Urban forests are an important element in the spatial and functional structure of cities. In a highly transformed urbanised environment, they create enclaves of a natural character, also playing an important role in shaping the urban landscape and in the lives of residents (Tyrväinen et al. 2005; Kaliszewski 2006; Kim 2016). The forest cover has a beneficial effect on the local climate (mainly in reducing temperature and wind strength) and also contributes to reducing the level of air pollution (McPherson et al. 1997; Tyrväinen et al. 2005). Urban forests are also important centres of local biodiversity, creating places of refuge for many species of plants and animals, including rare and endangered species (Alvey 2006; Lepczyk et al. 2017; Kowarik et al. 2019).
Urban forestry is defined as ‘an art, science and technique of management of trees and forest resources in urban ecosystems and surrounding them with the aim to secure psychological, sociological, economical and aesthetic advantages for municipal communities’(Miller 1997). Urban woodland means a forested ecosystem of natural, semi-natural or man-made origin, used for a variety of purposes (Bell et al. 2005). The concept of the ‘urban forest’ itself is defined in various ways (Konijnendijk et al. 2006). In a broader sense, it includes all woody vegetation in the city, that is, forests, parks, gardens and roadside trees (Costello 1993; Konijnendijk 2003; Randrup et al. 2005). In a narrower sense, this concept refers only to typical forest ecosystems, without gardens, parks and street trees (Konijnendijk 2003; Rydberg and Falck 2000; Gundersen et al. 2005; Randrup et al. 2005).
Forests in cities may have different characters. In a broader sense, urban forests are all the forests within the legal boundaries of urban areas (Konijnendijk 2003). This also includes suburban commercial forests, often having the character of dense forest complexes with relatively little diversity of tree stands (Jaszczak and Wajchman 2014; Fojcik et al. 2019). It seems more appropriate, however, to adopt a narrower definition for the concept of ‘urban forests’, that is, that they are enclaves of forests in strictly urban areas (Konijnendijk 2003). This approach reflects their specificity resulting from the development of specific urban conditions (including the specific climate related to the urban heat island effect, air pollution and disturbed soils) (Sieghardt et al. 2005). Such forests are usually isolated from larger forest complexes and occupy relatively small areas (Konijnendijk 1997). They usually arise in wasteland and develop largely through spontaneous succession. The trees may partly come from plantings, especially in previously reclaimed areas or in former parks (Keil and Loos 2005; Olszewski 2009). They are usually not used commercially.
With regard to urban forests (in the strict sense), the occurrence of species of alien origin deserves special attention. The spread of alien species is one of the negative effects of urbanisation (Bierwagen 2005; Arson et al. 2015). Especially in the last century, there has been an increase in the number of alien woody species in the urban flora and also in the forest areas (Mavimbela et al. 2018; Kowarik et al. 2019). They include invasive species that reproduce intensively and spread quickly, and even belong to the category of transformers, changing the features and conditions of ecosystems (Richardson et al. 2000). Urban forests, often exposed to disturbance, fragmentation and alien propagule pressure, are particularly susceptible to the penetration of alien plant species (Wagner et al. 2017; Becerra and Simonetti 2020).
The share of alien tree species in the forest structure is particularly important. The aim of this study is to determine the scale of this phenomenon in Katowice by analysing the share of alien species in the structure of stands in the local urban forests. The following research hypotheses were formulated: 1) the share of alien species in the tree stand structure of the urban forests in Katowice is significant; 2) invasive species play an important role among alien tree taxa and 3) differences in habitat conditions on the forest edge (in the ecotone zone) and inside the forest affect the occurrence of alien tree species.
Katowice (50.288815 N latitude, 18.943135 E longitude) is the main city of one of the largest urban agglomerations in Central Europe and the most urbanised region in Poland (Upper Silesian Industrial Region) (Krzysztofik 2021). Until recently, Katowice was a large mining and industrial centre, but in recent decades, the economy of Katowice has been transforming from the heavy industry of steel and coal mines into one of the most attractive investment areas for modern economy branches in Central Europe. The city encompasses an area of 165 km2 and has over 290,000 inhabitants (Województwo śląskie… 2019). The city area is characterised by a diverse relief, consisting of hills (exceeding 300 m above sea level), intersected with river valleys in the form of tectonic depressions and erosion channels (Szaflarski 1976). Over the last 200 years, humans and their economic activities have become an important factor modelling the local relief, resulting in the blurring of natural landforms and the appearance of new ones (embankments, workings, sinkholes, heaps, etc.) (Barciak et al. 2012). More than 38% of the city’s area is covered by forests, so it is one of the most forested cities in Europe (Pauleit et al. 2005). Dense forest areas cover the southern part of the city. These are mainly commercial forests of a mixed forest nature (Fojcik and Stebel 2010).
