Radial growth response of Douglas-fir provenances during extreme drought across a climatic gradient in Central Europe

Extreme droughts are occurring more frequently in Central Europe, resulting in elevated tree mortality and the need for reforestation and forest conversion. The Douglas fir (Pseudotsuga menziesii) is a tree species native to the West Coast of North America. It is considered a potential candidate for future climate-resilient forests. However, in the past, this non-native crop tree species was selected for productivity rather than stress resistance.Here, we conducted a comprehensive analysis of 524 tree cores from 50 Douglas-fir provenances that were grouped according to their climate at origin and sampled across six sites in Central Europe. The sites were established between 1975 and 1980 and covered a precipitation gradient. The tree ring data were used to estimate drought response indices for the extreme global change-type droughts in 2003 and 2018.We observed a high variability both in radial increment and radial growth patterns between the provenances, which were lowest for the northern-interior provenances across all sites but unrelated to any of the climatic data covered. As expected, extreme drought events significantly reduced the radial growth of all provenances, with the northern-interior provenances showing the strongest reductions, and therefore the lowest growth resistance. However, the recovery of radial growth is equal to the coastal origins in 2003 or even significantly higher in 2018. This combination of low resistance and high recovery is consistent with a higher tree-ring plasticity. Furthermore, we found a positive relationship between resilience and resistance, in contrast to the general trend recently reported for conifers.In conclusion, we found that interior provenances showed a higher plastic growth reaction in response to extreme drought. While Douglas-fir appears to be a promising candidate for future climate-resilient forests, further validation is needed to confirm whether interior provenances are better drought adapted than coastal provenances.