Vegetation: Liability for a Tree That Falls Across the Property Line

I'm curious here as to how/when/where responsibility is drawn for a tree that falls across the property line.

Let's look at a couple of scenarios all starting with a healthy tree:
1. Tree falls across property line onto neighbors yard without hitting anything. Each party is responsible for cleaning up their own property, correct?

2. Tree falls across property line and lands on neighbors building, so it's not on the ground but supported by building. Is each party still responsible for their own property and damages?

3. Current OP situation where tree fell across property line and is now leaning against a tree across property line. Now, it's suddenly all the responsibility of the property owner where the tree came from? Isn't this the same as the building situation? Does it depend on the resulting 'angle' that the tree is laying, ie if the tree is almost horizontal but still caught in the lowest branches/crotch of the neighbors tree is then the neighbors problem but if it's almost upright it's the original owners problem?

Not trying to be argumentative here, just trying to understand how responsibility is determined for these situations and why being caught up in another tree seems to change what I thought was responsibility for fallen trees?

Vegetation: Liability for a Tree That Falls Across the Property Line