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Article

The Impact of Adjacent Road on Vascular Plant Species Composition in Herbaceous Layers of Peucedano-Pinetum and Tilio-Carpinetum Urban Forests in the City of Warsaw (Poland)

by
Beata Fornal-Pieniak
1,*,
Filip Kamionowski
1,
Marcin Ollik
2,
Paweł Szumigała
3,
Barbara Żarska
1 and
Karolina Szumigała
3
1
Department of Environmental Protection and Dendrology, Institute of Horticultural Sciences, Warsaw University of Life Sciences—SGGW, Nowoursynowska St. 166, 02-787 Warsaw, Poland
2
Department of Biometry, Institute of Agriculture, Warsaw University of Life Sciences—SGGW, Nowoursynowska St. 166, 02-787 Warsaw, Poland
3
Department of Landscape Architecture, Faculty of Agronomy, Horticulture and Bioengineering, Poznań University of Life Sciences, Dąbrowskiego St. 159, 60-594 Poznań, Poland
*
Author to whom correspondence should be addressed.
Forests 2023, 14(12), 2401; https://doi.org/10.3390/f14122401
Submission received: 27 October 2023 / Revised: 6 December 2023 / Accepted: 7 December 2023 / Published: 9 December 2023
(This article belongs to the Special Issue Urban Forestry and Sustainable Cities)
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Abstract

:
The research was conducted in two types of urban forests: Peucedano-Pinetum and Tilio-Carpinetum. The aim of the study was to determine the differences in plant species compositions in the herbaceous layer of urban forests with different habitat fertility adjacent to the road in the northern part of Warsaw (Poland). Seven transects were laid out in each type of forest, with 10 plots spread out from the edge zone (forest border) to the interior of the forests. The size of each plot was 100 m2. The other seven transects were located within the forest, 150 to 200 m away from the forest’s edge. The field research included phytosociological relevés carried out on the existing transects. The indoor studies included an analysis of forest community disturbance. Furthermore, an analysis of abiotic environmental conditions using ecological indicator numbers was carried out. Ecotones of Peucedano-Pinetum are more likely to be colonized by the species inconsistent with the habitat due to processes that increase habitat fertility. The Tilio-Carpinetum forest is more easily colonized by invasive plant species than the Peucedano-Pinetum. The range of road effects can be determined as an area located within 90 m inward of the forest in the case of the Peucedano-Pinetum community and 100 m in the case of the Tilio-Carpinetum community. The presented research is important for formulating directions about how to manage the forests and their surroundings in terms of shaping forests of more natural character, with species more consistent with a forest habitat.

