1. A person should not plant a tree within four Amos of his neighbor's field.
2. This law (#1) applies only in Eretz Yisrael.
3. One is not allowed to cut down a fruit tree.
4. Rava bar Rav Chanan refused to cut down his fruit tree (because of the law in #3), even though it was causing damage to Rav Yosef's field.
5. Ula: It is forbidden to plant a tree within 16 Amos of someone else's field.
A BIT MORE
1. This is so that he not have to plow into his neighbor's field when he plows the ground next to his tree.
2. This is because it was customary in Eretz Yisrael to use a plow that plowed four Amos. In Bavel, where they used a plow that was only two Amos wide, one needed to distance his tree only two Amos from his neighbor's field.
3. If a person cuts down a fruit tree, he may die. Rebbi Chanina said that his son, Shichechas, died young because he cut down a fruit tree.
4. However, he told Rav Yosef that he could cut it down himself.
5. Ula therefore said that one may not even bring Bikurim from such a tree. Since the fruits received their nourishment from someone else's field, the fruits are considered stolen.