Young trees and trees in the stands
Trees growing in stands often reach great heights with small trunk diameters. A basic safety calculation may yield values less than 1.0 if reduced wind exposure resulting from the sheltering effect of neighbouring trees is not factored into the calculations. In these scenarios the creation of clearings within the stand or heavy thinning, can remove the shelter and increase wind loads before the remaining trees have the chance to adapt by developing larger stem diameters.
In young open grown trees with high degrees of slenderness, the calculated basic safety factor can often be less than 1.0. In this case, the load-bearing capacity of the stem is increased (beyond that represented in the SIA model) because the outer areas of the trunk cross-section are pre-stressed or subject to growth stresses. This is an interaction of form and material: in the case of a solid wood cross-section, the interior of the trunk supports the tree’s own weight, while the exterior is under tensile stress as a result of cell growth. The load-bearing capacity of the trunk cross-section increases because green wood first fails under compressive loading in the outermost fibres, which is alleviated by the prestressing (Wessoly & Erb 2014).