Field research was conducted in 2021 in Katowice in the northern part of the city. The research covered 30 sites (Tab. 1), which were single urban forests, defined as small wooded areas (with an area of at least 1 ha), isolated from larger complexes of managed forest, the vegetation of which is currently developing spontaneously, even if some of the trees come from old, purposeful plantings (as part of reclamation or park development). During the field research, conducted on the entire area of individual forests, species of tree stands (over 3 m high) were inventoried, and the following information was recorded:
occurrence of native tree species with the dominant species marked, occurrence of alien tree species, including their abundance in individual forests (=stands); three categories of abundance were adopted: 1 – single specimens (up to 10); 2 – not very numerous specimens (11–30) and 3 – numerous specimens (over 30).
o determine the potential of the alien species, the presence of undergrowth (up to 3 m) was also recorded, and its abundance was determined according to the scale described above. In addition, attention was paid to the location of trees of alien origin, whether they occurred deep in the forest or in the forest steppes; due to the relatively small area of the analysed forests, an area of up to 20 m into the forest was assumed as the ecotone.
List, area and location of the analysed urban forests in Katowice
No. | District | Area (ha) | Localisation | |
---|---|---|---|---|
N | E | |||
1 | 2 | 3 | 4 | 5 |
1. | Muchowiec | 2.7 | 50.241908 | 19.033019 |
2. | Muchowiec | 3.7 | 50.242854 | 19.028191 |
3. | Szopienice (Borki) | 8.0 | 50.278260 | 19.085934 |
4. | Szopienice (Borki) | 3.0 | 50.273858 | 19.092457 |
5. | Szopienice (Borki) | 2.5 | 50.271047 | 19.097414 |
6. | Szopienice (Borki) | 9.0 | 50.275485 | 19.097414 |
7. | Szopienice (Borki) | 2.0 | 50.271403 | 19.102413 |
8. | Burowiec | 2.4 | 50.273186 | 19.078531 |
9. | Dąbrówka Mała (Norma) | 1.7 | 50.272089 | 19.062395 |
10. | Pniaki | 5.2 | 50.280015 | 19.056901 |
11. | Dąbrówka Mała | 3.4 | 50.282319 | 19.068961 |
12. | Osiedle 1000-lecia | 4.8 | 50.277237 | 18.968822 |
13. | Osiedle 1000-lecia | 3.7 | 50.272749 | 18.967893 |
14. | Osiedle 1000-lecia | 1.4 | 50.271082 | 18.972764 |
15. | Osiedle 1000-lecia | 4.2 | 50.270299 | 18.985690 |
16. | Załęska Hałda | 1.8 | 50.250420 | 18.994464 |
17. | Załęska Hałda | 2.6 | 50.254042 | 18.997575 |
18. | Załęska Hałda | 2.0 | 50.247090 | 18.984634 |
19. | Kokociniec | 3.5 | 50.231153 | 18.958849 |
20. | Kokociniec (Płochy) | 3.9 | 50.233992 | 18.966832 |
21. | Osiedle Paderewskiego | 5.0 | 50.245109 | 19.048359 |
22. | Wilhelmina | 4.9 | 50.249884 | 19.093892 |
23. | Nikiszowiec | 3.9 | 50.246454 | 19.083292 |
24. | Dąbrówka Mała | 7.5 | 50.287081 | 19.070160 |
25. | Dąbrówka Mała | 7.4 | 50.290373 | 19.061810 |
26. | Józefowiec (Kolonia Alfred) | 7.1 | 50.290229 | 19.013960 |
27. | Józefowiec (Kolonia Alfred) | 1.9 | 50.292573 | 19.008059 |
28. | Brynów (Park Kościuszki) | 3.0 | 50.239455 | 19.002028 |
29. | Osiedle Witosa | 1.7 | 50.262051 | 18.963812 |
30. | Obroki | 3.7 | 50.266550 | 18.957426 |
The general frequency of occurrence of alien tree species was determined on the basis of the number of forests (tree stands) in which they were recorded (expressed in %), as opposed to the abundance of occurrence in individual forests, characterised by the abundance scale described above. The nomenclature of species, their systematic affiliation and current status in the national flora were adopted according to Mirek et al. (2020).
