# Stand Structure Impacts on Forest Modelling
## 2 Factors Driving to Stand Structure Dynamics  
Stand structure is dynamic in space and time and is influenced (and also influences) water and nutrient availability and uptake, light, climate, species, tree spatial distribution and interactions among individual trees. The higher the water and nutrient availability (and uptake), the higher the growth of a tree, due to the increase of leaf area index [9, 10]. It also determines the aerial/root ratio. This ratio increases with the increase of the availabilityof water and nutrients, as the tree does not need to invest in the root system to uptake water and nutrients [11; 12]. Inversely, in sites prone to drought and/or poor quality sites, trees have to invest in the development of the root system to ensure the uptake of water and nutrients [13; 14; 15; 16; 17; 18]. Yet, water deficits can be mitigated by water hydraulic redistribution or by their use in a more efficient way [14; 19]. Frequently, mixed stands with sociable species tend to have a more efficient use of water and nutrients due to the different uptake patterns, thus promoting complementarity rather than competition [20]. In short, aerial/root ratio tends to increase with the increase of water and nutrients availability. The social status of a tree (i.e., their position in the canopy) determines the amount and quality of light reaching the crown. While the trees in the upper canopy receive more direct sun light, those in the lower canopy layers receive more diffuse light [21], the former have, in general, higher the leaf area index and growth. This is due to the direct proportionality between absorbed photosynthetically active radiation (APAR) and crown area, while inverse proportionality is observed between absorbed photosynthetically active radiation and crown length [22]. The absorbed photosynthetically active radiation and, consequently, the growth, is frequently higher in mixed stands than in pure ones [22]. This is due to different tree species phenology, which results in a reduction of competition [22]; different tree species growth dynamics and physiology (e.g., tolerance to shade) that result in the stratification of foliage [23]; wider range of the distribution of the foliage in the canopy [24]; wider range of variability in tree allometric relations, such as height, shape and crown [22]; and higher leaf area index and absorbed photosynthetically active radiation as a consequence of the differentiation in the horizontal plane [25]. The concentration of the crown in one layer in the canopy, especially in pure stands, results in higher competition for space and absorbed photosynthetically active radiation, and the crown sway and branch abrasion can originate in the reduction of the crown dimensions [22]. Additionally, species' traits originate different allometric relationships for the individual trees, for example shade tolerance. Shade intolerant species in the lower canopy layers tend to reduce their crown (both width and length) and eventually die due to the lack of direct sun light, because of the shade cast by the trees in the upper canopy layers. Inversely, shade-tolerant species reduce their growth and the length of the crown but increase their diameter [26; 27]. The forest microclimate, which is affected by stand structure and macroclimate, can too influence tree allometry. This is the result of the amount of water reaching the soil and temperature, which affects the soil-water evaporation rate, thus affecting water uptake and growth patterns of the trees [28; 29]. Density influences tree allometry: the higher the density and the higher the irregularity of the spatial arrangement of the trees, the higher the variability in tree' dimensions. This is basically due to two types of interactions: complementarity and competition. Mixed stands with sociable species tend to have complementarity interactions due to the different species traits [30]. Inversely, in pure stands, especially monolayers, competition tends to be higher as individuals have the same traits.