Scientific insights into tree selection underutilized in urban tree management |
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A competition between politicians, developers and managers: who can plant the most trees?
Henrik Sjoman, senior researcher at Malmö University, is glad that we are now planting trees en masse, but he also sees that we often choose the wrong species. And this while there is plenty of scientific evidence available that shows which species are suitable. In this interview, he outlines the challenges.
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It's clear that in recent decades we've made significant progress in how we value greenery and trees in urban environments. Sjoman even goes so far as to say that we've learned a new language and a new narrative around this. In that narrative, greenery and trees are no longer questioned.
What we still need to learn, however, is how to make better species choices. Sjoman says, "We see a lot of trees fail. And once a tree is planted and doesn't thrive, it's unlikely to ever be replaced. That's what research shows."
According to Sjoman, the main reason so many trees fail is because tree managers and politicians remain overly focused on quantity. Politicians, developers and managers often seem caught in a competition to see who can plant the most trees. That's commendable in itself, but it shifts the focus away from quality. We've all seen plans to plant a hundred, a thousand or even a million trees. The result is an urban landscape where species are chosen without considering their ecological and climatic suitability for the specific site.
The goal should be to plant trees that have the opportunity to grow large. We all know this: only large trees with enough crown volume can provide ecosystem services.
According to Sjoman, we need to develop more knowledge about unusual species—those found only in botanical gardens or exclusive arboreta. These could very well be tomorrow's urban trees.
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The result is an urban landscape where tree choices have not been aligned with their suitability for the environment.
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Distribution
According to Sjoman, there are more challenges on the road to a healthy and balanced tree population.
"We conducted research in the Scandinavian cities of Helsinki and Oslo. It turned out that linden makes up nearly half of the tree population. Sadly, that's not just a Scandinavian problem—you see this same pattern worldwide in nearly all cities."
A homogenous tree population poses the risk of mass loss if that one dominant species is hit by disease. There are well-known examples. In Malmö, large numbers of elms had to be felled, and the same story is familiar in Amsterdam. In Melbourne, thousands of trees died from an infection caused by a small bark beetle: Polyphagous shot-hole borer.
According to Sjoman, this is what happens when you put all your eggs in one basket.
Asian longhorned beetle
Insects we need to watch closely include the Asian longhorned beetle (Anoplophora glabripennis) and its Chinese cousin, the citrus longhorned beetle (Anoplophora chinensis). These species have already been spotted several times in Europe, including the Netherlands, and pose a serious threat to our green infrastructure.
The damage is caused by beetle larvae, which feed on the wood of healthy trees—often fatally. Both beetles are far from picky. According to Sjoman's research, the Asian longhorned beetle thrives on 137 different tree and shrub species. The citrus longhorned beetle is even less selective: it feeds on 188 species.
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The citrus longhorned beetle feeds on 188 different species.
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Sjoman: "I've studied the impact of these beetles on the tree population in Stockholm. Depending on the severity of infection, 40 to 60 percent of the trees could be affected. It's not a question of if we'll be impacted—just when."
Native
Sjoman observes a global trend where tree selection is increasingly based on the native versus non-native debate. While he understands the rationale, he strongly opposes relying solely on this criterion.
"Risk diversification is what matters most, and that cannot be achieved with native species alone—especially because in Western Europe we have a relatively limited range of native tree species. If you plant only native trees, you inevitably end up with just one or two species or families dominating the landscape. That makes the population highly vulnerable to pests and diseases."
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"There's a lot of knowledge about tree species available, but much of it barely rises above barroom conversation."
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Knowledge
There is plenty of knowledge available about where trees can be used. But at the same time, Sjoman sees that much of it barely rises above anecdotal chatter.
Many well-known books and websites frequently consulted by tree managers contradict each other. When authors present completely opposing views on the same species—and those views aren't always based on scientific evidence—it becomes difficult to convince landscape architects and urban planners to choose less traditional planting material.
According to American dendrologist Michael Dirr—a highly respected figure—Liriodendron tulipifera prefers moist, well-drained loam. But according to Mitchell & Jobling, the same tree prefers dry, sandy soil.
Sjoman points out that such contradictions are common in the literature used by professionals daily.
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You're buying something to enjoy for the next hundred years, yet you don't think carefully about what you truly want.
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Ferrari
This lack of consistency in commonly used references, says Sjoman, erodes the sector's credibility.
"We should base ourselves much more on peer-reviewed scientific literature. And the right information is definitely out there."
He also believes that tree managers themselves are partly to blame for how they place orders with nurseries.
"We focus mostly on height, width, whether the tree flowers or has fall color, and whether it prefers sun or shade. That's about it. That's like going to a car dealership and saying you want something with four wheels. You might end up with a Ferrari—or a third-hand Lada."
Sjoman finds this astonishing. "You're buying something to enjoy for a hundred years, but you don't think carefully about what you really want."
And that's despite the fact that information—such as a species' drought tolerance—is readily available.
Drought is one of the key stresses that urban trees face. On that point, Acer monspessulanum, or Montpellier maple, is the undisputed champion. That species originates from the Balkans and thrives on dry steppes.
Koelreuteria paniculata also performs well under dry conditions.
According to Sjoman, we should especially consider spring drought tolerance. The year 2024 was an exception, but it's likely that dry springs will become more common in the future.
Reference books don't always speak with one voice - an example
Aesculus flava:
Prefers deep, moist, well-drained soil (Dirr, 2009)
Good tolerance for hard surfaces (Van der Berk et al., 2002)
Tolerates drought but prefers wet to moist soils (Hightshoe, 1988)
Grows in river valleys and lower slopes (Krüssmann, 1986)
Naturally occurs in stream valleys and lower slopes (Grimm, 2002)
Strongly recommended for dry, sandy soils (Mitchell and Jobling, 1984)
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This article was originally published on May 2, 2025 on the Boomzorg website.
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