Tree Forks in Urban Areas: Biomechanical Aspects, Structural Defects, and Management Strategies

The presence of tree vegetation in urban areas is important to the environmental, ecological, and social well-being of contemporary cities. Trees, in particular, play a fundamental role in mitigating the effects of urban heat islands, improving air quality, and promoting biodiversity. However, their growth and morphology are strongly influenced by the artificial conditions of the urban environment, characterized by limited space, compact substrates, and often incorrect pruning practices.

In this context, tree forks (two or more woody axes, with similar diameters, inserted at the same point), the main branches that determine the structure and stability of the tree, take on particular importance. Their formation, evolution, and response to mechanical stress are central to the sustainable management of urban greenery. Studying tree forks not only allows us to better understand the adaptive strategies of plants in human-influenced environments, but also to improve arboriculture practices and prevent structural failures that can pose risks to public safety.

Managing urban tree populations requires a thorough understanding of the biomechanical mechanisms that regulate tree stability. Tree forks are critical structural points, as their geometry directly influences wind resistance, the propagation of internal defects, and the transmission, dissipation, and distribution of wind forces on the canopy.

In urban environments, where trees are exposed to extreme environmental and management conditions (reduced root space, frequent pruning, water stress, pollutants, weak architecture), the likelihood of weak or defective forks increases significantly.

From a biomechanical perspective, the stability of a fork depends on the continuity of the reaction wood and the ability of the tissue to transmit longitudinal stresses. Analysis conducted using sonic tomography has shown that the presence of internal discontinuities, embedded bark, or eccentricity of the wood reduces the load-bearing capacity of the branch junction point compared to a healthy joint.

Fracture generally occurs by sliding along the plane of embedded bark or by longitudinal traction of the outer wood, with fracture patterns varying depending on the species and age of the tree.

The symptoms of bifurcations appear important; these symptoms can determine the risk of a bifurcation failing and determine the criteria for intervention or resolution. One expert on bifurcations is D. Slater (Duncan Slater: "Branch junctions: a classification system for arborist").

This English researcher highlights the symptoms of bifurcations: species type, bifurcation shape (U- or V-shaped), diameter ratio of the two axes, presence or absence of a ridge, internal anatomy at the bifurcation point, possible presence of cracks, joint containing other materials, presence of wood decay. Regarding species, some are more likelihood to bifurcation failure; for example, beech is very susceptible, plane is less prone.

Fork shape: the narrower it is (V-shaped), the greater the risk of failure. U-shaped bifurcations are less likely to fail.

Diametric ratio between the two axes. The higher the ratio between the two axes (secondary axis inserted at the bifurcation/main axis inserted at the bifurcation), the greater the probability of failure. Ratios ≥ 1 are symptomatic. Presence of a ridge, an outgrowth at the upper point of the fork. If present, the likelihood of embedded bark is lower, making the fork less likely to collapse. If the ridge is absent, the likelihood of embedded bark, not visible from the outside, is greater.

Presence of cracks, if present below the fork, are to be noted. These are partial subsidences and openings of the fork, detectable as longitudinal fissures in the bark tissue.

Presence of wood decay, which aggravates the condition. Wood decay is a group of fungal diseases. These parasites degrade lignin and cellulose, molecules that plants use to form their supporting tissues.

As an example, a survey carried out by the author of the book (Slater) is reported for a bifurcation in Beech (V-shaped joint, very tight), therefore with a high risk of breakage (Photo 1).

Another expert on branch junctions is Professor Edward Gilman of the University of Florida. In his manual "Prescription Pruning Qualification," he identifies symptomatic junctions, and he indicates the intervention criteria (photo by E. Gilman - Photo 2).

Other authors, for example, indicate how to perform a bifurcation survey, either as a follow-up during an inspection or to monitor the situation over time. For example, Simon Cox, in his "Aerial Inspections: A Guide to Good Practice," provides an example of a bifurcation assessment on a beech tree, including verification of the risk of breakage. The inspection was performed visually using only a tape measure and measuring the diameters (Photo 3).

