Cascading effects of insufficient seed cold stratification across early life-stages in three temperate trees: impaired phenology and reduced seedling fitness under climate warming.

BACKGROUND AND AIMS: Global warming is shortening winters in temperate ecosystems, limiting cold stratification for seeds with physiological dormancy (PD). The consequences of incomplete dormancy release can cascade beyond germination, potentially affecting fitness in subsequent life-stages, yet this remains understudied in trees. Here we assess the effects of insufficient stratification on germination, leaf emergence and seedling performance in three broad-range temperate tree species. We further evaluate how reduced cold stratification under climate change may drive shifts in these traits across the species distribution. METHODS: We sampled six Abies alba, six Acer pseudoplatanus and four Fagus sylvatica populations across their native distribution. 10 287 seeds of the three species were sown in climatic-controlled chambers at three different temperature regimes and two stratification treatments. We assessed the success, timing and synchrony of germination and first leaf emergence. We further monitored seedling growth and biomass, as well as root and leaf mass fractions. We analysed the effects of unfulfilled stratification in response to sowing temperature and the influence of populations' climate using generalized linear mixed effect models. We projected shifts in the studied traits under a SSP5-8.5 climate change scenario accounting for reduced cold stratification across the species native distributions. KEY RESULTS: Missing cold stratification delayed germination time in all species and decreased F. sylvatica germination and leaf emergence proportions. It further constrained Ab. alba and F. sylvatica seedling growth, particularly under warm temperatures, irrespective of phenological changes. Spatial projections depicted important germination phenology changes across species distributions, uphill increases of Ab. alba seedling growth, and a decrease of F. sylvatica early fitness towards its western distribution. CONCLUSIONS: Cold stratification improved seedling development and their performance under warming beyond regulating germination. Unfulfilled cold stratification in a warmer climate may therefore constrain temperate tree species seed regeneration even if germination proportions or phenology remain unaffected.