Preservation of Old Urban Trees

Trees are an essential component of urban green spaces, with a wide range of positive effects on people and the environment. Older trees make a comparatively outstanding contribution, not only for aesthetic reasons, but also for ecological reasons (air quality, CO2 sequestration, ...) and in particular as biodiversity hotspots and habitats for countless, often endangered animal species. However, older street and park trees are not only naturally more prone to damage (due to their age), but above all due to their specific urban "living conditions": collision damage and construction sites lead to rotting of the trunk and crown base, while unprofessional pruning measures lead to rotting in the crown - in each case with a potential risk of breakage later on. Roots, in turn, are damaged, especially during ground and shaft work, which leads to rot that can endanger stability. Soil compaction, in turn, not only restricts vitality, but also reduces the trees' ability to defend themselves against damage and pests.

On the one hand, German legislation, which is primarily based on EU regulations, encourages cities and municipalities to preserve old trees for as long as possible. On the other hand, however, the people responsible for monitoring "traffic safety" in the municipalities are personally liable in the event of damage caused by breaking or toppling trees, depending on how their behavior is legally assessed. There is therefore an enormous need for technical procedures to inspect trees and for methods to assess the results obtained with regard to breaking and stability. Initially, the so-called VTA method ("Visual Tree Assessment") was considered the standard here, especially in conjunction with drilling resistance measurements to determine the so-called residual wall thickness to radius ratio ("t/R"). However, it quickly became clear that the frequently published specification in this regard ("drill between the roots") was incorrect and misleading for the old urban tree and that the so-called "t/R>1/3 rule" has no relevance for these trees anyway for biomechanical reasons. The formulas of the "engineering" method of "statically integrated estimation" (SIA), which emerged in the 1990s as an alternative to VTA, were adopted from metal construction and proved to be fundamentally wrong and misleading for the structural assessment of trees. The "Allometric self-referencing" method was therefore developed. It not only takes into account the anisotropic material properties of the wood, but also the specific properties of old trees. Together with correctly executed drilling resistance measurements and acoustic tomography, this combination not only allows a reliable assessment of the breaking and stability of trees, but also reduces the costs of maintaining the tree population.

The URBORETUM project aims not only to simplify and accelerate the application of the above-mentioned technical examination methods, but also to develop largely automated evaluation and assessment methods to enable local authorities to assess their tree population more quickly and cost-effectively, but also with legal certainty, and to preserve it for longer at lower cost.