The domestic pine in an urban environment

Domestic pine (Pinus pinea). Lately this conifer has been the subject of discussion due to the repeated crashes in the urban environment (in particular in Rome). After the many incidents that occurred with passers-by, almost always fatal, a hard line was undertaken aimed at limiting any (and inevitable) new disasters.

Elements of physiology of Pinus pinea

To understand what is at the basis of these sudden subsidence and how useless many of the pruning interventions that we see more and more often are, we must start from the physiology of the domestic pine, very different for example from another family of conifers, the Cupressaceae.

The development of a tree was effectively divided into 10 stages: (1-4) growth in height, (5-6) growth in volume, (7-8) maintenance and renewal of the structure, (9-10) reorganization of the same.

In the early stages of growth, we can say that the trend of Pinus pinea is rather compliant with the standard if not for the already visible tendency to develop a rounded crown. Generally in the growth phase in height, the apical dominance causes the plant to take the shape of an arrow (upwards) to then subsequently guarantee the conditions for volume growth.

Passing through this second phase and gradually reaching the maintenance stages of the structure, the apical dominance is interrupted and the classic umbrella shape is thus formed. It is here that the first real and important difference with most trees appears.

Usually, at this stage of development, the tree tends to block the growth of its lateral branches by developing new and vigorous branches. Through these structures the process of continuous and progressive renewal of the hair is started, leading to the maintenance of the same. Our pine vice versa does not have the possibility to develop these ramifications and will tend to grow its lateral branches indefinitely until their inevitable collapse. Not being therefore able to renew its vegetation, the pine will end up depleting the green mass causing its own death in a purely mechanical way. As you can imagine the last two stages of the reorganization of the structure are precluded, being impossible for the plant to reconstitute its crown.

We find another important difference in the root system. Famous for its superficial roots, the Pinus Domestica immediately develops an important taproot root (which develops in-depth). This taproot will remain active and functional until the death of the plant, while it is literally abandoned to the ground in the other taproot species.

The importance of the root system

To understand the importance of the taproot it is good to dwell for a moment on the buttresses and the dissipation of energy.

The buttresses are special anatomical structures that regulate the translocation of loads towards the anchoring roots, acting as a mechanical consolidation element between the trunk and the root system. The following figure is visible at the height of the tree collar.

In the case of the Pine, on the other hand, we have a very rigid trunk that ends on the ground in a linear way with the total absence of buttresses. It is therefore flexibly linked with the roots, compensating for the lack of the structures just described with a taproot very developed and mechanically active for all phases of life. Moreover, to overcome this structural rigidity and effectively combat the wind thrust, another strategy adopted is that related to achieving an increasingly aerodynamic shape with increasing size.

We can, therefore, say that the taproot acts in the case of the domestic pine as a dissipator of all the energy produced by the canopy with the thrust of the wind, thus representing a vital element for the stability of the plant.

What happens in an urban environment

We now come to the heart of the matter to understand what are the problems related to the management of Pinus pinea in an urban environment and why we hear more and more often about sudden crashes.

To find the causes, however, we must go back to the breeding of individuals destined to form road trees.

Due to the processing of the sod and the transplants to which the pine is subjected during the breeding phase, the regular growth of the taproot and the horizontal roots is constantly hindered.

Moreover, the latter, once the tree is planted, often tend to follow the circular shape of the root ball, continuing their development in this direction until they become choking roots.

In addition to these conditions, the implanted pine frequently finds itself having to deal with very difficult soil (soil) conditions.

Often it is a compacted and asphyxiated soil, perhaps near a road surface. Due to the exceptional propensity for adaptation of the domestic pine, thickened roots and nodules develop in the vicinity of hard and strong pressure elements (such as asphalt or artifacts) that form real plaques. These particular plastic conformations are the basis of road breaking but also the primary source of stability.

Often the root systems are literally devastated by work on the sidewalks or the road surface. Therefore, not having a taproot system, the only anchoring system is thus weakened, thus becoming the main cause of sudden collapses.

To date, there are numerous containment and protection systems of the root system in an urban context, still unknown in the planting age of the current adult specimens of Pinus pinea.

Further harmful interventions such as reloading earth on the tree collar (lethal for all species) or sudden changes in humidity such as the often installation of irrigation systems following the creation of turfgrass, often add up to the rest.

Wrong pruning

The race against time towards the safety of the highest possible number of specimens certainly does not encourage careful analysis and conscious interventions.

As has already been explained, our pine is not able to rearrange its foliage and therefore tends to constantly rise, prolonging its branches to the breaking point. Prunings aimed at "unraveling" the tree by depriving it of the lower branches, can make sense in the juvenile phase in which by nature it tends to lose the lower vegetation favoring the development of the upper one.

By eliminating important branches in adulthood, the plant will tend to emit new vegetation in the higher parts, increasing further and unnaturally the size. Raising the point of application of the wind forces will increase the lever and thus the forces exerted on the root system, already weak in itself.

Moreover, the removal of low branches that have developed the right elasticity to resist the wind will expose the less flexible internal branches to non-calculated forces. Less vegetation means less aerodynamic capacity on the part of the tree and no greater safety as is often thought. Finally, a large part of the roots that previously served these hair sections ceases to function and dies, further weakening overall stability.

The only interventions to be adopted against Pinus pinea in adulthood must be aimed at lightening the loads and reducing the levers (pruning thinning), and containment of the crown through return cuts, and the always necessary dry.

In summary, preferring a correct thinning while keeping the structure of the lower stages as unaltered as possible, is definitely the right way to go.