1. Introduction

The anthropopressure factors, especially intensified in urban areas, lead to the degradation of forest ecosystems. The effects related to the impact of anthropopressure can be modified by the progressing climate change, which is especially visible in urban environments. As a result of climate change, the species composition of many vegetation communities is being modified [1,2]. The urban heat island has a negative impact on living organisms, including plants [3], as it increases their water demand [4] and reduces their drought resistance, which is common in the urban agglomerations of Poland. Studies have shown that the occurrence of the urban heat island phenomenon facilitates the development and expansion of species that do not belong to these habitats [5,6,7]. Roads are one of the main anthropogenic elements of the urban infrastructure. The roads, as the linear structures that enable transport, facilitate the spread of non-forest species to the neighboring forest ecosystems [8,9,10].
An increase in the proportion of non-forest species can also be observed on the roadsides [11]. It may result from the road operation and maintenance process, including frequent vegetation mowing, the presence of pedestrian traffic, and vehicle stopping. This type of regular activity stops at the forest boundary, where the shrub layer overdevelops and the proportion of bushes/tree species increases [11,12]. Road maintenance may also affect the soil salinity, manifested by the occurrence of species tolerant to this factor [8,11] and the disappearance of species sensitive to salt. The increase in pH causes the activation of heavy metals and increases their absorption by plants [8]. It can be particularly destructive in coniferous forests, which are naturally associated with acidic soils [13], but, at the same time, this also increases the overall number of plant species that prefer basic soils [14].
The soil reaction is also affected by the materials used for the construction of the road structures. Their gradual destruction due to use and weather exposure, including the leaching of chemical compounds and trace elements, may result in an increase in pH and heavy metals in the soil [15]. These elements and compounds can be accumulated in plants [15]. Non-native plant species are also planted and sown along the roads, and they may migrate to adjacent ecosystems [10]. During road usage, several factors, which are related to the intensity of road traffic noise, air, and soil pollution caused by the abrasion of the road surface and vehicle components, as well as fuel combustion and ground vibrations transferred from the surface of roads with heavy traffic, can also affect these forests [11].
Dust pollution settles on plants at the border of the forest, affecting the amount of light that reaches the vegetation and also increasing the nitrogen content in the soil [10,16], which in turn affects habitat fertility for plant communities and their species composition. Foot traffic along the road and into the forest damages chamephytes, which are highly sensitive to trampling, while this increases the occurrence of geophytes, hemicryptophytes, and therophytes, which are more resistant to squashing [8].
It has been reported that the impact of roads on the herbaceous layer can reach 20 m inwards the forest [8,11] or less [17,18], but it was also reported that these effects can reach between 30 and 100 m inwards the forest [11]. These data show that the optimal zone, in which research on the impact of a given harmful factor on an urban forest must be conducted, should be located about 100 m inward of the forest, starting from the place in which that factor is occurring. Ultimately, the proximity of roads leads to a change in the physiognomy of the forest border (the so-called ecotone zone). A forest ecotone is a transition zone between a forest and a non-forest, natural, or anthropogenic ecosystem [19,20,21]. However, the above description is accurate mainly in the case of natural ecotones [11]; in ecotones along the border of anthropogenic habitats, ecosystem degradation and a lack of edge effects occur [11,22]. The degradation of the ecotone reduces its functionality and enlarges the risk of further deterioration due to increased susceptibility to degeneration [23]. An example of a negative effect is the increased threat from the expansion of species considered invasive [11,24]. The forest ecotone and the “friendly” development of a non-forest zone adjacent to the forest complex should perform a protective function (i.e., the buffer zone), separating the forest from the non-forest vicinity, thus protecting it against the entry of alien species [11,22].
The occurrence of the forest species in the ecotone supports the forest interior [11,25], as these species have the ability to recolonize the forest complex [11]. The functions of forest ecotones are particularly important because of the necessity to maintain well-developed forest habitats [26], thereby protecting the forest landscape.
Habitat fertility determines the type of plant community that develops in a given area, and it is connected with different resistance to anthropopressure. Literature studies conducted by Andres [27] demonstrate that the plant communities from fertile and moist habitats are more resistant to mechanical damage and, at the same time, are more easily colonized by alien species than the communities from dry and poor habitats. Further, the research on invasive species proves that the communities from fertile and moist habitats are more easily colonized by the invasive species than those from dry and low-fertility habitats [28].
Habitat fertility, especially in marginal forest zones, can also be modified by the enrichment of organic substances contained in waste and feces [29], as well as by nutrients [16]. This leads to the appearance of nitrogen-loving plants that enhance biomass production [8].
The greatest threat to the stability of native plant communities is posed by the alien species that penetrate the natural or close to natural ecosystems, e.g., forests. However, not every alien species is invasive [30,31]. The habitats affected by human activities are more susceptible to penetration by alien species than pristine habitats, as they have a reduced ability for self-regulation [32].
Herbaceous plants react dynamically to the habitat changes within a short period of time. Therefore, they are suitable indicators of changes in the plant associations [33]. The aim of this study was to determine the differences in plant species compositions in the herbaceous layer of urban forests with different habitat fertility adjacent to the roads in Warsaw (Poland).
The following research hypotheses were formulated:
  • The adjacent road has a significant and specific impact on plant species composition in the herbaceous layer of urban forests.
  • The share of plant species inconsistent with the forest habitat decreases with the distance to the adjacent road, while the plant species composition stabilizes in the forest interior.
  • The specificity of changes in plant species composition caused by adjacent roads depends on the fertility of the forest habitat.
The subject of the article was undertaken due to the need to strengthen the protection and improve the management of forests located in cities where anthropopressure occurs continuously. Forest habitats are degraded because of various anthropopressure factors, e.g., landscape fragmentation, ongoing investments, as well as the effects of usage of these forests by the residents [11,34].

2. Materials and Methods

2.1. Study Area

The research was carried out in two types of urban forests in Warsaw: a variant of a more fertile Tilio-Carpinetum Tracz. 1962 habitat (52°19′00.7″ N 20°55′11.1″ E)—Młociny Forest and a poorer variant of Peucedano-Pinetum Mat. (1962) 1973—Nowa Warszawa Forest (52°19′06.9″ N 20°54′52.5″ E). Their locations are shown in Figure 1.
The city of Warsaw is located in a moderate climate zone [35]. The average annual precipitation is 550–600 mm. The average annual air temperature in Warsaw is 9–10 °C, for July (the warmest month) is 19–20 °C, for January (the coldest month) −1–(−2) °C. This area receives 1800–1850 h of sunshine annually. All of the above data are for the 1991–2020 multi-year period [36]. The growing season of the vegetation in Warsaw covers 220–225 days [37].
The Młociny Forest covers 102.23 ha [38], and the tree stand is mostly 40–180 years old, with specimens up to 280 years old [39], dominated by deciduous trees that strongly shade the ground during the growing season, such as Carpinus betulus L., Quercus robur L., Acer platanoides L., Betula pendula Roth, Tilia cordata Mill., Tilia platyphyllos L. The Nowa Warszawa Forest covers 288.50 [ha]; the tree stand was mostly planted after World War II, and it mainly consists of coniferous trees—Pinus sylvestris L. [39], with less dense canopy cover, providing a more illuminated environment for the herbaceous layer.
The forests are adjacent to the national road No. 7, with heavy traffic—59,212 vehicles per day [40] in the northern part of Warsaw. This road was built after World War II, and it has an asphalt surface with two carriageways consisting of two traffic lanes (eastern carriageway) and three traffic lanes (western carriageway). Carriageways are separated by a central reservation. There are no pedestrian walkways; the verges are about 4–10 m wide, covered by grasses partially damaged by vehicles. The following criteria were used to select forests for this research out of all urban forests of Warsaw: the location of forest adjacent to a road with heavy traffic, two types of forest habitats differing in fertility, and the optimal size of forest patches with a relatively similar habitat. The schematic location of both selected forests, in relation to the road, is shown in Figure 2.

