This text is taken from the Courrier de la Planète of October 11. Click here to subscribe.
“Street” trees, the ones we see squeezed into city sidewalks, would live better lives than their companions planted in parks. A team of Quebec researchers is digging into the roots of this beautiful problem. Despite the diseases that are decimating the ash and elm trees, the city trees are doing well. Very well. Even better than trees in parks, noted Alain Paquette, holder of the Urban Forest Research Chair at UQAM.
“It arouses a lot of reactions, because everyone thinks exactly the opposite, enthuses the enthusiast. In five cities, on several species and with hundreds of samples, we discovered that, systematically, street trees are not different from park trees and when they are different, it is in favor of street trees . »
They draw this conclusion from the small “carrots” of trees, these cylinders of wood taken from the trunks, to calculate the size of the rings of these. The bigger the tree ring, the rainier and better the year has been for the tree. The thinner the tree ring, the more the tree has suffered from the drought.
“Street trees tend to have more uniform growth from year to year, they are less connected with climate variability”, states the professor.
How to explain the vitality of these trees that grow in the middle of concrete surfaces? The hypothesis put forward by the researcher is that the trees drink directly from water leaks from the aqueduct and sewer network. “About a quarter of the water between the pump and the tap is lost in the pipes,” he says. So our hypothesis is that they have access to water that trees in parks do not have access to. »
On the contrary, the trees in the parks grow on very compact soils, which makes access to water difficult, emphasizes Alain Paquette. This observation could lead to other subjects of study: could the repair of underground water pipes to seal these leaks cause stress to street trees and harm their growth? This is a question on which it will be necessary to look into, indicates the biologist.
Restore fairness
In recent years, Montreal has accelerated tree planting. Ten days ago, the City boasted of having reached a canopy index of 25% for the Montreal agglomeration in 2019, three years earlier than expected, despite the devastation caused by the emerald ash borer.
Last week, the Montreal Metropolitan Community (CMM), which has 82 municipalities, including Montreal, however, released its own data. They show that between 2017 and 2021, the canopy index has rather decreased due to the devastation caused by the emerald ash borer and deforestation for the benefit of agriculture and the construction of homes.
We also note a disparity between the data collected by the CMM and those of the City of Montreal, which does not observe such a decrease in the canopy index on its territory.
Anthony Daniel, planning advisor at the Service des grands parcs, du mont Royal et des sports at the City of Montreal, believes that these differences are attributable to the methodology used. The resolution used [par la CMM] is much lower than ours. In addition to having validated the data with a very good resolution camera, we crossed this information with the LiDAR data [Light Detection and Ranging] and we had a geomatics technician going through the images with a fine-toothed comb. »
The City of Montreal is working with the boroughs to develop master planting plans in order to find the best strategies for increasing the canopy. The City also wishes to diversify the urban forest and avoid the massive loss of trees linked to pests and diseases such as those that have affected American elms and, more recently, ash trees. “We want to find the right tree in the right place,” says Anthony Daniel.
Alain Paquette believes that the challenge for a large city like Montreal is to reduce inequity in the distribution of green infrastructure. Data published in 2012 also revealed significant differences between the various Montreal boroughs.
The boroughs least well endowed with trees were former cities, namely Saint-Léonard (9.43%), LaSalle (10.04%), Anjou (10.19%), Saint-Laurent (10.80%) and Lachine (11.34%).
Tree of Life
It is well known that trees have many virtues: they produce oxygen, absorb atmospheric pollutants and provide shade, which helps to cool urban environments. In this regard, they are an effective tool in the fight against climate change.
But they offer many other advantages. Contact with green spaces reduces anxiety and stress, points out Dr. Claudel Pétrin-Desrosiers, president of the Quebec Association of Physicians for the Environment (AQME).
“Some studies show a decrease in crime and community tensions,” she adds, citing a study in disadvantaged neighborhoods in Brazil. “The list of benefits is really long. »
“Some publications show a faster postoperative recovery in people who simply have trees in their hospital window,” confirms Alain Paquette.
In children, a large Canadian study has shown that the abundance of trees in and around the schoolyard has a “significant effect” on performance in standardized exams. In other words, “those who had access to more greenery had better results”.