1) A TREE THAT OBTAINS ITS NOURISHMENT FROM ANOTHER PERSON'S FIELD
QUESTION: Ula says that when one planted a tree within 16 Amos of a neighbor's field, he does not read the Parshah of Bikurim when he brings his first fruits to Yerushalayim, because the tree is a "Gazlan," for it obtains its nourishment from the field of someone else. The Gemara attempts to disprove Ula's ruling from the Mishnah in Bikurim (1:11) that teaches that when a person buys a tree with the ground beneath it, he is obligated to bring the first fruits as Bikurim and to read the Parshah of Bikurim ("Mevi v'Korei"). The Mishnah implies that even when a person buys a tree with a minimal amount of land beneath it in someone else's field, he still may read the Parshah of Bikurim. The Gemara asks a similar question from the Mishnah later (81a, and Bikurim 1:6) which teaches that when a person buys two trees in his neighbor's field without buying the land beneath them, he is "Mevi v'Eino Korei," he brings Bikurim but does not read the Parshah of Bikurim. This implies that when he buys three trees, he is "Mevi v'Korei" because the land beneath them is acquired together with the trees by the buyer. It is clear from there that a person can be "Mevi v'Korei" even when his trees are within 16 Amos of his neighbor's field.
In both cases, the Gemara refutes the proof by saying that the Mishnah means that the buyer can be "Mevi v'Korei" only when he indeed acquires 16 Amos of land around the tree that he buys.
The reason that Ula gives for not bringing Bikurim when one's tree is near someone else's land is that the tree is a Gazlan -- it grows as a result of obtaining, or "stealing," nourishment from another field. However, when a person purchases a tree in another person's field, he certainly is permitted to let his tree take nourishment from that field, because the seller sold to the buyer the right to keep the tree in his land. Why, then, does the Gemara question Ula's ruling from the Halachah of a person who purchases a tree in another person's field?
ANSWERS:
(a) RASHI (26b, DH v'Ein) writes that one does not bring Bikurim from a tree that is a "Gazlan" because the Torah specifies that Bikurim may be brought only when the fruits grow "me'Artzecha" (Devarim 26:2), from the person's own land. (See also the Mishnah in Bikurim 1:2, which includes a thief in its list of people who may not bring Bikurim because the verse says that the fruits of Bikurim must come from "Admascha" -- "your land" (Shemos 34:26).)
TOSFOS (DH Gazlan) explains that according to this, Ula's reasoning for exempting the owner from Bikurim might not be related to the Aveirah that he did by letting his tree steal nourishment from another field. Rather, even when one is permitted to maintain a tree near another person's field, one still would not bring Bikurim from that tree because the fruit was not nourished from the person's own land, "me'Artzecha." Acquiring permission to obtain nourishment from someone else's land does not qualify as "me'Artzecha." This is why the Gemara questions Ula's ruling from the case in which a person buys a tree in someone else's field. Even though the buyer is permitted to let his tree derive nourishment from the other person's field, it is not considered "me'Artzecha" according to Ula.
According to Tosfos, why does Rebbi Yochanan argue with Ula (on 27b) and obligate the owner of the tree to bring Bikurim? Rebbi Yochanan asserts that at the time of the division of Eretz Yisrael, Yehoshua stipulated that people may let their trees nourish from the land of others. Even if the tree nourishes from someone else's land with permission, it still should not be considered "me'Artzecha," and the owner should not be able to bring Bikurim from the fruit of that tree!
Tosfos answers that Yehoshua stipulated that having the right to obtain nourishment from someone else's field should be considered as though the tree grows "me'Artzecha." Tosfos might mean that while permission establishes a Kinyan which commits the landowner to allow the owner of the tree to obtain nourishment from his land, permission alone is not enough to make the land considered "me'Artzecha." Yehoshua instituted that the owner of the tree even has a Kinyan ha'Guf l'Peros -- a Kinyan on the land itself for the purpose of obtaining nourishment from the land. This makes it "me'Artzecha."
(b) RABEINU CHANANEL (cited by Tosfos), the YAD RAMAH, and the TOSFOS RID explain that Ula exempts the owner of the tree from Bikurim because of the Aveirah of stealing. Bringing Bikurim from such a tree would be comparable to stealing an animal and bringing it as a Korban, in which case the Korban is invalid (see Sukah 30a).
How does Rabeinu Chananel explain the question of the Gemara from the Mishnah regarding one who buys a tree in someone else's field?
1. Perhaps the Gemara's inference from the case of one who buys a tree in someone else's field is that the Mishnah refers to the minimal land around the tree as "Karka'o" -- "its land," implying that this is all the land the tree needs for nourishment, and not more.
