The Gemorah says that Ezra made a takana to read the tochacha before Shavuos so the year and its curses should end. Shavuos is called Rosh Hashana because of the mishna in Rosh Hashana that says that we are judged for the fruits of the tree on Shavuos. The first mishna in Rosh Hashana lists four days called Rosh Hashana: 1 Nissan, 1 Elul, 1 Tishrei, and 1 or 15 Shevat, but not Shavuos. The second mishna lists four times of judgement, one of them being Shavuos.
However, it is not listed as a Rosh Hashana, only as a time of judgement. How can the gemorah in megilla say that Shavuos is therefore called Rosh Hashana?
The question is greater if one looks at the Rosh. The Rosh's girsa in the gemorah is that Atzeres is Rosh Hashana La'Ilan--the exact words used in the mishna to describe Tu B'Shvat! How can it be? Shavuos is not Rosh Hashana La'Ilan, but merely the time of the judgement of the fruits!
A.G., Chicago, USA
In fact the Gemara in Rosh Hashanah 7b quotes a Beraisa that describes Shavuos as Rosh ha'Shanah for Shtei ha'Lechem. Rashi (DH Rosh ha'Shanah l'Shtei) explains that this means that Shevuos is the start of the year for bringing Menachos from the new crop. Presumably this also has a connection with the fact that the Shtei ha'Lechem come to appease Hash-m at the time of the judgement on the fruit of the tree (Rosh Hashanah 16a). The Gemara in Rosh Hashanah explains that it is not listed in the Mishnah (Rosh Hashanah 2a) as a Rosh ha'Shanah for one of the following reasons:
(1) it is only Rosh ha'Shanah from the time of bringing the Shtei ha'Lechem and the Mishnah only lists those days that are Rosh ha'Shanah from the beginning of the day (i.e. from nightfall).
(2) The Mishnah only lists those that do not need an act to make them Rosh ha'Shanah, whereas Shevuos is only Rosh ha'Shanah through the Hakravah of the Shtei ha'Lechem.
(3) The Mishnah only lists Rosh ha'Shanah that falls on Rosh Chodesh.
Tu b'Shevat is Rosh ha'Shanah for Ilanos as far as Ma'aser is concerned, that one cannot take Ma'aser from one year's crop for the crop of a different year. This is a different type of Rosh ha'Shanah than Shevuos which is Rosh ha'Shanah for judgement on fruit of the tree, and for bringing Menachos from the new crop.
Dov Freedman