Natural dynamics of forests and their importance for biodiversity and ecosystem functions

Managed forests differ principally from old-growth forests by the scarcity of ageing and decaying trees beyond the rotation age as well as by the more radical disruption of stand continuity between consecutive forest generations. The natural dynamics due to age-dependent mortality and external disturbance in unmanaged stands results in a heterogeneous spatial pattern of differently sized patches of different forest development stages and in the formation of gaps. Forest stands, which follow natural forest dynamics since at least several centuries, are extremely rare in Central Europe. In a case study in the Harz Mountains, Germany, we had the opportunity to study such a forest stand (Bracken old growth forest). The objective of our study was the comparative analysis of plant diversity (ground vegetation, epiphytes) as well as of selected ecosystem properties (aboveground biomass, ecosystem carbon pool) in five age-dependent forest development stages (regeneration, initial, climax, over-mature, and decay stages). While the ground vegetation did hardly differ between the forest development stages, the species richness of epiphytic bryophytes and lichens on standing trees was significantly greater in the over-mature and decaying stages than in the earlier ones, which are also found in managed forests. The species richness on downed deadwood (originating from the preceding forest generation) was highest in the regeneration and initial stages. The aboveground biomass and carbon pools were higher in the over-mature and decaying stages than in the other forest development stages. The pool of soil organic carbon was consistently high across the forest development stages. Our results confirm the great importance of the over-mature and decaying stages for plant diversity and for the capability of forests to store high amounts of carbon.