2. Because "Lu You is paying attention to phenology. He not only pays attention to phenology, but also uses it to predict farming time. "
3. The word "Ye" cannot be removed. The word "leaf" here is a continuation of the above, because people in Tang and Song Dynasties sang "early spring" in phenology, and besides "picking green trees", there are also "migratory birds", so the word "leaf" was used.
4. "Must" cannot be deleted. "Must" plays an important role here, emphasizing that "phenology varies with time and place".