Bava Batra, Chapter Two, Mishnah Fourteen
In mishnah fourteen we learn that if the branches of a tree encroach on the public domain, they too may be cut down.
1) If a tree stretches into the public domain enough must be cut away to allow a camel and its rider pass by.
a) Rabbi Judah says: A camel laden with flax or bundles of branches.
b) Rabbi Shimon says: Every tree [must be cut away] according to the plumb lines measure, because of impurity.
There are three opinions in our mishnah with regards to removing branches that overhang onto the public domain from a tree originating in private property. The first opinion is that they must be removed up until a height that will allow a camel and its rider to pass by. Rabbi Judah says that the branches do not need to be removed all the way to the height of the rider for he can duck. They only need to be removed to the height of the camels load which obviously cannot duck to avoid the branches. Rabbi Shimon says that all of the branches which overhang into the public domain must be removed because of the laws of impurity. According to the Jewish system of purity and impurity if a pure thing is overshadowed by the same object overshadowing an impure thing, the impure thing will convey impurity to the pure thing. In other words, if a bone which imparts impurity were to be underneath branches and a person were to be there as well, the person would be impure. In order to prevent such a situation, Rabbi Shimon does not allow branches to grow into the public domain.