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.

To investigate the relationship between intrinsic water-use efficiency (iWUE) and forest growth of different afforestation species and their response to meteorological factors in the Saihanba area, and to provide a scientific basis for forest management in the context of global warming.

This study took Larix principis-rupprechtii, Pinus sylvestris var. mongolica and Picea meyeri in the Saihanba area as the research objects, and analyzed the relationship between iWUE and BAI and their responses to indicators such as temperature and SPEI, based on the climatology of the annual rings of the tree and stable carbon isotope techniques.

The results of the study showed that the water use efficiency of all three tree species showed a gradual increase over the 43-year period from 1975 to 2018 and had similar interannual trends, but the rate of increase varied, from fastest to slowest were Picea meyeri, Pinus sylvestris and Larix principis-rupprechtii, which rose at rates of 9.70, 5.75 and 4.40 μmol·mol^-1·(10 a)^-1; BAI of Larix principis-rupprechtii showed a significant negative correlation with iWUE (P < 0.01). There was a positive correlation between BAI and WUE_i in Picea meyeri, but it did not reach a significant level (P > 0.05) and there was no significant correlation between BAI and iWUE of Pinus sylvestris; The iWUE of all three species was significantly and positively correlated with the average temperature in July and the growing season of the year, negatively correlated with the rainfall in July and the growing season, and significantly and negatively correlated with the SPEI in all periods.

Combining the changes in iWUE of the three species and their relationship with BAI, Picea meyeri is more adaptable to future warm-drying climate change in the region, and Larix principis-rupprechtii is relatively less adaptable.