Increases the lifespan of urban trees by decreasing the frequency of redevelopment and increasing the rate of survival

Development and the Lifespan of Urban Trees

Tree Life Expectancy and the Cycle of Urban Development

Since trees can live longer than buildings, development has an enormous impact on the lifespan of urban trees. While mature trees increase property value, they’re susceptible to damage from nearby development. Soil compaction from heavy equipment or storing materials, damage to roots from excavation, and regarding the lot to change site drainage can all negatively impact tree health in ways that may be fatal but go unnoticed for a few years. In addition to indirect damage, trees are often situated too close to the footprint of the new building to save.

Development Frequency and Tree Survival Rate

The life expectancy of a privately owned tree is tied to two things:

  1. Development Frequency (years) – how often trees are threatened by redevelopment.
  2. Survival Rate (%)- the care taken to protect trees during development. Measured as the likelihood of surviving redevelopment.

Strategies impacting development frequency and survival rate are essential for establishing a mature urban forest. Robust building codes, maintenance, and flexible zoning impact the lifespan of the building and, thus, the frequency of redevelopment. When redevelopment inevitably happens, zoning rules such as building setback requirements, tree protection bylaws, and a cultural appreciation for trees increase the likelihood that on-site trees will survive. The longer we can stretch the gap between redevelopments, the longer the trees can live without the threat of the axe. The more heritage trees we’ll have.

A Thought Experiment

Imagine a property that’s redeveloped every 40 years – which, as it turns out, is about the average lifespan of a home in North America. Suppose that 50% of the on-site trees are removed when development occurs. Assuming no other causes of death, what’s the average life expectancy of a tree growing under these circumstances? About 80 years. While 80 years seems like a long time, it’s important to remember that 80 years is a fraction of the lifespan of many urban species.

Let’s create an Average Tree Life Expectancies table using various Tree Survival Rates and Development Frequencies. To keep things simple, I’m ignoring all other causes of death – old age, disease, etc. – other than tree removal during a redevelopment. This oversimplification skews the average age upwards but allows us to draw conclusions. A reminder:

  • Development Frequency (years) – how often trees are threatened by redevelopment.
  • Survival Rate (%)- the care taken to protect trees during development. Measured as the likelihood of surviving redevelopment.
Development and Lifespan of Urban Trees, Life Expectancy vs Development Frequency
Tree Life Expectancy at Various Development Frequencies

The first thing you’ll notice is that unless the development frequency is low or the tree survival rate is high that the average life expectancy of a privately owned urban tree is bleak. To achieve an average age of 100 years, we need tree survival rates higher than 80% or development frequencies of at least 100 years.

What Increases the Lifespan of Urban Trees? Increasing Development Frequency or Tree Survival Rates?

If you double the development lifespan, you’ll double the average life expectancy of the trees around it. If you triple the development frequency, you’ll triple the age of the trees, etc. So a development frequency of 100 years will have trees with a life expectancy five times as long as lots with a development frequency of 20 years. There’s a one-to-one correlation. If you add 30 years to the life of a building, you’ll add 30 years to its trees. The lesson is if you what to find old trees look for old buildings. Building maintenance is tree maintenance.

However, the big lesson is that life expectancy scales exponentially with Survival Rate. It’s a case of increasing returns. While increased tree survival from 10% to 20% increases the lifespan of urban trees life by 2.8 years, an increase from 80% to 90% adds another century to a site’s tree’s expected life. So, when it comes to protecting urban trees, increasing the likelihood that a tree will survive development is more impactful than decreasing the frequency of development.

Plant Long-Lived Trees, Construct Long-Lived Buildings , Tree Survival and Development, Heritage Trees, Alberta Edmonton

What’s Missing From the Numbers?

I want to acknowledge some assumptions. I’m assuming that the percentage of trees that survive the development of a site is consistent across time – it’s not. I’m also not taking the life expectancy of different species into account. While a two-thousand-year-old tree may need to survive 40 development cycles, short-lived trees may only experience one or two developments in their lifetime. I’m ignoring natural causes of death, such as disease, damage, extreme weather, climate change, and carelessness. The life expectancies charted above are optimistic. How optimistic? I’m uncertain. It would be great to have data on tree removal, but the City’s tree inventory does not include privately owned trees. If the City required permits for private tree removal, we’d retain many trees while tracking age, size, and species data. This data would allow us to make better-informed policies.

Edges Cases – When Trees Survive Development Zero and One-Hundred Percent Of The Time

While no tree is safe from redevelopment, trees living in parks and cemeteries come closest. Digging up green spaces (and the dead) is unpopular but not unheard of, as demonstrated by two 99 Percent Invisible episodes, The Modern Necropolis, Life and Death in Singapore.

Upon redevelopment, trees with a zero percent survival rate likely include trees growing along freeways or arterial roads. While the frequency of development is low, the odds of surviving the widening of the road or the construction of an overpass are near nil.

Future Forests

We’ll never have old trees unless we plant trees capable of living long lives. Old trees provide the most significant benefit, so trees planted today are for the benefit of the future. With luck and care, future cities will contain ancient versions of the trees growing among us. The trees aren’t for us; they’re for the future. It’s our job to grow, protect, and nurture what we have so that others may benefit.

A 40-year-old human cutting down a 400-year-old tree for a 40-year development involves a special kind of short-sightedness. Cutting down old trees robs the future of ecological wonder and a deeper connection to place and time. Any tool that extends the development frequency or increases the survival rate will impact the number of old trees in the future. I’ve argued that fitting development to the landscape – including existing trees – would make for more exciting places.

Takeaways – How To Increase the Lifespan of Urban Trees

  • Advocate for municipal protections such as private tree protection bylaws that increase tree survival rate during redevelopment.
  • Require residents and developers to apply for a permit to remove trees above a specific size.
  • Require developers to create a tree protection plan before construction.
  • Advocate for strong building codes that increase the life expectancy of our buildings.
  • Advocate for regular building maintenance; support programs that provide funding to upgrade or retrofit existing structures.
  • Advocate for flexible zoning and the repurposing and reuse of existing buildings.

Check out the Alberta Tree Register for a working list of Alberta Heritage Trees.