In the analysed stands of urban forests in Katowice, a total of 50 woody species were recorded, including 31 native species and 19 of alien origin (Tab. 2, 3). The vast majority were deciduous trees – 90.3% of the native trees and 100% of the alien trees. In individual forests, the stands ranged from 10 to 30 species, including 5–20 native species and 5–14 alien species (Fig. 1).
The share of native species in the analysed urban forests in Katowice
No. | Tree species | Number of forests | % of forests | Number of forests where species prevaile |
---|---|---|---|---|
1. | 4 | 13.3 | - | |
2. | 28 | 93.3 | 3 | |
3. | 22 | 73.3 | - | |
4. | 10 | 33.3 | 2 | |
5. | 2 | 6.7 | - | |
6. | 30 | 100 | 16 | |
7. | 8 | 26.7 | - | |
8. | 21 | 70 | - | |
9. | 22 | 73.3 | - | |
10. | 2 | 6.7 | - | |
11. | 9 | 30 | - | |
12. | 22 | 73.3 | 1 | |
13. | 2 | 6.7 | - | |
14. | 24 | 80 | - | |
15. | 1 | 3.3 | - | |
16. | 12 | 40 | 1 | |
17. | 3 | 10 | - | |
18. | 20 | 66.7 | 4 | |
19. | 30 | 100 | 11 | |
20. | 3 | 10 | - | |
21. | 27 | 90 | 3 | |
22. | 1 | 3.3 | - | |
23. | 29 | 96.7 | 1 | |
24. | 18 | 60 | 1 | |
25. | 2 | 6.7 | - | |
26. | 16 | 53.3 | - | |
27. | 25 | 83.3 | - | |
28. | 27 | 90 | 1 | |
29. | 8 | 26.7 | - | |
30. | 7 | 23.3 | - | |
31. | 6 | 20 | - |
The share of alien species in the structure of the analysed urban forests in Katowice
No. | Tree species | Tree stand | Undergrowth | ||
---|---|---|---|---|---|
number of forests | sum of abundance categories (average) | number of forests | sum of abundance categories (average) | ||
1. | - | - | 1 | 1 (1) | |
2. | 24 | 44 (1.8) | 25 | 55 (2.2) | |
3. | 2 | 2 (1) | - | - | |
4. | 14 | 15 (1.1) | 17 | 18 (1) | |
5. | - | - | 1 | 1 (1) | |
6. | 17 | 31 (1.8) | 18 | 33 (1.8) | |
7. | 19 | 22 (1.2) | 26 | 46 (1.8) | |
8. | - | - | 1 | 1 (1) | |
9. | 12 | 12 (1) | 1 | 1 (1) | |
10. | 26 | 53 (2) | 29 | 80 (2.8) | |
11. | 14 | 21 (1.5) | - | - | |
12. | 4 | 7 (1.7) | - | - | |
13. | 4 | 7 (1.7) | 1* | 1 (1) | |
14. | 1 | 1 (1) | - | - | |
15. | 15 | 24 (1.6) | 2* | 3 (1.5) | |
16. | 6 | 8 (1.3) | 1* | 1 (1) | |
17. | 15 | 18 (1.2) | 7 | 8 (1.1) | |
18. | 4 | 4 (1) | 1 | 1 (1) | |
19. | 19 | 42 (2.2) | 28 | 58 (2.1) | |
20. | 29 | 78 (2.7) | 29 | 71 (2.5) | |
21. | 3 | 3 (1) | 3 | 3 (1) | |
22. | 1 | 1 (1) | 2 | 2 (1) |
Suckers.
Among the native species in the forest stand, the most common were
The share of alien species in the stands of individual forests was significant, as they constituted from 15% to 44% of the total number of species (Fig. 4). Among the 20 trees most frequently included in the tree stands, as many as six are species of alien origin (Fig. 5).
The local potential of alien woody species can be determined using the abundance index of their occurrence in the understory (Tab. 3). The highest average abundance index values were obtained by
Although some of the alien species were not spontaneous (some were planted), preferences for inhabiting the forest interior or its edges can be noticed. There are three groups of species:
species recorded only in the forest ecotone zone ( species occurring more often in the ecotone zone ( species occurring both on the edge and inside the forest (
No alien tree species was observed exclusively inside the forest.
