Proximal genetic origin and diversity patterns of the nine Italian Douglas-fir seed stands

The introduction of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] provenances in Europe has been very successful across many countries and ecological environments and is currently considered a valuable option for future European forestry. However, many plantations have been established regardless of the origin of seeds or seedlings used and growth trends have rarely been linked to genetic data, particularly in old-growth stands. Nonetheless, Douglas-firplantations in Europe have sometimes been used as seed sources, disregarding any information on their genetic structure or geographic origin. In this work, we examined nine Douglas-fir plantations in italy of unknown origin and already included in the Italian National Register of Forest Basic Materials in the source-identified and selected categories. The sampled trees were genotyped by 13 well-known nuclear microsatellites and traced back to their proximal origin using a reference database compiled for the native range, including 38 natural populations. A predominance of coastal provenances was found across Italian seed stands, with seven out of the nine stands being assigned to the coastal area from Washington State and the Oregon Coast (Genetic Cluster I) of the native range. The remaining two stands were assigned to the Rocky Mountain variety from British Columbia and Idaho (Genetic Clusters X and VII–VIII). Italian stands were, on average, characterised by high levels of genetic richness and observed heterozygosity. The comparison of ecological conditions between Italian stands and the species’ native range highlighted that Douglas-fir in Italy was mainly established in warmer and drier conditions, further improving our understanding of its ecological plasticity. This supports its potential relevance to Italian and European forestry in a changing and uncertain future climate.