Plant Communities in Harsh and Favorable Environments: Characteristics of Their Organization, Their Dominant Structure and Its Relationship to Species Richness

It is believed that plant communities in harsh and favorable habitats are organized in different ways. The former are mainly organized by abiotic environment and random processes (S model), and the latter are mainly organized by competition (C-S-R model). It can be expected that the characteristics of the organization affect the structure of the dominance of plant communities and its relationship to species richness. The goal of our study was to test this assumption on the example of herbaceous plant communities of relatively stable habitats of several regions and high-altitude belts of the western Caucasus (alpine snowbeds, heaths and meadows, subalpine meadows and marshes, low mountain meadows, grass layer of forests, steppes). The aboveground biomass of communities at the time of its maximum development was used as a criterion of the degree of environmental severity. The relative participation of species was estimated as the ratio of their biomass to the total aboveground biomass on the site (D) or to the biomass of the community remaining after the removal of the biomass of higher-ranking species (K) from it. Under the structure of dominance, relative participation in community of the first rank species (degree of dominance) was understood, as well as the ratio of the values of D and K for two or three species with the largest participation in the aboveground-community biomass. The following results were obtained: (1) If the dominance structure is estimated based on D, then it varies widely and is almost unrelated to the production of the habitats (environmental conditions). (2) In C‑S-R-communities, the K values for species of first rank (K1) are on average higher than for less significant species (K2 and K3); in S-communities, they are on average approximately the same. Moreover, if we consider C-S-R- and S-communities separately, then the (K1 – K2) values are not related to the aboveground biomass of communities. The assumption is made that (K1 – K2) value can be considered as the index of the degree of expression in the dominants of C-strategy features. (3) The spatial variability of the parameters of the dominance structure is higher in areas of harsh habitats than in favorable ones. (4) C-S-R-communities are characterized by a closer relationship between the degree of dominance and species richness than S-communities. Our results suggest that this is due to the more intensive interspecies competition in C-S-R-communities and to the C-strategy of dominant species.