Urban trees for a cooler future - Growth patterns of urban trees in a changing climate
Urban trees provide important environmental services and are indispensable for the regulation of a city's climate, whilst growing in stressful conditions with low water and space availability. However, compared to their forest counterparts, little is known about urban trees' capability to cope with climate change. Due to the heat island effect, Montreal has already experienced 1.4°C higher temperatures, creating increasingly challenging conditions for urban trees. Planting resilient trees is necessary to minimize the efforts needed to ensure their health and performance and the provisioning of environmental services. Through dendrochronological analyses, we have compared the growth rates of four common urban tree species Norway maple (Acer platanoides), silver maple (Acer saccharinum), lime (Tilia cordata), common hackberry (Celtis occidentalis). Trees were sampled in the metropolitan area of Montreal across an urban gradient, from trees growing in pavement pits to lawns in residential areas, and in parks.
Contrary to expectations, in the last decades, the least accomodating conditions of the urban center have been showing an increase in growth rates. Both Acer species growing in pavement pits have shown a significant increase in growth rates, reaching the ones achieved by park trees. Climate correlations indicate a strong positive effect of autumn temperature, indicating an increase of the vegetative period in this area and therefore an increase in growth. Eastern Canada has recorded an increase in temperature as well as precipitation, explaining the consistent increase in growth in the last decades. Dendrochronological results proved to be a necessary assessment to understand the complex system of urban forestry.