Soil nutrient heterogeneity alters competition between two perennial grassspecies

Differences in root foraging behavior between species have been well docume nted, but the effects of these differences on belowground competitive abili ty are only beginning to be studied. Here we report the results of a compet ition experiment in homogeneous and heterogeneous soils between two species that differ in their ability to acquire nutrients from patchy environments . The perennial grasses Festuca rubra and Anthoxanthum odoratum have compar able growth rates, but results of previous studies with isolated plants des ignated the latter species as the more effective forager, probably due to i ts higher physiological plasticity (stronger increase in nutrient uptake ra tes per unit root mass in enriched nutrient patches). We introduced nutrien t soil heterogeneity at two spatial scales. In a fine-grained heterogeneous treatment, the nutrient-rich patches were smaller and more concentrated th an in a coarse-grained heterogeneous (checkerboard) treatment. Overall, the level of nutrient availability in these heterogeneous treatments was simil ar to a third treatment with homogeneous soils. The species were grown in m onocultures and mixtures in a standard replacement design during two growin g seasons. In the homogeneous treatment, the relative competitive ability of F. rubra was higher than of A. odoratum, but F. rubra's advantage declined in the he terogeneous environments, and the relative competitive abilities became the same. Both species produced higher root densities in the richer patches of the fine-grained heterogeneous treatment, but not significantly so in the coarse-grained heterogeneous treatment, and these responses were the same f or both species. The root activity of the species was assessed by determini ng the amount of strontium captured by a species at the end of the experime nt, after injecting SrCl2 in the nutrient-rich patches and in equivalent lo cations in the homogeneous treatment. F. rubra acquired significantly more Sr in the mixtures in the homogeneous treatments, but in the coarse- and fi ne-grained heterogeneous treatment the nutrient acquisition abilities of th e species were equal. These results suggest that a more effective root fora ging behavior confers a higher competitive ability in heterogeneous environ ments. Moreover, the results on root-proliferation and nutrient-acquisition ability suggest that a higher physiological (rather than morphological) pl asticity is critical in obtaining a long-term competitive advantage. Analys es of shoot size inequalities suggest that competitive interactions were si ze-symmetric in homogeneous soil and size-asymmetric in the heterogeneous t reatments. However, in the long term, competition becomes more size-symmetr ic in the heterogeneous soils, consistent with the increasing importance of physiological plasticity.