Urban Resilience and Residential Greenery—The Evidence from Poland

: Social distancing and movement restrictions associated with COVID-19 have highlighted the role played by easily accessible greenery at one’s own residence, which is of key importance to people’s physical and psychological wellbeing. The main focus of this study is to provide insights into residential green areas in Wrocław, Poland in terms of knowledge, investment, trends and associated issues, as examined from the perspective of the potential to boost urban resilience. Sixty-six representative multi-family construction projects typical of Central European cities were analyzed. The study is divided into three stages: I—pre-construction (selection of locations), II—construction of a housing estate (clearance of vegetation, number of trees planted, use of pro-ecological solutions), and III—post-construction (monitoring of the condition of green areas by means of the Normalized Difference Vegetation Index (NDVI) and data from Sentinel-2 satellites). The results highlighted the insufﬁcient use of pro-ecological solutions (green roofs, fountains or ponds) and shortage of woody plants in residential green areas. Their implementation should be included in any strategies for Wrocław’s transformation toward a sustainable post-pandemic city. Furthermore, the main ﬁndings of the study also revealed an imbalance between the amount of vegetation cleared vs. vegetation planted during construction and unequal access to high-quality greenery for local residents. Although some positive trends in spatial planning were observed compared to previous studies, two construction projects were implemented in ﬂoodplains. NDVI analyses indicate the poor condition of residential green areas; moreover, the change in NDVI for the period 2015–2020 attests to the poor standards of vegetation maintenance. The results of this study suggest that the potential of residential green areas for boosting urban resilience has not been fully utilized in Wrocław. It is highly recommended to implement monitoring of the condition of green areas by means of NDVI and to introduce detailed provisions on the shaping of green estate areas in the Local Development Plans to enhance Wrocław’s resilience. The main ﬁndings being presented expand the directions of methods of monitoring residential areas’ conditions in terms of applied landscape research and boosting urban resilience.


Introduction
"Sun, Space, Green" were listed as major elements of the urban environment in La charte d'Athènes-the most important urban manifesto of the 20th century [1].Despite the passage of time, this statement is still valid in the 21st century.Unfortunately, maintaining this balance is becoming increasingly difficult in contemporary European cities because of intensified urbanization, which results in a shortage of greenery [2] and open, unsealed spaces [3], combined with intensifying climate change [4].The importance of this issue is highlighted by the provisions that have been set out in a number of major European and international documents, including the Paris Agreement [5], the United Nations (UN) 2030 Agenda for Sustainable Development [6], World Health Organization (WHO) Reports [7,8] and the blueprint of a new European Union (EU) adaptation strategy [9].

The Concept of Resilience
The assumptions in strategic documents that set out guidelines for global and local initiatives are underpinned by the concept of resilience.This concept was first formulated by Holling [10] as a theory that describes the behavior of ecosystems and explains whether and how ecosystems can handle stress and disturbances caused by external factors [11].For the purposes of this paper, the adopted definition recognized by the United Nations Office for Disaster Risk Reduction [12] referred to "the ability of a system, community or society exposed to hazards to resist, absorb, accommodate to and recover from the effects of a hazard in a timely and efficient manner, including through the preservation and restoration of its essential basic structures and functions".
The increasing awareness of the need for resilience coincides with the growth in the number of harmful factors and threats to the environment.The crisis related to the COVID-19 pandemic has exposed the need to prioritize adaptation efforts [4].There is an urgent need to reconsider how lifestyles, the use of public spaces by residents, and the functioning of towns and cities have changed; in other words, how resilient is Europe to unprecedented threats that may also be expected because of further climate change?

Urban Green Spaces (UGS)
Within the context of the global economic crisis triggered by COVID-19, there is a need for a more efficient identification of profitable, low-cost alternatives for gray infrastructure [13].This is also of paramount importance to meeting the challenges arising from more frequently occurring natural disasters, progressive urbanization, loss of biological diversity, and climate change [14].This puts UGSs at the center of attention as a key factor in improving city dwellers' quality of life [8,15].To date, researchers have failed to agree on a universal definition of the term 'UGS' [7,16].UGSs consist of municipal parks, forests and gardens, as well as a number of other public, semi-public or private areas that provide city dwellers with various Ecosystem Services (ES) [7,8,[17][18][19], including residential green areas [15,20,21].The principal environmental benefits of UGSs are identified as improvements in air and water quality, combating urban heat islands (UHIs), preventing floods and reducing surface runoff [4,[22][23][24].Numerous studies have also shown that UGSs contribute to improving the physical and mental health of residents [25][26][27].Even where restrictions on movement have been introduced, residential green areas are nearby and offer opportunities for physical activity.Researchers have also shown that the view from ones' window is important for maintaining mental balance.A view of green areas, especially residential ones, may reduce stressful feelings [27][28][29].This is extremely important in the context of the impact that the COVID-19 crisis has on mental health.Studies in the USA have indicated that anxiety and depression symptoms were three and four times more common, respectively, in 2020 than in the second quarter of 2019 [30].
Most frequently, the available UGS-related data relates to parks, urban forests and public gardens [31][32][33].The number of studies concerning residential green areas is rather small [20,21], and therefore, there is a research gap in the detailed analyses of this type of UGS within the context of increasing urban resilience.Moreover, studies conducted in European cities have identified several problems regarding residents' access to residential green areas [18], which constitute an urgent issue related to environmental justice in densely populated areas [33,34].In the case of Poland, the studies carried out so far have primarily focused on UGSs on a larger scale [35][36][37], informal greenery [15], urban forests [38], more sustainable spatial planning in the context of environmental protection, and the development of residential areas in urban regions [39].Few studies have been devoted to residential green areas, although researchers dealing with this topic [40][41][42] have not considered the need to raise urban resilience.
To fill this research gap, this study concentrated on residential green areas with a view to increasing urban resilience: Wrocław in Poland was used as a case in point, as it is representative of other Central European cities.For the purposes of this study, 'residential green areas' is defined-after Battisti [20]-as mainly semi-public green areas regularly created during the construction process, closely associated with residential buildings.With a view to making the study comprehensive, the data acquired were related to both the vegetation that was cleared and planted during the construction process and its maintenance after completing the construction of the estate.Therefore, the analyses consider the state of development of a given plot before and after the project's completion, as well as the subsequent five-year period during which the condition of greenery maintenance was monitored by means of the Normalized Difference Vegetation Index (NDVI), based on data extracted from Sentinel-2 mission satellites.

