If somebody rents a field from his friend for a few years, which we understand to mean less than seven years, he is not allowed to plant flax in that field. Flax weakens the land, takes away all of its strength, and the land does not recover until the end of seven years. Since he rented the land for less than seven years and is going to return it to the owner in that short amount of time, he would return it to him in a weaker state than he received it. Therefore, you are not allowed to plant flax in a field you are renting for a short term. Likewise, a person who rented a field with sycamore trees would not be allowed to cut beams of wood, branches off of the sycamore tree. A sycamore tree takes a very long time to grow, and it would take seven years to grow back fully. If you cut the branches off, they would not grow back by the time the land was returned to the owner. So that is all about the short-term rental.
If a person had the field for longer than that—say, if you rented it for seven years—Shanah rishonah yizra'enah pishtan ve'yesh lo bikorot shikmah. During the first year of the rental, you'd be allowed to plant flax because by the end of the seven years, when you're about to return it, the soil would have its benefit fully restored. It would return to its normal strength when you gave it back to the rightful owner. You'd also be allowed to cut down beams of sycamore tree. By the time the rental period was over, the field of trees would have grown back, and you would be giving it back to the owner in its original state.