This research examined how a beneficial fungus that helps Douglas fir trees grow has spread from North America to other parts of the world. The study found that this fungus, Suillus lakei, can only expand to new areas where Douglas fir trees are present, and its spread is largely due to human activities like ornamental plant trade. Impacts on everyday life: • Shows how human activities like gardening can unintentionally spread organisms globally • Helps predict where beneficial fungi might establish in new regions • Provides insights for managing non-native species in gardens and forests • Demonstrates the importance of understanding plant-fungal partnerships in forestry • Highlights the role of climate in determining where organisms can survive