Distribution of trees in medieval areas of East-Central European cities ‒ regularities and peculiarities

Trees in the densely built-up historical cores of cities increase the aesthetic values of the cityscape as well as lower the effect of the urban heat island. The research aimed to determine the spatial distribution of tree populations in the medieval parts of cities. The investigation included three cities in Central and Eastern Europe: Poznań and Lublin in Poland and Lviv in Ukraine. The oldest parts of these cities share similar origins, the same continental biogeographical region, and comparable climatic conditions. This study considered indicators of occurrence, density, species composition, and spatial distribution of trees. The study identified different types of management concerning trees. In Poznań, trees are actively introduced in public spaces. In Lublin, there is the largest share of trees in the backyards and self-seeding is common. In Lviv, there are new plantings in private and public spaces. Lviv is distinguished by its large share of native trees, and in Poznan and Lublin, new plantings include artificially shaped, small spherical crowns.


Introduction
The historical areas of cities are currently more oriented towards services for visitors than services for residents. A high density of cultural heritage sites attracts tourists and a growing number of catering establishments and shops being created for the benefit of visitors. This is particularly true in the intensely built-up, oldest parts of cities, where the spatial layout and many buildings were constructed in the Middle. The public spaces of streets and squares are subject to competition between pedestrians, vehicles, and the owners of premises who want to use them for catering services. At the same time, there is great pressure to renovate or build new facilities (Kubus 2007). The permeable surface covered with vegetation, including trees, is therefore relatively small. There have been increased restrictions on cars in recent years, which has pushed vehicles outside the oldest part of the cities (Szczepanek and Trzaskowska 2019). This has created an opportunity to change the structure of the public space by introducing trees and other elements of green infrastructure.
In the rich literature regarding the role of trees in cities, few studies focus on the oldest parts of cities, which are characterised by high building density and high cultural values which have resulted in an intense tourist traffic, and the associated wide range of services, especially catering. These characteristics also result in an ambivalent attitude towards the presence of trees, especially along the streets (Mullaney et al. 2015).
Trees provide economic, social, health, visual, and aesthetic benefits to cities (Roy et al. 2012). Research documents the role of trees and greenery in compact urban developments in reducing the urban heat island effect (Bowler et al. 2010;Gillner et al. 2015;Norton et al. 2015). On the other hand, trees in a compact urban built-up area have disadvantages, including maintenance costs, light attenuation, infrastructure damage, and causing allergies (Avolio et al. 2015). Another point is that trees hide historic buildings, obscure views and panoramas (Orzechowska et al. 1996). The management and planning of the historical parts of cities is a growing challenge (Bruce and Creighton 2008).
In the Middle Ages, trees played only a marginal role in cities. There were some trees in the private gardens of the ruling class and monastery gardens, but they were mostly fruit trees, rather than ornamental trees (Lawrence 1993).
This research aimed to capture the similarities and differences between Poznań and Lublin in Poland and Lviv in Ukraine based on comparative studies. These are East-Central European cities established during the Middle Ages under the Magdeburg Rights in the 13th to 14th century (Poznań -1253, Lublin -1317, Lviv -1356 when they were part of the Kingdom of Poland. At that time, the type of location, which defined the organisation of the city's selfgovernment as well as being a planned, regular layout of streets with dense buildings, was very popular (Lück 2014;Szende 2016;Sakun et al. 2019). This has left its mark on the spatial structure of contemporary development and helps to compare the number and distribution of trees against the urban fabric. These cities are situated within the same continental biogeographical region (European Environmental Agency 2016), which means the same set of native species. The similarity in the climatic conditions of the cities under study (Table 1).

