• Martin Ford

Thrive not Survive in the City

How do trees manage to live in City conditions?

Trees in this climate (particularly native trees) grow by extending their root systems and crown from May to the end of July/early Aug IF they have a consistent water supply AND an aerated soil. They self-restrict their growth to these months to be able to respond/adapt to the extremes of cold winters and hot summers - often accompanied with extended dry spells. In a City the soils, drainage, compaction levels, and nature's water supply are all compromised and directed by how we engineer our hardscapes, buildings, and road construction. These conditions affect every City tree which is not part of a specifically retained naturalized woodland or wetland.

Knowing that there is only this 3 month 'window' of growth for trees it is essential to recognize that if at any point in those 3 months the tree's root system is unable to find water, then the tree stops growth for that year and will not restart if water becomes available. This means the tree cannot thrive and so ultimately has a very short lifespan and remains small. Thus, we lose not only the pleasure of having large trees near us but also the benefits they provide in cooling the city and therefore our living conditions.

The 4 principal reasons trees have very much shorter lives in City conditions:

1. When trees’ roots are subject to hot summer temperatures, inconsistent water supply, and poor soil aeration then they take on the survival features of a Bonsai - they miniaturize to be able to survive. A natural example of this locally is the common White Cedar which can be found living on the vertical face of the Niagara Escarpment - some are now over 1000 years old! They are small but healthy because they grow for very short periods in the spring and their root systems have extended deeply into the cool rock face.


2. The consequences of ‘engineering’ city soils are both compaction and a drained/lowered water table. This results in trees’ roots being unable to reach water and/or roots drowning because the planting holes dug for the trees cannot drain. In compacted soils little air is available - it is essential that air be present in the soil for roots to thrive and explore. The only exception to this is wetland trees such as Elms and Silver Maples which are naturally adapted to low oxygen.

3. Planting too far apart leads to soils baking during the heat of summer and so roots don’t extend much, even into the limited topsoil provided - thus the root system remains small and the tree vulnerable. In nature, trees grow close together and shade the soil they share, creating cool, healthy growing conditions. Planting shrubs at the base of each tree will also help to cool the soils and enable the symbiotic relationships seen in natural environments.

Photo courtesy of The City of Portland website – Jan 25, 2017

4. Climate extremes such as extended dry spells and prolonged deep cold are magnified by City conditions - high speed and drying winds, variable water supply, unfriendly soils, isolation, over-pruning, and transplant shock from planting trees which are too large (and thus have too many roots severed when transplanted in wire baskets from the nursery), and other factors we’re only now beginning to understand.

Grenoble, France Photo courtesy of The Telegraph - June 10, 2018

So we have several choices if we want to have large and thriving trees in our city - one of which is to ensure that the tree has the conditions for its roots to grow optimally from May to July and so establish quickly (once its roots are deeply into the soil it will need little care). Another choice is to plant very young saplings or plant trees from seed so that they follow their natural potential and throw deep roots before growing a large canopy. There are many more ways and means we can have healthy trees which we will discuss at another time.

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