Here is what the Bartenura says: 'Rosh Hashanah for trees - regarding Ma'aser (tithes) of fruit, since one is not permitted to tithe the fruit of a tree that budded before Shevat onto fruit that budded after Shevat (i.e. one must tithe each one separately). Alternatively, the difference concerns the third year of the Shemitah cycle, during which Ma'aser Ani (the tithe for the poor, which replaces Ma'aser Sheni, which the owner takes to Yerushalayim and eats there) takes effect. There too, the fruit which budded before Shevat (and subsequently grew), requires Ma'aser Rishon (for the Levi) and Ma'aser Sheni to be taken; whereas the fruit that budded (and subsequently grew) after Shevat, requires Ma'aser Rishon and Ma'aser Ani.

be'Virchas Kol Tuv.

Eliezer Chrysler.