Stem dehydration, more than hydraulic failure, drives drought mortality in conifer seedlings with contrasting...

Climate change is intensifying drought frequency and severity, threatening forest regeneration and survival in many parts of the world. To elucidate the mechanisms underlying drought-induced mortality in conifer seedlings, we compared Japanese larch (Larix kaempferi (Lamb.) Carrière) and Sakhalin fir (Abies sachalinensis (F. Schmidt) Mast.), two major afforestation species with contrasting stem hydraulic vulnerability distributed in cool-temperate northern Japan. We combined measurements of hydraulic traits with a soil-drying experiment using potted seedlings, monitoring plant-water relations, gas exchange, non-structural carbohydrates and survival after rewatering. Compared with fir, larch exhibited higher maximum stem conductivity (Kstem) but greater cavitation vulnerability, with 50% loss (P50) occurring at -3.26 MPa, whereas in fir this occurred at -5.89 MPa. In both species, stomata closed before turgor loss or onset of hydraulic failure, and photosynthesis ceased well before seedling death. Non-structural carbohydrate concentrations remained stable, with starch converted to soluble sugars, indicating mobilization for osmotic regulation and metabolic maintenance rather than carbon starvation. In fir, seedling mortality occurred at a lethal water potential (Ψlethal) of -5.50 MPa, which coincided with ~80% loss of stem relative water content (RWC) despite substantial conductivity being retained, pointing to tissue desiccation as the primary cause of mortality. In larch, mortality (Ψlethal: -4.40 MPa) coincided with both ~80% loss of stem RWC and Kstem, indicating the co-occurrence of severe dehydration and loss of hydraulic function at death. In these two species exhibiting different traits related to stem vulnerability, lethal drought thresholds were more consistently predicted by stem RWC than by hydraulic metrics alone. Stem RWC therefore provides a robust integrative indicator of drought-induced mortality risk in conifer seedlings.