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Forest management is widely considered to have an impact on the understorey plant diversity, which is essential for sustainable forestry and nature conservation. However, contradictions among previous large-scale studies and meta-analyses raised the need for better-designed research. Such research should identify drivers controlling diversity and species composition, quantify the overstorey-understorey relationships and their dynamics in the temperate forests.

Two spatial scales (2.25 m(2); 314 m(2)) and stratified random sampling design were used to record under storey vegetation and overstorey and environmental properties along the whole lifecycle of managed Norway spruce (108 years) and European beech (140 years) stands, the most wide-spread mountain forest types in European temperate zone. The dataset also includes primeval fir-beech forests located in the same area.

Forest management significantly altered understorey diversity and species composition in both beech and spruce stands. Diversity measures changed considerably over the development of managed stands, exhibiting the rapid decline from species-rich forest clearings to poor stands of 30-40 years, later slowly recovering and reaching the average level of old-growth forests at the age of over 100 years. The impact of management was stronger at the smaller scale. It is caused by the fine-scale character of overstorey-understorey interactions, and it is weakened due to stand structure heterogeneity in larger scales.

Competition of overstorey trees was identified as the most important driver of understorey diversity and its temporal dynamics. It was also reflected in the litter amount, which negatively affected diversity especially in the beech forests. In contrast, the light amount was a more important factor in the spruce stands.

We conclude that comparisons between datasets that are heterogeneous regarding overstorey age and proportion of development stages, as well as meta-analyses based on such inconsistent studies, can lead to confusing conclusions and improper applications. As a practical implementation, we recommend using overstorey tree density as the most efficient measure for assessment, prediction and managing understorey plant diversity. (C) 2017 Elsevier B.V. All rights reserved.