Effect of fine-scale spatial variation of soil nitrogen on the performance of the sexes of Poa ligularis in patchy ecosystems of northern Patagonia

Abstract

In patchy environments of arid Patagonia, males of the dioecious grass Poa ligularis dominate in N-poor microsites, while females are more common in N-rich microsites. In order to explore functional differences related to spatial segregation of the sexes, we analyzed biomass allocation and tissue N concentration in males and females of P. ligularis growing in a range of soil N concentrations (sN). Based on the general patterns of responses described for plants from N-rich and N-poor habitats, we hypothesized that (1) females, which dominate in N-rich microsites, would increase biomass allocation with increasing sN, while males, frequent in N-poor microsites, would show a limited response and (2) tissue N concentration would display a wider variation in males than females in response to changes in sN. At three sites in northern Patagonia, we randomly selected 15 plants of each sex of P. ligularis growing inside shrub patches and 15 in the interpatch areas and evaluated the biomass and N concentration of aboveground (vegetative and reproductive) and belowground structures. Biomass allocation to belowground structures and N concentration in roots increased with increasing sN for both males and females. Aboveground biomass increased with increasing sN only in females. In the N-poorest sites, we found higher N allocation to tiller crowns with increasing sN in males relative to females. In both sexes, biomass allocation to sexual reproductive structures (panicles) did not change significantly with variations in sN. These results provide partial evidence on morphological and functional dimorphism in a dioecious species with spatial segregation of the sexes.