Don't Take Umbrage. Plant More Trees!

There are many documented health, psychological and environmental benefits that accrue as a result of robust, urban tree canopies. Tree canopies are a form of green infrastructure that equip cities to adapt to climate change.

Of late, I have wondered whether our urban landscapes are equipped to adapt to progressively hotter, drier summers. My hypothesis: cities may be underestimating the scale of this challenge by an order of magnitude. This hunch is backed by my preliminary research on the state of Portland, Oregon's tree canopy.

Portland, Oregon, is home to over 300 heritage trees, an incalculable number of tree huggers (including yours truly) and a tree canopy that is in a state of distress. For instance 42% of the city's street trees are considered vulnerable to disease and pests. According to a report published by the City of Portland's Urban Forestry Department, the city's tree canopy declined from 30.7% in 2015 to 29.8% in 2020. That .888% reduction equated to 823 acres of tree canopy coverage out of a total land mass of 92,680 acres.

The Connecting Canopies initiative, a collaboration by #TheNatureConservancy of Oregon, #TheBlueprintFoundation, and #IntertwineAlliance, has a stated goal of achieving and maintaining a 40% healthy tree canopy in the Portland metro area by 2030. That begs the question: how many trees would we have to plant and maintain in Portland over the next seven years to bridge the 10.2 canopy gap not to mention offset loss of diseased and dying trees?

To achieve a 40% tree canopy coverage, Portland will need to plant trees covering the equivalent of 9,453 acres by 2030 - 10.2 points times 92,680 acres.

Stocking rate refers to the number of trees that should be planted on an acre of land. There are reportedly 1.2 million trees planted on the city's 11,760 acres of parkland with a canopy coverage of 54.2%. These data points suggests that there is an average stocking rate of 188 trees per acre in the city's parks [1.2M divided by .542) divided by 11,760].

Portland would need to plant 1.78 million trees by 2030 to get on a glide path of achieving 40% canopy coverage - 9,453 acres times 188 stocking rate per acre.

1.78 million trees is a big number. Is it possible that my back-of-the-envelope scribbles resulted in a misplaced decimal point? To find out, I posed the following question on Microsoft Bing Chat with GPT: "How many trees would the city of Portland, Oregon, have to plant by 2030 to achieve a 40% tree canopy coverage?"

Bing Chat with GPT provided a detailed and nuanced response that factored numerous factors including tree survival rates and the median canopy size of Portland's street trees (400 square feet). The Wisdom of the Cloud estimated that Portland would have to plant 1.1 million trees to achieve the 40% canopy target. [Bing's formula: (40% - 29.8%) x (city area in square feet)/ (average canopy size). More specifically: (.102 x 4,417,840,000 SF)/ 400 SF. That's equivalent to roughly 161,000 trees per year. [Note: Bing Chat with GPT's responses varied each time I posed the question. However, the 1.1 million figure seems like a reasonable directional target.]

The City of Portland's Urban Forestry Department planted or gave away a total of 10,000 trees in 2021. To achieve 40% canopy coverage, the City of Portland will need to increase its investment in tree planting by at least a factor of 16X and invest an indeterminate amount to maintaining these trees - including street trees.

Portland's Urban Forestry Department is updating the city's urban forest plan. This is a golden opportunity to reset expectations concerning the scale of investment that will be required to achieve and sustain a vigorous, interconnected canopy - not just in Portland but throughout North America.

City officials should consider carving out urban forestry departments from parks and recreation departments and establishing them as stand-alone bureaus of equal standing with other infrastructure departments (e.g., water and sewer). As cities undertake the Herculean task of restoring urban tree canopies they should heed the adage: don't let the perfect be the enemy of the good. Let's get planting!