No it doesn't explain "the third part of the trees." The reference to a third of the earth and a third of the trees being burned up, which is the first trumpet, will take place literally. The wrath of God via the seals, trumpets and bowl judgments are going to be the worst time in the history of the world, dismantling all human government and decimating the majority of the earths population.
You people who believe and teach as you do, have no idea of the wrath that is coming upon t his earth! All of those events that will result from the seals, trumpets and bowl judgments will be literal plagues of wrath that God is going to pour out upon this earth and not something that happened spiritually in the past. When it says that "a third of the earth and trees will be burned up" that is exactly what is going to happen. When scripture says that a third of the earths population is going to be killed by those four angels and their army via fire, smoke and sulfur issuing from their mouths, that is exactly what is going to happen. When the scripture says that the sun is given power to scorch the inhabitants of the earth with intense heat, that is exactly what is going to happen, etc., etc.