Does root competition asymmetry increase with water availability?

Background: Below-ground competition as a major structuring force in plant communities can be either symmetric or asymmetric with important consequences for the coexistence of plants. It is a matter of controversy whether asymmetry in competition increases with resource availability or not. Aims: This study tested the hypotheses that root competition between mature Fagus sylvatica and Quercus petraea trees is asymmetric and that asymmetry is more pronounced in non-water-limited environments. Methods: Initially equal-sized tree fine roots were grown for 365-390 days in root growth chambers in situ and exposed to competition by either a conspecific or an allospecific root. Relative root growth rate was used to determine asymmetry in root competition. Different soil moisture regimes were considered by conducting a throughfall reduction experiment and by including data from earlier experiments with contrasting moisture regimes. Results: The competitive interaction between Fagus and Quercus fine roots is asymmetric; root morphology appears to depend on the presence of a competitor; and asymmetry in root competition increases with increasing soil moisture availability. Conclusions: Below-ground competition in temperate broad-leaved mixed forests can be as asymmetric as competition for light, with asymmetry decreasing with increasing water shortage.