2.2. Indoor Study and Field Research

Field studies were carried out in the years 2022–2023. Seven transects with 10 plots each, starting from the edge zone (border) of each type of urban forest, were distinguished. The size of a single plot was 100 m2. Phytosociological relevés (according to the Braun-Blanquet method) were made in each study plot, from the border (edge zone) to the interior of the forests. A total of 70 phytosociological relevés were made in each urban forest using transects, which were initially located at the forest edges (distance 0 m) until reaching 100 m inside the forests. The next seven transects with 10 plots were established in the interior of the forests. These transects were located between 150 m and 250 m inwards of each type of forest community, and phytosociological relevés were performed along them. Phytosociological relevés were made during spring and summer in the Młociny and Nowa Warszawa Forests. The locations of transects with plots are presented in Figure 3.
Vascular plant species were grouped into the phytosociological classes, in accordance with the Matuszkiewicz classification [41], and into types of vegetation such as forest, scrub, and grass (including meadow and grassland). The synanthropic species were also identified in accordance with Matuszkiewicz [41]. An analysis of alien and invasive plant species was carried out on the basis of literature related to vascular plants developed by Mirek et al. [42]. Consistent and inconsistent species with the Peucenado-Pinetum and Tilio-Carpinetum forests were singled out. Plant community disturbance was defined as a disruption manifested by the occurrence of inconsistent species [2], which were defined all as non-forest species, alien species to each forest type. Consistent species were represented by the forest plants characteristic for each type of the studied forests [42]. The life forms of species of vascular plants (according to Raunkiaer) were taken from the study by Zarzycki et al. [43]. In relation to the life forms appearing in the studied transects, the following abbreviations were used: F—phanerophytes, H—hemicryptophytes, G—geophytes, T—therophytes, pp—hemiparasites. The conditions of the abiotic environment were determined using indicator numbers according to Zarzycki et al. [43]. The following indices were used: L—light index, T—thermal index, K—continentalism index, W—soil moisture index, Tr—trophism index (fertility), R—soil (water) acidity index, D—soil granulometric index, H—matter content index (organic matter), S—index of resistance to NaCl content in soil or water, M—index of resistance to increased content of heavy metals in soil.

2.3. Statistical Analysis

Mean number of species per plot was compared with the use of the GLM three-way model. The following factors were taken into consideration: forest (two levels: the Nowa Warszawa Forest and the Młociny Forest), location (two levels: forest edge and forest interior), and gradient position (10 levels). Every three-factor combination was repeated 7 times. This allowed for the comparison of both forests, evaluation of the difference between the zones, and assessment of the border effects. Poison distribution was used as the dependent variable model. According to the same GLM model, the number of inconsistent, alien, invasive species, types of vegetation, and Raunkiaer life forms were compared. As additional information, the share of inconsistent species was presented. Species that belong to some types of vegetation were too scarce to undertake an analysis; thus, the significance of differences for shrub, meadow, grassland species, and chamephytes was not examined, and therophytes were analyzed in a simplified model without a gradient position. In order to analyze the environmental preferences, the average values of 10 Zarzycki’s indicator numbers were calculated for each plot. Then, separately for each studied forest, they were condensed using Principal Component Analysis. The significance of the differentiation along the found gradients was checked using a two-way ANOVA. For greater clarity of the graphs, the values for the same items in the gradient were averaged. Statistical analyses were performed with Statistica 13.1 [44]—GLM and R 4.1.2 [45]—PCA.

3. Results and Discussion

The total number of understory species remained relatively low and differed significantly (p < 10−6) between studied forests. In the Nowa Warszawa Forest, the average was 5.5 species per plot, and in the Młociny Forest, it was 8.2 species per plot (Figure 4).
We also observed a significant difference (p < 10−6) in the number of species between the forest edge and the forest interior. At the plot level, we noticed about two more species in the edge zone than in the interior of both forests. No effects were found for the position in the gradient or the interactions among factors (Table 1). It may generally be due to a low species number and their considerable variability. The effect of habitat disturbance was visible primarily through the number of inconsistent species (Figure 5).

