Several types of plant communities, relict by their nature, display stability of their floristic composition under modified climatic conditions. Such are cryophytic steppes of Chukotka (relicts of the Pleistocene cryoxerophytic phases), dryad-bearberry-Scots pine and ashweed-aconite-Siberian spruce forests on gypsum outcrops of North-Eastern Europe (relicts of the Preboreal and the Atlanticum, respectively), as well as several communities on the White Sea islands, etc. Communities of all types, both at the climax and seral stages, display such floristic conservatism to some extent. As follows from the analysis of spectra of geographical elements for vascular floras of South-Karelian forest communities, one should speak of conservative features of the whole successional sere rather than of the plant community of a given type. The stability of the species composition of each sere in the succession is maintained by its repeatability. Predictability of successions is due to and dependent on the stability of the gene pool of the species forming the communities, constancy of the landscape ecotopes they inhabited, and interspecies co-adaptations, including dominant tolerance of the accompanying species, complementarity of life strategies, and symbiotrophy.