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I have a slide I use in a lot of my talks. It’s a photo of Geof Donovan and a quote of his that I keep coming back to:
“Every other benefit is rounding error compared to public health.”
That line shocks people. It’s supposed to. Because for decades, we’ve made the case for urban trees through carbon, stormwater, energy savings, aesthetics. And those matter. But Geof’s work has shown, over and over, that the health benefits of trees are so large they make everything else look like a footnote.
This is a long episode. It might be the longest I’ve ever done. And if Geof didn’t have a meeting to run to, I could have talked to him for another two hours, and I think by the end, you’ll understand why.
Geof Donovan spent 23 years as a researcher at the USDA Forest Service, focused on quantifying the benefits of urban trees. He started broad — energy, carbon, crime, stormwater — but over time, he kept narrowing. Not because the other benefits don’t exist, but because the health numbers kept dwarfing them. He’s an economist. He follows the biggest returns. And the biggest returns are in public health.
In the spring of 2025, Geof took early retirement from the federal government and started his own consultancy, Ash and Elm Consulting — named, as it turns out, after Viking cosmology. Geof grew up in York, the old Viking capital of Britain, and in Norse mythology, the first man and the first woman were an ash tree and an elm tree. I did not see that coming.
But what Geof does now is the same thing he’s always done: help people make the business case for urban trees.
And let me start by saying, business case is an understatement.
When trees die, people die
You may have seen the headline. You may not know it came from Geof.
When the emerald ash borer swept through the Upper Midwest of the United States, it killed hundreds of millions of ash trees. Geof saw it as a natural experiment — a tragedy, but a scientifically rare one. If trees are good for health, then killing them at scale should be bad for health.
He was right. Geof found that the rapid death of ash trees led to an additional 15,000 deaths from cardiovascular disease and 6,000 from lower respiratory disease in the affected counties. About 21,000 excess deaths.
He tried to break the results. He added every confounder he could think of. He ran a negative control — accidental deaths, which trees couldn’t plausibly affect — and found nothing there. The results, as he put it, just sat there like a bump on a log.
And then he did the inverse. In Portland, a nonprofit called Friends of Trees had been planting street trees for three decades and — bless their hearts, Geof says — had kept meticulous records of every tree, every date, every location. Geof got access to that data and found the opposite pattern: tree planting was associated with decreased cardiovascular mortality.
Two very different studies. Different designs, different locations, different times. Consistent results. That’s when you start to think you’re onto something.
Babies are bigger. People are not dead.
One of the things Geof keeps having to correct is the assumption that tree-and-health research is fluffy. It’s about people feeling “a little less sad”.
No. As he puts it: “Babies are bigger. People are not dead.”
Geof’s 2011 study was the first to look at the natural environment and birth outcomes — ever. He found that women with more tree canopy within 50 meters of their homes were less likely to have an underweight baby. Since then, at least three dozen studies have confirmed the relationship between maternal exposure to greenness and birth weight. It’s now probably the most studied green-health outcome there is.
And it matters beyond birth. Being born underweight puts a child on a worse health trajectory for life — higher risk of diabetes, cardiovascular disease, high blood pressure. Lower school performance. Lower lifetime earnings. Before they’re even born, through no fault of their own, a child can be put on a different path. And we could have changed it for the price of some trees.
As I said, Geof is an economist. He doesn’t usually make moral arguments. But this one, he said, offends him. And the fact that the kids most affected are more likely to be poor, more likely to be members of an ethnic minority — he was blunt about that too.
I pushed him on what a pregnant woman should actually do with this research, practically speaking. His answer was simple: spend as much time in the natural world as you can. Plant something if you’re able. Even a window box. Even a houseplant. Green your immediate environment. And keep doing it after the baby arrives — especially in those first two years.
This connects to something I’ve been exploring with the Van Leer Foundation, which supports this season: the idea that the neuroplasticity pregnant women experience might be the optimal window for building new habits — including a nature habit. Build it during pregnancy, carry it into the postpartum period and beyond. Geof had no complaints about that idea.
The biodiversity piece
More recent work has taken Geof beyond canopy cover and into what kind of canopy matters.
In New Zealand, he found that kids living in greener neighborhoods were less likely to develop asthma. But here’s the twist: if that greenness was more biodiverse, the effect was even stronger. He thinks it’s because of the microbes. Plant leaves are covered in them, and the total leaf area on Earth is roughly twice the land area — a vast microbial habitat. More diverse plants support more diverse leaf microbes, and exposure to those microbes appears to be important for immune system development, especially in young children.
