Pinus sylvestris provenance climate differentiates roots, bacterial communities, and their relationships — Evidence from a common garden experiment

Adaptations among Pinus sylvestris populations to diverse climatic conditions have been studied primarily in the species' aboveground traits, while their belowground adaptations remain underexplored. Belowground traits are also crucial for shaping the communities of bacteria that affect tree development. To date, climate effects on intraspecific differentiation, belowground traits, and bacterial communities have been studied separately; here we integrate these aspects to better understand tree growth complexity. In spring (beginning of the growing season) and early autumn (mid-growing season), we investigated three groups of belowground traits (soil chemistry, root chemistry, root morphology) and the bacterial communities (soil bacteria and root-endophytes) associated with mature P. sylvestris trees originating from ten climatically distinct provenances, grown in a common garden. The belowground traits' diversification was partially explained by climatic conditions characterizing the P. sylvestris provenances. This suggests that some belowground traits (e.g., soil pH, soil total carbon and phosphorus concentrations, root glucose concentration) are determined by provenance climates. Bacterial communities differed among the studied provenances, with root-endophyte bacteria showing higher differentiation than soil bacteria. In both community types, these differences were larger in spring. We suggest that the higher bacterial community differentiation in spring resulted from differences in trees' belowground activity onset, regulated by inherited adaptations to provenance conditions in addition to the local weather conditions. The differentiation of hosted bacterial communities persisted even in mature trees after decades of growth in shared climatic conditions. Understanding the limits to P. sylvestris capacity for re-adapting to changing conditions will be important for climate-smart forestry.