Abstract
Population differentiation in variable environments is often related to the different local selection pressures that plants may experience. We examine the relation between reproductive success and morphological and phenological traits in woodland and open habitat populations of the tetraploid grass Arrhenatherum elatius. The reproductive correlates of morphological and phenological variation were estimated in woodland and open habitats, and on plants from each of three populations from woodland and three from open habitats grown in a garden environment at two light intensity levels. Genetic variation in these populations was estimated in uniform conditions. # Temporal and spatial differences in phenotypic selection on morphology and phenology were observed within and between the two sites. Plants from the two sites showed significant phenotypic and genetic differences, and there is a dramatic reduction in reproductive success in the woodland habitat and in plants from this habitat grown in the two garden environments. # In controlled conditions, selection patterns were similar to those observed in natural habitats, suggesting that light availability may be an important ecological factor involved in population differentiation in A. elatius. The presence of significant genetic variability for the studied traits indicates the presence of a genetic potential to respond to the detected selection pressures. # We argue that a clear understanding of patterns of local adaptation and phenotypic selection in this species requires that they are examined in the light of regional processes related to seed dispersal and relative abundance in open and woodland habitats.