Soil Nutrient Heterogeneity Alters Competition between Two Perennial Grass Species

Abstract

Differences in root foraging behavior between species have been well documented, but the effects of these differences on belowground competitive ability are only beginning to be studied. Here we report the results of a competition 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 comparable growth rates, but results of previous studies with isolated plants designated the latter species as the more effective forager, probably due to its higher physiological plasticity (stronger increase in nutrient uptake rates per unit root mass in enriched nutrient patches). We introduced nutrient soil heterogeneity at two spatial scales. In a fine-grained heterogeneous treatment, the nutrient-rich patches were smaller and more concentrated than in a coarse-grained heterogeneous (checkerboard) treatment. Overall, the level of nutrient availability in these heterogeneous treatments was similar to a third treatment with homogeneous soils. The species were grown in monocultures and mixtures in a standard replacement design during two growing 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 heterogeneous 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 for both species. The root activity of the species was assessed by determining the amount of strontium captured by a species at the end of the experiment, after injecting SrCl2 in the nutrient-rich patches and in equivalent locations in the homogeneous treatment. F. rubra acquired significantly more Sr in the mixtures in the homogeneous treatments, but in the coarse- and fine-grained heterogeneous treatment the nutrient acquisition abilities of the species were equal. These results suggest that a more effective root foraging behavior confers a higher competitive ability in heterogeneous environments. Moreover, the results on root-proliferation and nutrient-acquisition ability suggest that a higher physiological (rather than morphological) plasticity is critical in obtaining a long-term competitive advantage. Analyses of shoot size inequalities suggest that competitive interactions were size-symmetric in homogeneous soil and size-asymmetric in the heterogeneous treatments. However, in the long term, competition becomes more size-symmetric in the heterogeneous soils, consistent with the increasing importance of physiological plasticity.

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