3.1. Consistent, Inconsistent, Alien, and Invasive Plant Species

The data obtained for the category of inconsistent species were different in both forests. In the Nowa Warszawa Forest, we observed a very strong impact of the road. In the forest interior, it was difficult to find any species inconsistent with the habitat (average of 0.36 species per plot (Figure 5a)). In the forest edge zone, a clear dependence on the distance from the road was observed: the number of inconsistent species was very close to the road, and it systematically decreased to values similar to those recorded in the forest interior. It is worth noting that the species inconsistent with the habitat dominated at the forest edge, constituting 75% of the species pool directly at the road (Figure 5a). In the Młociny Forest, more habitat-inconsistent species in the forest interior were found (1.2 species per plot) (Figure 5b). The road impact was visible as well, but not so strong. The inconsistent species number decreased from 4 to 5 in the initial three positions (plots) to 1.5 in the last positions. The percentage of inconsistent species close to the road in the Młociny Forest was lower and did not exceed 50%. The number of alien (Figure 6) and invasive (Figure 7) species did not give such a clear picture, mainly due to their rare occurrence (Table 2).
However, it is worth pointing out two important facts. Firstly, in the Nowa Warszawa Forest, a smaller number of both alien (0.50 vs. 1.25) and invasive (0.47 vs. 0.99) species per plot was observed. Secondly, in the Nowa Warszawa Forest, the number of alien and invasive species was higher in the forest edge zone than in the forest interior, while in the Młociny Forest, this difference did not occur or was negligible; this confirms the significance of the interaction between the forest and the location. The effect of habitat disturbance was primarily visible through the number of inconsistent species (Figure 5). In the Nowa Warszawa Forest, a very strong forest edge effect was observed. In the forest interior, we hardly found any species inconsistent with the habitat (average 0.36 species per plot). In the forest edge zone, a clear dependence on the distance from the road was observed: the number of inconsistent species was high at the forest border and systematically decreased with distance to values similar to those detected. It is worth noting that species inconsistent with the habitat dominated at the edge, constituting 75% of the species pool directly at the forest border.
In the Młociny Forest, more habitat-inconsistent species in the forest interior (1.2 species per plot) were found. The edge effect was also visible but not as strong as in the Nowa Warszawa Forest. The number of inconsistent species decreased from 4 to 5 at the border to 1.5 in the forest interior. Their percentage also did not exceed 50%. Compared to inconsistent species, the analyses of the number of alien species (Figure 6) and invasive species (Figure 7) did not provide such a clear picture (Table 2). This is mainly due to their not frequent occurrence.

3.2. Vegetation Types

The herbaceous layer of both forests was dominated by native species with an admixture of synanthropic species (Figure 8). Other types of vegetation constituted only a small addition, not exceeding 10%. In the Nowa Warszawa Forest (Peucedano-Pinetum) (Figure 8a), the forest species number approximately remained at a similar level, except for the border of the forest (position e1), where the synanthropic plants number declined from a dominant position (57% of all species composition) to a minor admixture (7%—0.3 species per plot). Shrub, meadow, and grassland plant species occurred almost exclusively in the forest edge zone.
In the Młociny Forest (Tilio-Carpinetum) (Figure 8b), a similar pattern was observed. The number of forest plant species remained approximately at a similar level, except for the first positions in the forest edge zone, and the number of synanthropic species decreased with distance from the road. The difference was that synanthropic plant species had a smaller percentage at the forest border (48%) and did not disappear completely in the forest interior (0.94 species per plot—13% of all species number). The significance of the differentiation of forest and synanthropic plant species is presented in Table 3.

3.3. Life Forms

In the Nowa Warszawa Forest (Figure 9a), the understory plant layer was dominated by phanerophytes (2.4 species per plot—43%), followed by hemicryptophytes (1.6 species per plot—29%) and geophytes (1.1 species per plot—20%). The percentage of therophytes and chamephytes was below 5%. In the forest edge zone, a much higher share of geophytes was observed (1.7 vs. 0.5 species per plot) and more hemicryptophytes (1.9 vs. 1.4 species per plot). In the forest border (e1 and e2), phanerophytes gave way to other forms, especially hemicryptophytes.
The Młociny Forest (Figure 9b) was also dominated by phanerophytes (3.2 species per plot—40%), but there were more geophytes (2.5 species per plot—31%) and therophytes (1.1 species per plot—13%). Chamephytes were not recorded. There were no clear differences between the zones. At the forest border (edge zone: e1, e2), more phanerophytes were found (4.0 vs. 2.4 species per plot). There were no major changes at the forest border, except for a smaller number of geophytes. The significance of the differentiation of the Raunkiaer life forms is presented in Table 4.

