Effects of crown transparency on phloem formation and metabolomic profiles in Pinus sylvestris L. co-occurring with Juniperus communis L. at a drought-prone site

Abstract Key message Increasing crown transparency shortens phloem production in co-occurring Pinus sylvestris L. and Juniperus communis L. and shifts carbon allocation from growth to defense metabolism in Pinus sylvestris L.. Context Climate warming intensifies drought stress in temperate forests, altering carbon allocation and tree vitality. Crown transparency (CT) is a widely used indicator of canopy decline, but its relationship with secondary phloem development and metabolism remains unclear. Aims We examined how CT influences intra-annual phloem phenology in Pinus sylvestris L. and Juniperus communis L., and metabolomic profiles in P. sylvestris , at a drought-prone submontane site (Tyrol, Austria), testing whether increasing CT alters phloem phenology and metabolomic profiles. Methods We combined microcore sampling, Gompertz modeling, and untargeted GC–MS metabolomics across CT classes, linking anatomical development with biochemical composition. Results Phloem onset was CT-independent in P. sylvestris but delayed in J. communis with higher CT. Increasing CT shortened phloem formation and reduced cell production in both species. In P. sylvestris , xylem-to-phloem ratios decreased with CT. Low-CT trees showed higher hexoses, arabinose, and redox-active metabolites, whereas high-CT trees accumulated phenolics and showed reduced metabolic flexibility, indicating a shift toward stress-related metabolism. Conclusion CT links structural and metabolic stem responses and coordinates phloem phenology with carbon allocation strategies.