Comparison of vascular plants in herb layers of ecotones in urban and non-urban forests in Brzesko city (Polish Carpathian foreland)

Forests are very important elements in the ecological structure of cities and rural areas. They are reservoirs of flora and fauna biodiversity in the ecological system (Sanesi et al. 2017). Urbanisation is a process which causes many changes in the environment, such as habitat fragmentations, temperature increase and soil compaction (Patarkalashvili 2017; Wang et al. 2020). Anthropogenic activities mainly affect semi-natural and natural ecosystems (Faliński 1966; Kornaś 1968; Sokołowski 1972; Sudnik-Wójcikoska and Galera 2005). Plant cover is often altered by displacing and depleting native species and appearance of species incompatible with the natural habitat. This process is highly undesirable in natural habitats and often leads to irreversible changes in forest ecosystems as well (Lundholm and Marlin 2006; Gonzalez et al. 2010; Zhou et al. 2018; Fornal-Pieniak et al. 2019). Herbaceous plants are very dynamic with respect to habitat changes within a short period of time. Therefore, they are suitable indicators of changes in plant association (Hofmeister et al. 2013). It is important to identify consistent plant species, which are an important indicator of forest naturalness. The purpose of this study was to analyse the composition of vascular plant species in the herb layer of ecotone's selected urban forests and forests outside the city.

Field studies were carried out in the years 2019–2020, including two seasonal aspects, spring and summer, in eight forests located in Brzesko city and outside the city. Brzesko city has got a surface area of 11.83 km²; and has 16,819 inhabitatns. (https://krakow.stat.gov.pl/vademecum/vademecum_malopolskie/portrety_gmin/powiat_brzeski/brzesko.pdf). A homogeneous habitat according to the phytosociological aspect was the primary criterion for the selection of research objects. It means that all forests are represented the oak–horn-beam community (Tilio-Carpinetum Scamoni et Pass. 1959 em. Traczyk 1962), growing on fresh, rich soil. Urban forests (four objects) were located nearby single-family houses with accompanying greenery, and the other forests (four objects) were adjacent to meadows and pastures with a small share of arable land outside the city (Fig. 1). The study included an inventory and identification of vascular plants in the herb layer on transect which represented the forest ecotone – 150 m from the edge of forests into the forests. According to Wuyts et al. (2009) and Goznalez et al. (2010), forest ecotone with was about 150–200 m.

Figure 1

It distinguished 15 study plots (phytosociological records were made according to the Braun-Blanquet method (Braun-Blanquet 1951)), with an area of 100 m² in each forest. In total, 60 phytosociological records were made in urban forests and 60 phytosociological records were made in forests outside the city (available from author(s) on request). The identified plant species were grouped according to the degree of compliance with the natural habitat. We distinguished consistent species represented by forest species, which including tree, shrub, herb and grass species, which are typical for Tilio-Carpinetum habitat. The second group of species was inconsistent species such as tree, shrub, grass, synanthropic and associated species, not typical for oak–hornbeam forest. Associated species means the species which occur in more than one habitat (Matuszkiewicz 2014).

To compare the occurrence of individual herbaceous species, its frequency in individual forests was determined as the share of research plots where the species was found within all plots in a given forest. The analyses were performed using the STATISTICA 13.0 package. A simple c² test was also used to distinguish forest herbaceous species characteristic/diagnostic of urban and non-urban forests.

The mean number of vascular plant species (20) was higher (p < 10⁶) in forests outside the city than in urban forests (17). There was also a variation in the number of species between the forests (p = 0.013), with the number of species in all urban forests being lower than in all forests outside the city. No significant differences were found between urban forests and forests outside the city (p = 0.84) or between individual forests (p = 0.97). A list of 10 vascular plant species occurring significantly (p < 0.05) more frequently in forests outside the city than in urban forests is presented in Table 1.

The composition of vascular plant species in the herb layer was dominated by forest species, consistent with Tilio-Carpinetum habitat (from 80% to 90%), and grass species from 4% to 12%. Grass species means grass plants inconsistent with Tilio-Carpinetum community, typical for meadows and pastures. A greater proportion of grass species was found in forests outside the city than in the city. The share of synanthropic species ranged from 3% to 8%. A higher share of synanthropic and other species group, typical for areas with human impact, was observed in urban forests (Fig. 2). The analysis of the frequency of occurrence of individual species allowed to find those occurring only in forests outside the city. There were no species characteristic of urban forests. Some plant species as Carex pilosa L. and Lathyrus vernus (L.) Bernh. were found only in forests outside the city.

Figure 2

Urbanisation has got many negative impacts on natural habitats in cities. It causes modifications in the environment as habitat fragmentations, increasing temperature and soil compaction (Patarkalashvili 2017; Wang et al. 2020). The fragmentation of natural habitats leads to transformations and, as a result, to changes in plant species compositions in forests (Dearborn and Kark, 2010). The total number of plants species was higher in forests outside the city than in urban forests. Similar results were obtained by Kowarik et al. (2019).

A higher proportion of grass species was found in forests outside Brzesko city. There were meadows and pastures adjacent to the forests, which could have influenced penetration of inconsistent species from the surrounding areas into the forests. A slightly higher share of synanthropic species typical for urbanised areas and a smaller share of grass species were found in the urban forests of Brzesko city. The number of forest species typical of natural habitat Tilio-Carpinetum (consistent) was similar in both types of studied forests. It means that forests still ‘keep’ their natural habitat despite human impacts. It should be noted that in the case of the investigated urban and non-urban forests, we observed preservation of native forest plant species, for example, L. vernus (L.) Bernh. and C. pilosa L., which were found only in the herbaceous layer of forests outside the city. According to Dzwonko (2015), these species are very sensitive to changes in habitat and occur only in natural or slightly transformed habitats. Similar research results were obtained by Fornal-Pieniak and Ollik (2013) and Fornal-Pieniak et al. (2019). Some authors, including Palmer et al. (2008) and Speziale and Ezcurra (2011), pointed to the fact that reduction of native flora enables the propagation of non-native species on the natural habitat.

Surrounding landscape structures have probably higher impact on the collection of plant species not typical for Tilio-Carpinetum habitat in urban forests. It was confirmed in the present study. The inconsistent species (mostly synanthropic plants) along the road and walking paths are the factors for their invasion into the natural habitats (Lee et al. 2012; Gonzalez et al. 2010). These plants are mostly characterised by an easy seed dispersal mostly by wind, which could possibly explain their occurrence in the forests (Rehm et al. 2019). The higher frequency of disturbance could also have an impact on the collection of synanthropic plants in urban forests than in forests outside the city.