3.4. Characteristic of Abiotic Environmental Conditions According to Zarzycki’s Indicator Numbers

For the Nowa Warszawa Forest, PCA found two gradients accounting for 52% of the total variation. The first principal component (PC1: 27.5%) correlates with soil granulometric index (D), organic matter content (H), humidity (W), and trophy (Tr) (Figure 10). On the left, there are more fertile habitats on more loamy soils. The second principal component (PC2: 25.3%) correlates with NaCl resistance (S) and heavy metals (M). At the bottom, there are habitats with salt- and heavy-metal-resistant plants.
ANOVA confirms the difference between the forest edge zone and the forest interior zone (p = 0.00013 for PC1 and p < 10−6 for PC2), but no differentiation along the gradient was found (p = 0.6 for both PC). The interaction between position and location was only found at the boundary of significance (p = 0.02) for PC2.
The plots from the forest interior are located in the upper left corner; this means that there are species that prefer poorer conditions: sandier, drier, and less fertile soils. Plant species from the forest edge zone prefer more fertile sites. The plots from the forest edge are located at the bottom of the chart. This indicates the presence of plant species relatively resistant to NaCl and heavy metals. The plots from the initial part of the gradient are located lower, which is associated with more considerable pollution (Figure 11).
For the Młociny Forest, the first principal component accounts for 23.3% of the variation, while the second principal component only accounts for 16.7% of the variance. The third principal component explains the 13.9% variation. On the left, there are more fertile and moist habitats without plants resistant to salt. The first principal component correlates with trophy (Tr) and humidity (W) and negatively correlates with NaCl resistance (S). The second principal component correlates with the continentalism index (K) and negatively correlates with resistance to salt; then, the predominance of species characteristic for Eastern Europe is visible. The third principal component (not shown) is connected with soil pH (Figure 12). ANOVA confirms the difference between the forest edge and the forest interior (p < 10−6 for PC1 and PC2, p = 0.0003 for PC3) but no differentiation along the gradient.
Plots from the forest interior are located in the upper left corner, which means (opposite to the Nowa Warszawa Forest) that these plants prefer more moist and fertile sites than the ones from the forest edge. As in the case of the previous forest, in the forest edge zone, species resistant to salinity are found. The third principal component suggests higher soil pH in the forest interior. (Figure 13). The table with phytosociological relevés is published in Supplementary Materials Tables S1–S6.
Forest vegetation is heavily transformed by human activities, including road impacts. Most plants sensitive to the disturbance of habitats are observed in herbaceous layers [3,8,11,34,46]. The fertility of habitats is one of the most important environmental factors that shape the composition of plant species in forests. The scale of the spread of invasive species also depends on the habitat trophy [28], as well as the penetration by species inconsistent with the forest habitat [27,28]. In our study, the abiotic environment was analyzed according to Zarzycki’s indicator numbers. It was proved that the soil was mineral–humus, fresh, moderately poor to poor, and neutral to moderately acidic in the Peucedano-Pinetum forest, while in the Tilio-Carpinetum forest, the soil was mineral–humus, fresh, rich to moderately poor, and neutral. In the Nowa Warszawa Forest, the result of this analysis showed a neutral soil reaction (pH) in the forest edge (ecotone). It could be a result of the excessive development of shrubs in the edge zone observed in the field and described in the literature [11], which in turn causes bigger leaf falls and an increase in soil reaction in the long term [8]. This state of local soil conditions, related to the anthropogenic origin of the forest, cannot be ruled out here in planted pine stands, where the soil pH decreases gradually, deeper into its layers [47].
It was assumed that the stabilization of the forest species composition occurs where the forest core effect is visible. Such an interpretation makes it necessary to provide two parameters (in order to make a more reliable comparison of the obtained results with the literature data): the distance at which the strongest growth of forest species and habitat-consistent species occur. The stabilization of the forest species composition took place, where the number of synanthropic species decreased [2]. This stabilization zone begins at “the point” where the number of forest and consistent species is the highest, and the number of synanthropic species is the lowest. In our research, the limit of the road impact was determined as an area located within 90 m from the forest edge zone in the Peucedano-Pinetum community and 100 m in the Tilio-Carpinetum community. These results confirmed hypotheses H1 and H2 (i.e., that roads have a significant and specific impact on plant species composition in the herbaceous layer in urban forests; the share of inconsistent species decreases in the forest with the distance from the adjacent road and the plant species composition stabilizes in the forest interior).
There were more plant species preferring lower soil fertility in the Peucedano-Pinetum forest (Nowa Warszawa Forest) than in the Tilio-Carpinetum forest (the Młociny Forest) with a more fertile habitat, in accordance with previous studies [13]. The highest number of inconsistent species occurred at the forest edge in the Nowa Warszawa Forest. This is connected with higher trophism in the forest edge due to the adjacent road impact; deeper in the forest, inconsistent species number was gradually decreasing towards the forest interior. These results confirm hypotheses H1 and H3 (the proximity of roads significantly and specifically influences the species composition in adjacent urban forests; this impact’s specificity depends a lot on the forest habitat fertility).
Among the types of vegetation inconsistent with the forest habitat recorded in the herbaceous layer, the synanthropic plant species compose the largest group in both forests. While the research conducted by Fornal-Pieniak et al. [6] indicated a 20% share of synanthropic species, in the Nowa Warszawa Forest (Peucedano-Pinetum), their percentage reached 16%, and in the Młociny Forest (Tilio-Carpinetum), it reached 18%. The average share of synanthropic species in the forest edge zones in both studied forests slightly exceeded the percentage determined by the aforementioned researchers; in the Nowa Warszawa Forest, it was 25% (whereas in the forest interior, it was 7%), and in the Młociny Forest it was 22% (whereas in the forest interior, it was 14%). It is worth noticing that, in the interior of the Tilio-Carpinetum forest, a few more inconsistent plant species were found than in the interior of the Peucedano-Pinetum forest. The reason for this may be that inconsistent species are mostly synanthropic species, so they are connected with human activities, which concern mainly fertile and medium-fertile habitats. Therefore, these synanthropic species could probably more easily colonize more fertile forest habitats than poorer habitats, especially in conditions of permanent urban pressure. A greater percentage of synanthropic species in the forest edge zone with the poorer habitat is probably due to the aforementioned increased fertility of the habitat in the zone located close to the road. The greater share of synanthropic species in the Młociny Forest (also in the interior) is in line with expectations—for easier colonization of more fertile habitats by inconsistent species, as indicated in the literature [27,28]. These results also confirm hypotheses H1 and H3.
The highest decrease in the synanthropic species numbers in the herbaceous layer was observed 30 m from the edge of the forest adjacent to the road—in the Nowa Warszawa Forest, with a poorer habitat, and 40 m in the Młociny Forest, with a more fertile habitat. This distance is 1.5–2 times larger than the total impact of roads on rural forests studied by Mizera, Grajewski [8], and Czaja and Wilczek [11] and several times larger than in the studies conducted by Honnay et al. [48]. The distance from the forest border adjacent to the intensively used road up to the stabilization of plant species composition in the forest interior was set at 90 m in the forest with a poorer habitat and 100 m in the forest with a more fertile habitat.
The stabilization of forest plant species composition in the herbaceous layer occurs after 90 m from the forest edge in the forest adjacent to the road—in the forest with lower fertility, and after 100 m—in the forest with the higher trophism, which also confirms hypotheses H2 and H3. According to the literature, the road impact on plant species composition was up to 20 m [8,11] in rural forests. In the mentioned studies, however, forests adjacent to roads located outside large cities were investigated. Therefore, the greater distance of the road impact, in our research, may result from an increased exposure to many anthropopressure factors specific to the urban environment. However, the obtained research values are close to the distance from the forest edge to the forest cores in Warsaw forests—set at 93–113 m, studied by Fornal-Pieniak et al. [2].
In both studied forests, there were invasive plant species in the herbaceous layer. Quercus rubra L. and Prunus serotine (Ehrh.) Borkh, the most dynamically colonizing invasive species [49,50,51], occurred in the Peucedano-Pinetum and Tilio-Carpinetum forests. As expected, they were more numerous in the forest with more fertile habitat than in the poorer one; such a relationship has been described in the studies conducted so far [28]. A similar effect was caused by the presence of thickets of the Acer negundo L. and Reynoutria japonica Houtt. Handl.Pl –Kruidk. in the forest edge zone in the coniferous forest and Impatiens parviflora DC. in the forest interior zone in the oak–hornbeam forest. These species were also observed by Kołaczkowska et al. [50] during research on invasive plant species in the urban forests in Warsaw. Referring to the latter, typical invasive species from gardens and crops were also recognized, as Cotoneaster lucidus Schltdl., Juglans regia L., and ruderal species, Erigeron annuus (L.) Pers and Erigeron Canadensis L., which occurred in all these species episodically without a greater impact on the forest vegetation.

