Effects of thinning on the growth and the undergrowth diversity of Pinus sylvestris var. mongholica plantation in Saihanba area

The aim of this study is to understand the effects of thinning intensity on the growth and plant species diversity of Pinus sylvestris var. mongholica plantation in Saihanba area, and provide scientific basis for the scientific and reasonable management of the plantations.

Based on the on long-term monitoring plots, the DBH, volume, stand stock and species diversity of Pinus sylvestris var. mongholica plantations under different thinning intensities were studied.

After 3 years of tending thinning, the average annual growth of DBH (Diameter at breast height) and volume of each tree was significantly higher than that of CK, and the average annual growth of DBH of low (LT, 2 000 plant/hm²), medium (MT, 1 667 plant/hm²) and high (HT, 1 333 plant/hm²) thinning intensity increased by 22.22%, 48.15% and 81.48%, respectively, compared with CK (3 200-3 425 plant/hm²). The annual growth of volume was increased by 58.00%, 72.00% and 130.00%, respectively. Thinning changed the DBH class distribution of the stand, which changed from normal to lognormal. The annual growth of stock of the thinning forests was lower than that of CK, but did not reach a significant level (P>0.05), and there was a significant level between HT, MT and the control (P<0.05) in the annual growth rate, which were increased by 38.02%, 23.37% and 2.97% for HT, MT and LT respectively, compared with CK. The biomass of HT, MT and LT increased by 123.53%, 94.12% and 35.29% compared with CK, and the species number increased by 46.67%, 70.00% and 36.67%, respectively. The Simpson index, Shannon index and Margalef index increased with the increase of thinning intensity, but the differences among them did not reach a significant level; the biomass of understory plants was more sensitive to tending thinning than species diversity.

Thinning obviously promotes the growth of DBH, individual tree volume, growth rate of forest stock, as well as the biomass and species number of understory plants; considering the impact on the growth and diversity of the stand, the current suitable tending intensity is HT, under which the density is about 1 333 trees/hm².