Frequency Distributions of Vascular Plants in the Geolittoral Vegetation Along Two Rivers in Northern Sweden

The frequency distributions of vascular plants growing along 200 m-long stretches of the geolittoral belt between the headwaters and the mouth and confluence, respectively, of the Pitealven and Laisalven-Vindelalven rivers in northern Sweden were investigated. The geolittoral flora of the Pitealven river (totalling 220 species) included 32% 'rare' species (F 60%o). The corresponding values obtained for the Laisalven-Vindeldlven river flora (totalling 252 species) were 32%, 44% and 24%. The frequency distribution of the vascular plants along the Laisalven-Vindeldlven river fits Raunkiaer's Law of Frequency, but that for the Pitealven river does not, a difference explained by the existence of an important distribution boundary which runs across the latter river and which causes a reduction in floristic homogeneity. The distribution of 'rare' species is discussed with reference to the belt structure of the geolittoral vegetation. The highest number of 'rare' species, within the geolittoral as a whole, was recorded in the upper-geolittoral belt. For both rivers this number amounts to 79% of the total 'rare' flora, and these values decrease then to a very low value in the lower-geolittoral belt. The geolittoral flora along both rivers can also be characterized floristically on the basis of its 'constant' species. This characterization reflects very well the main habitat differences between the rivers, viz the 'constant' geolittoral flora of the Pitealven river is comparatively rich in species typical of dry soils, whereas that of the Laisalven-Vindeldlven river indicates mesic conditions and a high frequency of meadow communities.