Would have answered sooner, but didn't see the post!
Trees in Canada is probably the gold standard overall.
www.bookcity.ca
Obviously, this includes trees from the west which are not found in Ontario.
I included the Book City link above, but you can get it through Chapters/Indigo as well. However, stock seems quite limited, I only see a few floating around in York region.
So you'll want to check on timing.
This book (Trees of Ontario is also good, but not in print anymore so you have to find an old one online, its also a bit dated, being from 2001)
Note that I haven't dealt w/the site below, just including it so you can see an image and description of the book.