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. 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. …Building resilient forests will thus require not just more species, but carefully selected combinations and adaptive management.