*A shady stretch of sidewalk. (Source: Author.)*

The thing I love most about my neighborhood (aside from the people, of course) is also the most fragile.

When I first moved here, I had no idea that fully half of the stunning street trees would be gone within 10 years. I did not picture my future self lingering over old Google Streetview captures, longing for the days where there was a massive, mature tree in front of every home, and wistfully remembering a time when you could walk the full length of the block in a rainstorm and barely get wet.

Those days are gone, and that lush and endless canopy is just a bittersweet memory now. The reason for all that devastation? Dutch Elm Disease (DED).

Because virtually all the trees in my neighborhood were elms, when DED arrived on the scene, they were all under threat. But it could just as easily be the trees where you live, succumbing to emerald ash borer, oak wilt or laurel wilt, chestnut blight, or the Asian longhorn beetle.

What all these insects and pathogens have in common is their ability to devastate tree populations in short order. Sometimes trees slowly decline and eventually die, others succumb quickly and become unstable and hazardous. Death is a natural part of the life cycle of all living things. So why are these particular threats so dangerous?

The answer is that in towns and cities across North America, urban forests are often made up primarily of just one or two tree species. These are called “monocultures.” And when you have a huge population of one type of tree, a single disease spreads rapidly and can wipe out thousands of trees in short order.

When settlers began to rapidly develop my city of Winnipeg, Manitoba, the first monoculture they established was the American elm (Ulmus americana). In retrospect, it’s easy to see why. American elm is a native species that was already growing well here. It was an ideal street tree: adapted to our cold weather climate and tolerant of urban conditions. American elms were perfect because they lived a long time (easily more than a hundred years) and grew to great heights, which made them excellent shade trees. On a more superficial level, they were just beautiful. When planted in neat rows, their vase shape created a picturesque “cathedral archway.” There was basically nothing not to love about the American elm. Across North America, it was a go-to species.

That is, until DED showed up. For a while, the city declared a moratorium on planting elm, and worked on developing institutional expertise in DED management. Because of that, Winnipeg is today home to the largest remaining urban elm forest in North America. But in spite of that, the city has lost the vast majority of its elms and continues to lose about 5,000 on public property, and thousands more on private property, each year.

In “elm cities” where DED hit, thousands of trees needed to be removed, and quickly, to prevent the spread to healthy trees. Cutting down large, mature trees costs a lot of money and resources, that, assuming they’re even available, often come at the expense of regular tree maintenance, like pruning. The well-intentioned decision to plant just one type of tree triggered a cascade of costly consequences.

In the wake of all that devastation, cities might have asked themselves whether adopting a new monoculture was wise. But they didn’t, and in Winnipeg and many other places, ash became the new darling. As sick elms came down, they were replanted with ash trees. New neighborhoods got a monoculture of ash right from the get-go. This ill-fated succession repeated itself right across North America, only to be met with the arrival of the deadly emerald ash borer.

Years ago, my husband and I were out for a stroll in the neighborhood adjacent to ours. At the time, we lived in the basement of a triplex, but we were thinking about buying a house just for ourselves and our future family. This neighborhood we were walking through had a few things our current one didn’t: chalk on the sidewalk (kids!); beautiful, wide boulevards separating the street from the sidewalk; and street trees—so many giant, gorgeous trees. We happened on an open house that day, and got the keys to our new home a few months later.

Given how involved I’ve since become with trees, when I look back, it surprises me how little I really thought about them for the first few years we lived here. Of course, I loved them and thought they were beautiful and a valuable characteristic of the neighborhood. But the idea that they wouldn’t always be there never really crossed my mind. I took them for granted. They seemed…immutable? Invincible?

But once they started coming down and I saw how drastically it changed the street, I started to care, a lot. (I eventually joined a local tree committee with some neighbors to get trees re-planted, and then we spearheaded a city-wide coalition to get better urban forestry funding.)

