Hi Tim,
You are right that most of the large urban areas of Canada, being in the South, will be pretty much done with winter by March. For easily accessable tree-scapes I would recommend Algonquin Park which is a great place to explore with camping in mind and is a two hour drive north from Toronto. There would probably still be some snow in the trees in March. For truly spectacular trees, I would recommend the Queen Charlotte Islands. There are parks there as well that make camping easy for everyone but there is a lot less settlement on those islands, some planning is required. If you consider the Queen Charlotte islands at all, where, incidently you will definitely not see any snow, you can also consider going on to the Yukon. The two most accessable "gateways to the Arctic" are Whitehorse in the Yukon and Churchill in Manitoba. From Whitehorse, there is a very good highway north to Dawson city. The terrain is mountainous becoming more so, the farther North you go. There will be lots of snow - L O T S . From Dawson there is a gravel highway that is the only highway in the world to cross the Arctic Circle. About a three day trip in a campervan from Dawson to Fort MacPherson in the Arctic. There are very affordable daily flights from McPhoo (Fort MacPherson) to Tuktoyaktuk on the Arctic coast. Open vistas with loads of color and drama and lots of snow, will abound all the way north from Whitehorse but in very mountainous country.
If you go to Churchill, the countryside is mostly very flat. For sure there will be LOTS of snow here in March also. The only way to get to Churchill though, is by train (SLOW train, three days from Winnipeg) or by air from Winnipeg. There are no Highways in or out of Churchill but there is a lot to see and do around town and there are lots of flights to the high Arctic from Churchill. If you do not go to the Yukon, the place to see in the Arctic is Baffin Island. It is geologically younger than the coastal areas around Churchill and that means a profound difference in the look and shape of the rocks and formations.
Here is a sample of work by a local Toronto Photographer that is worth checking out.
For something a lot less ambitous but a lot of fun, I recommend a trip from Winnipeg west to the Cypress Hills. You could easily rent a campervan or something similar in Winnipeg and either go straight West to Cypress Hills (only 18 hour drive if no stops) or divert to the northwest to wander through the Riding Mountain and Turtle Mountain parks. You would not see much snow but a good variety of mixed forests, open prairie, small towns, rugged rolling terrain (they are not really mountains

) and when you get to Cypress Hills, some truly spectacular vistas and variety.
Whatever you plan on, let me know. I will be doing a trip of some sort myself at about the same time so maybe we could catch up briefly along the way somewhere.
Tim R