DYNAMICS OF THE ABUNDANCE AND SPATIAL-ORGANIZATION OF ISOETID POPULATIONS IN AN OLIGOTROPHIC LAKE

The paper presents the long-term dynamics of abundance and spatial org anisation of Isoetes lacustris L., Littorella uniflora (L.). Aschers. and Lobelia dortmanna L. populations in an oligotrophic lake. The inve stigations were carried out for 6 consecutive years in 35 plots locate d in the central (medium-deep), the deepest and the shallowest parts o f the habitats of the populations. The highest Littorella abundance va riations (CV = 75.4%) were found in the shallowest habitat, and the lo west (CV = 54.6%) in the deepest one. After the emergence of the shall owest habitats the population size in the deepest habitat parts droppe d to 23.4% of the mean long-term level, in medium-deep (central) to 13 .9%, and in the shallowest to 1.4%. In the central and in the deepest Isoetes population habitats the number of assemblages of this plant sp ecies, their area, density and distribution pattern remained almost un changed throughout the study years. In shallow habitats the settling r ate of Isoetes was slow, colonisation was low, and the population spat ial organisation pattern was very unstable. The fastest population reg eneration rate was recorded for Littorella, a slightly slower one for Lobelia, and the slowest one for Isoetes. In Littorella, Lobelia and I soetes populations there occur three complementary regeneration strate gies, determined by the variable habitat dynamics. Reed swamp eliminat es Littorella, but creates favourable-conditions for Lobelia.