The species composition and abundance of alien and invasive understory shrubs and trees in urban forests of Yekaterinburg

Abstract

The aim of this work was to analyze the species composition and abundance of alien (including invasive) understory shrubs and trees in urban forests of Yekaterinburg, an industrial city in the Middle Urals. The need for rigorous assessments of the current stage of alien plant naturalization determines the relevance of the research. We tested two hypotheses. 1. In urban forests of Yekaterinburg the largest numbers of alien understory species are Acer negundo L., Malus baccata (L.) Borkh., Amelanchier spicata (Lam.) C. Koch and Cotoneaster lucidus Schlecht, which are invasive for the Middle Urals. 2. Age and spatial structure of invasive plant populations indicate their active dispersal in urban forests of Yekaterinburg. The work was carried out in urban forests of Yekaterinburg (56°47'N, 60°34'E), a large city with a population of 1.5 million people, located in the southern taiga zone. We studied pine forests of natural origin located within the city limits. The plots were located in four forest areas of 40-150 ha (56°47'54''N 60°32'22''E) that were spatially separated from each other by roads or wastelands at distances of 100-200 m. The plots were selected in such a way that they could be, as much as possible, comparable in terms of characteristics of the relief, soil type and tree stand age. The age of the main generation trees of urban forests is 90-120 years. We selected sites without recent, largescale, and/or targeted anthropogenic disturbances of the soil cover (roads, excavations) and tree stand (fires, tree cutting). Aegopodium podagraria L. and Urtica dioica L. mainly predominate in the field layer of the examined forest areas. Also, shrubs are very abundant on the plots, first of all, Rubus idaeus L. with the average cover of 30-60%. Calamagrostis arundinacea (L.) Roth and Vaccinium myrtillus L. have preserved their dominance at some sites. These species dominate in original communities. We made 103 descriptions in the understory shrub and tree communities in February-March 2016. Species were identified in a leafless state on round plots of 400 square meters. The size category of understory plants was registered (large or small individuals). We selected plots and sites and marked them with paint on trees and carried out preliminary characterization of sites and stands in June and July 2015. A pine tree (Pinus sylvestris L.) was in the center of each plot, and each plot contained, on average, 12.5 (from 3 up to 23) pine trees. Woody plants were recorded on the plots in a leafless state. The living individuals of trees and shrubs, which were higher than the thickness of the snow cover, i.e. above 40-50 cm, were considered. The criterion for the presence of an individual plant on the plot was the entry of the stem into a circle with a radius of 11.28 m. The distances were measured with a laser range finder. The counting unit for trees was a monocormic tree or a multi-stemmed tree if the trunk was branching. The counting unit for shrubs was a multi-stemmed shrub or shoot if there was one stem. Trees and shrubs were recorded by registering their size category: large or small individuals. The criterion for categorizing trees and shrubs as large specimens is a stem diameter greater than 1 cm at a height of 1.3 m. All multi-stemmed shrubs also belonged to large shrubs. Both working hypotheses were confirmed. On the whole, we registered 16 species of trees and 30 species of shrubs. The total number of registered alien species exceeded the number of registered native species: 25 and 21, respectively (See Fig. 1). Almost all alien species in urban forests are reproducing. Although native species quantitatively dominate alien ones, the latter, especially invasive species, are better represented among small individuals than native species (See Fig. 2). 6 invasive species (Acer negundo, Amelanchier spicata, Berberis vulgaris, Cotoneaster lucidus, Malus baccata and Padus virginiana) account for 72% of large individuals and 87% of small individuals of alien species, and 27% of all woody species individuals (See Fig. 3). Malus baccata is the most abundant invasive species both among large and small individuals. The second place in the abundance among large individuals is occupied by Cotoneaster lucidus, and among small individuals it is by Acer negundo. When analyzing the spatial features of native and invasive species location, we established that small individuals of all undergrowth species were predominantly present in close proximity to large individuals. A positive correlation between the number of large and small individuals is more pronounced for the four most abundant invasive plants, compared to the four most abundant native trees and shrubs (See Table). We concluded that the species of trees and shrubs, which were characterized as invasive I-st (Acer negundo, Malus baccata, Amelanchier spicata) and II-nd (Cotoneaster lucidus) categories make up the majority of alien woody plants in Yekaterinburg urban forests. Thus, invasive species are appreciably more successful in reproduction than native species. The 6 invasive species (Acer negundo, Amelanchier spicata, Berberis vulgaris, Cotoneaster lucidus, Malus baccata and Padus virginiana) account for 27% of all woody species individuals. The ratio between small and large individuals (size categories) allows us to assume that the expansion of Acer negundo and Malus baccata in urban forests is most expressed nowadays and will be expressed in the nearest future. The abundance of Cotoneaster lucidus and Amelanchier spicata will increase in a less degree.