Vertical root distribution of individual species in a mountain grassland community: Does it respond to neighbours?

Vertical differentiation in root placement is one of the potential mechanisms of plant niche differentiation. It can be due to the remarkable plasticity of roots in response to nutrients and neighbours, but most data on it come from pot or garden experiments. The roles of vertical differentiation and of plasticity in it in the field are thus not well known. We examined species-specific root vertical distribution in a montane grassland using quantitative real-time PCR. We asked whether individual species differ in their rooting depths, whether such differences are associated with above-ground functional traits (such as height or specific leaf area), and whether they respond to the presence of a competitor. This response was assessed by comparison of species-specific vertical profiles between control plots and plots where the dominant species, Festuca rubra, had been removed. Vertical profiles of individual species varied considerably, from species with most root biomass concentrated in the uppermost (<2cm) soil layer, through species with uniform vertical distribution, to a species with roots predominantly below 8cm (Nardus stricta). Species at the fast end of the plant economy spectrum were more likely to place their roots in the uppermost layers. Grassland species, thus, exploit different parts of the below-ground resources in spite of their short stature, minor differences in height above-ground and shallow soil. While below-ground and above-ground biomasses of most species were higher in the removal plots, species rooting patterns did not change in response to the removal. The interspecific differences in vertical profiles were thus due to species' innate differences, not to plastic responses to the presence of the dominant species.Synthesis. The findings imply that vertical root differentiation in the field is strong and can contribute to niche differentiation. However, the role of root plasticity in natural systems may be considerably weaker than in artificial systems with few species and strong nutrient gradients. This absence of the plastic response in the field is likely to be due to a fairly homogeneous distribution of nutrients in the soil and to the predominantly symmetric nature of below-ground competition.