Jamieson, Fausset and Brown on Psalms 72:16

16. The spiritual blessings, as often in Scripture, are set forth by material, the abundance of which is described by a figure, in which a "handful" (or literally, "a piece," or small portion) of corn in the most unpropitious locality, shall produce a crop, waving in the wind in its luxuriant growth, like the forests of Lebanon.

they of the city ... earth--This clause denotes the rapid and abundant increase of population--

of--or, "from"

the city--Jerusalem, the center and seat of the typical kingdom.

flourish--or, glitter as new grass--that is, bloom. This increase corresponds with the increased productiveness. So, as the gospel blessings are diffused, there shall arise increasing recipients of them, out of the Church in which Christ resides as head.