Cuticle water permeability, thickness and microchemistry of conifer seedlings and mature trees at the treeline

The needles of conifer seedlings at the alpine treeline experience hotter near-ground microclimates than adult canopies, prompting the question about how their cuticles maintain water-barrier function under heat. We compared needle cuticle permeability, thickness and microchemistry between seedlings and mature trees of Larix decidua , Picea abies , and Pinus cembra to assess phenotypic plasticity. The minimum leaf diffusive conductance (g min ) was measured two to 24 hours after sample detachment during bench drying at temperatures between 25 and 43 °C. This procedure should force the stomata to close, providing an indication of the temperature dependent cuticle's water permeability. Raman imaging was used to assess microchemistry and thickness of the cuticles. At the study site, seedling's needles reached temperatures of up to 40 °C, which is 14 K higher than that of mature trees. Among species, L. decidua had the highest g min , followed by P. abies and P. cembra . Seedlings of P. abies and P. cembra exhibited lower g min at 43 °C, indicating greater cuticular resistance to water loss, unlike L. decidua . Raman imaging differentiated species-specific cuticle structures: L. decidua being thinner and aromatic/flavonol-rich than P. cembra with a thicker and more lipid/cutin rich cuticle and P. abies was intermediate. Seedlings lacked or had an outer wax layer <300 nm, though their internal aromatic layers resembled those of mature trees. Despite the rise in cuticular permeability above ~35 °C, evergreen seedlings maintained lower g min at 43 °C, consistent with acclimative plasticity of cuticle architecture and microchemistry under thermal extremes. These findings link microclimate-driven heat exposure to cuticle traits and highlights that species identity and developmental stage jointly determine water-barrier performance at the treeline. • Seedling needles reached 40°C, ~14 K higher than mature tree needles. • At 43 °C, evergreen seedlings maintain lower g min than mature trees. • Larix decidua had the highest g min , followed by Picea abies and Pinus cembra . • Cuticle thickness, microchemistry and structure were key for g min . • L. decidua cuticle was phenolic-rich, P. cembra had more lipids.