Abstract:

Objective: This study aims to systematically analyze the effects of initial planting density, site conditions, and thinning treatments on forest maturity age, clarifying the effects of thinning on the stand volume, net present value (NPV) and saw log production of Pinus sylvestris var. mongolica (Mongolian pine) plantations, so as to provide science-based guidelines for the management of Mongolian pine plantations. Method: Based on the fixed plot data of Mongolian pine, the study first developed a set of growth and harvesting models of Mongolian pine including individual tree diameter growth model and mortality model, height curve model, site index model, maximum-size density line coupled with taper equation, and then integrated them into a stand growth simulator. With the annual average volume, annual average large diameter timber volume, and maximum NPV as the criteria for quantitative maturity, technical maturity, and economic maturity, respectively, the simulator was used to simulate forest growth, and compare the quantitative, technical and economic maturity age (rotation length) of the plantations and the corresponding stand volume, NPV, and large diameter timber at mature stage under different thinning schedules. Result: The simulator was able to accurately simulate the development and thinning of Mongolian pine plantations, with an average relative error of 5% between the simulated values and the measured values of the sample plots. Under natural growth conditions, the quantitative maturity age of the stands at two initial planting densities (3 300 and 2 500 plants·hm^?2) was 44?45 a at SI = 15 m, 38?42 a at SI = 18 m, and 36?40 a at SI = 21 m. The economic maturity age at a discount rate of 3% was 50?54 a at SI = 15 m, 44?43 a at SI = 18 m, and 39?40 a at SI = 21 m. The quantity and economic maturity advanced with the improvement of site quality and the increase of initial planting density, but did not reach technical maturity within 80 years. For stands with medium-to-high site quality, light and moderate thinning delayed both quantitative maturity and economic maturity. The mean annual volume increment of the stands reached 91%–104% of that of unthinned stands at quantitative maturity, the total net present value (NPV) increased to 113%–141% of that of unthinned stands at economic maturity, and the production of large diameter timber advanced. Under severe thinning, the technical maturity for large-diameter timber was advanced to 60–65 years, with the mean annual volume of large-diameter timber at maturity reaching 106%–173% of that in unthinned stands at 80 years. Under this intensity, economic maturity was delayed by 4–7 years, with NPV at maturity being 102%–132% of unthinned stands. Under this intensity of thinning, the quantitative maturity in the stands with high initial planting density was delayed, while that with low initial density was advanced, yet the total stand volume at maturity was only 74%–89% of that of unthinned stands. When the residual stand reached quantitative maturity (their growth potential was fully realized), the mean annual increment volume of the stand was 93%–95% of that of unthinned stands, and the total stand volume recovered to 92%–128%. In contrast, for low-quality sites, after light and moderate thinning, the average annual volume growth of the stand at quantitative mature was 80% to 97% of the unthinned stand, and the NPV of the stand at economic mature was 94% to 107% of that of the unthinned stand. Although severe thinning advanced technical maturity to less than 60 years, the mean annual yield of large-diameter timber at maturity was only 45%–64% of unthinned stands at 80 years. Even when the residual stand reached quantitative maturity, the total stand volume was only 52%–61% of unthinned stands. Moreover, severe thinning on poor sites resulted in 50%–56% lower total stand volume at quantitative maturity and 18%–33% lower economic returns at economic maturity. Therefore, when large-diameter timber production is the management objective, mid-to-high site quality stands should be selected for moderate-to-severe thinning treatments. Particularly under heavy thinning regimes, close monitoring of the residual stands’ quantitative maturity age is essential to fully realize their productivity potential. Conclusion: In the context of efficient cultivation of plantation and construction of reserve forests, it is necessary to explore forest maturity under different thinning schedules based on simulation optimization technology, which can effectively improve stand volume, large diameter timber yield, and economic benefits.

Key words: Mongolian pine (Pinus sylvestris var. mongolica), plantation, thinning, growth simulation, forest maturity age