Differences in stem traits mediate the variation in trait relationships of grasses and non-grasses.

Stem of vascular plants has emerged as hub organ for its connectivity and providing mechanically support. The relationship between stem diameter and reproductive traits has been widely considered. Differences in stem traits of grass and non-grasses influence plant reproductive output and trait correlations as well, which has been neglected in recent studies. 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 Grassland, Northeast China. These size and architectural traits were coupled with a measure of leaf and seed traits. Except for bulk density, stem traits were size-dependent which explained much of the variation in leaf and reproductive biomass for both grasses and non-grasses. Fecundity was positively correlated with plant height in grasses, and with total branch length in non-grasses. The effects of stem traits, leaf biomass, and seed mass on plant seed number suggest that the specificity of stem architecture and the ubiquity of trait trade-off relationships combine to explain variation in herbaceous plant seed number, 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.