It’s what’s underneath that matters: Big roots, not height, enhance competitive ability in a grassland

It is widely recognized that a variety of plant traits such as height, leaf area, and biomass are associated with high vascular plant competitive ability. This conclusion is drawn from experiments conducted primarily in highly productive systems, such as wetlands. What traits are associated with competitive ability in dry, low productivity systems, like grasslands, have received comparatively little attention. Greater than 70% of grassland plant biomass may occur belowground, and competitive interactions are primarily for soil resources, not light. Thus belowground traits likely influence competitive ability. To determine the relative importance of above- and belowground traits on plant competitive response ability, we measured competitive response and trait values of twelve species in an Alberta, Canada rough-fescue grassland. Numerous traits were measured on individuals taken from the field but outside of the 192 competition plots. Preliminary analyses indicated that species identity explained a significant proportion of the variance in competitive ability. Subsequent multiple regression analyses of trait values revealed that larger specific root volume, specific root length, specific leaf volume and average root diameter positively correlated with plant competitive ability, while greater maximum height and specific leaf area were negatively associated with competitive ability. This study is among the first showing that specific belowground traits are positively associated with enhanced competitive ability. These results suggest that, in low productivity systems, plant traits conferring persistence during periods of low resources, like thicker leaves, combined with traits for belowground competition, are more important than traits for above ground resource capture.