# Stand Structure Impacts on Forest Modelling
## 4 Diameter at Breast Height and Basal Area
The model developedfor the uneven-aged stands outperformed that of the even-aged stands and the variables with the highest predictive ability were diameter at breast height, total height and site index. Ref. [38] modelled the diameter growth for _Pinus ponderosa_ uneven-aged stands with three levels of basal area in USA with field inventory data and a generalized growth and yield model (GENGYM). The function developed was compared with the models for even-aged stands frequently used by forest managers. The authors found that, at stand level, predictions were within the range of the precision of the measurements, while error at the individual tree level was 44% when compared with the even-aged functions; the model developed for uneven-aged stands improved in performance when crown ratio was included as an independent variable, and the error in basal area estimation was higher in the densest stand, probably due to the higher proportion of supressed (small) trees. It is likely that inaccurate estimates of diameter growth resulted in bias in the dynamics of stand structure in time, a point of importance where the target is attaining (or preserving) complex stand structures. Ref. [54] fitted diameter increment models for _Picea abies_, _Pinus sylvestris_, _Betula_ spp. and other broadleaved species in pure, mixed even-aged and uneven-aged stands in Norway with national forest inventory data and mixed models. The models for conifers, in general, outperformed those of the broadleaved species, probably because the latter comprised a group of species with different growth patterns. The stand structure was included in the model as a dummy variable, being able to differentiate the diameter growth patterns in even-aged and uneven-aged stands. One drawback encountered was the bias of the models, which might have been related to multicollinearity, due to the number of independent variables (between 10 and 13). Ref. [55] studied the effect of structure on the average basal area increment for six species in both even-aged and uneven-aged stands in Finland, with mixed models. The variability of growth due to species, stand structure complexity and site was accommodated in the models. The authors were able to detect similar net periodic annual basal area increment (PAI) in both structures, while for the even-aged structure PAI increased with the increase of growing stock, in the uneven-aged structure the relationship was curvilinear, and the stand structure complexity in even-aged structures enhanced PAI, while in uneven-aged ones it decreased (the latter probably due to the increase of the number of the small trees with low growth rates). Ref. [56] modelled the basal area for a natural oak forest with inventory data and nonlinear seemingly unrelated regression. Site quality, stand age, density and canopy layers were the predictive variables of the model. The models used either one canopy layer or three canopy layers, and the latter outperformed the former. The better performance of the model that included the canopy stratification was due to the different growth patterns in each layer. This is in conformity with Ref. [57]. Ref. [58] evaluated the effect of stand development stage on diameter growth patterns of _Pica abies_ in Finland with national forest inventory data and mixed models. The author reported that mean diameter of basal area weighted by diameter distribution (d\({}^{\text{Gm}}\)) described the development stage better than age, especially for the shade-tolerant species in low light levels (in shade). This is attributed to the species traits: shade-tolerant species are able to regenerate and survive supressed in the lower stand layers, not losing their ability to increase their growth rates after release. The same author demonstrated that mean tree size rather than age determines the patterns of growth, and for a certain d\({}^{\text{Gm}}\) the variation of the growth patterns is small. The influence of the silvicultural system on diameter increment was reported by Ref. [59].