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Increasing tree species diversity is often regarded as a way to make forests more resilient to climate change. However, a new international study led by the University of Freiburg shows that diversity can have both positive and negative effects on forests' drought resilience.

By analyzing tree rings from 1,600 trees in 68 different species mixtures across Europe, the researchers found that the effects of tree diversity on tree growth can shift from beneficial to negative during prolonged drought conditions. The results highlight that careful selection of species combinations and locally adapted management approaches are essential for developing truly climate-resilient forests.

The study is in the journal Global Change Biology.

Drawing on tree ring data from the world's largest network of tree diversity experiments, researchers found that diverse forests can initially support better tree growth during single-year droughts. However, as droughts persist longer within a year or extend over consecutive years, these positive effects can turn negative depending on local conditions. In some cases, mixing tree species strengthened forest resilience; in others, it increased stress and competition for water.

"Our findings make clear that increasing tree diversity is not a one-size-fits-all recommendation as we face increasingly intense and longer droughts," says Hern谩n Serrano-Le贸n, lead author and forest scientist at the University of Freiburg's Faculty of Environment and Natural Resources. "To improve the drought resilience of mixed forests, we need to match species mixtures and forest management strategies to local environments."

Data from the largest network of tree diversity experiments

To isolate the impact of tree diversity, the team used data from the TreeDivNet network鈥攏ine large-scale, planted tree diversity experiments spanning six European countries, from the Mediterranean to the boreal zone.

Researchers collected and measured over 1,600 tree samples from 68 different species mixtures. They then filtered these samples for quality, leaving 948 for the final analysis. This approach allowed them to compare how 21 tree species growing in monocultures or mixtures responded to drought under similar conditions such as age, density, and management.

Choosing the right species mix and adapting to local conditions is key

A key insight of the study is that both positive and negative effects of tree diversity can result from complex interactions among species. For instance, positive effects on drought resilience may stem from more efficient sharing or facilitation of water resources among different species. However, negative effects can also occur when increased diversity leads to stronger competition for limited water during drought.

Building resilient forests will thus require not just more species, but carefully selected combinations and adaptive management. "Our results highlight the urgent need to bring together scientific evidence and local forestry experience," says Serrano-Le贸n. "With climate change making droughts longer and more frequent, the composition鈥攏ot just the number鈥攐f tree species will be critical for healthy, resilient forests."