Competitive Ability and Adaptation to Fertile and Infertile Soils in Two Eriophorum Species

We investigated the role of competition in adaptation to varying nitrogen (N) levels for two Alaskan species of Eriophorum found naturally in low- vs. high-nutrient soils. In a growth-chamber experiment, seedlings were grown at a range of proportions (0, 0.5, 1.0) of the two species and at a range of densities (n = 1, 2, and 4 plants per pot) under low- and high-N treatments. In a field experiment, plants were reciprocally transplanted between low- and high-nutrient sites into both unvegetated (noncompetitive) and vegetated (competitive) plots. The two species responded similarly to density and nitrogen in pure cultures, but in mixtures the species from the high-nutrient site (E. scheuchzeri) showed a greater growth response to N than did the species from the low-nutrient site (E. vaginatum). Analyzed as a replacement series, the experiment showed a reversal in relative competitive ability (as measured by the relative crowding coefficient) of the two species between low- and high-N treatments at high density. Tissue N concentrations were higher and more responsive to N addition for E. scheuchzeri. Nitrogen use efficiency was higher for E. vaginatum in all treatments, while N uptake efficiency was higher for E. scheuchzeri in all treatments. In the field, survival, growth, and flowering responses to reciprocal transplanting sug- gested local adaptation of the two species to their respective home sites. In noncompetitive plots, the growth response mirrored that seen in the growth chamber. However, after 2 yr there was a clear reversal between sites in relative aboveground masses of the two species. The local species acquired more nitrogen than the alien species at both sites. We conclude that Eriophorum vaginatum and E. scheuchzeri possess traits typical of species adapted to infertile and fertile soils, respectively. The contrasting ecological re- sponses of the two species are due, at least in part, to differences in their nutritional physiology, including differences in nutrient use efficiency, nutrient uptake efficiency, and root: shoot ratio. In the field, these differences result in differential growth, reproduction, and survival, while in the growth chamber, the differences result in differential ability to accumulate biomass and reversal in competitive ability as a function of nutrient availability. This and other studies suggest that adaptations to low, as well as high, resource levels may confer improved competitive ability.

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