Study Area
Wrocław is located in the Dolnośl ąskie Province (south-western Poland) and covers an area of 293 km 2 (according to data from the Local Data Bank, as of 12 April 2021).Regarding land-use structure, green areas make up 42% of the city's area, of which 7.5% are legally protected (including a landscape park and Natura 2000 areas).The individual UGS components occupy the following areas: parks-826 ha, communal forests-931 ha, street greenery-548 ha, and housing estate green areas-541 ha [43].The city engages itself in climate-related activities at the local, national and international levels as part of the following initiatives:

Selection of Multi-Family Housing Estates
In order to select the most representative construction projects, use was made of the data in the list of decisions regarding planning permissions granted by the Department of Architecture and Construction of the municipality of Wrocław [44].Because of varied construction process durations and times needed for vegetation to develop, the years 2010-2012 were identified as the most suitable time period to be studied in order to minimize the risk of analyzing an uncompleted construction project or having an insufficiently long monitoring period [40].From among the construction projects enumerated in the document, only multi-family complexes were selected.The filtering criteria (entries in the tabular listing) were as follows: 1. new; 2. multi-family buildings with at least three flats and building complexes (only those with more than one multi-family building were selected).This is the most common type of neighborhood in Polish towns and cities, and it takes up the biggest part of urban space [40].Where no detailed address was provided (e.g., only the street name was given), verification was carried out using orthophotomaps from the years 2006,2007,2009,2011,2012,2014, all of which were available online [45] and materials made available by the respective developers.
In total, 66 multi-family housing estates in Wrocław were selected (the reference number of a given housing estate corresponds to the order in which the decisions were issued).Based on the data made available on the Internet [46] by the Surveying, Cartography and Municipal Cadastral Office in Wrocław, each estate was designated an address number, plot number, sheet number, and the total area of plots occupied by the estate.When establishing the territorial extent of estates that had been constructed in several stages, the following were considered: plot boundaries; the name of the investor corresponds to the name of the investment; and the composition factor.Where the decision was only applicable to a fragment of a complex, the entire estate was examined.Subsequently, basic statistical data were calculated.The next step was an analysis of 66 construction projects, the scope of which was determined by the identified threats to which Wrocław is exposed.

Analyses-Stage I
In order to examine the impact of a construction project on the city's resilience, the analyses were divided into three stages (Figure 1): I. development of the plot prior to construction, II.construction of a housing estate, and III.plot development following construction-a five-year period of monitoring the maintenance of greenery by means of NDVI.The analysis of the pre-construction period (stage I) focused on the selection of estate locations from the perspective of the following threats: floods and urban sprawl.The existence of a Local Development Plan (LDP) for the area constituted an important aspect of the initial legal conditions.From the perspective of sustainable spatial planning, the lack of an LDP is highly detrimental and necessitates the acquisition of special planning permission (granted for the purpose of a specific construction project).The maps, made available on the Internet [47] by the Polish Water Management Office, were used to assess the threat of flooding (the probability of flooding occurring once every 100 years was designated as Q 1%).The erection of multi-family housing estates on the outskirts of cities, in most cases with poor transport links, encourages urban sprawl.It also has an adverse effect on shaping the city landscape, which reduces the city's resilience.As part of the analysis of their locations within the context of urban sprawl, a 1-km-wide buffer along Wrocław's administrative boundaries was established, while the housing estates within this area were identified.
issued).Based on the data made available on the Internet [46] by the Surveying, Cartography and Municipal Cadastral Office in Wrocław, each estate was designated an address number, plot number, sheet number, and the total area of plots occupied by the estate.When establishing the territorial extent of estates that had been constructed in several stages, the following were considered: plot boundaries; the name of the investor corresponds to the name of the investment; and the composition factor.Where the decision was only applicable to a fragment of a complex, the entire estate was examined.Subsequently, basic statistical data were calculated.The next step was an analysis of 66 construction projects, the scope of which was determined by the identified threats to which Wrocław is exposed.

