daf bit: Bava Batra 81

[cellio]

The mishna has been discussing the purchase of things that produce, such as beehives. On today's daf it talks about buying trees on another person's land. (This was, apparently, a thing.) If a man buys two trees on another's land, the mishna says, he does not acquire ownership of the ground beneath them. What grows from the tree trunk and branches belongs to him, but what grows from the roots belongs to the landowner. If the trees die, he doesn't have the right to replace them (because he has no land rights). If the trees grow large and threaten other things in the field (for example due to excessive shade), that's the landowner's problem -- he can't trim the tree.

If, however, a man buys three trees (by implication, together), the rules change. In this case he does have land rights and owns everything the tree produces, including from its roots. If a tree dies he can replace it (because he owns the ground beneath it). However, if his tree grows too large and threatens other things in the field, the owner of the field may cut the branches. (81a)

It seems that in the case of two trees he's buying "tree outputs" but does not have full responsibility for the trees themselves. The owner of the field provides the trees and is responsible for making sure they won't damage the rest of his field. With three trees, however, it seems that he's buying a tiny little plot of land and is responsible for preventing harm; if his tree threatens his "neighbor" (that is, the owner of the field), then said neighbor can trim the tree (like property law where I live; if my tree encroaches my neighbor, my neighbor can trim it back to the property line).