Younger trees in the upper canopy are more sensitive but also more resilient to drought

As forest demographics are altered by the global decline of old trees and reforestation efforts, younger trees are expected to have an increasingly important influence on carbon sequestration and forest ecosystem functioning. However, the relative resilience of these younger trees to climate change stressors is poorly understood. Here we examine age-dependent drought sensitivity of over 20,000 individual trees across five continents and show that younger trees in the upper canopy layer have larger growth reductions during drought. Angiosperms show greater age differences than gymnosperms, and age-dependent sensitivity is more pronounced in humid climates compared with more arid regions. However, younger canopy-dominant trees also recover more quickly from drought. The future combination of increased drought events and an increased proportion of younger canopy-dominant trees suggests a larger adverse impact on carbon stocks in the short term, while the higher resilience of younger canopy-dominant trees could positively affect carbon stocks over time. The authors analyse the impacts of drought on tree growth for various species of various ages to assess the influences of forest demographic shift on future drought responses. The increasing proportion of young trees showing greater growth reduction to drought raises concern on future carbon storage.