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

1 The long-term benefits of root foraging in heterogeneous environments are unclear. The short duration of many previous studies may have overlooked t he effects of patch depletion and root turnover, which may limit the long-t erm rewards of root foraging for perennial plants. 2 The benefits of root foraging were investigated for Holcus lanatus and Na rdus stricta over two growing seasons. Shoot biomass of each species was me asured in homogeneous nutrient-rich and nutrient-poor treatments and in a h eterogeneous treatment consisting of a nutrient-rich and a nutrient-poor si de, 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 mon ths, nitrate levels were low in all soil types. 3 In heterogeneous treatments, Holcus was able to proliferate roots in the nutrient-rich side compared with the nutrient-poor side, but only at the hi gh overall level of nutrient availability. Nardus did not selectively place roots in the nutrient-rich side of the heterogeneous treatment at either n utrient level. 4 Root longevity, as determined by minirhizotron observations, revealed tha t roots of Holcus tended to be shorter lived than those of Nardus, and to l ive longer in nutrient-poor soils. 5 Initially, Holcus produced more shoot biomass in the heterogeneous treatm ents, at both overall levels of nutrient availability, than expected from v alues in the homogeneous treatments, but this advantage disappeared by the end of the first growing season and, after 2 years, shoot biomass in the he terogeneous treatments was much less than expected. At the high overall lev el of nutrient availability, Holcus shoot biomass was not significantly gre ater than that produced in the homogeneous nutrient-poor treatment. In cont rast, shoot biomass of Nardus in the heterogeneous treatment was similar to the expected value, both after the first and second growing seasons. 6 For Holcus, fast root proliferation and presumably a high nutrient uptake resulted in increased shoot biomass in the short term, but this was then c urtailed by rapid patch depletion and high losses due to a limited root lif e span. We discuss the implications for the long-term rewards of root proli feration in perennial species of heterogeneous environments.