16 But the leaf of this tree shall not fall off. There is no ground for wonder that its leaves do not fall off, seeing that its fruits will not drop to the ground, either because they are forced off by ripeness, or shaken off by external violence, but it will yield them, distributing them by an act of reasoned service. Now the spiritual significance of the leaves is made clear by a comparison based upon material objects. We see that leaves are made to sprout round the fruits about which they cluster, for the express purpose of protecting them, and of forming a kind of fence to the young and tender shoots. What the leaves signify, then, is the teaching of God's words in which the promised fruits are clothed. For it is these words that kindly shade our hopes, that shield and protect them from the rough winds of this world. These leaves, then, that is the words of God, shall not fall: for the Lord Himself has said: Heaven and earth shall pass away, but My words shall not pass away, for of the words that have been spoken by God not one shall fail or fall.
Source: Homilies on the Psalms (New Advent)