City trees are important: they clean the air, reduce heat islands, regulate the water cycle and offer enormous health benefits. Unbridled development, however, threatens the survival of the urban forest and the entire spectrum of its ecosystem services.
The size of these services is closely related to the importance of the canopy, the area covered by the treetops. It is generally characterized by an index that relates the sector covered by the peaks to the total size of an area.
A recent study of the natural canopy in the areas of Québec City, Beaupré, l’Île d’Orléans, Lévis, and other communities along the St. Lawrence River found an annual benefit of more than $ 1.1 billion Dollar is made.
Water supply, flooding reduction, air quality improvement, and carbon sequestration were among the ecosystem services – the benefits humans derive from the ecosystem – that were considered. In this context, several major cities have set ambitious goals for the extension of the canopy.
However, these goals face several significant challenges. Housing construction and infrastructure development and renovation tend to reduce urban roofing. Part of this reduction is directly related to the space required by the infrastructure, while another part is due to damage to trees during installation.
Construction work near existing trees can directly damage the aerial part of the tree and the underlying root system.
As a forest engineer and professor of silviculture and urban forestry, I studied the stability of trees against wind in natural, urban and rural environments. I was interested in the effect of species, soil type, and apparent defects on resistance to stem breakage and uprooting.
Building in a wooded environment leads to general changes in the growth environment of the trees. Opening the canopy exposes trees to stronger winds, which increases their water needs and can affect their stability.
Surfaces can become impermeable during construction in a wooded environment. This reduces the flow of water into the soil and promotes water runoff. Combined, these two factors can lead to water stress and ultimately stain trees a few years after they are built.
Construction work near existing trees can directly damage the air part of the tree, which is easily visible. Damage to the root system is much more subtle, but it can have more important consequences.
Roots play many roles, including accumulating reserves, anchoring them to the ground, and removing water and nutrients. Damage to the roots affects their ability to perform these various functions, which affects the general functioning of the tree.
During construction work near trees, excavations are often carried out that damage part of the root system. Studies have shown that pruning the roots can reduce resistance to uprooting – this effect can last for several years after the initial damage.
The extent of the effect is related to the extent of the root loss, a function of the distance from the trunk. Although the root extension can extend beyond the space under the crown, it is generally believed that the most important part is in this area. The effect of root pruning on tree stability also depends on the configuration of the root system.
The difficulty for the tree to replenish its water reserves after root damage can lead to the closure of stomata, the pores that control gas exchange during photosynthesis. This closure can then lead to a reduction in growth. After root destruction, the tree redirects the products of photosynthesis to rebuild the root system at the expense of the growth of the air part of the tree.
In some cases, the effects on growth and mortality are limited, but severely damaged trees take time to recover, which can make them more susceptible to other stresses. If the root loss is too great, the tree will fall over several years and eventually die. When the root system is able to rebuild itself, growth can resume its normal rhythm. Recovery is more likely if the digging is temporary and a material that is conducive to root development is reintroduced at the end of the work.
Further studies needed
Soil compaction by machines or the addition of soil above the original level can change the aeration of the soil and its water conditions. The diffusion of air to the roots and the diffusion of CO2 created by root breathing are then reduced, thereby impairing their proper function.
However, the effect of soil improvement will depend on the thickness and type of soil added. The installation of underground ventilation systems has often been recommended for large soil additions, but studies of their effectiveness are inconclusive.
In order to reduce the impact of the construction work, protective measures must be planned in advance and the construction site closely monitored. There are several standards and guides, but sometimes their recommendations differ. These measures are often based on a limited number of well-documented studies. Additional research is still needed to validate them and propose new ones.