EFFECT OF DISTURBANCES AND INTERSPECIFIC COMPETITION IN ISOETID POPULATIONS

The size, fecundity, life history and mortality of Isoetes lacustris L . and Lobelia dortmanna L. were studied in wave-disturbed and sheltere d habitats, as well as in aggregations of Phragmites australis (Cav.) Trin. ex Steud., Sphagnum auriculatum Schimp. and in gaps between them . In disturbed habitats the rosette size of Isoetes was smaller by 86. 6%, the roots were shorter by 76.0%, an individual was lighter by 92.5 %, had fewer (by 94.2%) macrospores, and its life history was 1 or 2 y ears longer. Most of the individuals died before attaining the late-ju venile stage, that is, at the age of no more than 2 years. In wave-dis turbed habitats Lobelia individuals had rosettes smaller by 65.2%, wer e each 50% lighter, but their root length and number of seeds in a fru it were the same. Their growth from the seedling stage to the generati ve-individual stage was completed within the same time period (4 or 5 years). Population mortality was only slightly higher than in sheltere d habitats. In Sphagnum aggregations sporiferous Isoetes individuals h ad rosettes smaller by 13%, roots shorter by 18%, were each less heavy by 20%, but their fecundity was the same (ANOVA, P = 0.14). Isoetes m ortality in Sphagnum aggregations was similar to that outside them. In reed aggregations, individual size, weight and number of seeds of Lob elia were greater, and the number of vegetative offspring in the popul ation was larger than that of generative offspring. The low-density re ed swamp provided favourable conditions for Lobelia.