While many real estate laws vary based on state definitions, trees seem to be one aspect of the law on which all states can agree. When it comes to trees on property lines, the laws are clear.
The location of the tree trunk determines ownership. If a tree trunk stands solely on one property, that individual owns the tree. However, if the tree trunk is on both properties, it is a boundary tree and both neighbours assume equal ownership. Trees with branches encroaching on a property with items other than a trunk--limbs, leaves, fruit and so on--are not considered boundary trees. The tree must be planted directly on the property line to be a boundary tree.
Tree Maintenance and Removal
In order to remove a boundary tree, both owners have to sign an agreement for the removal and share in the cost. The owners can split the cost and pay the tree-removal company directly or one owner can reimburse the other. Regardless, no removal of the tree or maintenance of the tree is the responsibility of one party over the other.
If a boundary tree becomes diseased or dies, it is no longer a requirement that both property owners agree to remove the tree. One owner can have the tree removed despite objections from the other in this case only. In this case, the owner who opts for the removal is the responsible party for payment, but can bill the neighbour or take the neighbour to small claims court for half of the balance.