Repetitive seasonal drought causes substantial species‐specific shifts in fine‐root longevity and spatio‐temporal production patterns in mature temperate forest trees

Summary Temperate forest ecosystems are exposed to a higher frequency, duration and severity of drought. To promote forest longevity in a changing climate, we require a better understanding of the long‐term impacts of repetitive drought events on fine‐root dynamics in mature forests. Using minirhizotron methods, we investigated the effect of seasonal drought on fine‐root dynamics in single‐species and mixed‐species arrangements of Fagus sylvatica (European beech) and Picea abies (Norway spruce) by means of a 4‐yr‐long throughfall‐exclusion experiment. Fine‐root production of both species decreased under drought. However, this reduction was not evident for P . abies when grown intermixed with F. sylvatica . Throughfall‐exclusion prolonged the lifespan of P . abies roots but did not change the lifespan of F. sylvatica roots, except in 2016. Fagus sylvatica responded to drought by reducing fine‐root production at specific depths and during roof closure. This is the first study to examine long‐term trends in mature forest fine‐root dynamics under repetitive drought events. Species‐specific fine‐root responses to drought have implications for the rate and depth of root‐derived organic matter supply to soil. From a root dynamics perspective, intermixing tree species is not beneficial to all species but dampens drought impacts on the belowground productivity of P . abies .