Climate change, weather, and geography shape seed mass variation and decline across western North America

Seeds are an essential reproductive strategy for most plants, and seed mass is predicted to respond to environmental factors, but we do not know if climate change-related size changes observed in other organisms will also be reflected in seed mass. We investigated the temporal, geographic, and environmental patterns that influence seed mass across 2,800 species of native plants from over 13,000 populations across the western US using information from two decades of collections from a national program, Seeds of Success. Most species exhibited low variation in seed mass, though some varied by up to 220% over their occupied area. Measures of climate change influenced seed mass in over 40% of species, which could be evidence of adaptation to new optimal values or maladaptive responses. Species with the most variation in seed mass were those collected across sites with a wide range of minimum temperatures, from more northern latitudes, and across a wide range of dates, and seeds were generally larger in areas where it was wetter and warmer. Seed mass declined for over half (55%) of species, and declined overall by an average of 0.012 mg each year, though there were many species-specific differences, including 17% of species increasing appreciably in mass. Relationships between seed mass and environmental variables varied widely in strength, identity, and direction among species, including those that often co-occur. These results support the need to continue native seed collections for conservation and restoration, as plant reproductive strategies may be shifting with changing climates.