.com/
cottonarboretum@comcast.net

aeschulus pavia May flowers
{{gwi:270485}}From CARB SHRUBS



aeschulus pavia Fall fruit
{{gwi:270486}}From CARB SHRUBS

just came across this
best,
mindy

styrax obassia in june(shasta viburnum visible lower left) {{gwi:270487}}From CARB TREES

i forgot- heptacodium is a wonderful small tree- but really just for its amazing autumn flowers and its shreddy bark.
best,
mindy