Environmental constraints on a global relationship among leaf and root traits of grasses

Uncertainties regarding the relationships between leaf and root. traits have impeded an integrated understanding of plant evolution and the efficient parameterization of ecosystem models. We measured key root and leaf traits of grasses from 77 sites in four grassland regions of the world (New Zealand, Australia, South Africa, North America). Within each region, the relationships among leaf traits paralleled those among root traits. Plants with low root or leaf N concentrations had roots or leaves with high tissue density, high lignin concentrations, low amount of mass that was soluble in a neutral detergent solution, large diameter/thickness, and were less enriched in N-15. Yet, whether comparing plants within a region or among all four regions, there was little relationship between root traits and leaf traits, except for a positive relationship between root and leaf N concentration and between root and leaf delta(15)N. At the global scale, factors such as soil freezing and the type of nutrient limitation appear to determine relationships among leaves and roots. C-4 grasses not only had lower leaf N concentrations than C-3 grasses but also lower root N concentrations. When compared at the same root N concentration, C-4 grasses had greater leaf N concentrations than C-3 grasses.