I am so grateful that you helped me figure it out! Thank you for your wonderful explanation. I agree on your opinion completely: a reflection on the transience of nature.
I also love Du Fu poem, especially some of his poems written in Chengdu which was one of the most prosperous areas of China, without being involved in the war at that time. For my part, his poetry has a perfect antithesis in a gorgeous writing style. Due to Du Fu's poems on his daily happy life in Chengdu, it seems that Chengdu has been a holy land in my mind: a peaceful but vibrant place with flowers in bloom and the river, the tower, etc.(Alhough I have never been to Cheng du in reality till now, I know nothing can be more beautiful than Du Fu's poems.)