Soil organic matter distribution and below-ground competition between Calluna vulgaris and Nardus stricta

1. We conducted an experiment to investigate the effect of organic matter distribution on below-ground competition between the dwarf shrub Calluna vulgaris (L.) Hull and the coarse upland grass Nardus stricta (L.). 2. The two species were grown alone, or in competition in pots where the substrate was either a sand layer overlain by a peat layer (layered), or the same volume of peat evenly mixed with sand throughout the pot (mixed). 3. Root length and allocation in the top, middle and bottom of each pot was measured, along with plant biomass. 4. Calluna allocated the greatest proportion of root length to the top of the pot, irrespective of the peat distribution. Nardus allocated a large proportion of root length to the bottom of the pot. Root allocation by Nardus to the top and middle of the pot was positively related to the distribution of peat. 5. Nardus grown alone attained greater shoot mass in the layered than in the mixed substrate, but this was not the case when Calluna was present. Calluna was the superior competitor in both layered and mixed-substrate pots, but more so in the latter. 6. This was attributed to (i) the ability of Calluna to exclude Nardus roots from the upper organic material in the layered substrate, and (ii) the greater plasticity of root allocation of Nardus , enabling it to avoid competition with Calluna.