The question on Ula from the Mishnah (which implies that when one buys three trees he is "Mevi v'Korei") is that the Mishnah implies that when a person acquires three trees he also receives the entire amount of land that the trees need, and that amount is only the land between the trees. According to Ula, he should acquire much more land -- 16 Amos around each tree. (M. Kornfeld)
2. The TOSFOS RID and the TORAS CHAIM explain that Rabeinu Chananel is giving a reason for why Ula exempts the owner of the tree even from bringing Bikurim, and not merely from reading the Parshah of Bikurim. Bikurim cannot be brought from stolen land because the verse says "me'Artzecha," implying that the owner of the fruit has permission to use the land.
However, even when no Aveirah is involved in the growth of the fruit, the owner of a tree would bring Bikurim but he would not read the Parshah because he is unable to say the verse, "... ha'Adamah Asher Nasata Li" -- "from the land that You gave to me" (Devarim 26:10), which implies that he not only had permission to grow his fruit on this land, but that the fruit was grown on land that he actually owns. This excludes a person who brings fruit which are nourished from (or planted on) another person's land from reading the Parshah of Bikurim. The Gemara is asking that according to Ula, why must the owner of a tree who also owns the minimal amount of land around it read the Parshah of Bikurim?
This approach requires further explanation. If Ula taught only the first Halachah -- that one does not bring Bikurim from a tree that steals, then how is one to infer from his words the second Halachah -- that the owner of a tree which nourishes from another person's field with permission does not read the Parshah of Bikurim?
The Toras Chaim answers that Ula's main point is that although the tree's theft is only the nourishment -- since it is planted on earth that actually is owned by the owner of the tree, nevertheless the tree is considered a thief and the Bikurim are not considered "me'Artzecha." By the same logic, if the nourishment is not from the tree-owner's own land (e.g. when the tree is planted in his land but it obtains its nourishment from a neighbor's land -- even with the permission of the owner of that land), then it should not be considered "ha'Adamah Asher Nasata Li" because of the nourishment that it receives from another person's field. The tree-owner therefore should not be able to read the Parshah of Bikurim.

27b----------------------------------------27b

2) PERMISSION FOR A TREE-OWNER TO LET HIS TREE OBTAIN NOURISHMENT FROM ANOTHER PERSON'S LAND
QUESTION: The Gemara rules that fruit of a tree planted near another person's land must be brought as Bikurim. The tree is not considered as though it steals nourishment from the neighboring land, because Yehoshua stipulated that a person's trees may obtain nourishment from the fields of others (see previous Insight).
If Yehoshua stipulated that one may plant his tree near the field of his neighbor, then why do the Tana'im (25b) argue about whether or not one is permitted to plant a tree within 25 Amos of his neighbor's Bor?
Moreover, why does Rebbi Yosi need to explain that it is permitted because the person plants the tree in his own land? The reason why it is permitted is the stipulation of Yehoshua, which should permit one to plant his tree even within 16 Amos of another person's field!
One might argue that Yehoshua's stipulation applies only in a case in which the neighbor has no Bor within 25 Amos. However, the Gemara earlier (18a) teaches that according to Rava, the Chachamim maintain that even if there is no Bor in the neighbor's property, one is still prohibited from planting a tree within 25 Amos of his property, because when the neighbor eventually digs a Bor the tree will damage it. According to Rava, under what circumstances did Yehoshua permit a person to plant a tree near someone else's property?
According to the opinion that Rava prohibited planting near a neighbor's field only when the field was designated for digging pits, Yehoshua might have permitted planting a tree next to other types of fields. However, according to the opinion that maintains that the Chachamim prohibited planting a tree next to any type of field, the question remains. (RASHBA to 25b)
ANSWERS:
(a) The RASHBA answers that the opinion that maintains that Rava prohibits planting a tree next to any type of field must follow the view of Ula who argues with Rebbi Yochanan and maintains that Yehoshua never made such an enactment.
(b) The Rashba cites the RAMBAN who writes that the enactment of Yehoshua applies only to a tree planted within a field of trees. The Rashba interprets this to mean that the enactment applies only to a field that is large enough to plant three trees within it. Accordingly, the Chachamim who argue with Rebbi Yosi might be referring to a smaller field, a field in which there is not enough space for more than two trees.
(c) Even the opinion that prohibits planting a tree near a field that is not designated for pits would admit that one is permitted to plant a tree next to a field which is specifically designated not for pits, such as an orchard. The enactment of Yehoshua applies in such a situation. (M. Kornfeld)
(d) According to TOSFOS, the enactment of Yehoshua does not merely permit one to plant a tree next to the border, but it states that the nourishment that the tree obtains from the neighbor's field is considered as though it comes from the land owned by the tree owner (see previous Insight). Accordingly, the enactment of Yehoshua might apply only with regard to bringing Bikurim, and only in cases in which one is permitted to have a tree next to a neighbor's field (for example, when the neighbor explicitly gave him permission to plant the tree there).
(This might be the intention of the Rashba as well.)