Vegetationsgürtel in den Wiesenbeständen des Gatačko Polje

Summary

Plant zones (belts) formed by single or few species, pressed together on restricted areas or bands on a carst field in the Hercegovina have been described.

Sometimes, similar belts, to which W. Koch 1926 referred to as “topographic units” appear on shores or gentle slopes around ponds or lakes.

In the carst field, however, they appear on meadows. These field are subject to an extremely fluctuating moisture regime due to the climatic conditions and the geographic position of this area. During the rainy season they are often transformed into lakes of some meters depth while in summer the drought entirely stops the growth of vegetation which turns to dry, yellow masses.

As the surface of the fields is seldom ideally flat, very changeable moisture conditions exist there which cause striking changes in the composition of the vegetable cover; from far away slight elevations of only some centimeters reveal themselves by their different floristic composition. Therefore, for the appearance of belts with their regular successions, as developed in the carst field in question, a slightly undulating surface is necessary as well as a soil poor in organic matter because its high capacity of keeping moisture softens considerably the extreme environmental conditions of these fields.

The mentioned clumps of few species may be observed in all carst fields of Bosnia and the Hercegovina but in no one they are so conspicously developed as in the Gatačko Polje, especially on its north-western part, where all the conditions wanted for their appearance are evidentally present. This field can, therefore, be considered as the key to their understanding.

On this area small hills can be seen surrounded by white rings of Narcissus on its moist basis, followed by a green belt of Bromus racemosus along their slopes while the hilltop is covered with yellow clumps of Crepis setosa. On this area the whole vegetation is likewise divided into striking belts composed of few species, which are always to be found again on sites with the same amount of moisture or depth of water level at the same time of the growing season.

The single zones and their succession are cited in the german text; the entire series, however, are not always present depending upon the nature of the slopes. On steep ones there is so to say not enough place for them to develop and one or even several zones may be absent. The zone of Scirpus lacustris is bound to sloughs usually water-filled while the first zone with a water level completely receding during the droughts is formed by a pure stand of Carex Goodenoughii. During the chief growing season, in june, the depth of the water level exceeds there 50 cm; at its margin, when the water level has reached about 15 cm, Heleocharis palustris takes the dominance. Under somewhat drier conditions follows Leucoium aestivum, a geophyte, which is, therefore very resistant against summer droughts. In the next zone with a water level of about 3–5 cm the aspect is dominated by Ranunculus acer, accompanied by few hydrophil species, while in the coming zone dry land is finally reached. There occur Narcissus angustifolius with the codominant Colchicum autumnale on a soil which contains a considerable degree of moisture. Under similar conditions a zone is formed by Alectorolophus major which borders sometimes the small brooks, crossing the fields. The next zone is already developed on dry soil. It is composed by Bromus racemosus which also takes vast areas if similar conditions are present. In drought years Bromus alone forms almost pure stands of a very monotonous aspect, while in wet ones it is buried by the higher growing Poa silvicola which evidentally needs more moisture in early spring. On its margin forms Heleocharis palustris a narrow, characteristic line, while the next zone is developed on some-what higher elevations. Moenchia mantica forms there a dense cover which gives from far away the impression of areas covered by snow. The last zone is composed of species adapted to constantly dry sites.

The single species, composing the mentioned zones, all belong to plant associations which form the meadows on the carst fields, but not even one represents a species, characteristic for the single communities.

The zones were compared to similar ones in Finland, described by K. Teräsvuori 1927, who explains their existance with a severe competition which takes place on sites with greatly changing environmental factors and not with the autoecological claims of the single species.

In a table the content of organic matter has been shown in the single zones that decreases regularly with the receding water level and in the same way changes the life forms of the plants. In wet zones they consist of hemicryptophytes, on sites with extremely changing moisture conditions geophytes are dominating while therophytes on dryplaces. On areas never subject to regular inundations, hemicryptophytes appear again but of a more xeric type.

The cover value of the vegetation is also subject to the degree of moisture during the growing season. It amounts to 100% in the wet and moist zone and deceases to 60% in the dry ones while after the first precipitations in autumn in increases to 100% all over the field due to the germinating winter annuals — in the first place to Bromus racemosus. By this time the whole field has a strictly mesophytic aspect where all traces of whatever zones has vanished with the exception of Colchicum autumnale which is than in full blossom.