Physiognomic changes in response to herbivory increase carbon allocation to roots in a temperate grassland of central Argentina

In most temperate grasslands, cattle grazing can promote physiognomic changes on plant communities, as well as changes in species growth patterns. Through these changes in vegetation, cattle grazing can generate changes in allocation to total, fine and coarse root biomass, which can further affect ecosystem processes. The objective of this work was to evaluate differences in root biomass, in the proportions of fine and coarse root biomass and in root C:N ratio across physiognomies from 0 to 30 cm. We selected plots of grazing lawns (associated to relatively high stocking rates), and plots of thin and thick tussock grasslands (associated to relatively low stocking rates) within the Pampa de Achala system in central Argentina. We found that total, coarse, and fine root biomass were highest in grazing lawns, intermediate in thin tussock grasslands, and lowest in thick tussock grasslands. Thick tussock grasslands showed the highest relative proportion of fine roots while root C: N ratio did not vary among physiognomies. Higher root biomass in lawns was probably caused by the dominance of species that allocate more biomass to roots than to shoots, as well as by the complementarity of soil resource extraction and by compensatory root growth in response to herbivory. In addition, and contrary to what was expected based on their above-ground attributes, thick tussock grasslands presented higher proportion of fine roots, which suggests a more acquisitive resource use strategy. However, root C:N from thick tussock grasslands did not differ from other physiognomies which might indicate there are not clear differences in below-ground resource use strategies. Our results show that grassland root biomass in this study system can increase in patches associated to higher cattle grazing due to changes in plant composition and physiognomy, while changes in root quality seem to be uncoupled from the above-ground traits.