The "secret" in the sentence "secret only known by both parties" means that they all have a premonition that the tree and kingdom will be destroyed.
At the end of the article, the king said that "the old man can't die", which shows the king's repentance. He seems to have realized that forgetting history will lead to the decline of the nation and the country.
There are two clues in this novel. Open lines are the words of kings, while dark lines are the fate of ancient trees. The ending ends with "the old tree was cut down" and "the speech disappeared", and the two lines intersect to deepen the theme.
This novel adopts symbolic techniques, with vivid and mysterious images, profound implications and thought-provoking.