City trees might be misleading us about climate change impact

Urban trees have become popular guinea pigs for studying how climate change might affect plant life in the future. Predicting these effects is complex as there are numerous variables – shifting temperatures, rainfall, soil conditions, and genetics.

To simplify the task, many researchers compare plants growing in cities with those in nearby rural areas. The idea is that cities form heat islands, where concrete and buildings trap warmth, raising local temperatures by a few degrees.

These spots offer a natural glimpse into how plants might respond in a warmer world. But recent research from scientists at MIT and Harvard University suggests this common method may be giving us a false sense of security – at least when it comes to forests.

City trees could hide climate risks

City trees, it turns out, aren’t telling the whole story. While comparing urban and rural plant life seems straightforward, the researchers found that the trees growing in urban heat islands often have less genetic diversity than their rural counterparts.

This hidden difference changes the way they respond to heat. In fact, it might be masking the full impact of climate change.

“The appeal of these urban temperature gradients is, well, it’s already there,” said David Des Marais, one of the researchers. “We can’t look into the future, so why don’t we look across space, comparing rural and urban areas?”

The data is readily available, which has made this approach popular among ecologists.

But even though they knew the method wasn’t perfect – differences in soil nutrients such as nitrogen were already a known issue – researchers still saw urban trees as a useful stand-in to test the possible effects of future conditions.

City vs. rural trees: Genetic mismatch

This latest study took a different approach. Instead of just comparing how trees grow in the city and the countryside, the researchers looked at their genetics as well.

Specifically, they focused on red oaks in and around Cambridge, Massachusetts, and in the Harvard Forest in rural central Massachusetts.

The project actually began during the pandemic. With travel limited, the researchers started studying trees close to home.

Over three years, they gathered temperature data, tracked when trees began to leaf out in spring, and analyzed the trees’ genomes. What they discovered was surprising.

“So, lo and behold, you think you’re only letting one variable change in your model, which is the temperature difference from an urban to a rural setting,” Des Marais explained. “But in fact, it looks like there was also a genotypic diversity that was not being accounted for.”

Urban red oaks were genetically different from rural red oaks. And those genetic differences made city trees more resistant to warmer temperatures.

As a result, urban trees didn’t show the same level of sensitivity to heat that rural trees did.

Because of this, previous studies comparing plant responses in cities and rural areas may have underestimated how forests will react to global warming.

How this affects climate predictions

“Initially, we saw these results and we were sort of like, oh, this is a bad thing,” Des Marais said. “Ecologists are getting this heat island effect wrong, which is true.” But there’s an upside: the problem is fixable.

“It’s not that much more work, because sequencing genomes is so cheap and so straightforward. Now, if someone wants to look at an urban-rural gradient and make these kinds of predictions, well, that’s fine. You just have to add some information about the genomes,” explained Marais.

The lack of genetic diversity in urban trees actually makes sense. For decades, growers have selected varieties that can survive harsh city conditions, including poor soil, pollution, and inconsistent drainage.

Over time, this trial-and-error approach has narrowed the genetic pool of city trees.

What this means for global assessments

The implications are bigger than a few city parks. These urban-to-rural comparisons are part of the toolset used by organizations, including the Intergovernmental Panel on Climate Change (IPCC), to make global predictions.

“When the Intergovernmental Panel on Climate Change (IPCC) releases its regular reports on the status of the climate, one of the tools the IPCC has to predict future responses to climate change with respect to temperature are these urban-to-rural gradients.”

“If these results are generally true beyond red oaks, this suggests that the urban heat island approach to studying plant response to temperature is under predicting how strong that response is.”

A better way forward

This study doesn’t throw out the idea of using urban environments to study climate change. It just asks researchers to look a little deeper.

Adding a genetic lens to these comparisons could make future predictions more accurate – and help us better prepare our forests for the changes ahead.

As climate pressures increase, getting these predictions right could be the difference between a thriving forest and one in decline.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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