Abstract:

Objective: To determine the effect of initial planting density on the dominant height growth of Chinese fir plantations and its effective range for indicating site quality, as well as to provide a scientific basis for the evaluation of dominant height growth and effective density regulation of Chinese fir plantations, the impact of initial planting density on the entire growth process of dominant height of Chinese fir plantations in various production areas was explored. Method: The eastern region in the middle subtropics (Fenyi, Jiangxi) and the central region in the middle subtropics of Chinese fir (Naxi, Sichuan) were used as the research objects to analyze the response dynamics of dominant height to initial planting density of Chinese fir plantations in different production areas and different stand growth stages. A total of 30 long-term positioning observation plots of Chinese fir density test forests (ranging from 2 to 30 years old) were used. Result: Dominant height was negatively correlated with the initial planting density, the lower the initial planting density was, the higher the dominant height became. In the early stage of stand, the growth trajectory of dominant height with different initial planting densities almost overlapped, and the difference among the densities was small. With the increase of stand age, the difference of dominant height growth among initial planting densities increased. At the mature stage, the effects of initial planting densities on dominant height varied with distribution areas, which was still obvious in eastern region in the middle subtropics, but gradually weakened in central region in the middle subtropics. The mean annual increment and current annual increment of dominant height were negatively correlated with the initial planting density, and the higher the initial planting density was, the smaller the peak reached and the earlier the peak appeared. Under the same stand age, compared with the central region in the middle subtropics, the eastern region in the middle subtropics with the overall better site condition had a higher differentiation degree of dominant height by the effect of initial planting density. Initial planting density had no significant effect on the dominant height growth in the range of low and middle initial planting densities (1 667–3 333 trees·hm^?2) or high initial planting densities (6 667–10 000 trees·hm^?2) after 5 years in eastern region in the middle subtropics and during 10–19 years in central region in the middle subtropics. When the initial planting density reached or exceeded 5 000 trees·hm^?2, the initial planting density had a significant restriction effect on the dominant height growth, compared with the relatively low initial planting densities (1 667–3 333 trees·hm^?2). Conclusion: With various initial planting densities, the growth process of dominant height in stands varies dramatically. The initial planting density has no significant impact on the dominant height growth of the forest within a specific range of initial planting density variations. When the initial planting density exceeds a certain range, there is a significant difference between high density and low density. At the standard age of 20 years, the maximum difference value of dominant height is 2.46 m among the density combinations with significant differences in eastern region in the middle subtropics, which exceeds one site index class. At this time, it is not accurate to use dominant height to indicate site quality. The influence of density should be corrected for the site index prediction of the stand with initial planting density higher than 5 000 trees·hm^?2.

Key words: Chinese fir, dominant height, initial planting density, mean annual increment, current annual increment