Climate Change, Weather, and Geography Shape Seed Mass Variation and Decline Across Western North America

ABSTRACT 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 2092 species of native plants from over 13,000 locations across the western United States 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 42% of species, which could be evidence of adaptation to new optimal values or maladaptive responses. Mean seed mass was phylogenetically conserved, while seed mass variation had little to no phylogenetic signal. Species with higher seed mass variation were collected across sites with a wider range of minimum temperatures, from more northern latitudes, and had longer growing seasons. Seeds were generally larger in warmer and wetter areas. Seed mass declined over time for 55% of species and declined overall by an average of 0.012 mg each year, though there were species‐specific differences, including 19% of species increasing appreciably in mass. Seed mass of C3 and C4 species responded in opposite ways to global change metrics of temperature and precipitation, potentially providing insight into global declines in C4 species. Relationships between seed mass and environmental variables varied widely in strength, identity, and direction among species, including among congeners and species 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.