Bava Metzia, Chapter 9, Mishnah 9. The Mishnah before us discusses the imbalance created when a tenant farmer occupies a field for only a short period of time: he depletes the soil in such a way that he produces a large profit in one year, while the landowner is left with an impoverished field whose value has dropped significantly - due to the overuse, misuse, or unfair use that the tenant farmer made of it during that short period of time.
The two examples in the Mishnah:
Growing flax - Flax greatly depletes and exhausts the soil. Planting it is indeed profitable in the first year, but afterward the field requires a prolonged period to recover, and the Mishnah uses a measure of seven years.
Chopping trees for beams - Even when the branches of the tree are chopped for beams, it takes about seven years for them to grow back.
Therefore, it is not fair that a tenant farmer should occupy a field for a year or two, chop the trees, and leave, while the owner remains with no wood on the trees.
Leasing for a few years:
"Hamekabel sadeh mechaveiro leshanim muatos" - The farmer leases the field to work it for a few years, meaning fewer than seven. Regarding this, the Mishnah establishes two laws:
"Lo yizraenah pishtan" - He does not have the right to sow flax in it, since this sowing depletes the field unfairly.
"Ve'ein lo bekoros shikmah" - He also does not have the right to chop the branches of the sycamore tree, for since he will leave within a short time, the owner will be left without wooden beams.
Leased for seven years:
"Im kiblah heimenno lesheva shanim" - When the tenant farmer committed in the contract to occupy the land for seven years or more, the law changes:
"Shanah rishonah zor'ah pishtan" - He is permitted to sow flax in the first year. Even though this sowing demands many resources from the soil, by the time he leaves - after seven years or more - the field will fully recover.
"Venotel bekoros shikmah" - He is permitted to take the sycamore beams in the first year. He may not cut down the trees themselves, since these will not grow back; but if he chopped in a way that allows them to regrow, the tree will return to its former strength within seven years, and when he leaves, the branches will be as they were, and the landowner does not lose from his principal assets.
In summary: The Mishnah balances the profit of the tenant farmer with the preservation of the landowner's field principal: in a short lease, of less than seven years, he is forbidden from sowing flax and taking sycamore beams, because the field will not have time to recover by his departure; while in a lease of seven years or more, both are permitted to him in the first year, since by the end of the period, the soil and the trees will return to their former strength.