Temporal asynchrony in fine-root biomass may contribute to shrub and grass coexistence in mixed patches

Abstract

We described seasonal changes in fine-root biomass of a grass and a shrub dominant species in a plant community characteristic of the arid Patagonian Monte and then we inferred to want extent the observed differences could contribute to the species coexistence. We selected representative plant patches of the natural vegetation arrangement consisting of one isolated plant of the dominant shrub Larrea divaricata (Ld), grass patches formed by one or more bunches of the dominant grass Nassella tenuis (Nt), and mixed patches consisting of one individual of L. divaricata with bunches of N. tenuis under its canopy (LdNt). We assessed the biomass and temporal changes in fine roots of each species in the upper soil (50 cm depth) of each patch type at three-month intervals during 2 years. The temporal series of fine-root biomass were compared among patch types and in relation to above-ground phenology, as well as climate variables (precipitation, arid index and air temperature). Seasonal changes in fine-root biomass showed similar cycles in the three plant patches with a maximum in spring. The maximum increase in root biomass in Ld and Nt patches occurred during the onset of reproductive growth in winter and spring, respectively. Fine-root changes in LdNt patches mimicked that in Ld patches. Precipitation inputs were significantly positively and temperature negatively related to fine-root changes in Nt patches. Fine-root changes in Ld and LdNt patches were related to the aridity index (positively) and temperature (negatively). We concluded that the observed asynchronies in the date of the largest increases in root biomass and its climate control between the studied grass and shrub species could contribute to the coexistence of plants of both life forms when they overlap their root systems growing in mixed patches. Mechanisms underlying the root patterns observed should be further explored.