4. Conclusions

The adjacent roads have significant and specific impacts on the species composition of the herbaceous layer of Peucedano-Pinetum and Tilio-Carpinetum urban forests. The spatial range of this impact was set at 90 m in Peucedano-Pinetum and 100 m in Tilio-Carpinetum in the marginal zones of a forest. The forest plant species (from different phytosociological classes) and synanthropic plant species dominated in the studied forests. The impact of adjacent roads on the plant species composition in urban forests mainly consists of the appearance of species inconsistent with the forest habitat. The percentage of inconsistent species decreases with an increasing distance from the forest edge zone. The plant species composition stabilizes in the forest interior, where forest species dominate with a small share of inconsistent species. In forest interiors, the share of inconsistent species was a little bigger in Tilio-Carpinetum than in Peucedano-Pinetum. Ecotones of Peucedano-Pinetum are more likely colonized by plant species inconsistent with the habitat due to factors connected with human activity along roads, increasing fertility in edge zones. The Tilio-Carpinetum forest is more easily colonized by invasive plant species than the Peucedano-Pinetum. The conducted research results give more knowledge and guidelines for further research, mostly focused on formulating directions pertaining to how to shape the surroundings of urban forests if the aim is to maintain more natural forests, especially regarding plant species composition with the predominance of forest species.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f14122401/s1, Table S1. Phytosociological table (the edge zone of the Nowa Warszawa Forest). Table S2. Phytosociological table (the interior zone of the Nowa Warszawa Forest). Table S3. Phytosociological table (the edge zone of the Młociny Forest). Table S4. Phytosociological table (the interior zone of the Młociny Forest). Table S5. Characteristics of species observed in the Nowa Warszawa Forest. Table S6. Characteristics of species observed in the Młociny Forest.

