Diversity of Plants in Thickets of Invasive Sorbaria sorbifolia: Differences in the Effect on Aboveground Vegetation and Seed Bank

Abstract

The diversity of aboveground vegetation of the herb–dwarf shrub layer and seedlings from the soil seed bank in the thickets of invasive shrub Sorbaria sorbifolia was evaluated in urbanized forests of the city of Yekaterinburg (the Middle Urals, Russia). Geobotanical releves and identification of seedlings from the soil seed bank were performed in different forest communities: thickets of invasive S. sorbifolia, thickets of native Rubus idaeus, and in forest communities without S. sorbifolia and R. idaeus. The effect observed in the presence of S. sorbifolia was as follows: the abundance of aboveground vegetation in the herb–dwarf shrub layer was lower by a factor of 10–20 or more; the taxonomic richness of the herb–dwarf shrub layer in the releve (α-diversity), by a factor of 6–8; and the total number of species recorded in the community group (γ-diversity), by a factor of 3. The disappearance of species under S. sorbifolia was selective. Relatively light-loving and relatively dry-loving species disappeared first, while more shade-loving and moisture-loving species were relatively resistant. The taxonomic richness and abundance of seedlings from the soil seed bank did not change depending on the abundance of S. sorbifolia. Approximately half of the identified seedling taxa were typical forest plants, while the others were only specific for the seed bank and did not occur in the vegetative state. It is concluded that the influence of S. sorbifolia on the vegetation of subordinate layers is due primarily to the direct competitive and habitat-transforming effect of the living S. sorbifolia phytomass.