An increase in the dominant species participation in plant cover (increase in their projective cover, biomass, share in the total grass stand biomass) leads to change in the occurrence of many associated species, which may effect on the degree of difference (similarity) in the species composition of communities located in various habitats, and, accordingly, on the results of their classification. We considered this issue using the example of non-forest communities with high and low participation of certain dominants.
The study area included the vicinity of the city of Maikop, the foothills and mountain ranges of the Western Caucasus (the basins of the Belaya and Bolshaya Laba rivers, 200–2500 m a. s. l.), as well as coastal areas of the Black Sea shelf and shallow areas of the Taman Bay of the Azov Sea (depths from 0.1 to 5 m). The objects of study were communities with varying degrees of participation of certain species, located in natural (semi-natural) and anthropogenic habitats of different types: 7 communities with the dominance of brown algae Ericaria bosphorica and Gongolaria barbata (Cystoseira sensu lato) (macrophytobenthos of the Black Sea), 6 — aquatic plants Zostera noltei and Z. marina (macrophytobenthos of the Azov Sea) and Solidago сanadensis (synanthropic communities), 5 each dominated by Calamagrostis arundinacea (subalpine meadows), Calamagrostis epigejos, Botriochloa ischaemum (low mountain meadows, synanthropic communities), Rubus caesius (edges and old fields) and Medicago falcata (synanthropic communities).
Within each community 25–30 plots (0.5×0.5 m) were established. A sample of aboveground biomass was taken in each plot. For each of them were determined: 1) the weight of the wet biomass in general (W), the biomass of dominating (Wd) and associated species (Ws); 2) the degree of dominance (D = Wd / W), 3) the number and composition of associated species. For marine bottom communities, the Wd reflected the joint biomass of Ericaria bosphorica and Gongolaria barbata as well as Zostera noltei and Z. marina, respectively. In addition, since macrophytobenthos dominants may effect both negativly (competition) and positivly (protection, substrate) on other species, their participation in communities was assessed through absolute (Wd) biomass.
From each series 10 samples with both the lowest (LD) and the highest (HD) dominant participation were selected. Data on species constancy in groups of LD biomass samples taken from 5 to 7 communities dominated by certain species were combined into one Table (infracenotic system, ICS), as well as data on species constancy in groups of samples with HD. The degree of differentiation of ICS with LD and with HD was assessed in two ways: 1) through the number of species considered as diagnostic for certain communities (the higher the number of such species, the higher the degree of differentiation of the ICS); 2) by visual comparison of the results of PCA-ordination of biomass samples with LD and with HD.
The results show that an increase in the participation of dominants in non-forest terrestrial and marine bottom plant communities leads to a decrease in the constancy of associated species. As a result, some species lose their diagnostic status, while others become diagnostic ones. The number of the first in most cases is much higher than the second. Therefore, groups of samples with HD are characterized by a smaller number of diagnostic species than groups of samples with LD, and, accordingly, less differentiation. To a greater extent, this is expressed in communities with the dominance of Rubus caesius, Calamagrostis arundinacea, C. epigejos and Medicago falcata, and to a lesser extent, in communities with the dominance of Solidago сanadensis, as well as in macrophytobenthos. In addition only about 40 % of the species identified as diagnostic in groups of samples with HD are diagnostic for groups of samples with LD, taken from the same communities. Including, in communities with the dominance of Solidago canadensis — only 20 %, Zostera noltei and Z. marina — 25 %, Rubus caesius — 33 %. The PCA-ordination of groups of samples with low and high dominance of Calamagrostis arundinacea, Rubus caesius, Medicago falcata and Cystoseira s. l. shows slightly higher differentiation of the former (with LD) than the latter (with HD). In communities with low and high participation of other dominants, the differences in this regard are not pronounced.
Thus, our results showed that groups of biomass samples with low and high participation of dominants, taken from the same communities, are characterized mainly by different numbers and composition of diagnostic species. This means that the results of the plant communities classification based on the ecological-floristic approach may depend, among other things, on the degree of species dominance in the sample plots. However, there is a circumstance that limits the significance of our results for the practice of syntaxonomic studies of terrestrial vegetation. It is associated with a significantly different size of sample plots that we used for biomass sampling (0.25 m2) and which are usually used for terrestrial communities (16–100 m2). In particular, if the influence of dominants on other species is indiscriminate, then it should be expected that its consequences for the community species richness, and, accordingly, the occurrence of species, will be clearly felt only in relatively small sites (Powell et al., 2011, 2013; Akatov et al., 2021, 2022; Afanasyev et al., 2022). The relevés of macrophytobenthos (sampling of biomass) are usually made on much smaller sample plots (0.1–0.25 m2). Therefore, for the practice of hydrobotanical research, our results may be more useful.
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Dec 27, 2023
Martin. G. De Kauwe + 3