Oh, thank you for this. I assumed the line was the river, but now that I think about it - I do have a few cedar trees (ones that were there when I started the town) below the river. But they're all on parts that are more northern. I bet that's the issue, as I tried planting a cedar tree again yesterday and it was dead today. I'll experiment again and try planting a regular tree in that spot now and see what happens.
If you have a chance, I'm interested to know if you were able to plant regular trees!