Tree trunk discoloration
It looks like a water mark. Was the area flooded?
No, never. Our house is about 20 yards from the neatest tree. And there are trees at different elevations that have the mark the same distance from the base. You will notice the one further down the hill has the same amount of dicoloration as the ones on the hill.
Snow was probably to that height and that would be my second speculation, but I haven’t seen that kind of snow effect and I do look at lots of trees. Snow may have shielded the bark from the bitter cold of last winter.