A decade of lost growth in old trees: aging shapes the impacts of drought and late frost events on European beech

• A new methodology for the study drops in growth is presented. • We tested it in different European beech stands with contrasting characteristics. • Droughts and late spring frosts have caused a decade of growth loss in old trees. • These discrete climate events are responsible for the long-term decline in vigor. • Tree age shapes tree growth response to extreme climate events. Studying growth declines and the factors that cause them, such as droughts or late spring frosts, is key to understanding their influence on forest productivity. However, most of the currently used methodologies to assess these events have drawbacks that can lead to erroneous conclusions. The increasing frequency and importance of these growth declines is linked to a higher climate variability and thus requires more effort to find suitable approaches to quantify their impacts on long-term tree growth. Furthermore, dendroecology generally focuses its efforts on the study of growth relationships with prevailing climatic conditions, giving little weight to the effect of occasional and discrete climatic events on medium- and long-term growth dynamics. Here, we develop a new methodology that consists in: (I) analyzing the largest growth reductions, (II) characterizing climate in those years, (III) identifying the change points in the tree growth function using Bayesian regression, and (IV) quantifying the impact of climate on short-, medium- and long-term growth trends using relative growth and cumulative growth loss indices. We studied the drops in growth suffered by European beech ( Fagus sylvatica ), caused by both droughts and late frosts. The study was conducted in stands with contrasting structural features (diameter, age) at the southwestern species distribution limit in the central Iberian Peninsula. Our results indicate that extreme climate events have caused a decade of growth loss in old trees (age ca. 100–330 years), and are the factor responsible for the decline of tree vigor. However, the relationships between prevailing climate conditions and tree growth were not significant, highlighting the importance of occasional and discrete climate events as the main drivers of growth. Tree age, rather than tree diameter, shapes tree growth response to extreme climate events such as droughts and late frosts.