For the fracture resistance of the trunk, a traceable progression of fibre compression up to the irreversible point of failure has been determined based on multiple tests in which trees were loaded to failure. The relationship between applied load and fibre compression is linear up to the point of primary failure. Therefore, extrapolation can be used to infer the force that would cause failure, even if only small loads and weak tree responses were recorded by the pulling test (cf. Detter & Rust 2013, Wessolly & Erb 2014).
For root stability assessment, regardless of the tree species, a clear correlation between the inclination at the base of the trunk and the applied load has been found. In the pulling test, the tree is loaded only up to a maximum stem base inclination of 0.25°, and usually well below this value for larger trees. Subsequently, the response of the tree to the overturning moment is extrapolated up to the point of failure using the so-called generalized tipping curve (Wessolly & Erb 2014). Recent research has confirmed a significant relationship between the low tilt responses of the tree in the pulling test and the anchorage of the tree in the soil (Detter & Rust 2018, Detter et al. 2019). Therefore, the expected load capacity of the root system of the studied tree can be reliably estimated from the results of the pulling test.