fulltext - DiVA

Notes on aquatic vegetation 209SPONTANEOUS IMMIGRATION.-The bulk of thewild flora has evidently immigrated to the areafrom the south-west or south-east, but old immigrantsmust first have arrived in the land abovethe ancient shorelines much farther north, andthen gradually have proceeded south as the presentcoastal area rose above sea-level. Other species havea genuinely northern distribution. Even W oodsiaalpina and Petasites frigidus, often regarded as"mountain plants", here reach the seashore.ANTHROPOCHORES.-Through the growing effectivityof modern communications, the importanceof man as an agent of dispersal has increased considerably.The weed flora of the cultivated landscapeand the many species dispersed along railwaysand roads are evidence of this. Even fairlylate immigrants into Sweden such as Thlaspi alpestre,Matricaria matricarioides and Senecio viscosus,are more or less frequent here. Polemochorous(LUTHER 1948) plants introduced through militaryactivity during World War II have been demonstrated(JuLIN 1958a). The occurrence of Peucedanumostruthium (JuLIN 1961b) and on the Finnish sideof the Torne river, Rorippa amphibia (HYLANDER1960) are probably results of long distance dispersalfrom the south through human agencies in amuch older time. Little survives from the ballastflora which must have occurred at the many smallharbours used in the old times. Two of these plantsare still there, Carduus nutans at the harbour ofHanhinkari and Senecio jacobaea on heaps of ballaston the island of Seskaro.LakesNOTES ON AQUATIC VEGETATIONBy Svante PekkariJudging from still incomplete knowledge, themajority of lakes within this area do not differmuch from other oligotrophic lowland lakes innorthernmost Sweden. Most are small and shallow,with brownish water derived from the surroundinglarge mires. The latter are most extensive in theeastern part, where the lakes are generally elongatedin the NNW to SSE direction. To the west, therelief is rugged (on a small scale) which makes thistrend less evident.Some of the lakes were lowered at about the turnof the century to provide areas for hay-making{from self-grown vegetation) but this use has declined.There are a few small farming hamlets _close to some of the western lakes, but their influenceis limited.Within the area of Karelian bedrock betweenHaparanda and Kalix there are five small lakesthat possess a flora and vegetation remarkable forthis high latitude (HEDLIN et al. 1957). They aresituated only between 15 and 3 m above the sea,from which they were consequently separated incomparatively recent time (from late Roman timeto about the 16th century A.D.). They have beenlowered repeatedly, and are now very shallow, themaximum depth being only 1.3 m. This has acceleratedthe filling-in by quagmires that havereduced the open water surfaces, below which grow,e.g., Stratiotes aloides, Ceratophyllum demersum,Potamogeton obtusifolius, P. zosterifolius and P.Friesii, a vegetation that has its nearest counterpartin Finland, in some lakes around Kittila (at astill higher latitude and about 150-200 m altitude,in Finnish Lappland) and in lakes and more or lessdetached bays of the Bothnian Bay near Kemi,with almost permanent fresh water. On the Swedishside we have to move nearly four degrees southwards,to the province of Medelpad, in order to findStratiotes again.This disjunct occurrence of Stratiotes in NorthFennoscandia, so widely separated from the moresouthern population, poses interesting problemsregarding its immigration. Only the female plantoccurs in Fennoscandia, and the occasional parthenocarpicfruits ripen - only rarely. This excludes otherActa Phytogeog.r. Suec. 50