Forest stand productivity derived from site conditions: an assessment of old Douglas-fir stands ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii ) in Central Europe

Key message Douglas-fir growth correlates with the climate, the soil moisture regime, and the soil nutrient status, reflecting a broad physiological amplitude. Even though planting this non-native tree species is suggested as a viable strategy to improve adaptiveness of European forests to a more extreme climate and to assure future productivity, the expected temperature increase may induce a decline in forest stand productivity for Douglas-fir in already warm and dry regions. Context Tree species selection is one of the most important forest management decisions to enhance forest productivity and stand stability on a given site. Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii ), a non-native species from north-western America, is seen as an important additional species option for adapting Central European forests to a changing climate. Aims This study assesses Douglas-fir forest productivity derived from site conditions. We investigate climatic and physico-chemical soil characteristics and productivity of 28 mature Douglas-fir stands growing on siliceous, as well as carbonate bedrock material in southern Germany and north-eastern Austria. Methods The importance of climatic and physico-chemical soil characteristics was analyzed with the machine learning method Random Forests . Results The results show that Douglas-fir growth correlates with climate, soil moisture, and soil nutrient availability derived from ten climatic and physico-chemical soil parameters. Conclusion The broad pH optimum between 4.5 and 7.2 reflects the broad physiological amplitude of Douglas-fir, and no significant differences were detectable between carbonate and siliceous bedrock. We also conclude that climate change may induce a forest stand productivity decline, because lower productivity with the highest mean summer temperature across our study range was observed at the warmest sites in Eastern Austria.