Analyses-Stage I
In order to examine the impact of a construction project on the city's resilience, the analyses were divided into three stages (Figure 1): I. development of the plot prior to construction, II.construction of a housing estate, and III.plot development following construction-a five-year period of monitoring the maintenance of greenery by means of NDVI.The analysis of the pre-construction period (stage I) focused on the selection of estate locations from the perspective of the following threats: floods and urban sprawl.The existence of a Local Development Plan (LDP) for the area constituted an important aspect of the initial legal conditions.From the perspective of sustainable spatial planning, the lack of an LDP is highly detrimental and necessitates the acquisition of special planning permission (granted for the purpose of a specific construction project).The maps, made available on the Internet [47] by the Polish Water Management Office, were used to assess the threat of flooding (the probability of flooding occurring once every 100 years was designated as Q 1%).The erection of multi-family housing estates on the outskirts of cities, in most cases with poor transport links, encourages urban sprawl.It also has an adverse effect on shaping the city landscape, which reduces the city's resilience.As part of the analysis of their locations within the context of urban sprawl, a 1-km-wide buffer along Wrocław's administrative boundaries was established, while the housing estates within this area were identified.

Analyses-Stage II
As part of the analysis related to stage II (construction of a housing estate), the clearing of vegetation was examined, checks were made to ascertain whether any pro-ecological solutions (such as green roofs, fountains or ponds) were adopted, and the final number of trees was estimated.The clearing of vegetation was analyzed to determine the percentage of the area in which vegetation had been removed in relation to the total housing estate area.To compare the conditions before the construction of a multi-family housing estate to those after construction (taking into account trees and bushes removed as a result of the construction in relation to the total housing estate area) as accurately as possible, measurements were recorded using Wrocław's Geographical Information System and the ArcGIS program using orthophotomaps from the years 2006,2007,2009,2011,2012,2014.The data needed to locate pro-ecological solutions and estimate the number of trees were obtained by comparing pre-and post-construction orthophotomaps, materials provided by the developers, and the Database of Topographic Facilities [48].In order to verify the accuracy of the data that had been gathered and to acquire photographic documentation, site visits were made in 2018.

Analyses-Stage III
In the post-construction period, the last stage of analysis consisted of monitoring changes in the development and condition of local greenery by means of NDVI.The NDVI, a well-known and commonly used indicator, is a simple yet effective measure for the quantitative determination of green vegetation.It is based on the contrast between the reflectance of red visible light and the near-infrared band, ranging from −1 to +1 and is calculated using formula (1) (for images acquired from Sentinel-2 satellites): where: The pigment in plant leaves, chlorophyll, absorbs visible light very effectively (from 0.4 to 0.7 µm) for use in photosynthesis.The cell structure of leaves, on the other hand, very effectively reflects near-infrared light (from 0.7 to 1.1 µm).The more leaves a plant has, the more these wavelengths of light are affected, which is why the values of the index are correlated with the amount of biomass and chlorophyll content [49,50].Negative NDVI values correspond to water, and values close to zero (from −0.1 to 0.1) typically correspond to arid areas covered by rocks, sand or snow.Low positive values indicate bushes and grasslands (ca.from 0.2 to 0.4), while high values signify temperate and tropical forests.Studies have shown that NDVI is a good approximant of the condition of urban greenery [7,[51][52][53], and it correlates with tree canopies better than with any other type of land cover [23].The NDVI is the most commonly used index for vegetation dynamics.It was conducted to examine the effects of climatic variability (rainfall) on NDVI for the period 1982-2015 in the Gojeb River Catchment (GRC), Omo-Gibe Basin in Ethiopia [54].The NDVI was employed as an indicator to reflect the response of the vegetation dynamics, with a view to ascertaining the impacts of varying climate conditions [55].Another project showed some MODIS NDVI data potential for mapping the percentage of tree mortality in forests subjected to regional bark beetle outbreaks and severe droughts [56].The NDVI has also been used in epidemiological studies [57].When using satellite data, it is important to consider the season and weather conditions in which the photos were taken because these impact NDVI values [7].The data used in the study came from the Sentinel-2 mission of the European Space Agency (ESA).Its objective is to monitor the variability of conditions on the Earth's surface.Sentinel-2 contains 13 bands that provide high-resolution multispectral data [58].At middle latitudes, satellites Sentinel-2A and Sentinel-2B take measurements more or less every 2-3 days.However, in any given year, only several reliable images with no clouds or only a few clouds are taken of any particular location.Therefore, to obtain valid data, satellite images with 0% cloudiness were selected [59].In addition, to verify the results of the NDVI, two reference areas (a square with a non-permeable surface and an urban forest) were selected to identify any abnormal deviation from the overall NDVI trends.

Results
In total, 66 multi-family housing estates built in the city of Wrocław by 48 economic entities were selected for the study.The range of estate areas is quite significant, from 0.10 to 8.72 ha.The average plot area amounted to 1.34 ha.