A follow-up study looked at childhood leukemia — the world’s most common pediatric cancer — and found that kids in more biodiverse areas were significantly less likely to develop it. In both cases, genus-level diversity mattered more than species-level, probably because leaves need to be structurally and chemically different enough to support truly different microbial communities.
This is where Geof’s work intersects with something I care about a lot: the City Nature Challenge and citizen science platforms like iNaturalist. The biodiversity data Geof used in New Zealand came from large-scale observation records — exactly the kind of data that community bioblitzes generate. It’s one more reason why documenting what lives in your city isn’t just a nice weekend activity. It’s research infrastructure.
Read more about the City Nature Challenge in Field Note 4 👇
He also studied ADHD and found something I wasn’t expecting. The strongest predictor wasn’t average greenness over a child’s life — it was the peak grayness. The most built-up, least green environment a child had lived in. Geof described the built environment as a kind of poison, and the peak dose of that poison did the most damage.
The ROI that sounds like a Nigerian prince scam
Geof has a line I love: the return on investment for urban trees is so high that even a Nigerian prince email wouldn’t dare claim numbers like that.
Property value returns in Portland: 10 to 12 to one. The return on tree planting in terms of health outcomes: around 1,700 to one. Ongoing work on greening school yards is showing 10 to 20 to one returns just through increased lifetime earnings from better test scores. The numbers are almost embarrassingly good.
And yet. Cities pour hundreds of millions into hospitals and mental health clinics and traditional health infrastructure — and Geof’s not saying they shouldn’t — but when I pushed him on what he’d say to a mayor who claims there’s no budget for trees, he didn’t hesitate: “Skim a tenth of one percent off your health spending. Put it into trees. That will be the most effective money you spend. Nothing else will even come close.”
Trees, he argues, are prevention. Like eating well. Like vaccination. Like speed bumps. We know prevention is the most cost-effective form of health spending. But people can intuitively understand a speed bump. The tree connection is weirder, less obvious, and so people dismiss it.
How we tell the story
Near the end, I asked Geof what he’d say to green professionals trying to translate this research into practice.
His answer surprised me a little. He talked about science, yes. But what he kept coming back to was storytelling.
He described asking audiences to close their eyes and remember losing a large tree in their childhood neighborhood. Most people can. And the feeling is visceral — a hole in the sky, a recoil. He sometimes reads poetry to audiences — there’s a Seamus Heaney poem where the death of a chestnut tree becomes a metaphor for the death of his mother. Geof hits people with the graphs first, then stops, takes a breath, and goes there.
“There’s a reason people write poetry about trees and not speed bumps.”
I think about this constantly. In my book, in my talks, on this podcast — the question is always: how do you take research this important and make people feel it, not just know it? Geof’s answer is that most of us already feel it somewhere. We just need someone to tap into it.
He also told me about a property developer who retained not just the required trees but additional ones on a housing development — and sold every unit in advance. The developer said two things made the difference: keeping the mature canopy and letting people paint their houses whatever color they wanted. The development looked like a community from day one, not a sterile subdivision. That story will do more for the next developer than any odds ratio ever will.
The finding Geof is most excited about right now came from his Portland tree planting study. He’d had the results for a while. They were good. But then he found a different way of expressing them.
Roughly one hundred trees in the ground equals one death averted.
He described a record-scratch moment in his own head. One hundred trees isn’t that many. Most cities could get hundreds of thousands in the ground without enormous effort. And the outcome we’re talking about isn’t reduced anxiety or improved mood — it’s the most fundamental health outcome there is. Not being dead.
Geof wants us to stop calling trees livable infrastructure. They’re survivable infrastructure. We didn’t evolve to live in cities. The built environment is, in a real sense, toxic to us. Trees dull that toxicity. Take them away, and the poison is back.
Even the word benefits feels wrong, we agreed. It implies something extra, something optional. The frilly bits on the edges.
But John Doe being alive is not a frilly bit.
Find Geof and Ash and Elm Consulting on his website.
Happy trails,
Nadina
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Sponsor note: The Bernard van Leer Foundation generously supports this season, and I’m honoured to be a Van Leer Fellow — exploring whether urban nature can truly fulfill our evolutionary need for wilderness during the most formative stages of life: pregnancy, early parenthood, and childhood. It’s a question that runs like a thread through every conversation this season, including this one.
