Tree Stability

Tree stability assessment aims to describe the propensity of a tree to incur structural failure. It represents one of the components that can be used in the risk assessment; the concept of risk, indeed, is conceivable as the combination of the potential for the fall of a tree or part of it in conjunction with the occurrence of a triggering event (“severity of damage”) and the probability that the falling can cause damage to things or people (“exposure”).

Tree stability assessment applied by PAN/De Rebus Plantarum is an accurate diagnostic analysis carried out through the application of specific recognized protocols associated with twenty years of experience. PAN/De Rebus Plantarum uses methodologies that are based on both deductive and inductive protocols. In the first case, we focus on the individual characteristics potentially correlated to the stability of the tree and we proceed with a probabilistic approach (comparison with tables or reference parameters); in the second case, the reactions of the tree are directly measured in its reference context. Depending on the single situations, on the purpose of the survey and the needs of the client, it is possible to choose the specific protocol to be implemented.

Once the stability survey has been carried out, it is possible to identify the interventions useful for the reduction of the danger and the temporal cadence of the subsequent monitoring.

Below is a brief review of the main diagnostic protocols for the evaluation of tree stability.

The “Visual Tree Assessment method” (VTA) consists of a detailed visual analysis of the tree, looking for possible indicators of structural defects or degradation processes that may affect the collar, the trunk, and the branches. A visual analysis may include the use of diagnostic tools to learn more about the condition of the tree and its defects. In particular, with the Resistograph, an instrument similar to an electronic sophisticated drill, it is possible to record the consistency characteristics of woody fabrics thanks to the measurement of the resistance encountered by its thin drilling needle during penetration.

In some cases, when the diagnostic picture obtained with the visual analysis is not clear or sufficient, it is advisable to proceed with a more accurate instrumental investigation on the points of the tree that showed defects related to a possible propensity to collapse.

Sonic tomography consists in the study of the propagation of sound through internal woody tissues. In particular, some special sensors are applied around the trunk section; these allow to evaluate the transmission time of the sound wave through the internal woody fibers and, therefore, allow to estimate its consistency. The tomograph is useful in the study of large trees and is a functional alternative in the case of trees with fungal diseases, which could spread through the holes of the electronic penetrometer.

“Static Integrated methods” (SIM) is an inductive instrumental investigation that allows evaluating the reactions of the tree, in its reference context, simulating the action of wind on the crown, in order to evaluate the risk of eradication or rupture. The response of the plant to the application of certain loads is measured with sophisticated sensors able to record the inclination and the degree of elongation or shortening of the wood fibers of the stem. Without causing injury, SIM application allows also to evaluate the root anchorage and therefore the integral stability of the tree.

An example: stability analysis in Villa Pisani National Museum

PAN/De Rebus Plantarum team at work with Visual Tree Assessment (VTA), sonic tomography and pulling test (SIM, Static Integrated Methods).