Genetic and environmental considerations for the utilization of Pinus pinaster Ait. provenances across a region lacking proper local genetic materials

Maritime pine (Pinus pinaster Ait.) covers vast areas and is of economic importance in southwestern Europe, particularly in Galicia (NW Spain). Galicia is a heterogeneous region with an Atlantic climate in the coast and a Mediterranean-like climate in the inland, where forest reproductive materials (FRM) with proper adaptation, productivity and timber quality are not available. Thus, there is a need for tailored FRM recommendations for reforestation in this region. P. pinaster is particularly sensitive to environmental variation and shows significant intraspecific genetic variability in this sensitivity, so understanding population responses to environmental variation becomes crucial for proper selection of FRM. Taking advantage of volume growth and stem straightness 13 years after planting, assessed on c. 7500 trees from 25 P. pinaster populations established in seven common gardens across inland Galicia, we analyzed intraspecific variation in sensitivity to climate, geographic, edaphic and site quality factors. We used Mantel correlations and factorial regression models to distinguish the environmental parameters explaining the observed population × site interaction. We also estimated population phenotypic plasticity across sites and the existing genetic relationship between growth and stem straightness to define the optimal selection strategy for productivity purposes. Results showed a quantitatively significant population × site interaction for growth whereas it was almost negligible for stem straightness. In the case of growth, no specific environmental factor was able to explain the population relative performance across test sites, being site quality the only significant factor but with low power to describe the patterns found. Population differences were maximized in higher site quality conditions, where the largest gains from planting selected populations would be expected. Tree growth and straightness were not genetically correlated. Based on these results, subdividing inland Galicia for deployment of P. pinaster FRM throughout specific selections for each subregion is not recommended. Selecting FRM based on average growth and its phenotypic stability should be also avoided, as both properties were negatively correlated. We recommend selecting FRM based on the results from high quality sites, where Atlantic origin populations with high levels of genetic improvement showed the fastest growth. The inclusion of selection criteria based on stem straightness did not alter recommendations given that fast growing FRM showed intermediate or slightly above-average straightness. The limited explanatory power of climate factors for the population × site interaction prevents adjusting recommendations in light of the projected climate change.