Historical mixed-severity disturbances shape current diameter distributions of primary temperate Norway spruce mountain forests in Europe.

• Current DBH distribution shapes are driven by historical disturbance regimes. • Low-severity disturbances showed a reverse J-shaped diameter distribution. • High-severity disturbance lead to a unimodal diameter distribution. • Time and severity of last disturbance are good predictors of DBH distribution shape. Natural disturbances strongly influence forest structural dynamics, and subsequently stand structural heterogeneity, biomass, and forest functioning. The impact of disturbance legacies on current forest structure can greatly influence how we interpret drivers of forest dynamics. However, without clear insight into forest history, many studies default to coarse assumptions about forest structure, for example, whether forests are even or unevenly aged. The aim of this study was to analyze the effects of past disturbances on the current diameter distributions of Norway spruce (Picea abies (L.) Karst.)-dominated landscapes throughout the Carpathian Mountains. Our dendroecological dataset comprises tree cores from 339 plots (7,845 total tree cores), nested within 28 primary forest stands, known to vary greatly in the severity of historical disturbances. Our analyses revealed that historical disturbances had a strong and significant effect on the current diameter distribution shapes at the plot level. We demonstrated that mixed-severity disturbance regimes were more frequent and create a complex pattern of diameter distributions at the plot and stand scale. Here, we show that high severity disturbance was associated with unimodal diameter distributions, while low and moderate severity was associated with the reverse J-shaped distribution. This is a result of complex disturbance patterns, with structural biological legacies. Our results will have important management implication in the context of tree size heterogeneity, biomass storage, and productivity as influenced by natural disturbances. Lastly, these results demonstrate that structural changes may arise as consequences of changing disturbance regime associated with global change. [ABSTRACT FROM AUTHOR]