Shevi'is, Chapter 4, Mishnah 10. In Parashas Shoftim the Torah forbids cutting down a fruit tree, and this is the prohibition of 'bal tashchis' - one may not bring the tree to ruin. However, if the value of the tree's wood exceeds the value of the fruit it produces, it is permitted to cut it down. This permission applies in the ordinary years, but not in the Shemittah year.
What is new in the Shemittah year:
In the seventh year, once the tree has begun to produce its fruit, a further concern arises: as we learned in the previous mishnayos regarding the sanctity of Shevi'is, the fruit itself is holy, and one may not cause it to be lost. This prohibition is separate from the laws of 'bal tashchis' and stems from the laws of Shevi'is. Hence the question of the Mishnah: from what stage is it forbidden to cut down the tree, even when its wood is worth more than its fruit - that is, from what stage in the production of the fruit is one no longer permitted to cause its loss?
The view of Beis Shammai:
The Mishnah establishes that every tree is judged according to the time when it began to produce fruit - "mishe'yotzi" - from when it brings forth. According to Rashi this refers to the emergence of the fruit, while the Rambam understands that it applies even from the point at which the tree began to sprout leaves. This is the view of Beis Shammai.
The view of Beis Hillel - each species on its own:
Carobs: from when the carob fruit reaches the stage at which it begins to hang down in a chain-like form, like a chain. From that moment it is forbidden to cut down the tree, since one who cuts it down is about to cause the ruin and loss of the fruit.
Grapes: from the moment a seed can be seen in them, that is, from when they have begun to form a seed inside.
Olives: from the moment they have begun to blossom.
The other fruit trees: from when they have begun to form fruit - as Beis Shammai says, that the fruit has started to take shape, or according to the Rambam, from when the tree began to produce leaves.
It emerges, then, that Beis Hillel disagrees with Beis Shammai only with regard to these particular fruits.
From when the permission returns:
Any tree whose fruit has ripened well beyond this stage, until it has reached the time when ma'aseros can be separated from it - as we mentioned based on the Mishnah in Maasros, that for each species there is a set time at which it is considered sufficiently ripe to be obligated in ma'aseros - may be cut down, since the fruit will not go to waste. Once the fruit has reached this level of development, it may be brought into the house, as we learned in the previous mishnah; it is considered ripe fruit, and cutting down the tree involves no loss.
"Kamah yehei ba'zayis v'lo yekutzenu":
Here the Mishnah asks about the other years as well: what quantity must an olive tree produce for it to be forbidden to cut it down? The Mishnah answers: "rova" - a quarter of a kav of olives. Once it produces this amount, it is forbidden to cut it down.
And this too applies specifically to an olive tree, which is more important and significant than the other trees. With any other tree, cutting it down is forbidden only once it produces a full kav of fruit - four times the measure stated for an olive tree, which is only a quarter of a kav.
Rabban Shimon ben Gamliel says: "hakol lefi ha'zayis" - everything depends on the quality of the olive tree. If it is a superior olive tree or a uniquely fine tree, it will be forbidden to cut it down even with less than a quarter of a kav.
In summary: In this mishnah we learned that besides the prohibition of 'bal tashchis' which applies in all years - and which is set aside when the tree's wood is worth more than its fruit - in the seventh year there is an additional prohibition because of the sanctity of the fruit and the ban on causing its loss. Beis Shammai sets a single measure for all trees, "mishe'yotzi," while Beis Hillel sets a time for each species: carobs from when they hang down in chains, grapes from when they form a seed, olives from when they blossom, and the other trees from when the fruit begins to form. Once the fruit has reached the time of ma'aseros, cutting it down is permitted again, since there is no loss. And as for the measure of quantity: for an olive tree a quarter of a kav, for the other trees a full kav, and according to Rabban Shimon ben Gamliel everything depends on the importance of the olive tree.