When beech strikes back: How strict nature conservation reduces herb-layer diversity and productivity in Central European deciduous forests

• European beech is considered to spread out in strictly protected European deciduous forests prospectively.
• We analyse the effects of increasing beech proportion on herb-layer diversity and productivity.
• In dark unmanaged stands, increasing beech proportion reduces herb-layer diversity and productivity.
• In managed stands, light availability is significant for herb-layer diversity and productivity.
• Forest management cessation may result in lower herb-layer diversity and productivity in many regions.

Current nature conservation policy is going to alter the tree-layer composition of many Central European deciduous forests. As a consequence of ceasing silvicultural management, the competitive ecosystem engineer European beech (Fagus sylvatica L.) will spread and the proportion of secondary tree species will considerably decrease. Thus, changes can also be expected in herb-layer diversity, productivity and composition. To predict these changes, we analysed relationships between tree- and herb-layer diversity and composition along tree-diversity gradients in the Central German woodland areas of the Hainich National Park (unmanaged) and the Göttinger Wald (managed by close-to-nature forestry).The flora of both study areas was dominated by typical forest species. In the Hainich, the herb-layer vegetation in beech-dominated stands was less diverse compared to stands richer in canopy species. Herb-layer productivity was also lower in beech-dominated stands. In the Göttinger Wald, herb-layer diversity and productivity were not related to tree-layer diversity and beech proportion, instead to the light transmissibility of the canopy layer. The different results can be explained by logging effects in the Göttinger Wald, which promoted the light factor as significant for herb-layer diversity and productivity. In the consistently darker stands of the Hainich, soil variables, particularly influenced by the ecosystem engineer beech, were decisive. Areas of high canopy diversity in the Hainich result from former management practices and can, in the future, be expected to decrease with the spread of beech.We conclude that forest management cessation will result in decreasing herb-layer diversity and productivity in many Central European deciduous woodlands, especially in landscapes where historical management practices led to unnaturally low proportions of beech. The short-term consequence of management cessation is a loss of herb-layer diversity due to reduced light availability; in the long-term, herb-layer diversity is expected to decrease due to the expansion of beech.If the key aim of nature conservation policy is to preserve and develop the typical diversity of near-natural beech forests, then a complete cessation of forest management can be advised. However, if the typical forest herb diversity of the semi-natural, century-old cultural landscape is to be maintained, close-to-nature forestry management is necessary.