Changes of Tree Duration and Forest Tree Species Structure in the Eastern and Western Zhejiang Province during 1994 and 2019

According to data of reexamination of permanent sample plots for continuous forest inventory during 1994 and 2019, quantitative analysiswas made on forest structure and composition in the eastern and western Zhejiang in terms of unit area number and ratio of standing trees, and themathematical expectation of tree duration (METD). The results showed that METD of the three tree species groups (pines, firs, and broadleaves) in both the eastern and western part had a "N"-shape, indicating increased first, then decreased and increased steadily. The turning points were at the first period (1999—2004). Tree duration in the eastern part was longer than that in the western part within 25 years. Whether in the eastern or the western part, trees duration of pines species was the lowest among the three groups, while broadleaves species was the highest. The tree species composition changes in the eastern and western part were similar, pine group declined, fir group had no change, and broadleaf group increased. The proportion of broadleaves species in the eastern part was lower than that in the western part in the first three periods (1994-2009), and slightly increased in the latter two periods (2009—2019). The unite area number of broadleaves trees in the eastern part was less than that in the western part. The result concluded that the duration of broadleaves species in the eastern and western part of Zhejiang province increased, more in the eastern part, unit area number and proposition of broadleaved tree increased, indicating more stable and better quality of forest.