Canopy cover
In both the inter-habitat and intra-habitat comparisons, higher canopy cover was observed for A. negundo in comparison with other species of woody plants in urban pine forests.
Inter-habitat comparison. We observed a slightly higher canopy cover in communities dominated by the ash-leaved maple compared to communities with other woody dominants. In a two-way ANOVA, the differences between plots An+ and An– in canopy cover were significant (Fplot type(1; 66) = 6.06; P = 0.0165), while the effect of the year (Fyear(2; 66) = 1.91; P = 0.1555) and the interaction between option and year were insignificant (Fplot type × year(2; 66) = 0.18; P = 0.8344). The absolute differences in the mean values of canopy cover between the plots were small (Fig. 2): 90 ± 1% in the An+ plots and 86 ± 1% in the An– plots.
Intra-habitat comparison. We found a higher canopy cover in areas dominated by A. negundo compared to areas with other tree dominants. In the one-way ANOVA, the differences in canopy cover between the plots dominated by native and alien tree species versus A. negundo were significant (Fplot type(2; 797) = 4.21; P = 0.0151). The absolute differences in the mean values of canopy cover between the options were small (Fig. 3): areas with native dominants showed 66 ± 1%, with A. negundo 70 ± 1% and with other alien dominants 70 ± 2%.
We found that native and alien woody species resulted in heterogeneous canopy cover (Fig. 4). Some alien species, such as Populus balsamifera, had thin canopies, while some native species, such as Prunus padus and Sorbus aucuparia, had dense canopies. Moreover, among the alien species, the canopies of A. negundo were not the densest. Some alien woody plants (namely Ulmus laevis, Acer tataricum, and Malus baccata) had higher average canopy cover than A. negundo.
Richness of ground cover
In both inter-habitat and intra-habitat comparisons, we observed reduced species richness of the ground cover on plots with a tree layer formed by the ash-leaved maple.
Inter-habitat comparison. We observed reduced species richness of the ground cover in communities dominated by A. negundo compared to communities with other dominant trees. In a two-way ANOVA, the differences in the number of species per 400 m2 were significant between plots An+ and An– (Fplot type(1; 66) = 23.44; P < 0.0001). The observation year (Fyear(2; 66) = 0.50; P = 0.6079) and the interaction between the factors were not significant (Fplot type × year(2; 66) = 0.45; P = 0.6401). The absolute differences in average values of species richness of the ground cover between the plot types were significant (Fig. 5): 17.1 ± 1.7 species per 400 m2 in the An+ plots and 28.8 ± 1.7 species per 400 m2 in the An– plots.
Intra-habitat comparison. We found that on the plots with alien woody dominants, including A. negundo, the number of ground cover species was lower in comparison to sites with native dominants (Fplot type(2; 797) = 28.58; P < 0.0001). We observed that absolute differences in average values of species richness between the plots with native woody dominants were 6.6 ± 0.1 species per m2; with alien woody dominants, excluding A. negundo, - 5.2 ± 0.2 species per m2; and with A. negundo - 5.3 ± 0.2 species per m2 (Fig. 6).
Plots were relatively homogeneous in terms of richness of the ground layer under the canopy regardless of whether the dominant species was native or alien (Fig. 7). Areas dominated by different species of alien plants did not differ in terms of ground richness. Among the native plants, two groups of species were distinguished according to the richness of ground cover communities: 1) underbrush shrubs Prunus padus and Sorbus aucuparia and 2) trees of the first and second tiers of Pinus sylvestris, Salix sp., Betula sp., Populus tremula.
Relationship between canopy cover and species richness of ground cover
In both the inter-habitat and intra-habitat comparisons, a negative correlation was found between canopy cover and the number of vascular plant species comprising ground cover.
Inter-habitat comparison. In the three-way GLM using the factors ‘plot type’, ‘canopy cover’ and ‘year’, the species richness of the ground cover significantly depended on the main effects ‘plot type’ (Fplot type(1; 60) = 13.61; P = 0.0005) and ‘canopy cover’ (Fcover(1; 60) = 6.02; P = 0.0170). No other effects, including interaction effects, were significant. This means that the angle of inclination of the lines (coefficients b in the equation y = a + bx) describing the relationship between canopy cover and the number of species of ground cover on plots dominated by the ash-leaved maple and other tree species did not differ (Fig. 8, b - d). An increase of 10% in canopy cover induces a decrease in the number of ground cover species by 5.82 ± 3.30 species per 400 m2 (P = 0.0866) in the An+ plots and by 4.77 ± 2.66 species per 400 m2 (P = 0.0820) in the An– plots. The estimates for species richness of the ground cover in the absence of the shading effect of trees (coefficient a in the equation y = a + bx) in the plots An– and An+ were similar: in the An– plots a = 70.05 ± 23.09 species (P = 0.0046) and in the An+ plots a = 69.29 ± 29.60 species (P = 0.0252).
Intra-habitat comparison. In the second study’s two-way GLM with the factors ‘plot type’ and ‘canopy cover’, the number of ground cover species on the site significantly depended only on the main effects Fplot type(2; 794) = 25.73 (P < 0.0001) and Fcover(1; 794) = 12.43 (P = 0.0004). The ‘plot type × canopy cover’ interaction was not significant: Fplot type × cover(2; 794) = 1.69; P = 0.1855; this indicates that we did not establish a difference in the angle of inclination of the lines describing the relationship between the canopy cover and the number of ground cover species per m2 on plots with different woody dominants (Fig. 8, a). However, the species richness of the ground cover changed with the growth of the canopy cover depending on the dominant plants. A significant decrease in species richness with an increase of canopy cover was observed in plots with a dominance of both native (with a 10% increase in canopy cover by 0.27 ± 0.07 species per m2; P < 0.0001) and alien plants (with a 10% increase in canopy cover by 0.38 ± 0.13 species per m2; P = 0.0035). On plots dominated by A. negundo, the number of ground cover species did not change significantly with an increase in canopy cover (with an increase in canopy cover of 10%, the number of ground cover species per m2 decreased only by 0.04 ± 0.11; P = 0.7465).
In the plots dominated by A. negundo, the species richness of the ground cover was reduced even at the lowest shading levels: in the equation y = a + bx, a = 5.51 ± 0.79 species per m2 (P < 0.0001). On plots dominated by native or alien plants—excluding A. negundo—initial levels of ground cover richness were higher: under native plants, a = 8.40 ± 0.46 species per m2 (P < 0.0001); under alien plants, a = 7.92 ± 0.93 species per m2 (P < 0.0001).