Study sites
Contemporary Poznań is the Polish city of an agglomeration with approximately 900,000 inhabitants. There are nearly 540,000 inhabitants within the administrative borders of the city, an area of 262 km 2 (Statistical Office in Poznań 2019). The city lies in the Middle European Lowland, along the Warta River. Poznań was an important political and religious centre in the second half of the tenth century. In 1253, the new city was established on the left bank of the Warta River under the Magdeburg Rights. It is estimated that in the fifteenth century there were no less than 4,000 inhabitants (Gąsiorowski 1988). The medieval part of the city is currently inhabited by approximately 3,000 permanent residents (based on evidence of the houses and attributed residents, i Kartografii 2016), which shows that the population has remained more or less stable from medieval times.
The medieval city covered an area of nearly 22 hectares, the layout of streets has remained almost unchanged, despite the demolition of nearly all the former initial buildings and city walls. Some 1.5 million tourists a year today visit Poznań, however, the oldest part of the city is the biggest attraction and the gastronomic centre. Car traffic has been eliminated from the area of the Old Market Square and several adjacent streets, and the remaining streets are limited to local traffic.
The Lublin agglomeration in Poland is inhabited by over 650,000 people. Within the administrative borders of the city, there are approximately 320,000 inhabitants in an area of 147.5 km 2 (Statistical Office in Lublin 2019). The city is situated on hills cutting through the valley of the Bystrzyca River and its tributaries. A settlement complex was originally formed on five hills near the confluence of rivers. The town charter under the Magdeburg Right of 1317 regulated the town's layout and method of governance. The newly established town was inhabited by about 2,500 people in the 14th century (Szczygiel 2017). The current population is similar to that of medieval times: the inhabitants of this part of the city, according to official data, is estimated at 2,400.
The medieval city is relatively small covering an area of seven hectares. The layout of streets has remained almost unchanged, despite the demolition of nearly all initial buildings and city walls. Currently, renaissance buildings prevail. Over 1 million people, for whom the oldest part of the city is the main attraction, visit Lublin annually. Car traffic has been eliminated from the Market Square and several adjacent streets, and the remaining streets are limited to local traffic. Contemporary Lviv is the largest agglomeration in western Ukraine with a population of about 800,000. The administrative area of the city covers 182 km 2 , where about 725,000 people live (Derzhavna sluzhba statystyky Ukrainy 2019). Lviv is situated on the European watershed separating the Baltic and the Black Sea basins. The medieval part of the town is situated in the Peltwa River Valley, to the east of its riverbed, which is now included in an underground channel.
According to Zubrzycki's (1844) estimation, some 4,000 people were living in the city centre of Lviv in the early fifteenth century. Compared to medieval times, the population has therefore increased slightly; and some 2.2

Methods of investigation
The research covered an area limited by a range of internal walls of the medieval cities. The course of the walls, which were depicted on historical engravings and archival maps, was superimposed on contemporary cartographic pictures, photographs, and satellite images. We started with an analysis of archival drawings, and studies of trees in cities. Although the oldest engravings cannot be treated literally, they were undoubtedly published as documentary material that gives an approximation of reality (Kulesza 2012). Utilized satellite images come from publicly available OpenStreetMaps data. They were the basis for making working maps with basic land cover types, i.e. buildings, sealed streets and areas, unsealed areas, low greenery, and trees. The QGIS tool was used to process the data, which allowed to calculate the proportions of the main elements of the urban fabric in the studied areas of the cities. Field research was conducted over two summer seasons in 2018 and 2021, which included the mapping of trees with their basic attributes (species name, size, age group). Due to the lack of source documentation on tree planting, non-invasive tree age assessment methods and age tables were used (Łukaszkiewicz 2010). A distinction was made between mature and younger trees, which were considered to have been planted after 1990, that is after the beginning of the political and socio-economic transition in both Poland and Ukraine. Spatial databases were created for trees in the individual cities. This helped to determine and then compare the tree saturation rate. The research was carried out in available public and private spaces (backyards), where permissions were obtained from the owners. The research included the identification of the species composition by native and non-native cultivated species or kenophytes in cases of self-sown trees (Rutkowski 2004; European Environmental Agency 2016). Each of the specimens was assigned to one of the main types of locations within the city. Three aggregated categories were defined in the public space: streets/alleys, places/ squares/pocket parks, and unmanaged greenery (rubble, bluffs). Private yards were distinguished as a separate group, but publicly accessible church areas were classified as parks. The results are detailed in tabular form for each of the studied cities, and a comparative analysis made.

