## 2 Previous Research and Background
### Thinning  
Thinning can be seen as the primary steering technique in preparation for the final harvest. Although many types of thinning have emerged throughout the history of forestry, the main objective of all of them is to remove some trees to accelerate growth in the remaining future crop trees. Notably, this enables the remaining trees to allocate the newly available resources mainly into their basal stem [2; 40], which is economically the most valuable part of the tree.  
Furthermore, by applying selective thinning, hence selecting the most promising future crop trees, the log quality can be significantly enhanced [41; 42]. Thus, thinning provides an opportunity to increase the amount of good-quality timber in the final harvest while producing additional income from sales of the removed trees. Since the main objective of many forest owners is to maximize the net present value of their forest area, thinned stands outperform unmanaged stands [4; 5; 6].  
One of the main concerns of performing thinning is the enhanced risk of damage to the forest due to high wind or snow. Various studies have shown that right after thinning, a forest stand's stability is lower due to the higher roughness of the tree crowns [43]. Although this is true for the time right after thinning, the risk of injuries decreases with time until no additional risk is present anymore [43; 44]. Nonetheless, thinning in Norway spruce stands has been recorded to reduce the h/d ratio of trees (height-to-diameter ratio) [45]. The h/d ratio is an indicator for the individual stability of a tree, where low h/d values are equivalent to high tree stability [46; 47; 48]. Thus, newer research suggests that early heavy thinning might even increase the stability of a stand [49]. Therefore, timeliness is crucial, as delaying thinnings results in an increased risk of damage to the stands [46; 50].  
Optimal thinning schemes differ by the tree species, any previously applied measures, and the density of the standing trees. Due to these variations, it is essential to define the type of thinning that is being used throughout this study. The study area is predominantly stocked with Norway spruce-dominatediferous forest. The researchers in [51] and, then later, those in [6] found that the highest economic return on investment is produced via selective crown thinning with a selection of target trees for this typical forest type. The Austrian Federal Forest adopted this thinning type as the standard thinning scheme, and thus, nearly all thinnings are planned as such. Therefore, when we refer to thinning in this study, it is equivalent to this specific thinning type.  
Thinnings of Norway Spruce stands of the Austrian Federal Forest are executed 2-3 times during the lifetime of a stand, where 1/4 to 1/3 of the actual stocking volume is harvested each time. The first thinning is carried out typically at around 13-19 m top height, followed by a second thinning at approximately 20-30 m top height, with a third thinning at good-growing sites. The exact timing of the procedure depends on the density of the standing trees, which is related to conditions such as the timing of the previous thinning and the growth performance at the site.