# Improving a Process-Based Model to Simulate Forest Carbon Allocation under Varied Stand Density
## 4 Discussion
### Effects of Stand Density on Carbon Allocation and Stand Growth  
In our study, more carbon was allocated to the stems, and less was allocated to the nutritional organs, especially the fine roots, as the density increased. With the increase in density, the percentage change of the stem carbon allocation was from \(-0.9\%\) to \(1.9\%\), the change in leaf allocation was from \(1.0\%\) to \(-3.3\%\), and the change in fine root allocation was from \(4.0\%\) to \(-5.2\%\). With the increase in density, the leaf and root allocation slightly decreased and the stem allocation slightly increased, because the near-unidirectional nature of the light in the closed canopy was related to the position of the leaves at the top of the canopy, which are crucial for light interception [5]. However, other studies have reported contradictory results. For example, stem growth has been shown to increase significantly under uninhibited conditions and is the highest under medium shade conditions [45]. With an increase in competitive pressure, trees invest more carbon in their photosynthetic organs to compete for limited light resources [3]. In stands with a low stand density, trees are usually not inhibited by light, water, or nutrition conditions. Under these conditions, trees allocate more resources to nutritional organs for increasing photosynthesis and absorbing more nutrients in order to grow fast. When the forest cover rate or density is high, the competition for resources such as light, water, and nutrients restricts tree growth. Under these conditions, to better collect light, trees allocate more NPP to the stems for achieving the highest canopy. During water or nutrient stress, the allocation of carbon to fine roots increases to maintain water absorption and growth [4]. Therefore, competition for light under high density conditions increases the proportion of stem carbon allocation. We found that competition induced by a high stand density negatively affected DBH growth. The growth range of DBH decreased with the increase in stand density. Our results are consistent with those of previous studies [46; 47; 48]. Most previous surveys of other tree species found similar results [49]. This occurs because, under the condition of canopyclosure, the competition between individual trees for nutrition restricts the growth of DBH. To compete for more nutrients, nutrient organs such as foliage and fine roots grow fast, while the stems grow relatively slowly. In accordance with previous studies, we also found that density had little effect on height. According to a study on the impacts of different planting densities on growth factors and the physical and mechanical properties of _Populus_ wood, [PERSON] et al. [46] found that planting density had no significant effect on height growth. After a 7-year study on the effects of density on the growth of self-thinning _Eucalyptus uptohylla_ stands, [PERSON] et al. [50] found that density had a smaller effect on tree height than that on DBH. A study on eight different afforestation densities of southern type clones of _Aigeiros_ poplars showed that there was no significant difference in tree height among poplar stands with different densities before 7 years of age; however, there were significant differences when the stand age was more than 8 years old [51]. At lower densities, trees grow faster to dominate at the stand. As the density increases, trees compete for more sunlight by increasing their height. Therefore, regardless of a high density or low density, trees need to increase their height for better survival. The results of this study show that the biomass of poplar stands first increased and then decreased with the stand density, and the growth rate of the biomass slowed. Unlike our results, [PERSON] [52] found that the biomass decreased with the increase in density. This discrepancy may be attributed to the low density (severe thinning) and lack of competition between trees. The yield increased rapidly with the increase in density.