This natural neighborhood asset, calmer of traffic, cooler of homes and pavement, booster of property taxes, and diverter of stormwater had a vulnerability hidden within its hardworking, uniform canopy. Everything was fine, until it wasn’t.

The dots were easy to connect. DED had a foothold in the area. Elms were susceptible to DED. And the street trees in my neighborhood were virtually all elms.

My block was home to 60-odd street trees, the majority of them American elms that had been planted about a hundred years earlier. We were in big trouble. Every time one of these giants came down, there would be a jagged hole in the canopy. Because of the way that DED spreads, it often kills chains of trees, so we wound up with long, open stretches.

A front yard that had always been a shade garden was now in full sun. A home that had been comfortable all summer now needed air conditioning. A walk down the street on a sunny afternoon had stretches of blissful canopy cover rudely interrupted by scorching rays. Even in winter, stretches where many big trees were gone were now noticeably windier.

A neighbor of mine had always said, “This neighborhood’s just not the same without its trees. If my boulevard tree dies, I’m outta here.” I knew what he meant, but didn’t think he meant it. (Turns out, he did.)

*A street that’s lost most of its trees. (Source: Author.)*

I am happy to report that, at last, Winnipeg has learned from its mistakes. It now has a policy that, when planting new trees, no one species should exceed more than 25% of the tree population. Forward-thinking urban forestry departments across North America have embraced biodiversity guidelines such as the 10-20-30 rule, which says that an urban tree population should include no more than 10% of any one species, 20% of any one genus, or 30% of any family.

Today, Winnipeg’s public tree inventory is much more varied, but the legacy of the elm and ash monocultures are still prominent.

*(Source: Trees Please Winnipeg Coalition.)*

I wouldn’t exactly call it a happy ending for Winnipeg, but we’ve at last learned from the past. Sadly, places across North America were seduced by the siren song of the monoculture: think ash trees in Detroit, olive trees in Sarasota, or poplars in Calgary—most places have one or two species that make up a huge percentage of their urban forest. If a species-specific threat hasn’t yet come along, it’s probably just a matter of time. A recent study predicted that in the U.S., wood borers of maple and oak trees were the highest-risk future invader.

Monocultures may be easy (take what you know works and replicate it) and look great (those oh-so-Instagrammable “tree tunnels”) but in fact, they’re quite fragile. No one knows what pathogen or pest will come along next and devastate a particular species. How ecosystems will be impacted by the changing global climate is another serious unknown. In essence: monocultures are one big, expensive, risky bet.

Some invasive insects and diseases don’t simply kill trees, they also leave them unstable and hazardous, posing a real threat to people and property. When a monoculture is hit with a lethal threat, it’s insult added to injury. Not only does the city lose out on all those incredible economic, health, and environmental benefits, it also has to shell out big bucks to deal with all those dead and dying trees, control the pest, and settle claims related to property damage.

In many neighborhoods, including mine, Winnipeg’s tree monoculture has an additional weakness: They were also all planted right around the same time. It’s the urban forest equivalent of being “built all at once to a finished state,” where all the trees are not only susceptible to the same threats, but will also reach the decline phase of their natural lives all around the same time.

I would take an intact canopy over the current situation any day. But we have to work with where we’re at now, and I really believe that we can find a silver lining here, too.

The catastrophic tree loss we’ve experienced in Winnipeg provides an opportunity to get an abundance of different species planted at staggered times. Should those trees be lucky enough to die of old age, their human neighbors will be able to remember them as individuals, and not one of thousands lost in any given year.

The parallels between monocultures in nature and in cities are obvious. Whether it’s planting only one type of tree, or zoning for only one use, too much of the same creates a precarious position.

Whether we’re talking about people, or housing, or trees, the lesson is this: diversity is beautiful, diversity is resilient, diversity is the way of the future. A diverse urban forest may not be neat or tidy, but it reaps the benefits of a thousand small bets.

The Hidden Lessons of Catastrophic Tree Loss

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