Mixing tree species does not always make forests more drought-resilient

(Press-News.org) Increasing tree species diversity is widely suggested as a way to help forests withstand climate change – especially prolonged droughts. But a new international study led by the University of Freiburg, published in Global Change Biology, shows that simply mixing more tree species does not always boost forests’ resilience to drought. In fact, the effects of diversity on tree growth can shift from beneficial to negative as droughts drag on.

Longer droughts change how tree diversity affects tree growth

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 Hernán Serrano-León, 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 – nine 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.

Researchers combined dendrochronology (studying annual tree rings) with advanced X-ray tomography, providing detailed measurements of tree growth. This work benefited from a close international collaboration within the MixForChange (https://mixforchange.cirad.fr/) and CAMBIO (https://www.cambio-treediversity.com/) projects, both focused on the climate adaptation potential of mixed-species forest plantations.

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-León. “With climate change making droughts longer and more frequent, the composition – not just the number – of tree species will be critical for healthy, resilient forests.”



Further Information:

Original publication: Serrano-León H, Blondeel H, Glenz P, Steurer J, Schnabel F, Baeten L, Guillemot J, Martin-StPaul N, Skiadaresis G, Scherer-Lorenzen M, Bonal D, Boone M, Decarsin R, Druel A, Godbold DL, Gong J, Hajek P, Jactel H, Koricheva J, Mereu S, Ponette Q, Rewald B, Sandén H, van den Bulcke J, Verheyen K, Werner R, Bauhus J (2025) Multi-year drought strengthens positive and negative functional diversity effects on tree growth response. In: Global Change Biology. 10.1111/gcb.70394 (https://doi.org/10.1111/gcb.70394)

Hernán Serrano-León is a forest scientist at the Chair of Silviculture, Faculty of Environment and Natural Resources at the University of Freiburg. His research focuses on sustainable forest management in the face of global change, with a particular focus on the relationships between tree diversity and forest ecosystem functions.

TreeDivNet is the world's largest network of tree diversity experiments, including two experiments – IDENT-Freiburg and BIOTREE – managed by the University of Freiburg.

END