The LORD Almighty, who planted you, has decreed disaster for you, because the house of Israel and the house of Judah have done evil and provoked me to anger by burning incense to Baal. The trees of the earth represent the nation of Israel, a third of which chose to accept God's gift of salvation and follow Him. The olives that is mentioned above is the remnant of the nation of Israel which survives because of their acceptance of Christ as their savior In the early years of the Church, the Church was made up almost entirely of Jews who had accepted the gospel. So that leaves only the grass to be explained, so why is it said that all the green grass was burned up. Remember that the evil will be thrown into the eternal lake of fire to burn and suffer forever.
All men are like grass , and all their glory is like the flowers of the field.
The grass withers and the flowers fall, because the breath of the LORD blows on them. Surely the people are grass . That last verse was pretty clear, the grass represents the people of the earth who reject God's gift of salvation and are rebellious against Him. This death is spiritual and not physical. So to summarize this vision, I believe it means that after Christ ascended into heaven the Church is established and the gospel is spread.