Stage I
The housing estates differed widely, particularly in the southern and western parts of the city (Figure 2).In the city center, which has been identified as the most susceptible to climate change, there are eight housing estates.Within the established area (a 1-km-wide buffer along the city's administrative boundaries), a total of 22 housing estates were selected, i.e., 33% of the examined construction projects.These projects intensify urban sprawl, which is very well attested to by housing estates numbered 11, 53 and 61, all of which are built among cultivable fields, with one access road leading to or from the center of Wrocław (Figures 3 and 4).Analysis of the risk of flooding indicated that two projects (constituting 2% of the studied estates) had been constructed on floodplains (Nos.15 and 63).The initial legal conditions for building these estates were investigated.In the case of as many as 17% of them, there was no LDP in force.
only several reliable images with no clouds or only a few clouds are taken of any particular location.Therefore, to obtain valid data, satellite images with 0% cloudiness were selected [59].In addition, to verify the results of the NDVI, two reference areas (a square with a non-permeable surface and an urban forest) were selected to identify any abnormal deviation from the overall NDVI trends.

Results
In total, 66 multi-family housing estates built in the city of Wrocław by 48 economic entities were selected for the study.The range of estate areas is quite significant, from 0.10 to 8.72 ha.The average plot area amounted to 1.34 ha.

Stage I
The housing estates differed widely, particularly in the southern and western parts of the city (Figure 2).In the city center, which has been identified as the most susceptible to climate change, there are eight housing estates.Within the established area (a 1-kmwide buffer along the city's administrative boundaries), a total of 22 housing estates were selected, i.e., 33% of the examined construction projects.These projects intensify urban sprawl, which is very well attested to by housing estates numbered 11, 53 and 61, all of which are built among cultivable fields, with one access road leading to or from the center of Wrocław (Figures 3 and 4).Analysis of the risk of flooding indicated that two projects (constituting 2% of the studied estates) had been constructed on floodplains (Nos.15 and 63).The initial legal conditions for building these estates were investigated.In the case of as many as 17% of them, there was no LDP in force.

Stage II
The biggest vegetation clearance area was recorded for housing estate No. 50-1.54 ha; the smallest for estate No. 64-0.01 ha; the average amounted to 0.07 ha.No vegetation was cleared on 38% of the housing estates.After calculating the area of cleared vegetation, this was divided by the total area of the relevant estate.This provided vegetation-clearing rates [%] for all construction projects (Figure 5).The biggest share of the housing construction projects (38%) had a rate of 1-10% (Figure 6), while the highest rate, at over 20%, was

Stage II
The biggest vegetation clearance area was recorded for housing estate No. 50-1.54 ha; the smallest for estate No. 64-0.01 ha; the average amounted to 0.07 ha.No vegetation was cleared on 38% of the housing estates.After calculating the area of cleared vegetation, this was divided by the total area of the relevant estate.This provided vegetation-clearing rates [%] for all construction projects (Figure 5).The biggest share of the housing construction projects (38%) had a rate of 1-10% (Figure 6), while the highest rate, at over 20%, was

Stage II
The biggest vegetation clearance area was recorded for housing estate No. 50-1.54 ha; the smallest for estate No. 64-0.01 ha; the average amounted to 0.07 ha.No vegetation was cleared on 38% of the housing estates.After calculating the area of cleared vegetation, this was divided by the total area of the relevant estate.This provided vegetation-clearing rates [%] for all construction projects (Figure 5).The biggest share of the housing construction projects (38%) had a rate of 1-10% (Figure 6), while the highest rate, at over 20%, was recorded in the case of 6 estates (9% of all multi-family housing complexes).The highest clearing rate, 50%, was calculated for housing estate No. 57.Regarding the studied projects, pro-ecological solutions supporting small-scale water retention were rare.Only four estates (6%) had a pond (estates Nos. 17 and 53) (Figure 7) or a fountain (estates Nos.16 and 48).Furthermore, rooftop rainwater harvesting was investigated, which found that green roofs were constructed on 18 of the analyzed estates (27%).There were no trees at all on 11 estates (17%).
Sustainability 2022, 14, x FOR PEER REVIEW 8 of 18 recorded in the case of 6 estates (9% of all multi-family housing complexes).The highest clearing rate, 50%, was calculated for housing estate No. 57.Regarding the studied projects, pro-ecological solutions supporting small-scale water retention were rare.Only four estates (6%) had a pond (estates Nos. 17 and 53) (Figure 7) or a fountain (estates Nos.16 and 48).Furthermore, rooftop rainwater harvesting was investigated, which found that green roofs were constructed on 18 of the analyzed estates (27%).There were no trees at all on 11 estates (17%).recorded in the case of 6 estates (9% of all multi-family housing complexes).The highest clearing rate, 50%, was calculated for housing estate No. 57.Regarding the studied projects, pro-ecological solutions supporting small-scale water retention were rare.Only four estates (6%) had a pond (estates Nos. 17 and 53) (Figure 7) or a fountain (estates Nos.16 and 48).Furthermore, rooftop rainwater harvesting was investigated, which found that green roofs were constructed on 18 of the analyzed estates (27%).There were no trees at all on 11 estates (17%).

