Increased recovery in coarse‐root secondary growth improves resilience to drought in transition forests

Interaction of global change drivers affects forest resilience. Land‐use changes (land abandonment) and climate change (a higher frequency and intensity of droughts) are interacting in the Mediterranean Region. Components of resilience in secondary stem growth have been widely studied but, despite the importance of root systems in forest functionality and resilience, non‐previous studies have assessed them in coarse roots. In this study, we use Juniperus thurifera tree‐ring chronologies in coarse roots and stems to assess biomass allometry and tree resilience to drought events comparing two stages of a forest expansion gradient (mature forests and transition zone) in Alto Tajo Natural Park. We extracted cores of stems and coarse roots in 48 trees distributed in different developmental stages and calculated cross‐sectional area increments, root‐stem allometric relationship and resilience components for both organs in each individual for two drought events (2005 and 2012). Stem and root growth as well as its allometric exponent were higher in the transition zone than in mature forests. Both organs exhibited a trade‐off between resistance and recovery in mature forests but maintenance of higher values in the transition zone. Resilience did not show differences between organs being higher in the transition zone than in mature forests. However, relative resilience in roots in the transition zone was higher than in mature forests, without differences in stems between stages. Finally, the 2012 drought event showed a higher impact on the components of resilience than the 2005 drought event. Synthesis. This study extends the knowledge of root response to drought events and highlights the potential of land‐use legacies to reduce the negative impact of climate change by promoting increased root recovery after drought events in trees established in past agricultural lands. Land‐use legacies reduce the negative impact of climate change by promoting increased root recovery after drought events in trees established in past agricultural lands. Moreover, we found different biomass allocation during drought events in each stage of forest expansion gradient: more investment in roots than in stems in mature forests (optimal partitioning) and non‐differential investment between organs in transition zone (allometric partitioning). Thus, assessing secondary growth of coarse roots is essential for better understanding tree resilience to drought events.