Trees in the City: We Need More Diversity & Density To Mitigate Climate Change

There’s a famous story of the renowned ethnobiologist Wade Davis being asked to do research on potential positive impacts of trees in the forest. Dr. Davis is said to have replied “They suck out carbon dioxide and turn it into oxygen-isn’t that good enough for you?”

The same can be said about the importance of tree canopy in cities. With climate change and more intense heat domes and atmospheric rivers trees provide shade, ground stability, and lower temperatures.

Viewpoint Vancouver has documented the City of Vancouver’s wrongheaded decision to allow the axing of trees on private property that were under 30 centimeter. Prior to that, trees above 20 centimeter were protected. That decision made to “speed up permits” resulted in  400 trees being slaughtered in the first  six months of 2021 , suggesting that the City was losing 800 trees a year on private property, four times more than estimated.

That’s how much we value trees. That decision was to be reversed this fall.

It is well documented that trees boost physical and mental health, capture and store carbon dixoide, scrub toxic particulate matter from trees, and cool local temperatures nearly a degree for every ten percent in forest cover.  There’s huge financial benefit for a “tree rich city”.

That’s part of what a group of American scientists discovered when researching the importance of diverse tree planting and canopy in this article that looked at five million trees in 63 U.S. cities.

These scientists were engaged in discovering why biodiversity in city trees was not championed, and questioned whether local climate impacted species diversity.

In the 20th century, planting of street trees and picking of street trees was the purview of the Park Board even though the street boulevards are the jurisdiction of the City Engineering Department. In Vancouver the same street trees were planted block by block. Some of them, like the cherry trees on Heather Street are beyond their peak and are declining.

This study looked at how trees by species can be spatially arranged for diversity and for local climatic conditions. Throughout the 63 cities the researchers found that the number of introduced street trees varied. To look at diversity the researchers also counted the trees that “just occurred” that were native as data about the capacity of the area for “diverse communities of birds, butterflies and other animals”.

It was sobering to hear that urban trees were planted street by street  by species in 98 percent of cities, and almost half of all the cities planted trees that were introduced to the region and certainly not native. The data shows that this decreases the resilience of the urban forest and makes it much more susceptible to disease.

Aysha Khan writing in Next City notes that urban forests which cool urban heat sinks also have an “environmental and health justice” component. Just as transit,  sidewalk and cycling infrastructure is lacking in communities differentiated by  income and demographics, the same happens with city tree planting. That needs to be directly addressed by reviewing tree inventories.

Sustainable decision making also include a prudent approach to maintaining and bolstering tree canopy on streets and on private property, developing resilience to climate change and encouraging diversity. Urban environments are hard on street trees and on private property trees, and their growth may not be as vigorous.

For a list of plants and trees that are native to the Vancouver area  you can click on this link. Of course with climate change not all plants are appropriate, and look for the municipalities to educate citizens to water street trees as more extreme temperatures  predominate.

In a warming climate, trees in cities  need to be treated as the special cooling and pollution mitigating marvels that they are. You can take a look at this very short video from the City of Stirling in Western Australia that outlines some of the benefits of tree canopy.