Stem size and architecture affects herbs reproductive output but not trade-off relationships.

1. Stem of vascular plants has emerged as hub organ for its connectivity and providing mechanically support. The relationship between stem thickness and reproductive traits has been widely considered. Stem architecture influences plant reproductive output and trait correlations as well, which has been neglected in recent studies, especially for herbaceous plants. 2. Measurements of plant height, stem biomass, volume, basal diameter, total branch length and bulk density were used to describe the different stem size and architectures of 46 species of grasses and non-grasses in the Songnen grasslands. These size and architectural traits were coupled with a measure of leaf and seed traits. 3. Except for bulk density, stem traits were size‐dependent which explained much of the variation in leaf and reproduction biomass for both grasses and non-grasses. Fecundity, however, was positively correlated with plant height for grasses, and with branch length for non-grasses. 4. The effects of stem traits, leaf biomass, and seed size on plant fecundity suggest that the specificity of stem architecture and the ubiquity of trait trade-off relationships combine to explain variation in herbaceous plant fecundity, and it is therefore important that phylogenetically determined morphological differences in plant architecture should be incorporated into studies of interspecies competition and coexistence relationships, also the trait-based community dynamic.