The purple saxifrage, Saxifraga oppositifolia, in Svalbard: two taxa or one?

Abstract

Several studies have demonstrated high levels of genetic (DNA), ecophysiological, ecological, and morphological variation within the species purple saxifrage, Saxifraga oppositifolia, in Svalbard. It has recently been proposed that S. oppositifolia is represented by two conspicuously different subspecies in this archipelago: ssp. reptans, a late-flowering, prostrate ecotype of snow-protected, damp habitats, and ssp. pulvinata, an early-flowering, cushion-like ecotype of dry, wind-exposed heaths and ridges. It has also been suggested that the subspecies may be differentiated at the tetraploid and diploid levels, respectively, which would promote reproductive isolation. These hypotheses are tested by examining variation in morphology, ecology, and pollen size and stainability in 150 plants of S. oppositifolia growing in 50 vegetation samples at four sites in the Kongsfjorden area. Although analyses of the various data sets demonstrated the large variation within the species, the material could not be separated into distinct groups. The morphological variation was continuous along local ecological gradients. The pollen grains were fully stainable and the pollen diameter data showed a unimodal distribution, suggesting that the plants analysed represent only one ploidal level. These results reject a hypothesis that the morphologically intermediate plants are hybrids between two taxa at different ploidal levels. Thus, the conspicuous variation in S. oppositifolia in Svalbard probably results from local, in situ ecoclinal differentiation. Although this variation clearly is without taxonomic significance, it is important in the broader context of arctic conservation biology and the potential impact of global warming on arctic vegetation.