Answer:
The main theme in this poem is brevity of life.
Step-by-step explanation:
The writer wants to show how short life is, comparing it with the beauty of nature. He decides to enjoy his life from now on, because he is conscious it is beautiful and short as the trees. Throughout the poem he uses the trees to show the different stages of the life. For example at the beginning of the poem he uses an image from a cherry tree in bloom, which suggests the beauty of life and its importance.