Stage III
The condition of the greenery was monitored using NDVI for the years 2015-202 From among the available satellite data, photos that had been taken in the spring or su mer were selected because they were much more suitable for identifying the condition the greenery than those taken in January or February when vegetation is almost com pletely dormant.The mean NDVI for all housing estates in the years 2015-2020 amount to 0.249.The averaged NDVI values obtained are correct for vegetation in Wrocław (low positive values correspond to bushes and grassland).For a clearer presentation of the sults, the rate was calculated for each housing estate as a sum of the differences betwe successive NDVI values in successive years (2015-2020).The results were juxtaposed w the reference areas: a square with a non-permeable surface (Wolności Square in Wrocła and an urban forest (Pilczycki Forest in Wrocław).The biggest NDVI rise was record for housing estate No. 18 (0.265), while the most noticeable drop was recorded for esta No. 56 (−0.488); these figures indicate the best improvement and worst deterioration the condition of the local vegetation, respectively, among the housing estates in questio The changes in NDVIs for housing estates Nos.18 and 56, as well as for the referen areas, are presented in graphs (Figures 8 and 9).The changes in NDVI for each housi estate in the years 2015-2020 are shown on a graph (Figure 10).Based on the data obtain for all estates, the following ranges were established: The results are shown on a map (Figure 11).Three estates (nearly 5%) had results th fell in the range, indicating a deterioration in the state and condition of their vegetatio which meant a drop in the city's resilience.All of these were located in the south Wrocław.

Stage III
The condition of the greenery was monitored using NDVI for the years 2015-2020.From among the available satellite data, photos that had been taken in the spring or summer were selected because they were much more suitable for identifying the condition of the greenery than those taken in January or February when vegetation is almost completely dormant.The mean NDVI for all housing estates in the years 2015-2020 amounted to 0.249.The averaged NDVI values obtained are correct for vegetation in Wrocław (lower positive values correspond to bushes and grassland).For a clearer presentation of the results, the rate was calculated for each housing estate as a sum of the differences between successive NDVI values in successive years (2015-2020).The results were juxtaposed with the reference areas: a square with a non-permeable surface (Wolności Square in Wrocław) and an urban forest (Pilczycki Forest in Wrocław).The biggest NDVI rise was recorded for housing estate No. 18 (0.265), while the most noticeable drop was recorded for estate No. 56 (−0.488); these figures indicate the best improvement and worst deterioration in the condition of the local vegetation, respectively, among the housing estates in question.The changes in NDVIs for housing estates Nos.18 and 56, as well as for the reference areas, are presented in graphs (Figures 8 and 9).The changes in NDVI for each housing estate in the years 2015-2020 are shown on a graph (Figure 10).Based on the data obtained for all estates, the following ranges were established:
The results are shown on a map (Figure 11).Three estates (nearly 5%) had results that fell in the range, indicating a deterioration in the state and condition of their vegetation, which meant a drop in the city's resilience.All of these were located in the south of Wrocław.
The results of all these analyses (stages I-III) are presented in tabular form, attached as Supplementary Material (Table S1).The final figures indicate that 19.7% of the construction projects had a negative impact, 13.6% had no impact and 66.7% had a positive impact on Wrocław's resilience (Figure 12).The results of all these analyses (stages I-III) are presented in tabular form, atta as Supplementary Material (Table S1).The final figures indicate that 19.7% of the struction projects had a negative impact, 13.6% had no impact and 66.7% had a po impact on Wrocław's resilience (Figure 12).The results of all these analyses (stages I-III) are presented in tabular form, attac as Supplementary Material (Table S1).The final figures indicate that 19.7% of the struction projects had a negative impact, 13.6% had no impact and 66.7% had a pos impact on Wrocław's resilience (Figure 12).

Discussion
Resilience denotes the ability of a system to create new structures and paths of operation in response to changed external conditions [60].Consequently, an evaluation of a construction project in the context of increasing resilience or adapting to climate change should consider development both before and after construction.An attempt was also made to compare the findings of this study with the results obtained by other researchers dealing with housing estate green areas.
Bradecki and Twardoch [41] adopted an architectural and urban planning approach to analyze 41 residential projects (including single-family housing) in the Metropolitan Association of Upper Silesia for the years 2000-2011 (with an area of 0.08-4.4ha).The study method enabled a simultaneous description of the intensity rates, as well as an overview of a given estate.The research team, led by Battisti [20], used a socio-environmental approach in their work.The study focused on housing estate green areas in estates with unfavorable social situations.In 2017 and 2018, a total of 32 plots in eight research areas in Berlin were examined.Housing estates (with areas ranging from 9.6 ha to 35.5 ha) were constructed during various periods, ranging from the 1920s to the 1980s.The study focused on a variety of tree species, considering their air-cleaning potential and allergenicity, as well as on the presence of elements of landscaping, including the use of pro-ecological solutions.
Szulczewska et al. [40] applied an urban planning approach using ecological and spatial indicators.A total of 18 Warsaw housing estates (with an area of 5-7 ha) erected from 2007 to 2009 were examined.These were selected based on their varied values (ranging from about 20% to nearly 70%) of the Polish ecological-and-spatial indicator known as the Ratio of Biologically Vital Area (RBVA).This study aims to determine the minimum percentage of green areas needed to maintain good environmental conditions.
The results of scientific research have been compared on several levels: (A) Bardecki and Twardoch (2013) recorded the presence of pro-ecological solutions in only 5% of estates.In Berlin, only one bioswale was found (12.5%), and there were no

