Module demography does not mirror differentiation among populations of Stellaria longipes along an elevational gradient

Available evidence suggests that vegetative and reproductive plant growth should decrease with climatic severity. This was tested first with observations of Stellaria longipes ramets established naturally along an 1100 m gradient of elevation in the Rocky Mountains of Southern Canada, and secondly with ramets collected from the same sites but grown in climate-controlled cabinets under long warm days. Growth cabinet plants yielded three times as many new modules and biomass, at a rate twice as fast as that of field plants. In addition the mortality of field-grown modules was approximately 40 times that of growth cabinet plants. This points to a very high `cost' of living in a natural environment. Contrary to anticipated results, the considerable change in habitat generated by the gradient in elevation was not matched by an equally dramatic gradient in vegetative or reproductive module growth in the field. Plants grown in cabinets also failed to show any clinal trends in module growth. Measures of vegetative module growth, such as rate of module accumulation and biomass per module, were generally conserved among sites. By contrast, production of reproductive modules was highly variable, both within and among sites. The failure to predict module behaviour from climatic severity may have been caused by unanticipated complexities of the gradient such as soil moisture and soil pH which were distributed independently of elevation. On the other hand, differentiation in growth among ecotypes along the gradient may be explained by changes in the characteristics of individual modules, not the demography of the modules that make up the plants.