"Tree", "Trees" is a symbol of the "Plantings" of God, i.e. people capable of perceiving the Revelation of the Truth of His Will.
"The Tree of Knowledge of Good and Evil" is a symbol of the totality of the Divine Laws of Nature (Genesis), of the material, physical world, which contain Good - when following the Commandments of God, and Evil - when departing from their execution, when perverting them.
"The Tree of Life" is the Wisdom of the eternal spiritual Divine Laws of true Life, His precepts, the Commandments, the comprehension, implementation of which opens to man the way to spiritual immortality, to Eternal Life.
"Tree", "Trees" bearing fruit is a symbol of righteous people living according to the Will of God, bringing the "fruits" of righteousness by their proclamation of the Will of God to other people. 
Negative Meaning.
"Fruitless trees" are ungodly people who do not bring forth the good "fruits" of righteousness by their sinful lives, who do not do good works to save themselves and their neighbors from sin.