Agree with White Ash as posted on other thread. Resin

Foliage-wise these do not look much like suckers from any stock I have seen used here. They do look like Japanese cherry seedlings - something I have seen numbers of coming up on more than one local site (where seed source trees were fully visible nearby). Try pulling a small one up and seeing if it pops out with its own small branching root system or if it turns out to be coming from a large horizontal root. One that leads back to a much larger source tree in the immediate vicinity.

Update: Didn‘t someone once say there are no failures in gardening only experiments? This was definitely an experiment. As it turns out, my balcony was far too windy for this sapling but I didn’t notice in time to move it to a more sheltered spot before some branches were torn off. I put it back in the ground. Fortunately the experiment will continue with a rhodie and different conifer that seem to handle the conditions better. I don’t know what they are. I found the conifer in a small pot in my Dad’s garden. It was about a foot tall at the time. The rhodie was planted stealthily a few months ago in an area of my apartment building‘s landscaping that contains several other (but different) rhodies. If the person had done a better job, no one would have noticed until it bloomed. Apparently that sort of thing happens periodically.

That is a very good guess! Those are native to this area. I will have to pay attention to the way it flowers. I do remember the leaves changing orange-ish in the fall. Could this be a young black willow?