Unique corner posts for firewood shed?

Are you thinking something like this?

 

Wedges and sledges, you could split those larger diameter poplars in to more manageable pieces.
fresh cut poplar splits real easy. I've done it with some 16"-18" trees. Would make some pretty cool looking posts in my opinion.

 

I wasn't at first, but I like the idea! Maybe cut to 12'" lengths and cribbed. Like the corner of the picture below. Moisture would not be trapped then.

 

My wife's uncle had a a pretty large work shop/garden shed he had built with his father in the 70's that looked like the picture above. Also had a friend years ago that had a cabin with walls made of 10" long 4-6" rounds set in concrete. Looked great and was well insulated.

 

What your looking to do would work. Tulip is both a good and bad choice for it. Tulip is quite a stable wood in my opinion. Doesn’t check, twist or crack like crazy. That’s good.
The bad part is it rots near the ground. Old sawmill guy told me keep it 2 feet off the ground and it makes great barn siding.

So as others have said. The first piece should have a waterproof bond break from the ground.
The next sequential pieces I would cut large blind V shaped grooves into the end butts. Like old fashioned brick these are called frogs. A frog aligned top and bottom filled with mortar will create a dual key way that will stop lateral movement.
You could use enough mortar to create a visible joint or just fill the frogs and leave a line. As long as the cut was sequentially mated of course. Personally I’d go with a full joint. Raked deep,,,, bark on or off? You’ll lose it eventually.

Wanna get crazy go with a V cut like a roof. Solidly locked together and water would have a tendency to run out.
EDIT: on second thought forget the above idea. Any shrinkage with a v shaped butt cut would have a tendency to split the round. Not good

Biggest concern is rot.