Psalms 1:3

All who delight in and meditate on God’s law will prosper like a flourishing fruit tree (cf. Psalms 92:12-14). Their fruit will appear at the proper time, not necessarily immediately, and their general spiritual health, represented by the leaves, will be good. Usually the fruit God said He would produce in the lives of most Old Testament believers was physical prosperity (cf. Deuteronomy 28:1-14). The fruit a Christian bears is mainly a transformed character and godly conduct (cf. Galatians 5:22-23). In both cases it is God’s blessing on one’s words and works. His prosperity is from God’s viewpoint, not necessarily from the world’s.

The most important part of a tree is its hidden root system because it draws up water and nourishment that feeds the tree. Without a healthy root system a tree will die, and without a healthy "root system" a believer will wilt. Fruit, in biblical imagery, is what is visible to other people, not just what is hidden within a person. It is also what benefits other people, what others can take from us that nourishes them (cf. John 15:1-11). In contrast, leaves are what others simply see and admire.