Had some excitement this afternoon when a friend brought me a branch of a tree from Regina to identify. I recognized the novel presentation of the berry/seed but could not recall where I had seen it before. However, my wife soon turned it up in "The Native Trees of Canada" by R.C. Hosie as a Basswood, Tilia americana L. Further research revealed it is found in the deciduous forests from New Brunswick to South-eastern Manitoba. I located a list of vascular plants of Saskatchewan from the University of Saskatchewan at Saskatoon that indicated the occurrence of the tree in Sask is extremely rare. I am told that there are three trees, 2 on a road divider and one in the yard known to my friend. The tree owner became curious because she has never seen the tree in another location in Regina. I really feel chuffed, as they say !
We had one in our farmyard in Manitoba - come to think of it, I never saw another in anyone else's yard nor ever found it in my (lacking) explorations of southeastern Manitoba.
Tilia are commonly planted here. If your reference(s) does not include foreign species and garden hybrids, tell how to distinguish them all from one another then maybe you are calling one of these T. americana by mistake - not that they couldn't be that species, which is common in cultivation. However, regional guidebooks do tend to ignore garden forms or be so limited in geographic scope that many groups are represented by only a few species at best. If one or two Tilia are all that fall within the scope of the guide(s) you used then it will not delve into how to tell these from others in the same genus.
Tilias are very common where I live as well but Tilia americana is the only linden that survives the Canadian prairie winter. And then very rarely east of Brandon. I would love to hear from others like Dan who have seen this tree in the three prairie provinces.
Cultivar selection in zone 2b will be extremely limited - pretty much what is available on this page from Jeffries Nursery. If the age of the tree is moderately old, it would be easy to eliminate all but a few choices.
My friend estimated the height of the tree at about 25 feet/8metres but I did not get an estimate on the trunk size.
Almost certainly Tilia americana then. The one in our farmyard was about 6m high 13 or so years ago, should be about 8m now, I'm guessing with an age of ~35-40 years.
Some of the Asian species occur in very cold areas of eastern Siberia - I'd guess that Tilia amurensis is also hardy to zone 2 (and T. mongolica too, but that's very distinctive in leaf shape)
http://www.cnr.vt.edu/DENDRO/DENDROLOGY/syllabus/factsheet.cfm?ID=88 This page has a nice description that matches the branch and observations I assessed.
Tilia americana is not the only basswood hardy enough to survive in Regina - Tilia cordata is also planted there, even as a boulevard tree, so it is obviously quite hardy as well. Various selections of hybrids between the two species are also used there (as in the boulevard of the avenue near our old house). We lived in Regina for many years, and spent lots of time enjoying Wascana Park and the tree plantings therein. Greengoose, if your friend is interested, I'd suggest she study the trees in Wascana, as I seem to recall specimens of both T. americana and T. cordata there. Some of the old trees there are even labelled (or at least were at that time). Unfortunately, there does not seem to be a good "tree list" available for Wascana Park - on the internet, anyway.