Author Contributions

Conceptualization, B.F.-P. and F.K.; methodology, B.F.-P., F.K. and M.O.; formal analysis B.F.-P., F.K. and M.O.; investigation, B.F.-P. and F.K.; resources, B.F.-P., F.K., B.Ż., F.K. and P.S.; data curation, B.F.-P., F.K. and M.O.; writing—original draft preparation, B.F.-P., F.K. and M.O.; writing—review and editing, B.F.-P., F.K. and M.O.; visualization, F.K., M.O. and B.F.-P.; supervision, B.F.-P., P.S., K.S. and B.Ż.; project administration, B.F.-P. and P.S.; funding acquisition, B.F.-P. and P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This publication was funded by being co-financed within the framework of the Polish Ministry of Science and Higher Education’s program: “c” in the years 2019–2023 (No. 005/RID/2018/19)”; amount of the financing: 12,000,000.00 PLN.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Location of the studied urban forests in Warsaw, Poland (own study).
Figure 1. Location of the studied urban forests in Warsaw, Poland (own study).
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Figure 2. Schematic representation of the selected urban forests adjacent to the road.
Figure 2. Schematic representation of the selected urban forests adjacent to the road.
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Figure 3. Localization of transects with plots = positions (white squares) in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpnetum) (own study).
Figure 3. Localization of transects with plots = positions (white squares) in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpnetum) (own study).
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Figure 4. Number of herbaceous plant species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position (study plot) in the transect.
Figure 4. Number of herbaceous plant species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position (study plot) in the transect.
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Figure 5. Number (a) and share (b) of inconsistent species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position in transect.
Figure 5. Number (a) and share (b) of inconsistent species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position in transect.
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Figure 6. Number of alien species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position in the transect.
Figure 6. Number of alien species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position in the transect.
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Figure 7. Number of invasive species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position (study plot) in the transect.
Figure 7. Number of invasive species per plot located in the Nowa Warszawa Forest (Peucedano-Pinetum) and the Młociny Forest (Tilio-Carpinetum) in relation to the location and position (study plot) in the transect.
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Figure 8. Average number of plant species belonging to a specific vegetation type: in Peucedano-Pinetum (the Nowa Warszawa Forest) (a) and Tilio-Carpinetum (the Młociny Forest) (b); e—forest edge zone, i—forest interior zone.
Figure 8. Average number of plant species belonging to a specific vegetation type: in Peucedano-Pinetum (the Nowa Warszawa Forest) (a) and Tilio-Carpinetum (the Młociny Forest) (b); e—forest edge zone, i—forest interior zone.
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Figure 9. Average number of plant species belonging to specific life forms according to Raunkiær in the Nowa Warszawa Forest (Peucedano-Pinetum (a) and the Młociny Forest (Tilio-Carpinetum) (b); e—forest edge zone, i—forest interior zone; F—phanerophytes, T—therophytes, G—geophytes, H—hemicryptophytes, Ch—chamephytes.
Figure 9. Average number of plant species belonging to specific life forms according to Raunkiær in the Nowa Warszawa Forest (Peucedano-Pinetum (a) and the Młociny Forest (Tilio-Carpinetum) (b); e—forest edge zone, i—forest interior zone; F—phanerophytes, T—therophytes, G—geophytes, H—hemicryptophytes, Ch—chamephytes.
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Figure 10. Projection of the mean Zarzycki’s indicator numbers on two first principal components by the Nowa Warszawa Forest (Peucedano-Pinetum) (L—light index, T—thermal index, K—continentalism index, W—soil moisture index, Tr—trophism index (fertility), R—soil (water) acidity index, D—soil granulometric index, H—matter content index (organic matter), S—index of resistance to NaCl content in soil or water, M—index of resistance to increased content of heavy metals in soil).
Figure 10. Projection of the mean Zarzycki’s indicator numbers on two first principal components by the Nowa Warszawa Forest (Peucedano-Pinetum) (L—light index, T—thermal index, K—continentalism index, W—soil moisture index, Tr—trophism index (fertility), R—soil (water) acidity index, D—soil granulometric index, H—matter content index (organic matter), S—index of resistance to NaCl content in soil or water, M—index of resistance to increased content of heavy metals in soil).
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Figure 11. Projection of plots on two first principal components by the Nowa Warszawa Forest (Peucedano-Pinetum). Plots of the same zone and gradient positions have been averaged. The positions in the gradient have been encoded as color brightness.
Figure 11. Projection of plots on two first principal components by the Nowa Warszawa Forest (Peucedano-Pinetum). Plots of the same zone and gradient positions have been averaged. The positions in the gradient have been encoded as color brightness.
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Figure 12. Projection of mean Zarzycki’s indicator numbers on two first principal components by the Młociny Forest (Tilio-Carpinetum) (L—light index, T—thermal index, K—continentalism index, W—soil moisture index, Tr—trophism index (fertility), R—soil (water) acidity index, D—soil granulometric index, H—matter content index (organic matter), S—index of resistance to NaCl content in soil or water, M—index of resistance to increased content of heavy metals in soil).
Figure 12. Projection of mean Zarzycki’s indicator numbers on two first principal components by the Młociny Forest (Tilio-Carpinetum) (L—light index, T—thermal index, K—continentalism index, W—soil moisture index, Tr—trophism index (fertility), R—soil (water) acidity index, D—soil granulometric index, H—matter content index (organic matter), S—index of resistance to NaCl content in soil or water, M—index of resistance to increased content of heavy metals in soil).
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Figure 13. Projection of plots on first two principal components by the Młociny Forest (Tilio-Carpinetum). Plots of the same zone and gradient positions have been averaged. The positions in the gradient have been encoded as color.
Figure 13. Projection of plots on first two principal components by the Młociny Forest (Tilio-Carpinetum). Plots of the same zone and gradient positions have been averaged. The positions in the gradient have been encoded as color.
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Table 1. GLM table for herbaceous plant species number.
Table 1. GLM table for herbaceous plant species number.
DfWald’s Statisticsp
forest173.1280.000000
location147.7430.000000
position912.2080.201852
forest location10.1170.732306
forest position95.2840.808891
position location95.0620.828851
forest position position95.6630.773144
Table 2. The significance (p-value) of the differentiation of inconsistent, alien, and invasive species.
Table 2. The significance (p-value) of the differentiation of inconsistent, alien, and invasive species.
Inconsistent SpeciesAlien SpeciesInvasive Species
forest0.0026080.0000000.000000
location0.0000000.2511180.834035
position0.0000000.6329010.962144
forest location0.0000010.0074100.000122
forest position0.6528840.8420020.938210
position location0.0000010.9564470.885380
forest position position0.0303880.8549680.493661
Table 3. The significance of the differentiation of forest and synanthropic species.
Table 3. The significance of the differentiation of forest and synanthropic species.
Forest SpeciesSynanthropic Species
forest0.0000000.173080
location0.1621110.001332
position0.0768330.000000
forest location0.0875110.639040
forest position0.8095360.446829
position location0.0394120.000027
forest position position0.9048870.699433
Table 4. The significance of the differentiation of Raunkiaer life forms.
Table 4. The significance of the differentiation of Raunkiaer life forms.
FTGHCh
forest0.0000230.0000000.0000000.001263x
location0.0000010.0033240.0003300.015597x
position0.154161x0.1019120.346650x
forest location0.0019530.0264220.0000550.190243x
forest position0.884503x0.7299410.435176x
position location0.430430x0.3350580.804377x
forest position position0.941205x0.6730270.245284x
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Fornal-Pieniak, B.; Kamionowski, F.; Ollik, M.; Szumigała, P.; Żarska, B.; Szumigała, K. The Impact of Adjacent Road on Vascular Plant Species Composition in Herbaceous Layers of Peucedano-Pinetum and Tilio-Carpinetum Urban Forests in the City of Warsaw (Poland). Forests 2023, 14, 2401. https://doi.org/10.3390/f14122401

