Big trees are essential in big cities
Montreal, 17 October 2020: Locals enjoy the nice autumn weather in the Mont-Royal park.
Alison Munson, co-holder of the Research Chair on the Urban Tree and its Environment (CRAUM) and Anaïs Paré, research professional for CRAUM, analyze the effects of the presence of trees in large cities on the level of physical and mental health of individuals, but also in relation to the environment and climate regulation.
Trees are important elements of our urban landscape. With more than 50% of the world's population living in cities, it would be unimaginable to do without the many ecosystem services (benefits to residents) that they provide to us.
We have had proof of this in the previous months when sanitary measures were most restrictive: urban parks saw their attendance rate increase dramatically. And this is no coincidence! The presence of trees has the effect of promoting the physical and mental health of individuals, and this is among other things what may explain the fact that city dwellers have felt the need to find themselves in green spaces.
< p>In addition, the trees we see every day on private land, in streets or in parks provide a multitude of environmental and climate-regulating benefits, including noise reduction in cities, carbon capture , and contributing to the slower infiltration of water into soils.
Although all of a city's tree heritage plays a role in the quality and quantity of ecosystem services provided, trees on an individual scale do not all have the same characteristics, and therefore do not all have the same characteristics. same ability to deliver ecosystem services. It is therefore relevant to ask which trees are the most effective in delivering these benefits, why, and what management practices would promote them.
We are members of the Research Chair on urban tree and its environment from Laval University, which aims to find solutions to help the long-term survival of trees in urban environments.
Which trees provide the most benefits?
In general, taller trees have a better ability to store carbon (thus, capture carbon from the air to reduce CO2 in the atmosphere), reduce air pollution, and prevent more effectively the stormwater runoff. Indeed, trees with a larger trunk diameter have a greater woody biomass (amount of wood), which allows them to store more carbon than smaller trees. Similarly, the interception of precipitation and air pollutants would increase with greater canopy size (the crowns of dominant trees) and total leaf area (total area of all leaves) associated with greater height. Large trees are therefore generally more effective than smaller ones in providing these regulation services, which are essential in urban areas and especially in a context of climate change.
The inclusion of tall tree species in plantations also brings significant economic benefits. One study reports that the annual net benefit of planting tall tree species is 44% greater than that of a medium-sized tree species, and 92% greater than that of a small essence. Moreover, according to this same study, it would take less than five years from the time the tree is planted for the net benefits of these trees to outweigh the net costs. This can be explained in particular by the fact that tall trees have the effect of increasing real estate prices and land values of the land where they are located, in addition to reducing energy costs related to heating and cooling. air conditioning through microclimate regulation.
However, the aerial or underground spaces available in urban areas do not always allow the use of large-scale trees. Under these conditions, smaller trees can also make an interesting contribution.
How to manage our urban forests optimally?
As mentioned earlier, tall trees play a vital role in the delivery of ecosystem services. But the ability to deliver these services is conditional on one thing: the trees must be in good condition! Those in poor condition will have less ability to provide ecosystem services, since poor conditions impede growth, slow carbon sequestration and can also lead to canopy dieback.
In urban areas, it is not uncommon to see hostile environments that could hinder the growth and proper development of trees. Lack of space for the root system, soil compaction, limited soil moisture, the use of de-icing salts and air pollution present challenges for the survival of young plantations. As a result, several landscaping practices favorable to the growth and development of trees exist. Here are some examples:
- Choosing the right tree in the right place. Some tree species will be more suited to certain climates, or more tolerant than others to limited amounts of space, for example. There are several guides to dictate planting choices based on the characteristics of the environment.
- Avoid too frequent pruning of large trees, which would have the effect of significantly reducing the leaf area and woody biomass of individuals. One of the keys to reducing the need for pruning is in particular to choose a tree species adapted to a given location.
- Officially recognize the value of the ecosystem services provided by large trees to introduce policies that would support the conservation of these.
The importance of taking concrete action
Although it was pointed out that tall trees were generally more effective than smaller ones in generating certain ecosystem services, in a context where the climate is changing and where the resilience of forests is paramount, it is important not to put all our eggs in one basket and not only to plant species of large-scale trees.
Moreover, at the scale of a forest, the characteristics that are positively correlated with the production of ecosystem services are in particular vertical heterogeneity (the number of vegetation strata, ranging from herbaceous to dominant trees) and the shrub species richness, which is the number of different species present.
Finally, what must be remembered above all is that large trees are extremely important, and that we are winners in deploying efforts to preserve them. In addition, the planting of tall tree species should be encouraged, since there is often a tendency in cities to plant small-scale species. Thus, concrete actions can be taken today to make the most of urban trees now and in the long term.
Forest Ecology , urban ecology, urban soils, Université Laval
Research professional for the Research Chair on the Urban Tree and its milieu (CRAUM) of Université Laval