Synergistic interactions: Population origin, chemical traits, and endophytic fungi shape white spruce adaptation across environments

White spruce ( Picea glauca ), a dominant species in Canadian boreal forests, is increasingly threatened by climate change. Assisted migration, which involves relocating populations to areas with more suitable climates, has been proposed as a proactive management strategy to help vulnerable plant species in the face of climate change. However, it is not known whether assisted migration will impact the plant’s natural defence against herbivores. White spruce relies on toxic secondary metabolites, such as terpenes, along with symbiotic associations with endophytic fungi, to defend against many herbivores like the eastern spruce budworm. We explored if relocating to new climates, especially across long distances, influences chemical defences and fungal endophytes of white spruce. Our results showed that the population origin influenced terpene concentrations and the composition of endophytic fungi. In short-distance transplantations, population origin explained 31 % of the variation in terpene concentrations and 33 % of the variation in endophytic fungal communities. In long-distance transplantations, population origin explained 32 % of terpene variation and as much as 66 % of fungal community variation. Regression analyses further showed that, in short-distance transplantations, several terpene concentrations were significantly associated with elevation differences between origin and test sites as well as climatic variables from the source populations. In contrast, in long-distance transplantations, terpene concentrations were primarily associated with geographic differences (latitude and longitude) and mean annual precipitation (MAP). These findings highlight the ecological and genetic factors shaping white spruce adaptability to environmental changes. By elucidating the interactions between the population origin, chemical defence, and fungal endophytes, this study provides insights for developing forest management strategies that enhance the resilience and conservation of forest trees in response to climate change and pest pressures. • Terpene concentrations vary with geographic differences between sites and origins in assisted migration. • Terpene concentrations at short and long distances are shaped by different geographic and climatic factors. • Endophytic fungal communities are shaped by origin populations across transplant scales. • Terpenes and endophytes guide assisted migration and spruce management.