Trees in historical times
The oldest picture of Poznań (Braun and Hogenberg 1617), shows the buildings and regular layout of streets within medieval walls from its northern perspective. An analysis of this picture reveals that trees are drawn inside almost all quarters of the buildings. The picture also depicts some backyards with signatures crops, probably vegetables. There were no trees seen along the streets or in the central square, which, was perhaps due to their communication and commercial functions.
Dense buildings in the medieval Lublin meant that only small clusters of trees were found in the Dominican and Jesuit monastery areas and near the castle (Fijałkowski and Kseniak 1982;Lerue 2015). Vegetation, including single trees, could be found in courtyards. Small street sizes, the communicational and commercial function of the streets, and the central square lead to the assumption that there were no trees in the medieval city. In paintings from the seventeenth century depicting the appearance of the city, there is no high greenery within the walls, although vegetation is visible just outside the walls in the monastery gardens. Photographs from the second half of the 19th and early 20th centuries record the noticeably higher number of trees than presently occurs in the medieval area of Lublin.
An analysis of the oldest map of Lviv from 1777 (Atlas ukrainskykh istorychnykh mist 2014) reveals that only one tree was marked within the inner walls of the medieval city. Other trees appear to be located in the zone between the inner and outer city walls and other peripheral parts of the city. This can also be seen in maps and watercolour paintings from the eighteenth and nineteenth centuries. The lack of greenery within the boundaries of the medieval town results from the subordination of its spatial structure to residential, craft, and commercial functions. Gardens with fruit trees can be distinguished in the northern area of the city, within the inner boundaries of medieval Lviv, and only on the map of 1849 (Kadastr Lvova 1849).

Urban fabric
The city areas studied are characterised by the presence of similar elements in the urban fabric. Spatial patterns are similar in Poznań and Lviv with a regular layout of perpendicular streets that extend from the central square.
The urban fabric of Lublin is a bit different, where the streets run from the former castle and are connected by parallel blocks forming a fan layout. The spatial specificity of the old Lublin is related to the location of the city in the area of the former settlement structure and reflects a more diversified relief.
The hermetic development of the frontages of the quarters formed by the streets facing the central square in four directions is a characteristic feature of the spatial structure of the medieval part of Poznań (Fig. 1).
The buildings cover 45% of the area, and fortified streets and squares 35%. Unlike in the other two cities, there are large unpaved areas within the built-up quarter. On the outskirts, there are larger areas overgrown with managed green. The total area of unpaved surfaces occupies 20% of the studied part of the city. The spatial distribution of trees is more dispersed in Poznań than in the other cities.
The area of medieval Lublin is the smallest of the cities under consideration (Fig. 2.). Slightly more than half of the area is covered by buildings (52%), and sealed squares and The contemporary urban fabric on the satellite image within the medieval part of Lublin streets occupy 43%. The area of greenery, only 5%, is concentrated in three larger fragments.
Currently, in Lviv, 54% of the area is covered by buildings, and 43% of the area is sealed streets and squares. The proportions of the main elements of land cover are almost identical to those in Lublin, while the spatial layout of the medieval city (Fig. 3.) is very similar to that of Poznań. The remaining impervious areas are mostly in the peripheral parts.
The structure of land cover is similar in the city areas in this study, which results from the preservation of the historical urban shape (Fig. 4.).
Poznań has a certain distinctiveness, where larger biologically active areas have survived on the outskirts and inside quarters of buildings, which together occupy almost 20% of the area. There are also two unsealed plots of land in Poznań, which are presently used as car parks. In Lublin and Lviv, the unpaved surfaces are in small scraps of areas that do not significantly affect the structure of the land cover. An important structural element limiting the possibility of planting high vegetation in the soil is the developed underground, it means infrastructure and building elements. Fig. 3 The contemporary urban fabric on the satellite image within the medieval part of Lviv Fig. 4 The main elements of land cover structure -comparison between Poznań, Lublin, and Lviv

