# Stand Structure Impacts on Forest Modelling
## 4 Diameter at Breast Height and Basal Area
In stands of _Picea abies_ and _Pinus sylvestris_ in Sweden, the effect of the shelterwood system, with two densities and no filling, were studied with forest inventory and stem analysis using generalised linear models (GLM). The authors reported that the model was able to capture the variability in diameter increment for the three treatments. Similarly, Ref. [60] evaluated the effect of three forest systems (clearcut with standards, basal area retention and no harvest) on basal area growth, with linear mixed models. The model enabled the identification of the variability of growth. In the no harvest treatment, the trees of the upper canopy layer (dominant and codominant) were the ones with the highest growth, while in the lower layers' tree mortality occurred. The clearcut with standards resulted in a high density of regeneration individuals with high growth rates. In the basal area retention treatment, regeneration and recruitment occurred at a slower rate than in the clearcut one. The growth differences were due to the available growing space, which were higher in the clearcutting system than in the basal area retention one. Ref. [61] studied the diameter increment of adult trees of _Abies alba_ with increment cores in two areas of different elevations in France with linear mixed models. The authors' main results were: competition explained 30% of the variability of diameter growth; a correlation between climatic variables (e.g., temperature and precipitation) and growth both from the present and former years; drought in summer (present and past years) reduced diameter growth; yearly spring temperature tended to increase diameter growth; drought that occurred in the previous years had a positive effect on diameter growth; variability in diameter growth per site (temperature, precipitation, relative humidity and elevation) was influenced by either the species plasticity or genetic adaptation; and growth reaction to summer precipitation had high variability, due probably to the variability (heterogeneity) of the micro edaphic variation. Ref. [62] (and references therein) demonstrated that low temperatures were the limiting factor of growth when elevation was high, while in the Mediterranean climates the limiting factors were drought and high temperatures. The authors observed that _Fagus sylvatica_ growth diminishes with the increase of elevation because it is very sensitive to spring frost, while _Abies alba_ had optimal potential growth at middle elevations, probably due to its sensitivity to the water deficits in summer. Ref. [63] evaluated the effect of drought events on the diameter increment of _Picea abies_ and _Fagus sylvatica_ stands in duplets with linear mixed effects models. The two species have different strategies to water stress. The mixed models were able to identify the variability in diameter growth, both the reaction per species and of both species, either under a drought event or not. Ref. [64], using increment cores and generalised additive models, analysed the effect of climate on the diameter increment of _Picea abies_. The model was able to capture the influence of precipitation and temperature on diameter growth. Precipitation promoted growth was denoted by the positive linear trend of the predictor (especially from May to July), whereas temperature had a non-uniform behaviour. Several regression methods have been used to model diameter growth. Ref. [37] studied the effect of spacing on the diameter growth of _Pinus ponderosa_, _Pseudostuga menziesi_ and _Sequoidendron gigantum_ in the USA with data from field plots and linear regression. The authors observed that there was a positive correlation between spacing (and, thus, growing space) and diameter at breast height for all species, which was probably due to the low competition levels between individuals. Another spacing trial was used by Ref. [36] with _Sequoidendron gigantum_ in the USA with field data plots and generalized linear mixed-effects models.