Tree Life Expectancy and the Cycle of Urban Development
Since trees can live longer than the buildings, they cohabit the landscape with, development has an enormous impact on the life expectancy of a tree. Developers prefer blank-slates, so levelling the site is common and preferred. Bigger buildings fetch bigger profits, and while mature trees also increase property value, they’re susceptible to damage from nearby excavation, soil compaction, and regrading.
Development Frequency and Tree Survival Rate
The life expectancy of a privately owned tree is tied to (1) how often development happens and (2) the care taken to protect on-site trees during construction. Robust building codes, routine maintenance, flexible zoning, and the housing market influence the former. Setback requirements, market demand, and a cultural appreciation for the value of trees impact the latter. Both can be influenced by strong planning and bylaws that provoke pause or shed light on the benefit of mature trees.
A Thought Experiment
Imagine developing a property every 40 years – which, as it turns out, is about the average lifespan of a home in North America. Let’s imagine that 50% of its trees get cut down at the beginning of each redevelopment. Assuming no other causes of death except the axe, what’s the average life expectancy of a tree growing under these circumstances? The answer: 80-years. While 80-year may seem like a long time, it’s important to remember that 80-years is a fraction of the lifespan of many urban species.
Using various Tree Survival Rates and Development Frequencies, we can create a table of Average Tree Life Expectancies. To simplify the math, I’m ignoring all other causes of death – old age, disease, etc – other than tree removal during a development cycle. While this oversimplification skews the average ages upwards, I believe that we can still draw some conclusions from the numbers.
Tree Survival Rate (%) – the likelihood that a tree will survive the development of its site.
Development Frequency (Years) – How often a site is developed.
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 to be at least 100 years.
What’s more important? Development Frequency or Tree Survival Rates?
If you double the lifespan of development, you’ll double the average life expectancy of the trees around it; triple the development frequency and 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. This suggests that if you what to find old trees, look for old houses. Maintaining our buildings is maintaining our trees.
In contrast, the average life expectancy scales exponentially with each added increase in Survival Rate. It’s a case of increasing returns. While increased tree survival from 10% to 20% increases the life expectancy 2.8 years, an increase from 80% to 90% adds another century to a site’s trees.
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.
What’s Missing From the Numbers?
let’s acknowledge my 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 species like Mayday (Prunus padus) may only live long enough to experience one or two development cycles. I’m ignoring natural causes of death such as disease, damage, extreme weather, climate change, and carelessness. The result is that 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 only shows exiting trees, and privately owned trees aren’t mapped. If the City required citizens to apply for a tree-removal permit, we could more easily track data around the age, size, and species.
Surviving Development Zero and One-Hundred Percent Of The Time
While no tree is one-hundred percent safe from development, trees living in parks and cemeteries come closest. Digging up green spaces and the dead is unpopular but not unheard of, as demonstrated by at least two 99 Percent Invisible episodes; The Modern Necropolis, Life and Death in Singapore.
Trees with a zero percent survival rate at development likely include those 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.
The longer a species can live, the more confident we can be that it will meet an untimely demise, but we’ll never have old trees unless we plant them.
Since trees can live longer than people, the majority of their benefits will happen outside of our lifetime. The trees aren’t for us; they’re for the future. Future cities can contain ancient forests. Forests that are already here. It’s our job to plant, protect, and nurture this forest.
A 40-year-old human cutting down a 400-year-old tree to build a development with a 40-year life expectancy 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. I’ve argued that fitting development to the landscape – including around existing trees – would make for more interesting places. In either case, any tool that extends the development frequency or protects trees during construction could have a meaningful impact on the number of old trees living within our cities.
How To Grow An Old Tree Takeaways
- Advocate for municipal protections that increase the odds that trees will survive development.
- Reduce the frequency of redevelopment
- Advocate for strong building codes that increase the life-expectance 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.
through routine building maintenance, strong building codes, and flexible zoning. ex. Studio 96
Support the protection and upkeep of heritage buildings ex. Edmonton Historic Resources