Discussion
Resilience denotes the ability of a system to create new structures and paths of operation in response to changed external conditions [60].Consequently, an evaluation of a construction project in the context of increasing resilience or adapting to climate change should consider development both before and after construction.An attempt was also made to compare the findings of this study with the results obtained by other researchers dealing with housing estate green areas.
Bradecki and Twardoch [41] adopted an architectural and urban planning approach to analyze 41 residential projects (including single-family housing) in the Metropolitan Association of Upper Silesia for the years 2000-2011 (with an area of 0.08-4.4ha).The study method enabled a simultaneous description of the intensity rates, as well as an overview of a given estate.The research team, led by Battisti [20], used a socio-environmental approach in their work.The study focused on housing estate green areas in estates with unfavorable social situations.In 2017 and 2018, a total of 32 plots in eight research areas in Berlin were examined.Housing estates (with areas ranging from 9.6 ha to 35.5 ha) were constructed during various periods, ranging from the 1920s to the 1980s.The study focused on a variety of tree species, considering their air-cleaning potential and allergenicity, as well as on the presence of elements of landscaping, including the use of pro-ecological solutions.
Szulczewska et al. [40] applied an urban planning approach using ecological and spatial indicators.A total of 18 Warsaw housing estates (with an area of 5-7 ha) erected from 2007 to 2009 were examined.These were selected based on their varied values (ranging from about 20% to nearly 70%) of the Polish ecological-and-spatial indicator known as the Ratio of Biologically Vital Area (RBVA).This study aims to determine the minimum percentage of green areas needed to maintain good environmental conditions.
The results of scientific research have been compared on several levels: (A) Bardecki and Twardoch (2013) recorded the presence of pro-ecological solutions in only 5% of estates.In Berlin, only one bioswale was found (12.5%), and there were no fountains or ponds.Regarding the housing estates in Wrocław covered in this study, fountains and ponds were present in 4 of them (6%), while green roofs were present in 20 estates (30%).In total, pro-ecological solutions (increasing retention by creating green roofs or ponds and fountains) were adopted on nearly 35% of the estates (complex No. 48 has both green roofs and a fountain).In this regard, the obtained percentages were higher than those determined in other studies.
(B) Bardecki and Twardoch [41] established that 26% of the studied residential areas had no trees.Elsewhere, the vegetation principally consisted of plants that had been growing before construction started.A similar situation occurred in the estates examined in Wrocław, even though there were no trees at all in 16.7% of the areas examined.However, another 22.7% of multi-family residential complexes had only 1-10 tree specimens.The studies conducted by Szulczewska et al. [42] in Warsaw had more positive results because none of the studied areas had trees.However, in the case of three estates (16.7%),only single young trees were discovered, with no mature trees present.The analyses carried out in Berlin [20] provided the most satisfactory and detailed results.Not only was there no plot without trees on it, but all of them had from 12 to 32 tree species growing on them (a total of 523 trees).
(C) Data relating to legal aspects were included in one publication.Only 40% of the building complexes investigated by Bardecki and Twardoch [41] were built with an LDP in force.In this respect, Wrocław's examined estates are in a much better situation because 83% of them were created based on an LDP.However, this result is still insufficient to ensure a consistent improvement in the city's resilience.
The results indicate a slightly smaller intensification of adverse phenomena in the shaping of housing estates.This may be caused by the steadily improving standards of designing residential complexes (in extreme cases, the difference in construction dates may amount to 12 years).Interest in using pro-ecological solutions is on the rise, albeit on a small scale.The conclusions remain very similar, so the problems are the same, e.g., an insufficient area of greenery, especially tall greenery, or the erection of housing estates on land without an LDP.However, one should bear in mind the differences in time, place and scope between the individual studies, as well as the examined areas.The conclusions of this study are consistent with the findings and recommendations of Szulczewska et al. [42] and Bardecki.Analysis of the performed vegetation clearings and an estimation of the number of trees left on the housing estates covered by the study have shown that the problem of declining green spaces in the city is still relevant.Regarding the lack of trees, the result was better than in the case of estates in Gliwice but worse than those in Warsaw or Berlin.The results of the examination of the initial legal conditions are more optimistic.Most of Wrocław's housing estates were created in accordance with the relevant LDPs in force, unlike those in Gliwice in 2013.This indicates a positive trend in spatial planning.The fact that there is an LDP for a particular area does not mean that its provisions and level of detail are sufficient in terms of the protection and planning of urban greenery.
The COVID-19 pandemic has raised questions about future public space design.It has put a strain on many global cities and the various consequences thereof [61][62][63].The findings are directly in line with the latest studies on what today's post-pandemic cities should be, which support the idea of the 15-min cities "the engines of recovery," wherein residential neighborhoods are closely integrated with greenery [64,65].Moreover, this research's recommendations of introducing more trees and widely using pro-ecological solutions (such as green roofs, fountains or ponds) are a way to fight boredom in public spaces, which has been recognized by scientists as one of the solutions for mitigating the negative effects of social distancing and lockdown [61].Numerous publications have indicated that enormous opportunities have been created using NDVI analysis when examining urban green areas [3,23,51].Researchers emphasize that the Sentinel-2 satellites provide high-resolution images (10 × 10 m), which means that UGSs are depicted more accurately than in the case of the Landsat OLI satellite, such that various elements such as lawns, groups of trees and bushes are now more easily differentiated [53,55].The results obtained by Moreno et al. [53] in the town of Temuco indicate that the average NDVI amounts to a park (0.63) or town square (0.38).The average NDVI for Wrocław's studied housing estates for the years 2015-2020 was lower (0.249).Studies conducted in Sophia (Bulgaria) and Bratislava (Slovakia) showed that the adopted methodology could be used to detect urban greenery in family and apartment housing areas [66].However, it should be remembered that these studies covered entire cities, while the analyses carried out in Wrocław were restricted to specific selected plots.Furthermore, one of the objectives of the latter study was to determine the standards of the maintenance and conditions of vegetation for a selected period.The NDVI value was important, but the main focus was changes to such value in the years 2015-2020 in order to monitor the condition of housing estate green areas and the standards of their maintenance.This study's findings allow for the conclusion that to boost Wrocławs' resilience, it is highly recommended to implement close and continuous monitoring of the condition of green areas by means of NDVI and to introduce detailed provisions on the shaping of green estate areas in LDPs.Collectively, our results and recommendations concerning the shaping of residential greenery appear consistent with research into what today's cities should be after the pandemic [61][62][63][64][65].The suggested approach will help achieve sustainability goals and ensure equal access to high-quality greenery for local residents, thereby enhancing their well-being.The key findings being presented will contribute to the literature and practices by expanding the directions of methods monitoring the condition of residential areas insofar as applied landscape research and boosting urban resilience are concerned.Wrocław's problems are typical of modern housing estates; therefore, the same approach can be applied in other cities to improve the state of residential greenery, hence better urban resilience.
Certain limitations and challenges regarding this research must also be considered.Despite the better spatial resolution of Sentinel-2 imagery, an oversized measurement unit (10 × 10 m) can still be a problem.A significant problem is the limitation regarding the frequency of retrieving fully usable imagery.This may cause some deviation from the overall NDVI trend due to prolonged drought or rain, which we have not been able to identify.It must also be stressed that NDVI is a dynamic indicator influenced by both abiotic and biotic factors that are more sensitive, especially to urban ecosystems [59].