AMA Style

Fornal-Pieniak B, Kamionowski F, Ollik M, Szumigała P, Żarska B, Szumigała K. The Impact of Adjacent Road on Vascular Plant Species Composition in Herbaceous Layers of Peucedano-Pinetum and Tilio-Carpinetum Urban Forests in the City of Warsaw (Poland). Forests. 2023; 14(12):2401. https://doi.org/10.3390/f14122401

Chicago/Turabian Style

Fornal-Pieniak, Beata, Filip Kamionowski, Marcin Ollik, Paweł Szumigała, Barbara Żarska, and Karolina Szumigała. 2023. "The Impact of Adjacent Road on Vascular Plant Species Composition in Herbaceous Layers of Peucedano-Pinetum and Tilio-Carpinetum Urban Forests in the City of Warsaw (Poland)" Forests 14, no. 12: 2401. https://doi.org/10.3390/f14122401

APA Style

Fornal-Pieniak, B., Kamionowski, F., Ollik, M., Szumigała, P., Żarska, B., & Szumigała, K. (2023). The Impact of Adjacent Road on Vascular Plant Species Composition in Herbaceous Layers of Peucedano-Pinetum and Tilio-Carpinetum Urban Forests in the City of Warsaw (Poland). Forests, 14(12), 2401. https://doi.org/10.3390/f14122401

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