Example of a fork in a beech tree, the tree is shown in the photo below (Photo 4).

Given the frequency with which tree forks occur in urban environments, numerous dedicated studies have been conducted, with in-depth analyses and monitoring of various field situations. Another researcher who has studied forks is Lothar Vessoly; in "Tree Statics and Tree Inspection," he discusses several types of junctions (Photo 5).

In the case reported below, it was not the wind, however strong, that caused the collapse of one of the trunks, but rather a structural defect, consisting of a symptomatic bifurcation, with included bark, a high ratio between the two trunks, and a V-shape. The collapse could have been foreseen since it was symptomatic. Measures could have been implemented to reduce the risk (photo by E. Gilman - Photo 6).

There are also documents, based on scientific data but intended for general public use, on the proper care of trees. For example, the Georgia Forestry Commission has produced a handbook for private tree owners. It contains simple tips for proper tree care, including identifying a single trunk as the best structure for wind and weather resistance (Photo 7).

Another example for understanding forks is the "Arborist's Certification Study Guide," the manual for preparing for the arborist exam.

Numerous examples of forks with a high risk of breakage are reported; here's a published example (Photo 8). 

Other references concern the wind resistance of plants; some species are very resistant, others very little. In windy areas, the choice of plants to plant will focus on species with marked wind resistance. For example (photo by E. Gilman), during Hurricanes Jeanne and Charley, two species performed very differently, although the wind speeds in the two cases were comparable: Pinus clausa survived by 4%, Taxodium distichum survived by 95% (Photo 9). Another useful example to understand the problem, in the photo a plant with structural defects, such as bifurcations and included bark, not very resistant to wind (photo E. Gilman - Photo 10). In this case, a bifurcation collapsed during a wind event. The culprit was not the wind, but the pre-existing structural defect. The defect could have been seen from the ground, and the risk of failure could have been estimated before the wind event.

This step is only possible by knowing the symptoms of bifurcations. Plants with a single trunk are more resistant to wind (photo E. Gilman - Photo 11). Example of a symptomatic junction in Beech, with a high risk of collapse: very narrow bifurcation and cracks at the base of the bifurcation (Photo 12). 

Generally, reducing the risk of bifurcations is possible. For example, depending on the severity, interventions can include branch selection, thinning, cabling, reduction.

Branch selection from a bifurcation can be performed when the branches inserted into a junction have a small diameter. Otherwise, the wounds inflicted by pruning are large, increasing the risk of infection by pathogenic fungi. The objective in this case (selection) is to restore the crown; with multiple selections of woody branches, the final trunk is achieved, aligned with the underlying trunk.

Thinning is a type of pruning aimed at reducing the force of the wind. It is performed with cuts of a smaller diameter, resulting in a less dense crown; trimming the branch tips is not required. Bracing, on the other hand, involves tying the tops of the branches that make up a bifurcation with appropriate patented systems. If there is a high risk and replacement is not possible, the reduction is possible. Other tree fork management systems are implemented before planting, choosing species that are unlikely to collapse or have a natural tendency to form large branches (e.g., plane and linden). Wind-resistant species should be chosen, and, as a maintenance management system, large-diameter pruning should be avoided, as these can easily form forks or multiple woody axes in the same spot, thus altering the species' specific architecture. Investigation criteria include monitoring, at least annually if visual, and instrumental assessment (with tomography).

Branch junctions are key elements in analyzing the structural stability of trees in urban environments.

Understanding them requires an integrated approach, combining knowledge of wood anatomy, biomechanics, plant physiology, and advanced arboricultural techniques. Defect prevention and targeted management of critical junctions are fundamental tools for ensuring public safety, the longevity of tree populations, and the sustainability of urban greenery.

Luciano Riva

Leggi tutti gli articoli della rivista