Augustine of Hippo

"And he shall be like a tree planted hard by the running streams of waters" [Psalm 1:3]; that is either Very "Wisdom," [Proverbs viii] which vouchsafed to assume man's nature for our salvation; that as man He might be "the tree planted hard by the running streams of waters;" for in this sense can that too be taken which is said in another Psalm, "the river of God is full of water." Or by the Holy Ghost, of whom it is said, "He shall baptize you in the Holy Ghost;" [Matthew 3:11] and again, "If any man thirst, let him come unto Me, …

Exposition on Psalm 1 ↗

Ambrose of Milan

And it shall be like a tree that is planted by the courses of water, which will yield its fruit in its season. And its leaf shall not wither, and whatever it does, it shall prosper. What is this blessedness that is compared to the tree, unless we understand it to be in paradise, that blessed place, the tree of life produced from the earth in the midst of other trees? …

Commentaries on the Twelve Davidic Psalms ↗

Jerome

And he shall be like the tree which is planted beside the running waters, which shall give its fruit in its season, and its leaf shall not fall off. Many interpret it simply, and say thus: Just as a tree, if it is planted next to the waters, must needs be green, and is not dried up, because its roots have moisture whereby they live: so whoever meditates on the law of God, the meditation on the law itself makes greenness and life for him. …

Homily 1, on Psalm 1 ↗