But wait! Didn't we say that the New Year of the trees is on the first of Tishrei? Yes, and yet there is no contradiction. The Tishrei New Year is to count the number of years in the life of a tree. For the first three years, the tree is "orlah," and its fruit cannot be used. In the fourth, its fruit is taken to Jerusalem and eaten there.
By contrast, Tu Bishvat is for separating tithes. In different years, there are different kinds of tithes. There is tithe for the poor and tithe to be eaten in Jerusalem. Tu Bishvat serves as a demarkation telling the year the fruit belongs and therefore the tithe that should be taken from it.
Art: The Large Tree by Paul Gauguin