Are fine roots of both shrubs and perennial grasses able to occupy the upper soil layer? A case study in the arid Patagonian Monte with non-seasonal precipitation

Abstract

We tested whether both shrubs and grasses are able to develop similar active fine-root systems in the upper soil layer of the arid Patagonian Monte ecosystem with non-seasonal precipitation. We selected in the field shrub patches consisting of one isolated modal plant of the dominant shrub Larrea divaricata Cav., grass patches formed by one or more bunches of the dominant grass Stipa tenuis Phil. (15 cm diameter), and mixed patches consisting of one individual of L. divaricata with bunches of S. tenuis under its canopy. We assessed the biomass, regrowth, and activity of fine roots (diameter <1.4 mm) of each species in the upper soil (50 cm depth) of each patch type at 3-month intervals. We also measured the N concentration in fine roots to estimate the relative contribution of each species to fine-root biomass of mixed patches. We injected Li+ in the soil as a chemical tracer to detect fine-root activity of each species in the upper soil. Fine-root biomass was higher in mixed patches than in grass patches while fine-root biomass in shrub patches did not differ from the two former. We did not find differences in fine-root regrowth among patch types. Li+ injection provided evidence of active fine roots of both species in the upper soil when it was wet. N concentration in fine roots suggested the prevalence of fine roots of L. divaricata in the upper soil of mixed patches. Our results support evidence of the ability of fine roots of both the shrub and the grass species to occupy the upper soil. These findings did not support the two-layer model (H Walter, Ecology of tropical and subtropical vegetation, Oliver and Boyd, Edinburgh, 1971) and provide evidence of this model would be less applicable to arid ecosystems with non-seasonal precipitation. Further, our results highlighted some issues deserving more research such as the outcome of belowground competition between neighboring plants of both contrasting life forms, the eventual limited fine-root carrying capacity of the upper soil, and differences in fine-root lifespan between species of both contrasting life form.