Why does the Torah go out of its way to tell us specifically that "all the trees in the field" (23:17) as well as the field and the Me'aras haMachpeila transferred to Avraham's possession?
In the parsha sheet I mentioned a Parashas Derachim which resolves an apparent contradiction in Rashi. Commenting on "ger v'toshav", Rashi writes that Avraham told the Bnei Cheis that if they refuse to sell him land he will claim a pre-existing right of ownership (i.e. he is already a toshav) as Hashem had promised all of Canaan to him. Yet, in Parhas Lech Lecha, when the shepherds of Lot claimed the right to graze in other people's pastures based on Hashem's promise of Canaan to Avraham, Rashi explains that their claim has no merit because Avraham did not yet have possession of the Land. What's the difference between Avraham's claim to being a toshav and the shepherd's claim of grazing rights?
The Parashas Derachim answers that property rights consist of 1) rights to the guf hakarka, the actual land; 2) right to peiros, use of the land for farming, grazing, etc. Hashem's promise to Avraham gave him immediate ownership of the guf hakarka, but the right to use the land was retained by its inhabitants until Yehoshua's conquest. The shepherds of Lot were stealing the peiros of the land, and these did not yet belong to Avraham. Avraham needed a burial plot which utilized only the guf hakarka, and that he already owned.
Had Avraham taken property based on his right of being a toshav, the land alone would have been his. Now that Avraham bought property, he gained the rights to peiros in addition to the property itself. Perhaps the Torah stresses specifically the trees to highlight this difference and emphasize the acquisition of peiros in addition to the property.