fulltext - DiVA

Notes on aquatic vegetation 211Fig. 2. Storforsen, the mightiestcataract in the still unfetteredPite river, gives an impression ofviolent power, its water white fromfoam, its spray rising high andwetting the shores with a constantfog. A heap of floatingtimberis piled up on a shallowerpart of the riverbed. Aug. 25,1962. Photo G. Einar Du Rietz.a reduction of the number of vascular hydrophytesfrom the sheltered muddy substrates of the broadsto the boulder bottoms of the rapids, devoid ofhigher plants, the last to endure being Potamogetongramineus, Ranunculus peltatus, Myriophyllumalterniflorum and Sparganium spp. (sterile), rootedin gravel between the boulders and swinging in thecurrent-they attain full size in late summer whenthe flow is much reduced.VASCULAR AQUATIC VEGETATION.-The helophyticvegetation is low but generally well developedat least at moderate current and absence of hardgrazing. However, Phragmites and Scirpus lacustris,though they are often dominant in non-fluviallakes, are mostly totally absent in the rivers andin their broads. This absence, also noted fromother Swedish rivers before their flow was controlledand the extremes reduced, is almost certainly dueto the great amplitude, especially the low water inwinter exposing the bottoms to deeply penetratingfreezing (cf. LoHAMMAR) .The most prominent helophytes are insteadEquisetum fluviatile, Eleocharis palustris, Carexaquatilis, C. rostrata, Alisma plantago-aquatica,Lysimachia thyrsiflora and Arctophila fulva var.pendulina. The latter interesting species (also mentionedin JuLIN's part) is only found along theTorne river and on the marine shores close to itsestuary. It may form a broad belt next to the freewater, contrasting through its bright verdure withthe sombre green of the other helophytic vegetation.The occurrence of Butomus umbellatus-commonin the Torne river-is also remarkable. It grows inthe river itself exclusively as a sterile plant-hardto recognize at first-with long floating under-waterleaves only bending their tips occasionally abovethe surface at low waters. It is also found in theestuarian archipelago, both on muddy substratesand firmly attached between boulders in exposedsituations. On the Kalix river it occurs verysparsely, and only on muddy bottoms.The isoetid vegetation is often well developed, inparticular that of shallow muddy bottoms especiallyfound in the broads, where many smalltherophytes (annuals) escape too strong competition:Subularia aquatica, Callitriche verna, Elatinehydropiper, E. triandra, Limosella aquatica andCrassula aquatica, etc. The hard conditions inwinter (see LoHAMMAR, in this volume) keep awaythe perennials with the exception of Eleocharisacicularis, I soetes echinospora and Ranunculusrep tans. Lobelia dortmanna , found in lakes in thearea, is completely absent in the rivers.The nymphaeid vegetation is dominated bySagittaria sagittifolia, S. natans and Sparganiumspp. In contrast to the lakes, the rivers are poor inwater lilies.The prevailing lotio vegetation is formed byelodeids, growing often very long in an inclinedActa Phytogeog.1·. 8-uec. 50