SPECIALIZATION, PLANT STRATEGIES, AND PHENOTYPIC PLASTICITY IN POPULATIONS OF STELLARIA LONGIPES ALONG AN ELEVATIONAL GRADIENT

Using differences between the alpine and prairie ecotypes of Stellaria longipes, we investigated whether characteristic levels of phenotypic plasticity could be explained in terms of either Grime's C-S-R model or Taylor and Aarssen's specialization hypothesis. Seven populations of S. longipes were located along an elevational gradient, and fine-scale differentiation between the alpine and prairie forms was quantified genetically and morphologically. Analysis of eight enzyme systems revealed two distinct clusters separating high-elevation populations from low-elevation populations. Morphologically, means and plasticities of traits previously used to describe the tundra ecotype showed parallel population grouping to that of the isozyme analysis. Analysis of environmental parameters showed that growth change and amount of phenotypic plasticity had significant associations with increased wind and temperature stress at high elevations and high interspecific competition at low elevations. This is consistent with the idea that the parallel patterns of separation observed for isozymes, morphology, and plasticity are a result of selection pressures consistent with the C-S-R model. In reciprocal transplants, tundra plants with low plasticity were selected against in a prairie environment. However, no clear advantage was observed for either ecotype in the alpine environment. The high plasticity of the prairie ecotype is likely adaptive for competitive habitats and does not bear a cost of low performance in the unfavorable alpine habitat, as predicted by the specialization hypothesis. The relatively poor performance of the alpine ecotype may reflect a restriction rather than specialization to stressful alpine environments. However, selection for the tundra ecotype in its native habitat may have been detected in experiments carried out over a longer period than those of this study.