Conclusions
The study has shown that the role of housing estate green areas in increasing the resilience of cities has been marginalized.This is especially true with respect to the use of pro-ecological solutions (at only about 32%) and the number of trees on an estate (with no trees at all in nearly 17% of all estates).The provision of sufficient green areas in housing estates must be recognized as the starting point for sustaining healthy and resilient cities.Therefore, it is extremely important to take note of the vegetation that exists on a plot before starting a project and, if possible, to protect it.Considering the moderate condition of vegetation and poor standards of its maintenance, as confirmed by analyzing NDVIs, it must be pointed out that housing estate areas have great but insufficiently used potential in boosting cities' resilience.The worst results were noted in housing estates in the southern, most intensively developed part of Wrocław, where rapid urban residential land expansion has caused inequalities in terms of access to residential greenery.At the same time, some positive trends and improvements have been observed when compared with studies conducted in previous years, mainly regarding the existence of LDPs.The insufficient use of pro-ecological solutions and shortage of woody plants are very alarming, considering the challenges confronting Wrocław and many other European cities in connection with the COVID-19 pandemic and climate change.
The results from three estates (nearly 5%) indicated a decline in the city's resilience; 37 estates (56%) were neutral, and 26 (slightly over 39%) were positive in this respect.Due to the COVID-19 pandemic, the need for action has become even more urgent to ensure that the physical and mental health of city residents can be maintained.The key contributions of this work comprise recommendations for a more resilient, post-pandemic city design and better management of residential greenery.These require an inclusive multi-stakeholder approach with an emphasis on landscape architecture and sustainable urban planning, which includes the following:

•
Deliberate wide-ranging use of pro-ecological solutions by practitioners, with encouragement from local authorities; • Protecting (in the legal records of LDPs) existing vegetation (maintaining a balance between cleared vs. planted greenery during construction);

•
Ensuring that city dwellers have equal and easy access to high-quality greenery at their place of residence; • Implementing continuous monitoring of the condition of Wrocław's green areas using NDVI by local authorities; • Granting discounts and bonuses for high-quality, appropriate greenery at housing estates.
Considering this study's limitations, future research on urban resilience and residential greenery should focus on using an integrated, holistic approach.Combining NDVI with various indicators, e.g., NDMI (Normalized Difference Moisture Index), can determine ecosystem resiliency to drought.Future studies should build upon the latest insights from theorists and practitioners on the interface between COVID-19 and city design.Using socioenvironmental vulnerability mapping will detect socially disadvantaged neighborhoods exposed to environmental stressors that require the most urgent intervention.Furthermore, there is abundant room for further progress in improving the technical aspect by obtaining easy and extensive access to various satellite images or lower-altitude imagery (UAV/drones) with better spatial resolution.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/su141811317/s1. Table S1: Assessment of the studied housing estates and housing estate greenery in terms of increasing the resilience of Wrocław.

