Climate change drives declines in seed germination potential, particularly in woody species and warmer, drier landscapes of a temperate bioregion

Climate change is accelerating species losses in ecosystems across the world. Seed germination is a critical, climate-dependent phase of the plant life cycle; however, the ecological determinants of germination climate niches within diverse landscapes and across functional types (FTs) are still not well understood. In this study, we characterized seed germination temperature and water availability niches for 28 species that represent different FTs (tree, shrub, grass, forb) and vegetation types (grassy woodland, dry and wet forests) within a temperate bioregion (Sydney, Australia). We tested whether ecological determinants, specifically species climate of origin, seed traits, FT and vegetation type explain germination niches and predicted spatial and temporal patterns of germination potential across the landscape under high and low emission scenarios. We found wide variation in thermal and hydric germination niches among species. Optimal germination temperature (thermal niche) was predicted by FT, climate of origin and seed traits, such that shrubs, cool-origin species, and species with large seeds had significantly cooler optimal temperatures for germination. We also quantified spatial and temporal changes in germination potential to identify vulnerable areas and FTs. We found strong species-specific seasonal patterns in germination potential with future climate shifts affecting FTs differently; germination of woody species declined more than forbs. Future germination potential was predicted by historical climatic conditions, with warmer and drier localities being more vulnerable. Overall, our findings demonstrate that species germination responses to climate change depend on FT, seed traits, and species climate of origin, with woody species and warmer, drier parts of the landscape emerging as being particularly vulnerable to declines in recruitment. Our study provides a mechanistic understanding of germination responses to temperature and water availability, enabling predictions of vulnerable species and areas for conservation under climate change, and inform large-scale ecosystem restoration approaches through improved species selection and sowing times.