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. While Douglas-fir (Pseudotsuga menziesii) is considered a potential candidate for future climate-resilient forests, in the past this non-native crop tree species was selected for productivity rather than stress resistance. Here, we performed dendrochronological analyses in mature trees from 50 Douglas-fir provenances across six sites in Southern Germany, covering a precipitation gradient of 542 mm yr−1. Because the provenances were not equally distributed across the gradient, they were divided into three groups according to their climate at origin. We observed high variability both in radial increment and radial growth patterns between the provenances, which were lowest for the northern-interior provenances but unrelated to the precipitation gradient. As expected, the extreme drought events in 2003 and 2018 significantly reduced the radial growth of all provenances, with the northern-interior provenances showing strongest reductions and therefore lowest growth resistance. However, the recovery of radial growth was equal to the coastal origins after 2003 and even higher after 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 reported for conifers. In conclusion, northern-interior provenances showed a higher plastic growth response to extreme drought. While Douglas-fir appears to be a promising candidate for future climate-resilient forests, at least at our relatively wet sites, further validation is needed to confirm whether interior provenances are better drought adapted than coastal provenances

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