Urban forests, by definition, have a different character compared to large, dense, managed forest complexes. Urban forests (sensu stricto) are small, semi-natural woodlands, formed through natural regeneration on disturbed post-industrial sites or other neglected areas, usually not subject to systematic management measures (Konijnendijk 2003; Keil and Loos 2005; Randrup et al. 2005). This was the nature of the analysed urban forests in Katowice. Generally, these were deciduous forests with a visible share of alien species (only in some places, there was a small admixture of native coniferous species).
Many factors influence the structure of forests, including those in Katowice. On the one hand, these are general climatic and habitat conditions, and on the other, the genesis of the tree stand (planted or spontaneously formed) (McPherson et al. 1997). One of the most important habitat conditions is the type of soil. In urbanised and industrial areas (also in Katowice), anthropogenic soils dominate, shaped under the influence of human activity (Fajer 2008). Urban forests are often created on former urban and post-industrial wastelands (Keil and Loos 2005; Pietrzykowski 2016). Moreover, these are usually areas with an increased level of pollution (especially with heavy metals), which adversely affects the soil’s microorganisms and biological activity (Grabińska 1988; Olszowska 2016). These soils usually also have a more alkaline reaction (Romańczyk et al. 2015). All these factors have an impact on the formation of the local vegetation, including the urban forests.
The origins of urban forests, also in Katowice, may be different. They arise either as a result of intentional human activity and the planting of specific species, or they develop spontaneously as a result of the natural colonisation of wastelands by species of various origins (Keil and Loos 2005). They may also arise as a result of a combination of these processes – reclamation plantings are supplemented by the natural succession of other trees, something which is often observed in Katowice. Natural succession is evidenced by the large share of pioneer species such as
Native species usually prevail in the urban forest stands in Katowice. The most common species are
Particularly noteworthy, nevertheless, is the significant share of species of alien origin in the analysed stands. Among the 20 most frequently recorded trees, as many as six are alien species. Their presence in urban forests is facilitated by the fragmentation of tree stands, disturbances, earlier plantings and propagule pressure (Brothers and Spingarn 1992; Wagner et al. 2017). Therefore, they often constitute a significant element in this type of forests (Zipperer 2002; Kowarik et al. 2019). In the analysed stands, the most common alien species were
It is worth noting that among the recorded alien tree species, there were no coniferous taxa. This is due to the fact, among others, that alien coniferous trees, although relatively often used for planting, are among the relatively few trees that establish and spread spontaneously (Křivánek et al. 2006; Danielewicz et al. 2020; Pliszko and Górecki 2022). Alien species of the
The six most common trees of alien origin in the analysed stands are invasive species:
The establishment and spread of alien woody species is facilitated by climate warming (Theoharides and Dukes 2007; Robinson et al. 2020). In urbanised areas, the ‘urban heat island’ phenomenon also plays an important role, causing the growing season to extend (Kowarik 1995; Benedikz et al. 2005). Climate change affects the dynamics of alien species and increases the number of invasive species (Bellard et al. 2013; Robinson et al. 2020). Perfect examples of such changes are
In the urban forests in Katowice, most alien tree species occurred mainly or exclusively on the forest edges (in the ecotone). The ecotone is a zone with specific conditions – it receives more light, more wind and is warmer and drier than the forest interiors (Chen et al. 1999; Davies-Colley et al. 2000; Dovčiak and Brown 2014). Greater wind penetration also favours the influx of diaspores of wind-seeding species. This favours the success of alien species (Bierwagen 2005; Harper et al. 2005). This effect is particularly visible in smaller vegetation patches because fragmentation increases the proportion of the edge zone (Murcia 1995; Theoharides and Dukes 2007).
To better illustrate the character of urban forests in Katowice, it is necessary to mention the presence of other plants of alien origin that occur here. These include, for example, ornamental shrubs, most likely introduced as part of earlier recultivation (most often,
Research has confirmed the significant share of alien species in the tree stand structure of the urban forests in Katowice.
Invasive species are among the most common and abundant alien tree species in urban forests in Katowice (
Conditions prevailing on the edge of urban forests (in the ecotone zone) – with more light, warmth and dryness than the forest interiors, and greater wind penetration favouring the inflow of diasporas – favour the occurrence of alien tree species.