The productivity of mixed mountain forests comprised of Fagus sylvatica, Picea abies, and Abies alba across Europe

Mixed mountain forests of European beech (Fagus sylvatica L.), Norway spruce (Picea abies (L.) Karst), and silver fir (Abies alba Mill.) cover a total area of more than 10 million hectares in Europe. Due to altitudinal zoning, these forests are particularly vulnerable to climate change. However, as little is known about the long-term development of the productivity and the adaptation and mitigation potential of these forest systems in Europe, reliable information on productivity is required for sustainable forest management. Using generalized additive mixed models this study investigated 60 long-term experimental plots and provides information about the productivity of mixed mountain forests across a variety of European mountain areas in a standardized way for the first time. The average periodic annual volume increment (PAI) of these forests amounts to 9.3 m3ha-1y-1. Despite a significant increase in annual mean temperature the PAI has not changed significantly over the last 30 years. However, at the species level, we found significant changes in the growth dynamics. While beech had a PAI of 8.2 m3ha-1y-1 over the entire period (1980-2010), the PAI of spruce dropped significantly from 14.2 to 10.8 m3ha-1y-1, and the PAI of fir rose significantly from 7.2 to 11.3 m3ha-1y-1. Consequently, we observed stable stand volume increments in relation to climate change.