Modeling the productivity of forest plantations with the different spacing the trees

The main objective of this study was to assess the biomass productivity of short-rotation forest plantations depending on different spacing between trees. Our simulations were focused on the birch plantation with the initial density of 2500 trees$\cdot$ha$^{-1}$, which could be reached by using two planting schemes: with the distances between trees in a row and between rows, respectively, 2 and 2 m or 1 and 4 m. We used the model EFIMOD-fbp-REG, and the soil-climatic data relevant to the Eutric Podzoluvisols disposed in the Republic of Mariy El in the Eastern part of European Russia. The results showed greater biomass of trees in the planting scheme of 1$\times$4 m. With the scheme of 2$\times$2 m, increments of diameters and heights of most trees decreased at the age of 15–20 years, and then the total number of trees strongly reduced. Intensive death of trees with the scheme of 2$\times$2 m was due to the procedure simulating the self-thinning of trees, which was used in the model. It is based on the use of the threshold ratio of an annual biomass increment of the tree to the total tree biomass, $I_P/B_T$. With the planting scheme of 2$\times$2 m, the almost simultaneous closing of crowns and root areas of neighboring trees in rows and between rows was the cause a competition for light and for available soil nitrogen, and resulted in a sharp reduction of biomass increments. As a consequence, the value of $I_P/B_T$ was below the threshold. With the planting scheme of 1$\times$4 m, step-by-step (or gradual) closing of crowns and root areas of trees (first in the rows and then between rows) reduced the competition effects, that ensured a higher growth rate of the tree stand in the next years.