In the gemara, Ulah says that a tree absorbs nourishment from the ground for a 16 amah radius around the tree (the gemara concludes that it is 16.5, and rashi points out that it is more like 16.6). Therefore, if reuven had a tree growing on his property within 16 amos of shimon's property, he is a gazlan and cannot bring bikurim. Rashi understands that these are 2 independent statements. First, he is a gazlan for nourishing his tree from shimon's property without permissions. Second, even if he has received permission from shimon he is exempt from bikurim because his fruits don't qualify as אשר תביא מארצך. However, Rashi 27a d.h. ha'koneh, when he explains that one who acquires 2 trees brings bikurim but doesn't read the parsha because he cannot make a declaration of פרי האדמה אשר נתתה לי. The gemara 81b explains that according to the Rabbonon, when someone purchases 2 trees it is a safeik whether he acquires land, so out of doubt he must bring bikurim, but cannot read the parsha because it is michzi k'shikra. Nonetheless, rashi holds that he is exempt from bikurim, but would be allowed to bring it if he would like (so long as he makes sure to be makdish it to avoid chulin in the azara, and to separate terumos and ma'asros, as the gemara explains 81b).
Rabbeinu Chananel cited in Tosafos holds that it is assur for him to bring bikurim due to the issur gezel - אני ה' שונא גזל בעולה. Rabbeinu Chananel understands that the statement of Ulah reads together, it is gezel and therefore he cannot bring bikurim.
The gemara says that the measurements of Ulah aren't precise, but he is machmir. Accroding to Rashi Ulah is being machmir by minimizing the measurement. Ulah comes to say a leniency that he is exempt from bikurim, but is machmir by limiting the leniency to a smaller measurement. But according to Rabbeinu Chananel, Ulah comes to say an issur that one is not allowed to bring bikurim. Therefore, by minimizing the measurement he is implying that if it is further than 16 amos, he is allowed to bring, which is not accurate because the tree receives nourishment for more than 16 amos. Tosafos is bothered by this question and explains that Ulah was only coming to say the halacha within 16, but not to imply that outside 16 amos one may bring bikurim.
Tosafos points out that the conclusion of the gemara 27b which allows him to bring bikurim because - על מנת כן הנחיל יהושע את הארץ, means something different in rashi than in Rabbeinu Chananel. According to Rabbeinu Chananel it simply means that Yehoshua stipulated these rights which avoids the issur gezel. But, according to Rashi we aren't dealing with the issur gezel, we are dealing with the fact that it is not ארצך. Therefore, rashi will have to say that the stipulation of Yehoshua was not merely to give permission, but to literally consider it to be the land of Reuven for this purpose. It would seem that the t'nai of yehoshua would even consider it to be his land to allow him to read the parsha of האדמה אשר נתתה לי, which is the opinion of the Rambam (but tosafos rid holds that the t'nai only helps to bring but not to read the parsha). Also, Rashi in his commentary on the line of the gemara when it mentions the t'nai of yehoshua is difficult. Rashi writes - התנה עמהם שלא יקפידו על כך. This approach of not being makpid which implies mechila would work for rabbeinu chananel, but it wouldn't be enough for rashi to be considered ארצך?