Species composition
In Poznań (Table 2.), non-native Acer platanoides 'Globosum" and Aesculus hippocastanu prevail out of 27 identified species and account together for almost 40% of all the trees. A subdominant position is held by native Acer platanoides and non-native Platanus xacerifolia. These four species together make up some two-thirds of all the trees in the medieval section of Poznań. The large proportion of these species results from new plantings (since 1990). They are found mainly in streets and squares, but Aesculus hippocastanum is the most common species in yards. Trees of other species occur in Poznań individually or in groups of several specimens.
Lublin trees are characterised by high number of species; 85 specimens in total were assigned to species (Table 3.). The three most numerous ones, Acer platanoides (15), Robiniapseudoaccacia (10), and Sambucus nigra (8), together account for about 35% of the total number; there are no clearly dominant species. The greatest diversity is found in the backyards, where ornamental trees and shrubs are planted; in these areas, short-lived, non-native tree species dominate. Self-seeding specimens (Sambucus nigra) can also be found in the backyards. Lviv trees are characterised by a significant share of noble long-lived trees belonging to native species (Table 4.). The most numerous of them are Tilia cordata (47), Acerplatanoides (23), Fraxinus excelsior (21), which, together with other native, cultivated species, account for over 60%. They are a valuable component of the greenery in this part of the city because, in addition to their aesthetic qualities, they are resistant to the stresses of the urban environment. Among other planted species, Thuja occidentalis (42) is the most abundant ornamental evergreen tree, probably because it is easy to care for. Of this group, Aesculus hippocastanum (12) and Prunus domestica (10) are also found in significant numbers. The most common species of trees across the cities is Acer platanoides. Another feature the cities have in common is a lack of fruit tree species (excluding some low valued Prunus domestica), which implies a complete change in the function of the trees compared to preindustrial periods.

Age of trees
The differences between the cities reveal a different approach to the introduction of tall, new greenery into the built-up areas (Fig. 5). The proportion of younger trees (up to about 30 years old) is highest in Poznań (45%), although it is only slightly lower in Lviv (40%). In Lublin, mature specimens dominate, and younger specimens comprise only about 22%.
Younger specimens of Robinia pseudoaccacia 'Umbraculifera' have been planted in greater numbers in Lublin. Eight of those specimens are found in the town square by the theatre (in the less frequented part, close to the old walls) and four Acer platanoides 'Globosum' in the yards. Other trees are single specimens from seven other ornamental species, all planted in backyards. New plantings in Poznań, with single exceptions, include Acer sp. and Platanus xacerifolia which were planted along two streets and in pocket parks and squares. The species structure of the new plantings in Poznań and Lublin shows that tree size is decisive in the choice of species, and native or non-native status does not play any significant role. In Lviv, about half of the new plantings are non-native Thuja occidentalis, which are predominantly found on church grounds. Among the cultivated non-native species, there are also six trees of Aesculus 1 3 hippocastanum, introduced as complementary plantings in the Market Square. A few younger trees of native species are found in Lviv (24 trees). Eight Salix alba were planted in the courtyard of the polyclinic, and seven Crataegus levigata were planted along one of the streets. Other single specimen species have been planted in the last few decades. Among them is Robinia pseudoaccacia 'Umbraculifera'.The analysis of the species composition of younger trees in the three cities shows that short-lived non-indigenous trees or ornamental forms of native species are planted.

Spatial distribution of trees
Despite the similarities in the spatial structure of the medieval parts of the cities, both the indicators of the number of trees per unit area as well as their distribution between different elements of urban space differ. The greater proportion of soil covered with vegetation in Poznań is in contradiction to the lowest tree saturation index. Lublin (13.0 trees/ha) and Lviv (11.3 trees/ha) had a twofold greater tree saturation index than Poznań (5.9 trees/ha). Differences between the cities in the total number of trees are not reflected in the variety of the observed tree attributes. The spatial distribution of trees between public and private space is very different. Each city has a specific spatial distribution of trees, as illustrated in Fig. 6. Poznań has the most balanced distribution of trees between different locations. Compared to the other two cities, it is distinguished by having the highest share of street trees. This is the result of planned planting trees along two selected streets in recent decades. In Lviv, there is a slightly higher density of trees in similar locations, whereas in Lublin the share of trees along traffic routes is marginal, but a significant percentage are grouped in the Cathedral and Theatre Squares. Lublin is distinguished by having the highest proportion of trees in its yards. This indicator is almost three times higher than in Poznań and six times higher than in Lviv. This shows a higher proportion of outbuildings used in Lublin for housing purposes and higher standards of backyard care.
In Lviv, there is a small proportion of trees in yards (9%), which is related to the high proportion of the sealing area within building quarters. In Lviv, 67% of the trees are located on squares and plazas. This is a distinctive feature of the city: a significant number of trees growing in church grounds and monastery yards, which have traditionally been a place for burials and garden crops, can be considered a factor conducive to a larger number of trees. There are also many trees in the Old Town Square. This is related to the policy of the municipal authorities to preserve the historical features of the landscape in this part of the city, including trees (Lvivska Miska Rada 2018). The markets in Poznań and Lublin are completely devoid of trees, although there is a considerable number in plazas and pocket parks. In Poznań, this accounts for nearly 40% of the trees, which are mostly located on peripheral squares. In Lublin, trees in this location account for 26%, and most grow in squares near the cathedral and at the theatre.

