Seed Size-Number Trade-Off Exists in Graminoids but Not in Forbs or Legumes: A Study from 11 Common Species in Alpine Steppe Communities

Seed size and number are two important components of plant reproductive traits. Previous theoretical studies have suggested that resource limitations lead to a strong trade-off between seed size and seed number. However, empirical evidence from natural communities remains scarce. In this study, the relationship between seed size and seed number was tested at the community level and in three functional groups—graminoids, forbs and legumes—in a natural alpine steppe community in the Tianshan Mountains. The role of limiting resources in reproduction and in determining trade-off patterns was also examined by treating the reproductive biomass and allocation of each species as a resource pool for producing seeds. Our results showed a significant negative relationship between seed size and seed number at the community level, which indicated that a trade-off between seed size and number existed and that the species that produced large seeds produced fewer seeds and vice versa. This trade-off was detected for the graminoid group but not for the forb or legume group. Moreover, the graminoid group presented lower reproductive biomass and allocation than the forb and legume groups did, indicating that the graminoid species were more strictly limited by reproductive resources. Our study provides evidence of a seed size‒number trade-off in a natural alpine steppe community, and the important role of reproductive resources in determining the trade-off.