Montreal, like many large cities, is adopting the strategy of increasing green spaces to adapt to global warming, particularly to counteract heat islands, increase biodiversity and canopy cover. There is also potential to increase carbon sequestration in soils and vegetation associated with these green spaces. We developed a project with the City of Montreal on a newly developed intersection, to test these benefits by proposing protocols to also decrease maintenance and to improve the durability of vegetation islands. Urban vegetation managers question the usual practices that generate year after year repetitive maintenance tasks of cutting grass, weeding and replacing dead trees and herbaceous annual plants. Our objectives were to propose an alternative protocol using mainly indigenous perennial plants, in particular mixes, that would be resistant to the urban microclimate and salt usage, would quickly cover the soil to block weeds, would not require replanting, and would improve local biodiversity, contribute to a cooler microclimate, sequester carbon in biomass and soil, and would contribute to more autosufficient urban ecosystems. We planted and followed different plant mixes for two years in a typical urban soil used for new green spaces along streets.
We have identified several species (out of over forty tested) that perform very well in this urban context. They were seeded and followed for aboveground height and biomass, cover, root development and carbon and nutrients sequestered in the biomass. Carbon sequestered in the soil was also measured, and a number of species show higher belowground development and contribution to soil carbon. The lack of information on germination rates, and properties of root systems of plants are a hindrance to the application of the principles of this protocol.
We also noted losses of soil carbon over the two years, that may be related to the texture of the substrates used. Organic matter levels were high, while soil texture was quite coarse (sandy loam), which means the organic matter (and soil carbon) are not protected by interaction with clay surfaces. This leads to potentially high losses of soil carbon over short time periods. We are doing measurements under similar contexts to validate if this is a more generalized problem.