Trees get a boost from diverse neighbors, especially in wet forests

Biodiversity benefits nature in many ways. A greater variety of plants and animals helps create forests that are stronger and better able to withstand pests, diseases, and changes in climate.

However, exactly how much biodiversity is needed – and in which places – has been somewhat unclear. A global study of forests by scientists at the University of Michigan brings sharper focus to that question.

The research suggests that biodiversity isn’t just good – it’s especially important in wetter places. For forest managers and conservationists, that detail could change how we approach tree planting and restoration around the world.

“Diversity matters everywhere we look. But in our experiments, we see it matters more in the wetter climates,” said Peter Reich, professor of environment and sustainability and director of the Institute for Global Change Biology at the University of Michigan.

Looking at trees, one by one

The researchers studied 15 forest experiments across the globe through the Tree Diversity Network – the world’s largest of its kind. The analysis included nearly 100,000 trees and close to 130 species in a wide range of climates.

The University of Michigan team collaborated with co-authors from more than 30 institutions across nearly a dozen countries.

The study was unusually detailed. Instead of treating each forest as one big data point, the scientists zoomed in. They looked at each tree individually, and then took note of its closest neighbors.

“What’s unique about our study is it has 15 long-term experiments with lots and lots and lots of trees,” said Reich. “Then, rather than looking at an entire community, we looked at every individual and eight or so of its neighbors and asked, ‘Are those neighbors diverse or not?’”

Forest biodiversity boosts tree growth

The results revealed a clear pattern. Trees in wetter climates grew more when surrounded by a mix of different species. The effect was weaker – or even nonexistent – in drier regions.

“This provides a more nuanced insight into how biodiversity works in different environments,” Reich said. “If you’re thinking of restoring or regrowing, it makes a lot of sense to plant diverse trees, especially in wetter areas.”

The “wetter” category is not limited to tropical rainforests either. By the study’s standard, forests in places like Michigan count as wet.

Benefits of forest biodiversity

According to the researchers, the benefits of diversity were not just about the number of species.

“We find a tree growth boost in diverse tree neighborhoods with different species and functional traits – for example, specific leaf area and wood density,” noted study lead author Liting Zheng, who conducted the research as a postdoctoral fellow at Michigan’s School of Environment and Sustainability.

“Our results highlight the importance of establishing and maintaining high neighborhood diversity in forest management – especially to maximize the potential of biodiversity as a nature-based climate solution in wetter regions.”

Extreme climate years

One surprising takeaway: the benefit of diversity didn’t increase during extreme climate years. It seemed more useful in typical conditions. That challenges the common belief that biodiversity is most helpful in tough times.

“Diversity appears to be better suited for the average condition, which matters for management,” Reich said.

“There’s thinking that biodiversity is really going to help you when you have those extreme years and some evidence supporting this idea, but in our data this was not the case.”

Diversity as a management tool

There’s still more to learn, especially about how older forests behave. The trees studied were still young – just 4 to 14 years old. But the results are strong enough to guide real-world decisions about how we plant and protect forests going forward.

For wetter areas, at least, one thing is now very clear: variety isn’t just the spice of life – it’s how trees grow best.

Given that tree-tree interactions are ubiquitous and likely to interact with climate in both young and old forests, the findings suggest that neighborhood diversity could be incorporated as a management tool to enhance forest productivity, concluded the researchers.

The full study was published in the journal Nature Ecology & Evolution.

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