Acfas Congress | Plant trees, but which ones?

The City of Montreal wants to plant 500,000 trees by 2030. Planting trees is good, but planting the right species in the right places is even better. This is the motto of a Quebec study that would make it possible to considerably improve the diversity of the city’s urban forest in order to deal more effectively with climate change.

Survive climate change

It’s kind of an open secret: trees store carbon and can cool an entire neighborhood if there are enough of them. But they still have to be able to survive the changing vagaries of a climate undergoing profound change. To achieve this, the key is to improve the functional diversity index of the urban forest in Montreal. This is precisely the subject of a report prepared by Quebec researchers, which was presented Tuesday at the Acfas congress. The objective was to answer three preliminary questions before starting a tree planting program: where can trees be planted, where should priority be planted and which tree species should be favoured?

It will be warmer and warmer in Montreal

Remember that the City of Montreal has set itself the goal of planting 500,000 trees on its territory by 2030, including in neighborhoods that are most vulnerable to heat waves. Montreal will need it, according to the Quebec consortium Ouranos, which specializes in the study of climate change. Average temperatures in major Montreal neighborhoods will rise by 3 to 6 degrees Celsius by the end of the century, according to Ouranos models. But planting trees will not be enough, it is also necessary to ensure that the right species are planted in the right places. The study produced by the firm Habitat for the David-Suzuki Foundation presents an “optimized” planting scenario which notably takes into account vulnerability to five climatic hazards: heat waves, droughts, heavy rains, floods and storms.

Improve diversity

The firm Habitat, founded by researchers Jérôme Dupras, Christian Messier and Andrew Gonzalez, was able to establish the functional diversity index of the urban forest in Montreal. This one currently has a rating of 3.7 out of a possible nine points. According to the report, the functional diversity of trees is based on their biological characteristics and “their ability to respond to various known stressors, such as drought, soil compaction or flooding”. “For example, maples are resistant to rains and floods, but vulnerable to episodes of droughts and high winds, while oaks have a higher drought tolerance,” the report says. According to the researchers, the diversity index of the Montreal forest could climb to 8.2 points by planting tree species “with complementary biological characteristics and vulnerabilities”.

An opportunity to do better

The study presents five families of tree species, which possess different characteristics, explained to The Press Sylvia Wood, director of research and development at the firm Habitat. “The important thing is to have better biodiversity in the urban forest,” she explains. Often, the same species of tree has been planted in a street. In some sectors, there are only maples while there are only lindens in other sectors. The species are not evenly distributed, but there needs to be more variety in each area. According to Sylvia Wood, the emerald ash borer that has hit the Montreal canopy hard represents a rare opportunity to improve the diversity of urban forest cover.

The dangers of the status quo

In the event that 500,000 trees were planted while respecting the same proportions of species that are currently found in Montreal, the functional diversity index would virtually stand still, falling from 3.7 to 3.9 points on a nine point scale. “Functional diversity is the most relevant indicator to consider when planting new urban trees and ensuring the long-term sustainability of this cover,” says the report, which was officially unveiled last November. “You have to quickly choose the right species [à planter] to cope with climate change”, adds Silvia Wood, considering that they do not all reach their adult size at the same speed.

Plant in less favored neighborhoods

In a “scenario of equity and climate resilience”, it is in the borough of Rivière-des-Prairies–Pointe-aux-Trembles where the most trees should be planted, i.e. close to 84,000 by 2030. According to the Habitat study, it is the borough of Saint-Léonard that would benefit from the greatest increase in its canopy, at 7.1%, followed by Montréal-Nord (6.6%) and Villeray–Saint-Michel–Parc-Extension (6.3%). A scenario that would partially compensate for an inequity observed in several scientific studies: there are generally fewer trees in the least favored neighborhoods in large cities.