Objective  The research aimed to study the relationship between initial thinning period and site of Chinese fir, and provide scientific basis for determining initial thinning period of large diameter timber cultivation.

  Method  The long-term observation data of 23-year-old Chinese fir plantation in Xiayang, Fujian Province of eastern China were used to study the variation law of annual DBH growth of target tree. According to the variation of annual DBH growth, the effects of site index and target tree with different plant number gradients on the first thinning period of Chinese fir plantation were analyzed.

  Result  (1) The site index significantly affected the DBH growth rate and annual growth of the target tree in the middle and young age of the stand. The higher the site index was, the faster the early growth rate of the stand was. (2) When the annual DBH growth decreased to 1 cm, the annual DBH growth of the 22 site index target tree was smaller than that of the 18 and 20 site index target trees; with the increase of forest age, the annual increment of DBH of target trees with different site indexes tended to be equal. (3) In the whole stand (CK), the first thinning period of Chinese fir plantation with the 18 and 20 site indexes should be at the ninth year of age; the first thinning period of Chinese fir plantation with 22 site indexes should be 10 years old. (4) When the target trees with different gradient numbers (T80, T60, T40) were selected, the initial thinning time of the 18 and 20 site index stands was selected at the 13th year; the first thinning time of the 22 site index was determined at the 11th year.

  Conclusion  The bigger the site index is, the earlier the stand enters the first thinning period. Based on the cultivation of large diameter timber, the first thinning period of Chinese fir plantation at site index 18 and 20 should be controlled in 9−13 years. The first thinning time of the 22 site index should be controlled in the early 10–11 years. The actual thinning time can be determined according to the actual situation of intensive forest management, site quality, economy, manpower and other conditions, but the thinning time in the 18 and 20 site indexes should not be later than the 13th year; the first thinning time of the stand in the 22 site index should not be later than the 11th year.