Why does Wu Tong express the artistic conception of sad autumn in ancient poetry?

"Wu Tong" comes from Mei Cheng's "Seven Hairs", "The tung tree in Longmen is a hundred feet high and has no branches ... basically it has been dead for half a lifetime", and it was later used as a metaphor for widowhood, so later generations also have the sentence "Wu Tong is half dead, and the white mandarin duck loses her companion after clearing the frost" (He Zhu's "Partridge Sky"). It inherits the exquisite and long-standing historical origin of the phoenix tree image, which itself contains the meaning of mourning, loneliness and loneliness, and has a strong China classical meaning. "It often appears in ancient poems, such as Lonely Wu Tong's Deep Courtyard Locking Clear Autumn (Li Yu) and Wu Tong Xiaoxiao (Li Qingzhao). As for the use of phoenix tree rain to express sadness, there are countless more. Lu You's poem "Qiu Si" also contains the sentence "Wutong builds a mine field, Sagan cries cold day and night".