Long‐term disadvantages of selective root placement: root proliferation and shoot biomass of two perennial grass species in a 2‐year experiment

Abstract

Summary The long‐term benefits of root foraging in heterogeneous environments are unclear. The short duration of many previous studies may have overlooked the effects of patch depletion and root turnover, which may limit the long‐term rewards of root foraging for perennial plants. The benefits of root foraging were investigated for Holcus lanatus and Nardus stricta over two growing seasons. Shoot biomass of each species was measured in homogeneous nutrient‐rich and nutrient‐poor treatments and in a heterogeneous treatment consisting of a nutrient‐rich and a nutrient‐poor side, at a high and a low overall level of nutrient availability at the same patch contrast. Large initial differences in nitrate concentrations in the soil solution between the soil types disappeared so that, after several months, nitrate levels were low in all soil types. In heterogeneous treatments, Holcus was able to proliferate roots in the nutrient‐rich side compared with the nutrient‐poor side, but only at the high overall level of nutrient availability. Nardus did not selectively place roots in the nutrient‐rich side of the heterogeneous treatment at either nutrient level. Root longevity, as determined by minirhizotron observations, revealed that roots of Holcus tended to be shorter lived than those of Nardus, and to live longer in nutrient‐poor soils. Initially, Holcus produced more shoot biomass in the heterogeneous treatments, at both overall levels of nutrient availability, than expected from values in the homogeneous treatments, but this advantage disappeared by the end of the first growing season and, after 2 years, shoot biomass in the heterogeneous treatments was much less than expected. At the high overall level of nutrient availability, Holcus shoot biomass was not significantly greater than that produced in the homogeneous nutrient‐poor treatment. In contrast, shoot biomass of Nardus in the heterogeneous treatment was similar to the expected value, both after the first and second growing seasons. For Holcus, fast root proliferation and presumably a high nutrient uptake resulted in increased shoot biomass in the short term, but this was then curtailed by rapid patch depletion and high losses due to a limited root life span. We discuss the implications for the long‐term rewards of root proliferation in perennial species of heterogeneous environments.