ObjectiveThinning program is crucial to the management of mixed forests. The diameter structure and growth changes of larch-spruce-fir stand after selective thinning were studied in order to help creating better thinning programs and improve stand quality.

MethodLarch-spruce-fir mixed forests including 12 selective thinning plots with different thinning intensities (light, 20%; medium, 30%; heavy, 40%) and 4 control (0%) plots were established. The effects of selective thinning and dynamic changes on diameter structure stand growth were analyzed based on stand inventory of the 5-11th year after thinning.

ResultWe found that medium thinning and heavy thinning had significantly increased the growth of basal area and stand volume, which conformed thinning program leading to faster stand growth because more space and nutrient were provided for standing trees after thinning. The diameter distribution (Weibull distribution) curves of control stand and medium thinning stand were center-skewed, which demonstrated small and medium trees dominated in these stands. While the diameter distribution curves of light thinning and heavy thinning stand were left-skewed which showed a higher number of bigger trees. The parameter c of Weibull function remained the same in unthinned stand, while it increased with time in thinned stands. This indicated thinning treatment optimized stand structure by removing inferior trees and increasing stand space. Simultaneously, thinning treatments could decrease tree mortality, however, heavy thinning increased the risks of wind throw.

ConclusionDifferent selective thinning schemes can be applied to the forest according to the management goal. For sustainable management, medium thinning faster the stand growth, which also resulted in a stable diameter structure with higher DBH range and less tree mortality, hence medium thinning may promote sustainable forest development.