Gods government over the earth.

Ps 1 3 He is like a tree planted by streams of water, which yields its fruit in season and whose leaf does not wither. Whatever he does prospers.

4 Not so the wicked! They are like chaff that the wind blows away.

5 Therefore the wicked will not stand in the judgment, nor sinners in the assembly of the righteous.

6 For the LORD watches over the way of the righteous, but the way of the wicked will perish.

The Psalms are not written to focus on the situations that David experienced in his life. But they are the doctrines of grace applied to the nation of Israel. You see this in verse 3 where there is a promise to those who meditate on Gods law. The Psalms are describing two governments. The government of God that was established in the creation ordinance. That is the tree of life in the garden that was planted near the river Euphrates was representing Gods kingdom that was established for man.
"And a river went out of Eden to water the garden,.... Before man was created, as Aben Ezra observes, this river went out of Eden and watered it on every side; but what river is here meant, is hard to say. It is more generally thought to be the river Euphrates, when that and the Tigris met, and became one stream or river, and as such entered and passed through Eden; and as it was parted into four rivers afterwards, in two of which they retained their names: the learned Reland (k) thinks, this river is now lost; but the learned writer before referred to thinks, as has been observed, that it is the river Jordan; see note on Gen 2:8 and which, as Pliny (l) says, was a very pleasant river:" http://biblehub.com/commentaries/gill/genesis/2.htm As you read in Ps 8 God created the earth and made it for man to rule through His laws, decrees, statutes, curses and covenants.Ps 8 6 You made him ruler over the works of your hands; you put everything under his feet:7 all flocks and herds, and the beasts of the field,8 the birds of the air, and the fish of the sea, all that swim the paths of the seas. Ps 115 16 "The highest heavens belong to the LORD, but the earth he has given to man."This is in the context of the righteous ruling the earth which will be renewed in the future... but the mans authority over the earth is established once again through the covenant of grace. The wicked worship dead idols and are like the chaff but the righteous rule from Israel ...Gods kingdom. So here the tree by streams of water is the man who establishes his rule through the words in the Psalms. The trees branches represent mans power that is established by these words on his lips.

The Psalms are not teaching that man establishes his authority through wise choices or legislation. But Gods law stands over all powers and establishes righteousness through his King. Even tho God gives man governmental authority it is never separated from His creation ordinance. That ordinance which gives each man who has these Psalms on his lips as a king over Gods kingdom. Gods kingdom is His perfect creation and recreation through allowing and preventing. 6 For the LORD watches over the way of the righteous, but the way of the wicked will perish. Or the Lord reorders all events to fall out for our good and for His glory. Ps 111 6 "He has shown his people the power of his works, giving them the lands of other nations."This is the working principle of God working through His word in redeeming a people for Himself This is done through His creative word. This is the principle of God smiling on His people when they are suffering. The tree is the person who not only overcomes the curse but through cursing the curse ...reverses the curse. So Gods kingdom authority grows while mans governments are turned upside down.