Bava Batra 81 - Buying Trees
If someone purchased two trees in the field of his fellow, he has not acquired any land - he has merely purchased the rights to their produce. Thus, if a tree dies, the buyer takes the dead wood, but he may not plant a tree on that site. Rabbi Meir, however, says that he has acquired land.
Therefore, if the branches of the trees grew until they extended over the seller's land, the seller may not trim them, even if they block the sunlight and thus damage his crops. Since the buyer gets no land, it is understood that the seller pledges his own land to provide for the needs of the tree.
If someone purchased three trees, he has acquired land, since he surely intended to buy a field of trees. Therefore, if the branches grew over the seller's land, the seller may trim them.