Gemara

(Rav Zvid): If Reuven made a Chazakah in the trees, and Shimon made a Chazakah in the land, each acquired what he was Machazik in.

Objection (Rav Papa): If so, Reuven does not own any land. Shimon can tell him to uproot his trees and leave! Rather, Reuven acquired the trees and half the land, and Shimon acquired the rest of the land.

Obviously, if Levi sold his land but not his trees, he kept land for himself. Even R. Akiva says that one sells generously only when he kept a pit for himself (he kept a path for himself only if he specified), for it does not harm the surrounding land, so the buyer will not tell him to remove it. Trees harm the land. If he would not keep land for himself, the buyer would tell him to uproot them!.

R. Akiva and Chachamim argue about one who sold trees, but kept the land. R. Akiva says that one sells generously; he sold also land. Chachamim say that one sells stingily, and he kept all the land for himself;

Even Rav Zvid (who said above that each acquired what he was Machazik in) could say that R. Akiva admits there, but argues here. There, Shimon can say 'just like I got only land, you got only trees.' Here, Levi sells generously (at his own expense).

Even Rav Papa could say that here, Chachamim argue and say that the buyer gets no land. When one sells everything, he is not generous to one and stingy to the other. Rather, he is generous to both. Here, the seller kept land for himself. We assume that he was stingy and sold only the trees.

71a (Rav Huna): Even though if Reuven buys two trees in Shimon's property, he does not acquire land with them, if Reuven sold his property and kept two trees for himself, he keeps land with them.

Even R. Akiva says that one sells generously only regarding pits, which endure. Trees weaken. If he would not retain land, the buyer could tell him 'uproot your trees and leave.'

(Mishnah): R. Shimon says, one who was Makdish a field was Makdish only (what is included in a sale of a field, and) grafted carob and sycamore stumps.

(Beraisa - R. Shimon): The reason is, they nurture from the surrounding Hekdesh land (he does not want to benefit from Hekdesh).

Question (against Rav Huna): If one leaves land for his trees, they nurture from his own property!

Answer #1: R. Shimon holds like R. Akiva (one sells and is Makdish generously). Rav Huna's law is like Chachamim (one sells stingily).

Objection: Chachamim say that one keeps a path to his pit. There is no need to say that he keeps land for his trees!

Answer: Rav Huna teaches that if they fall, he may plant new trees.

72b - Answer #2: R. Shimon addresses Chachamim according to their reasoning. I say that just like one sells stingily, he is Makdish stingily, and keeps land for himself. Granted, you say that he is Makdish more generously than he sells, but you should admit that he is Makdish only grafted carob and sycamore stumps (for they will nurture from Hekdesh, but not pits,...)'. Chachamim hold that he is Makdish as generously as he gives.

Rishonim

Rif and Rosh (3:26): The Halachah follows Rav Papa.

Hagahos Ashri: He does not really get half. Whether he buys one or three trees, he acquires rights to plant more after these dry up.

Rif (38a and Rosh 4:17): On Daf 37a, we might have thought that one keeps land for himself only if he kept three trees. Rav Huna teaches that even if he kept only two trees, he kept land. The Tana'im argue about one who sold trees, but kept the land. R. Akiva says that one sells generously, and he sold also land. Chachamim disagree. The Gemara rules like R. Akiva.

Rif (ibid): This is when he sold three trees.

Rambam (Hilchos Mechirah 24:8): If one sold land and kept the trees, he kept half of all the land. If he would not retain land, the buyer could say 'uproot your trees.' Similarly, if he kept only two trees, he kept land proper for them. If he would not keep land, the buyer could say 'uproot your trees and leave.'

Rebuttal (Ra'avad): The Gemara mentioned keeping half the land only when one sold, at once, the land to one buyer and the trees to another. Each gets half the land, for 'trees' includes the field. I say that even if one bought only two trees, he gets half of all the land. However, if one sold land and kept the trees, he kept only the land under and between them and around them for the width of a harvester and his basket.

Magid Mishneh: The Rambam understands that the Gemara literally means half the land. Others say that it means only rights to plant others in place of them. The Ri mi'Gash says that he gets based on the trees, and the width of a harvester and his basket around them. The Rambam says that he keeps half, lest the buyer say 'uproot your trees.' The same applies when he sold land and kept the trees. The Gemara did not say 'half' in this case, but the reason is the same, so we learn from what was explicitly taught. When he kept only two trees, it suffices to say that he gets land proper for them. This is one level above one who buys two trees, who gets no land. We find that one who buys (three) trees gets only what is needed for them, and one who keeps trees gets more, i.e. half the land. However, why must he keep half the land to stop the buyer from saying 'uproot your trees'? One who buys three trees does not get half the land, and he can plant new trees when these die! This is why the others explained differently.

Rambam (9): If one sold trees, but kept the land, the buyer gets land appropriate for the trees, like we explained.

Poskim

Rema (CM 216:6): Even if one who sold three trees kept the land between them, the buyer gets the trees' place, and can plant again if they dry up.

Gra (17): This is like the Rif, who says that the Sugya on 37a discusses three trees.

Shulchan Aruch (CM 216:11): If one sold land and kept the trees, he kept half of all the land. If he would not retain land, the buyer could tell him 'uproot your trees.'

Beis Yosef (DH Mochar): The Rashbam explains that even if he kept only two trees, he kept land needed for them to live and their needs, and if they dry up he can plant others in place of them. Rav Huna explicitly said that if he kept two trees, he kept land.

SMA (36): The Magid Mishneh thought that 'lest the buyer say 'uproot your trees'' explains why he needed to keep half for himself. The Rambam did not mean this. It suffices to keep the land needed for them! The Rambam merely adopted the words of the Gemara. The Rema brings the opposing opinion, which equates one who left trees for himself to one who buys trees. This opinion agrees that he keeps what is needed for them even when he keeps only two.

Prishah (9): The Rashbam holds that one who keeps two trees keeps only land needed for them, but not rights to plant more when they dry up. The Tur says that he has land, i.e. also to plant more when they dry up, just like one who buys three trees. This is like Tosfos.

Gra (26): The Rambam holds that 37a discusses one who kept at least three trees, like the Rif (38a) said. Rav Huna could have taught a Chidush about keeping three trees (that he keeps half the land, which is more than one who bought three). He taught about two, that even though one does not keep half the land, he keeps what is needed, unlike one who buys two. He explainss that the Reisha and Seifa on 37a-b discusses three trees. However, the Rashbam (37b DH Plugta) proved that it cannot discuss three, for Chachamim agree that one who buys three trees gets land!

Rema: Some say that he kept only land needed for them.

Shulchan Aruch (12): Similarly, if he kept only two trees, he kept land appropriate for them. If he would not retain land, the buyer could tell him 'uproot your trees and leave.'

SMA (37): Even though this is unlike the previous law, it says 'similarly', for also here one who kept (two) trees for himself is one level better than one who bought (the same number of) trees.

Gra (28): In the conclusion (72b), Rav Huna's law is even like R. Akiva.