QUESTION: The Mishnah (2a) states that the Rosh Hashanah for trees is the first of Shevat according to Beis Shamai, and the fifteenth of Shevat according to Beis Hillel. The Gemara seeks a source for Shevat as the new year for trees, and it answers that since most of the rain of the year has already fallen by this time it is considered the end of the year for trees. Even though the winter season is not yet over and more rain may still fall, since a majority of the winter season has passed this date is considered the end of the year for trees.
The Gemara's intention is unclear. It is true that once most of the year's rain has fallen the trees begin to blossom. Why, though, does that render Shevat the Rosh Hashanah for trees? Why does the time of blossoming determine the date of the new year for trees?
(a) RASHI maintains that the requirement to separate Ma'aser from the produce of such trees is only mid'Rabanan. Hence, it was the Rabanan who determined the dates for the new year for Ma'aser of trees (Rashi 13b, DH Mitoch; 23b, DH Mena Hani Mili), and they decided that it is appropriate for the Rosh Hashanah for trees to be the time when trees usually begin to blossom.
(b) TOSFOS (DH b'Echad) explains that the only appropriate time to be considered the new year is the first of Tishrei. The reason why the Rabanan established the Rosh Hashanah for trees in Shevat is because trees that blossom after Shevat grow exclusively from rainwater that fell after Rosh Hashanah.
Only after they absorb an amount of water equivalent to most of the year's rainfall do trees have enough water to blossom. Consequently, since most of the rain of the year falls by the fifteenth of Shevat, any fruits that blossom after that date blossom entirely from water that fell after Tishrei, and thus their Chanatah (blossoming) is attributed to the new year and the fruits they bear belong to the new year (which began with Tishrei). When a tree blossoms before Shevat, this indicates that it grew from the rain that fell before Tishrei, and thus it belongs to the previous year's produce. Accordingly, the Rosh Hashanah for trees is actually the first of Tishrei, and blossoming after Shevat is merely an indication that the blossoms grew from rain that fell after Tishrei. (This explanation has a source in the Yerushalmi, Rosh Hashanah 1:2.)
In this respect, the Ma'aser of fruit differs from that of vegetables (Yerek) and grains (Tevu'ah). The latter items belong to the year in which they were picked or in which they grew a third of their eventual growth, even though the water that nourished them came from last year's rains (see TOSFOS and TOSFOS YESHANIM).
(Tosfos apparently follows his opinion that the new year for trees which starts in Shevat affects not only Halachos which are mid'Rabanan but even Halachos which are mid'Oraisa, such as the laws of Shevi'is. See TOSFOS DH Mah Goren.)
(c) RABEINU CHANANEL understands the Gemara's question differently. The Gemara is not asking merely why Shevat was established as the Rosh Hashanah for trees. Rather, the Gemara is asking specifically why Beis Hillel says that the new year for trees is the fifteenth of Shevat. Why does Beis Hillel say that this Rosh Hashanah occurs in the middle of a month, when every other Rosh Hashanah occurs at the beginning of a month?
The Gemara answers that Beis Hillel maintains that the Rosh Hashanah for trees cannot occur until most of the rain season has passed. That season begins on the seventeenth of Cheshvan (see Mishnah, Ta'anis 10a) and continues until the end of Nisan, for a total of five and a half months. The beginning of Shevat cannot be the Rosh Hashanah for trees because by that time most of the rain season has not yet passed (because Rosh Chodesh Shevat is only two and a half months from the beginning of the season) and only a minority of the rain has fallen by that time. The Rosh Hashanah for trees must be the fifteenth of Shevat, which is after most of the rain season has passed.