Environmental disturbance in natural forest and the effect of afforestation methods on timber volume increment in Pinus sylvestris L. var. mongolica Litv.

The renewal of temperate coniferous forests has a high impact on the global climate, as well as the survival of forest wildlife. As an important native afforestation tree species in northern China, Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) is facing the dilemma of severe regeneration and forest growth. External factors, such as natural disturbances or human intervention, may affect the forest growth potential. Therefore, the present study explores whether environmental factors and afforestation methods affect the growth increment of Mongolian pine. To answer the question mentioned above, natural forest and two kinds of afforestation forests are included in this study. The growth increment is calculated by measuring basic growth parameters. A linear mixed-effect model is used to test whether surface fire, accompanying birch, edge effect, afforestation method, and tree age were determinants of the growth increment. With the model selection, the tree age combined with fire or afforestation is determined as significant effects on the growth increment. The effects of growth position and accompanying plants only influenced young pines. The effects decrease with age, implying the resource competition occurs at young ages only but not affect the adults of dominance trees. Since the growth rate of Mongolian pine varies with age, the environmental selection of timber volume is age-dependent. However, we also found that the growth increment slowdowns if too-fast growth investment in secondary growth at a young. Strengthening the competitive pressure of seedling afforestation is conducive to selecting individuals with a high growth increment and delayed growth cessation. Accordingly, fast-growing trees that survive fires are recommended as sources of seed-harvesting mother trees. Sowing afforestation that is easier to adjust the growth rate of seedlings is recommended than the seedling afforestation. Since external disturbances underlie the age-dependent selection of Mongolian pine growth, competition and environmental pressures at a young age are beneficial for prolonging the growth period and increasing timber volume accumulation. From this study, it can be seen that moderate fire disturbance and the selection of seed from healthy trees for tending are beneficial to the regeneration of temperate coniferous trees and the maintenance of healthy forests.