The meaning is that the 15th of Shevat is the New Year’s Day for tithing the fruit of trees. For most of the year’s rain has already fallen. The sap has begun to ascend through the trees, and fruit begin to take form on the trees from this time. Similarly, in the divine realm, it is the New Year’s Day for the fruit of the supernal tree which bestows its holy abundance on its fruit, the [upper worlds]. From them the shefa descends until it reaches the trees in our world below and the powers that oversee them.