Discussion
Our investigation has shown that the number of new plantings (since 1990) was low for all of the cities in this study, which shows that the policymakers do not favour an increase in tall greenery in cities. This is in line with trends found in the USA (Nowak and Greenfield 2012) and with the results of research for Wrocław in Poland (Krężel et al. 2014). Kronenberg (2015) points to the role of bureaucratic barriers in introducing trees to the heavily invested parts of the urban fabric. The practice in the studied cities is contrary to research findings that the cultural history of an area is positively related to its green quality and, in turn, to public health (Weimann et al. 2017). This was noticed in the nineteenth century and developed in the twentieth century, when trees were introduced into the squares and streets of historic parts of cities, and parks were created in place of city walls. In places, buildings destroyed during World War II were replaced with greenery and trees.
The transition towards a market economy facilitated investment opportunities in the area of dense development, Fig. 6 Distribution of trees according to aggregated urban structural elements in the considered parts of Poznań, Lublin, and Lviv which led to the removal of some trees and green areas. In recent years, this trend has been reversed thanks to planting small trees in the ground and introducing plants in pots (Fortuna-Antoszkiewicz 2019). Also in the analysed parts of cities small decorative trees are dominant, grafted onto the trunks of Acer platanoides 'Globosum' and Robinia pseudoaccacia 'Umbraculifera' among the trees planted in recent decades. This is unfavourable from an aesthetic and architectural point of view; the unnatural morphology of their crowns does not match the scale of the development or the character of historical spaces (Przesmycka 2017). Ameen et al. (2015) point out the advantages of sustainable urban design, which harmoniously combines the protection of historical heritage with the character and spatial distribution of green areas. The planting of small decorative tree forms has very limited natural-ecological significance compared to native forms (Gromke and Ruck 2007). These new forms of tree crowns are alien to the landscape of historic parts of cities.
When selecting tree species and size of forms, one should take into account their harmonious fit into the historical landscape of the city (Krause 2001; Ziemeļniece 2012; Fortuna-Antoszkiewicz and Łukaszkiewicz 2021). The protection of the visual values of the landscape, including vegetation, should be the basis for activities also within urban areas (Council of Europe Landscape Convention 2016).

Conclusions
Despite similarities in the urban fabric and common historical backgrounds, there are significant differences in the attributes of trees planted in the medieval parts of the cities under study. These differences prove that the main driving force of tree distribution is local practice. Each city represents a different type of approach to trees within its medieval sector: (a) Active in public space and passive in private spacerepresented by Poznań, where nearly half the trees are new, located in public spaces, along selected streets and squares. Planting in private spaces rarely occurred. (b) Restrictive in public space and permissive in private space -represented by Lublin, where there are numerous plantings within yards as private spaces and farreaching restrictions on introducing and restoring trees into the public space. (c) Non-restrictive -represented by Lviv, where there is a high proportion of new plantings in private plots near churches and public spaces as well.
As a result, there are large differences in the spatial distribution, species, and age of trees. This applies, in particular, to younger trees, which highlights the differences in the planting preferences of local authorities.
The results indicate that the existing trees mainly have aesthetic functions, with low ornamental trees predominating, which can also take the form of shrubs (Crateagus leavigata, Sambucus nigra, Magnolia sp.).
A new trend of planting small trees, grafted onto the trunks of Acer platanoides 'Globosum' and Robinia pseudoaccacia 'Umbraculifera' and characterised by unnatural crown shapes, was observed in the studied cities. This is aesthetically controversial and has no significant effect on climate change adaptation. The research findings document the approach to high greenery management in Central and Eastern Europe, however, the conclusions provide a rationale for also local policy concerning trees in other regions with similar natural conditions.
The results of this study indicate the need to extend research further in densely built-up cities, as well as to consider the social and cultural aspects, especially concerning the perception of trees in the medieval parts of cities, by residents, tourists, and business owners. This is all the more important because climate change is forcing adaptation projects, among which nature-based solutions play a crucial role. included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.