Seed Size‐Number Trade‐Off in Populations of a Cosmopolitan Species ( Oxybasis glauca ) From Xinjiang, China

ABSTRACT The trade‐off between seed size and number is a fundamental reproductive strategy critical for species survival and distribution. While this trade‐off is well‐documented at the interspecific level, its manifestation within a single species (intraspecific scale) remains surprisingly scarce. In this study, we examined the interplay between seed size (mass) and seed number in 48 natural populations of the cosmopolitan species Oxybasis glauca along a broad environmental gradient in Xinjiang, northwestern China. We confirmed a significant negative correlation between seed size and seed number, reinforcing the classic trade‐off strategy in which individuals produce either few, large seeds or many, small seeds. Our results suggest that the negative correlation may reflect a fundamental resource‐partitioning trade‐off, where reproductive biomass allocation to seed size appears to occur at the direct expense of seed number. Notably, seed number exhibited extraordinary phenotypic plasticity, whereas seed size remained evolutionarily conserved, suggesting that fecundity is the primary trait for adaptive modulation. Furthermore, our results reveal that broad‐scale geographical, climatic, and soil gradients had negligible predictive power for reproductive variation. Instead, reproductive success was governed primarily by fine‐scale, human‐mediated resource availability within local microhabitats, with seed size, plant height and reproductive biomass acting as the primary functional links that translate local conditions into seed production. This synergistic strategy, pairing a stable seed size with a highly plastic seed number responsive to microhabitat conditions, underscores the remarkable capacity of O. glauca to thrive across diverse and disturbed environments, accounting for its success as a cosmopolitan species.