What causes tree lines?
There are a bunch of possible answers;


On my own property I have such a line.  Turned out it was an old fence line.  One side was farmed intensely for 90 years, the other side left in bush.  The farmed soil is enough different that it doesn't grow good trees.  Even the grass is different.
Another place I have an abrupt line of willows.  That one turns out to be water level from the local creek.
A third one has an abrubt chaange in types of grass.  That one is caused by a clay seam coming to the surface.  Uphill the surface soil is sandy.  Water hits the clay, and moves downhill, coming out at the grass line.
I've seen another one, in the mountains, that I think is due to cold air drainage.  The valley floor and 20 feet up the sides are bush.  I think the cold air settles and kills young trees.