1) HOW TO ASSESS DAMAGE DONE TO A TREE
OPINIONS: The Gemara cites two versions of the ruling of Rav Papa and Rav Huna brei d'Rav Yehoshua. In the first version, they rule in accordance with Rav Nachman that we evaluate damage done to a Dekel (date-palm) based on damage done to something "sixty times" larger. In the second version, they evaluate the damage done to a Dekel based on "a small plot of land."
What is the difference between these two versions?
(a) The LECHEM MISHNEH (Hilchos Nizkei Mamon 4:14) explains that according to the first version, they evaluate the damage done to a single Dekel by considering the devaluation of sixty Dekalim out of which one was damaged. The second version maintains that we evaluate the depreciation that would be caused to this Dekel in an area of land sixty times larger than the land on which this Dekel stands.
The RAMBAM there rules that we evaluate damage done to the Dekel based on an area of land sixty times larger. The Lechem Mishneh explains that the Rambam rules like the second version in the Gemara.
(b) CHIDUSHEI RABEINU MOSHE KAZIS explains that the two versions do not argue, but they complement each other. Rav Papa and Rav Huna brei d'Rav Yehoshua evaluate the damage based on a plot of land (like the second version) which is sixty times larger (like the first version). Accordingly, the Rambam rules like both versions.
2) TWO TYPES OF DATE-PALMS
OPINIONS: The Gemara rules that the Halachah follows Rav Papa and Rav Huna brei d'Rav Yehoshua (who rule that we evaluate the damage based on an area sixty times larger; see previous Insight) when the Dekel that was damaged is a Roman date-palm. The Halachah follows the Reish Galusa (who rules that we evaluate the damage by itself) when the Dekel that was damaged is a Persian date-palm.
What is the reason to differentiate between two types of date-palms?
(a) RASHI explains that a Persian palm is much more valuable, and therefore we evaluate it by itself. Rashi's logic seems to be that we evaluate the damage based on a plot of land sixty times larger (when the fruit that was damaged was not completely ripe, see Mishnah 55b) in order to determine a fair value for the fruit. By evaluating the entire land, we include the value of the future growth that was expected from the fruit that was damaged, as Rashi writes later (59b, DH b'Shishim). When the fruit is of a superior quality, even when a person buys the tree by itself he takes into account the future growth of the fruit, and therefore it is not necessary to assess the damage based on a larger plot of land in order to gain a fair assessment of the damage.
The fruit of the Persian palm is much fatter and ripens more fully than the fruit of the Roman palm, as the Gemara teaches in Shabbos (29a, Rashi there, DH b'Armiyasa) when it says that for this reason the fruit of the Roman palm is not Muktzah on Shabbos since some of the flesh which is not fully ripened sticks to the pit after the fruit is eaten.
The RA'AVAD (cited by the Shitah Mekubetzes) writes that a Persian palm was so superior that it was not normally sold together with the land on which it stands. Since it is normally sold independently, we also evaluate the damage done to it independently.
(b) RABEINU YEHONASAN (cited by the Shitah Mekubetzes) explains in the opposite manner of Rashi. He says that the reason the Persian palm is evaluated by itself is that it is of such poor quality that if we would evaluate the damage based on the depreciation of the entire field, the value of the damage would be so minimal that the Nizak would not receive fair compensation. Therefore, we evaluate the damage based on the actual damage that was done in order for the Mazik not to get away with a trivial payment. (This conforms with the explanation of the Ra'avad cited in Insights to 58b, who explains that when the damage was less that a certain minimum value we do not assess the damage based on a larger field. Although we do not rule like those opinions, with regard to a Persian palm we do follow that ruling.)
Rabeinu Yehonasan seems to understand the Gemara in Shabbos (29a) as RABEINU CHANANEL and the ARUCH understand it. They explain that the pit of the fruit of the Roman palm is not Muktzah because it is so soft that it can be eaten, while the pit of the Persian palm is hard and cannot be eaten. This quality makes the Roman palm more valuable than the Persian palm.
(c) The ROSH explains that the Gemara here is not discussing damage done to the fruit of the tree, as the RAMBAM rules (Hilchos Nizkei Mamon 4:14). Rather, it is discussing damage done to the tree itself. Accordingly, the difference between a Persian palm and a Roman palm is the same as the difference between fully-ripe fruits and partially-ripe fruits, as mentioned in the Mishnah (55b). The Persian palm refers to a fully-grown tree; the owner could uproot it and replant it elsewhere, and it would take root in the second place. Therefore, it is comparable to fully-ripened fruits which can be picked and eaten in their present state. The Roman palm refers to a tree that is not yet completely grown. If it would be uprooted, it would not have strength to take root elsewhere. Therefore, it is comparable to fruit that is not fully-ripened, and the value of the damage is assessed based on an area sixty times larger.