Figure 1 .
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Figure 1 .
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Figure 2 .
Figure 2. Stage I of the study (pre-construction)-the locations of multi-family housing estates in relation to the established 1-km-wide buffer zone along Wrocław's administrative boundaries were presented.The conditions, in the form of existing LDPs and the construction of floodplains, were considered.

Figure 2 .
Figure 2. Stage I of the study (pre-construction)-the locations of multi-family housing estates in relation to the established 1-km-wide buffer zone along Wrocław's administrative boundaries were presented.The conditions, in the form of existing LDPs and the construction of floodplains, were considered.

Figure 5 .
Figure 5. Variability of the vegetation clearance rate in the examined housing estates.

Figure 6 .
Figure 6.Stage II of the study (construction)-for each housing estate, the vegetation clearance rate, the number of trees, and the use of pro-ecological solutions were all determined.

Figure 5 .
Figure 5. Variability of the vegetation clearance rate in the examined housing estates.

Figure 5 .
Figure 5. Variability of the vegetation clearance rate in the examined housing estates.

Figure 6 .
Figure 6.Stage II of the study (construction)-for each housing estate, the vegetation clearance rate, the number of trees, and the use of pro-ecological solutions were all determined.

Figure 6 .
Figure 6.Stage II of the study (construction)-for each housing estate, the vegetation clearance rate, the number of trees, and the use of pro-ecological solutions were all determined.

Figure 8 .
Figure 8. Variability in the Normalized Difference Vegetation Index (NDVI) for housing estat 18 in the years 2015-2020.Key: green-urban forest (reference area Pilczycki Forest in Wroc red-town square with a non-permeable surface (reference area Wolności Square in Wroc blue-housing estate No. 18.

Figure 9 .
Figure 9. Variability of NDVI for housing estate No. 56 in the years 2015-2020.Key: green-u forest (reference area Pilczycki Forest in Wrocław), red-town square with a non-permeable s face (reference area Wolności Square in Wrocław), blue-housing estate No. 56.

Figure 8 . 1 Figure 8 .
Figure 8. Variability in the Normalized Difference Vegetation Index (NDVI) for housing estate No. 18 in the years 2015-2020.Key: green-urban forest (reference area Pilczycki Forest in Wrocław), red-town square with a non-permeable surface (reference area Wolności Square in Wrocław), blue-housing estate No. 18.

Figure 9 .
Figure 9. Variability of NDVI for housing estate No. 56 in the years 2015-2020.Key: greenforest (reference area Pilczycki Forest in Wrocław), red-town square with a non-permeable face (reference area Wolności Square in Wrocław), blue-housing estate No. 56.

Figure 9 .
Figure 9. Variability of NDVI for housing estate No. 56 in the years 2015-2020.Key: green-urban forest (reference area Pilczycki Forest in Wrocław), red-town square with a non-permeable surface (reference area Wolności Square in Wrocław), blue-housing estate No. 56.

Figure 10 .
Figure 10.Changes in NDVI for the examined housing estates in the years 2015-2020.

Figure 11 .
Figure 11.Stage III of the study (post-construction)-changes in NDVI for the examined ho estates in the years 2015-2020.

Figure 10 .Figure 10 .
Figure 10.Changes in NDVI for the examined housing estates in the years 2015-2020.

Figure 11 .
Figure 11.Stage III of the study (post-construction)-changes in NDVI for the examined hou estates in the years 2015-2020.

Figure 11 .
Figure 11.Stage III of the study (post-construction)-changes in NDVI for the examined housing estates in the years 2015-2020.

Figure 12 .
Figure 12. Results of the final assessment of the impact of the studied housing estates on Wrocław's resilience in terms of UGSs.

Figure 12 .
Figure 12. Results of the final assessment of the impact of the studied housing estates on Wrocław's resilience in terms of UGSs.

Author Contributions:
Conceptualization, J.D.; methodology, J.D. and A.N.; software, J.D. and A.N.; validation, R.K. and J.D.; formal analysis, J.D. and A.N.; investigation, J.D. and A.N.; resources, J.D. and A.N.; data curation, J.D. and A.N.; writing-original draft preparation, J.D.; writing-review and editing, R.K., A.N. and M.Z.; visualization, J.D. and A.N.; supervision, R.K. and M.Z.; project administration, J.D.; funding acquisition, R.K. and J.D. All authors have read and agreed to the published version of the manuscript.Funding: The Article Processing Charge (APC) was co-financed by Wrocław University of Environmental and Life Sciences.Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.
Eurocities, Compact of Mayors, Global Climate Action (originally known as Non-state Actor Zone for Climate Action (NAZCA)), Covenant of Mayors for Climate & Energy and Local Governments for Sustainability.Wrocław faces many challenges typical of modern cities [43]: