The Impact of Anthropopressure on the Health Condition of Ancient Roadside Trees for a Sustainable City: Example of the Silver Maples (Acer saccharinum L.) Alley in Łódź (Central Poland)

Abstract

This pilot study aims to evaluate the state of the natural environment in the Silver Maples Alley (SMA) in Łódź, Poland, by using interdisciplinary research methods combining landscape architecture and environmental microbiology. The research focuses on the ecological condition of the trees in SMA, a historical monument consisting of about 100 century-old silver maples (Acer saccharinum L.). As part of the analysis, the study examines the area’s soil properties, microbiological composition, and air quality, providing a comprehensive approach to assessing environmental quality. Microbial analyses were conducted to determine soil pH, the presence of polycyclic aromatic hydrocarbons (PAHs), and the activity of Bacillus bacteria that produce biosurfactants for pollutant degradation. The results were compared with control sites with different Air Quality Index (AQI) values, including a park, a rural area, and a revitalized urban space. The findings support the hypothesis that environmental cleanliness correlates with the presence of pollutant-degrading microorganisms, particularly in areas with better air quality. This research contributes to understanding the role of green infrastructure, particularly old tree alleys, in urban ecosystems and public health. It also provides valuable insights into future management practices for historical green spaces. It highlights the need for interdisciplinary collaboration between landscape architecture, microbiology, and environmental sciences to address pressing sustainable development challenges.

1. Introduction

The tradition of planting trees along roads in open landscapes dates to the 16th century in Poland. Initially, these were primarily private roads connecting estates, manors, and farms. By the 17th and 18th centuries, avenues became a characteristic landscape feature throughout Europe, often monumental and representative, following French prototypes [1,2]. Consequently, from the mid-18th century, public roads in Poland also began to be lined with trees, and by the 19th century, this practice had become an established rule, shaping the modern landscape [3].

By the late 19th century, ornamental tree planting along roads was widely promoted in partitioned Polish territories. These trees were valued not only for their aesthetic appeal but also for their ecological benefits. Roadside afforestation served multiple functions: reducing soil erosion, providing habitats for various species, and enhancing scenic beauty. Promoting sustainable urban alleys reinforced these ecological benefits and encouraged further tree-planting initiatives [4,5].

In the southern region of Łódź, within the Mazovian Lowlands, old avenues and linear roadside trees continue to play essential spatial functions. They enrich the physiognomy of lowland areas, alleviating monotony while holding historical and cultural significance. Avenues often remain the only visible “[…] remnants of native nature, relics of the rural landscape” [5].

Since the mid-20th century, roads once characterized by tree-lined routes with low traffic intensity have transformed into busy transportation corridors. With increased motorization, modern road infrastructure development has placed aging roadside trees—including ancient avenue specimens—under considerable stress [2,6,7,8,9]. Their ability to adapt to rapidly changing environmental conditions is limited [9,10].

Anthropogenic pressure introduces numerous stress factors that are often irreversible for older trees. Earthworks, road modernization, surface expansion, underground infrastructure installation, vibrations from traffic, pollution, and soil degradation cause significant damage to both above-ground and below-ground tree structures. This degradation contributes to a decline in growth and, in extreme cases, leads to a phenomenon known as the “death spiral” [8,11].

Given these challenges, maintaining mature and aging trees—often referred to as “veteran trees”—along urban roads is of paramount importance. Examples from cities such as Seattle, Bristol, and London demonstrate that urban strategies emphasizing sustainable development and tree preservation improve residents’ quality of life by increasing biologically active spaces [11,12,13]. Indicators such as the Biotope Area Factor (BAF) and Green Space Factor (GF) recognize mature trees as valuable components of the urban environment [6,14].

Mature trees significantly impact roadside greenery due to their biomass and environmental functions. Large and old specimens (“veterans”) provide the greatest benefits among all trees. A single mature tree often equates to several dozen newly planted saplings in terms of ecological impact [6,15]. Notably, aging trees play a crucial role in biocenosis and phytoremediation. Tree rows and strips along roads act as natural barriers, limiting (though not entirely preventing) the spread of particulate matter (PM). The turbulence created by tree canopies enhances PM deposition on leaves and shoots, with the highest efficiency observed in trees located 3.0–15.0 m from traffic emission sources. Trees in this zone accumulate over 2.5 times more dust particles than those further from the road [16,17].

Moreover, older trees demonstrate significant capabilities in assimilating gaseous pollutants from traffic emissions, including nitrogen oxides (NOx), carbon monoxide (CO), sulfur dioxide (SO₂), and ozone (O₃) [17,18,19,20]. Air pollution poses severe health risks, particularly from heavy metals (cadmium, copper, lead, mercury, nickel, and zinc) and polycyclic aromatic hydrocarbons (PAHs). These pollutants are toxic, and some exhibit mutagenic and carcinogenic properties, threatening human health and urban ecosystems. Unlike organic pollutants, heavy metals do not degrade, contributing to long-term environmental burdens. Chronic exposure to air pollution is associated with numerous health problems, including respiratory diseases and cardiovascular conditions such as atherosclerosis [13]. Given these concerns, this study analyzes such pollutants in a more extensive range.

Beyond air quality improvements, old trees contribute significantly to biodiversity. Research indicates that in the UK alone, over 2000 invertebrate species depend on old tree habitats, underscoring their ecological importance [21]. Additionally, aging trees in urban environments provide crucial habitats for microbial life, further influencing biodiversity, public health, and landscape aesthetics [22,23].

The condition of roadside trees—especially those in advanced senescence—is primarily influenced by habitat and spatial conditions, such as soil degradation. Urban and rural roadside tree stands are anthropogenic in nature, making them vulnerable to degradation. These trees remain at risk despite legal protections, such as the designation of tree alleys as natural monuments. Historical parks, streets, and roads provide numerous examples where such protections have failed to prevent degradation [11,15,24,25].

Modern arboriculture draws upon the pioneering work of Alex Shigo (1930–2006), who emphasized stimulating trees’ natural defense mechanisms and improving habitat conditions to enhance longevity and health [26]. As urbanization accelerates and environmental degradation intensifies, proactive measures for protecting and caring for old trees are essential. These trees are not merely historical artifacts but are integral to urban ecosystems. Protecting historical vegetation in highly urbanized areas is crucial for maintaining cultural and ecological values. Therefore, research and continuous monitoring of old trees should be incorporated into urban planning to ensure their long-term survival and maximize their environmental benefits.

Urbanization often leads to unsustainable landscapes, where vegetation is lost despite its vital role in mitigating pollution. A sustainable urban landscape requires nature-based solutions that restore ecological balance and enhance ecosystem functions [27,28,29,30,31,32]. One example of such a solution is green infrastructure, particularly pedestrian corridors and tree-lined avenues along highways. A case in point is the Silver Maples Alley (SMA), discussed in this paper, which highlights the urgent need for both scientific research and government intervention to address the health risks associated with aging alley trees and the presence of urban pollutants such as heavy metals, PM, and PAHs.

This pilot study aims to assess the condition of roadside tree habitats in the Silver Maples Alley (SMA) in Łódź using an interdisciplinary approach that integrates various research methods. By combining legal analysis, health assessments of street trees, soil quality evaluation (including pH and polycyclic aromatic hydrocarbons, PAHs), and microbiological studies of Bacillus bacteria with their biosurfactant-producing properties, we seek to gain a comprehensive understanding of the environmental status of urban greenery. Additionally, air quality analysis (Air Quality Index, AQI) will be used to verify the relationship between environmental pollution and the state of tree habitats. The findings will contribute to sustainable urban planning and developing conservation strategies for aging roadside trees.

Hypothesis

It is assumed that microbiological analysis of tissue samples provides a broader insight into the environmental quality of the Silver Maples Alley (SMA) in Łódź, complementing observations made using classical environmental protection and landscape architecture research methods. In such cases, the evidence of environmental degradation is reflected in microbiological studies and air quality analysis (Air Quality Index, AQI, presented in

Table 1. Average number of hemolytic strains per soil sample.

Area of Soil Sample Collecting	nS	nHS	nHS/nS
Silver Maple Alley (SMA)—case study	38	76	2.00
Wiedźmin Square (W)—control test no. 1	32	40	1.25
Kolnica (Kol)—control test no. 2	5	47	9.40
Kashubian Landscape Park (KAS)—control test no. 3	4	31	7.75
Assessing the strength of the relationship (linear correlation analysis)	r = 0.5731; p < 0.0001

Legend: nS—number of samples; nHS—number of hemolytic strains.

and Table S1), which corroborates these findings. Furthermore, the assumption can be made that an increase in environmental cleanliness correlates with a higher proportion of Bacillus bacteria producing biosurfactants capable of degrading contaminants.

2. Materials and Methods

2.1. Case Study

The Silver Maples Alley (SMA) monument is located along National Road No. 71, extending from the roundabout at Okólna Street in Łódź to the village of Skotniki near Zgierz, in the Łódzkie Voivodeship, Central Poland. The monument comprises approximately 100 century-old trees. The avenue was initially marked and planted with maples in the 1920s. At its inception, it extended from Skotniki through Łagiewnicki Forest to the borders of Zgierz. Following the incorporation of this area into the administrative boundaries of Łódź in 1991, the avenue was granted legal protection [33]. Regulatory changes and administrative boundary adjustments affecting Zgierz, Łódź, and Łagiewniki led to modifications in the protected area’s delineation about a decade ago. Currently, the most valuable section of the alley, spanning approximately 2 km, is marked with red information boards indicating its protected status. This segment begins near the Łagiewniki Church and extends to the boundary between Łagiewnicki Forest and Skotniki (Figure 1).

This case study was selected due to the demographic challenges faced by the Łódź agglomeration, which, according to the Central Statistical Office (GUS), is currently experiencing the fastest population decline in the European Union. Given this trend, ensuring the development of green infrastructure, including preserving historical street trees such as the SMA, is particularly important. These avenues serve as historical relics and contribute to public health and well-being. Green spaces play a crucial role in urban environments by promoting psychological well-being and potentially supporting demographic recovery, aligning with the broader EU’s sustainable development goals.

2.2. Materials

The research utilized the Map of Tree Crowns TM [35], a QGIS database containing a dendrological inventory of the Silver Maples Alley (SMA) from 2017 to 2018 [36], as well as legal resolutions and regulatory documents detailing tree parameters within the SMA between 1991 and 2021 [37,38,39,40].

2.3. Methods

The study utilized an interdisciplinary, mixed-method approach [41,42], combining landscape architecture and environmental microbiology, as outlined in the research framework (Figure 2), which we present below.

• Step 1: Literature query

The first step involved an analysis of the legal protection of the SMA trees based on local legislation, documentation reports, and a literature review on the historical and current importance and functions of avenue trees and silver maple species’ ecology and morphology characteristics.

• Step 2: Fieldwork

The next phase included fieldwork, consisting of a site visit (period May–July 2022) and an assessment of the SMA trees’ current age estimates based on maples (Acer sp.) age Majdecki’s tables [2] and tree breasts dendrological inventory database [36], health condition, and photographic documentation. Due to the poor health condition of the old trees, it was decided to examine the soil properties, as soil quality is fundamental for tree life. For this reason, soil (0.05 m depth) and decayed wood (0.01 m depth) samples were taken for microbiological analysis.

• Step 3: Laboratory studies

The laboratory phase focused on analyzing the soil properties, including the determination of pH and the presence of polycyclic aromatic hydrocarbons (PAHs), as well as identifying microorganisms that indicate good environmental quality—Bacillus bacteria known for producing biosurfactants (biologically active substances) that help in pollutant degradation (PAHs). To analyze the soil, 2 g of soil was used in the microbiological tests to measure the pH, and 5 mL of distilled water was added (at a ratio of 1:2.5 m/v). The mixture was stirred for 1 min and left to stand for 20 min, after which the pH value was measured using a Mettler Toledo pH meter. The surface activity of the biosurfactants produced by the Bacillus bacteria was assessed using the Drop Collapse Test (DCT). For this test, 10 μL of the culture supernatant was dropped onto a polypropylene plate and left to dry. The diameters of the dried drops were measured and compared to the positive control (a 5% water solution of sodium dodecyl sulfate) and the negative control (LB medium). Biosurfactants were isolated using the QuEChERS technique, as described by Paraszkiewicz [43] and Jarecki [44]. In brief, 5 mL of the post-culture supernatant was mixed with 5 mL of distilled water, then 10 mL of acetonitrile (ACN) was added, and the mixture was stirred for 2 min. Afterward, a salt mixture was added (0.25 g of disodium hydrogen citrate sesquihydrate, 0.5 g of sodium chloride, 0.5 g of trisodium citrate dihydrate, 2 g of magnesium sulfate anhydrous), and the sample was remixed for another 2 min. The mixture was centrifuged for 5 min at 5000× g, and the organic phase was collected into a separate test tube. A second 10 mL of ACN was added, and the extraction process was repeated. The biosurfactants were identified using MALDI-TOF/TOF-MS (AB SCIEX 5800 TOF/TOF, Sciex, Framingham, MA, USA) according to the method described by Bernat et al. (2016) [45]. Equal parts of the methanol-diluted organic phase and the matrix solution (10 mg/mL DHB solution in ACN) were mixed and placed onto the MALDI plate. The MALDI-TOF/TOF analysis was performed in positive ionization mode in the m/z range 900–2000.

• Step 4: Cameral studies

To assess the environmental quality of the SMA area, the collected samples were compared with control material from locations (control tests No. 1–3) with different Air Quality Index (AQI) values [46]. These included Kaszubski Landscape Park (KAS) in Pomorskie Voivodeship, Kolnica (Kol) in Wielkopolskie Voivodeship, and Wiedźmin Square (W) in the center of Łódź, which had undergone natural revitalization. The selection of these control sites was based not only on the good air quality observed in the Główny Inspektorat Ochrony Środowiska (GIOŚ) maps [47], considering primary air pollutants (PM2.5, PM10, benzo(a)pyrene, as detailed in Figure 3A–C and Table S1), but also on the availability of biological material (including Bacillus strains) from these sites at the Department of Industrial Microbiology and Biotechnology at the University of Łódź.

Then, the statistical test was carried out to show significant correlations between selected stains and AQI ranges. Table 1 presents the number of samples analyzed (nS) and the number of isolated hemolytic strains (nHS). The average hemolytic Bacillus strains in one analyzed soil sample (n HS/nS) were estimated. The obtained number was interpreted as a manifestation of the hemolytic strains in each studied area. The results indicate that in regions characterized by a high degree of anthropopression (SMA case study and W control test No.1), the average number of hemolytic strains is lower than in areas with a lower degree of anthropopression (Kol—control test No. 2, and KAS control test No. 3). To determine the strength of the relationship between a certain state of environmental pollution and the number of hemolytic strains, a linear correlation analysis was performed. The Statistica 13.0 package was used [48]. The result indicates that the relationship is positive and statistically significant, and its strength, according to the Guilford scale, should be described as moderate [49]. However, using the classification according to Stanisz [50], it can be interpreted as high.

Next, it was checked what percentage of the total number of hemolytic strains isolated in the tested samples were biosurfactant overproducers (nOBs) and what percentage of all hemolytic strains were Bacillus strains producing biosurfactants. The results of this analysis are presented in Tables S2 and S3. To assess the relationship between the number of surfactant overproducers recorded in the samples and the number of Bacillus strains, the studied sites were divided into 3 groups (1—sites with a high degree of contamination; 2—medium degree of contamination; 3—low degree of contamination). Spearman correlation analysis was then carried out.

• Step 5: Synthesis and conclusions

The final step in the research involved synthesizing the results (5a) and drawing conclusions (5b) on future directions for developing and managing SMA and other similar sites worldwide. The study also outlined potential interdisciplinary research opportunities within landscape architecture, green microbiology, and environmental sciences and discussed collaboration possibilities with stakeholders and local authorities.

3. Results

3.1. Legal Protection of the Alley

The Silver Maples Alley (SMA) in Łódź was designated a multi-object natural monument in 1991, referring to formal regulations and resolutions [37,38,47,51]. Initially, the City Council of Łódź included SMA among natural monuments, with the number of protected trees increasing from 248 to 300 in 2014 due to administrative changes [43].

Over the following years, trees gradually declined due to health deterioration, damage, and extreme weather events, including Hurricane Eunice and intense precipitation. The initial SMA tree stand of 300 old specimens had dropped by 2018 to 278, decreasing to 275 in 2021 and 271 in 2023 [37,38,39,40,51]. Figure 4 illustrates tree locations, inventory numbers, and characteristics, as documented in Annex 2 to resolution [38]. Due to the vastness of the alley, we decided to measure only the western part of the SMA because we observed the worst condition of these trees or their absence in the alley row.

3.2. Silver Maple Tree—Species’ Ecology and Morphology Properties

Silver Maple (Acer saccharinum L.) is a tree native to the eastern part of North America, predominantly found in riparian forests along river valleys and floodplains. Rapid growth and high ornamental value, including early spring development, nectar-producing flowers, attractive autumn foliage coloration, and a picturesque habit, characterize the tree. For these reasons, it has been used in urban green spaces since ancient times, for example, in 19th-century post-industrial cities (Warsaw, Łódź, Zgierz) in Central Poland. This species exhibits a broad range of habitat requirements. It prefers fertile and moist soils but can grow in dry soils, though it tends to decline prematurely under such conditions. It thrives best in sunny locations but can also tolerate partial shade. The species demonstrates high frost resistance. It is quite resistant to air pollution caused by smoke and dust. Due to its morphological characteristics—brittle and fragile branches and softwood easily affected by fungi—it is susceptible to strong winds. The impact of habitat conditions is evident in the tree’s growth form. A low soil moisture level affects silver maples by limiting trunk thickness growth and significantly deforming the tree crowns. Specimens with conical crowns (e.g., Acer saccharinum L. “Pyramidale”) gradually adopt a completely different silhouette, transforming into trees with broad, spreading crowns. For this reason, it is not currently used in alley plantings along streets and roads [15,23,52].

3.3. Bacillus Bacteria and Their Role in the Environment

A unique attribute of bacteria of the Bacillus group chosen for the research is the ability to survive in environments characterized by extreme values of temperature, pH, and radiation, low availability of water, and/or the presence of toxins. This trait is due to the ability to produce survivors. Bacillus bacteria are classified as psychophiles, mesophiles, or thermophiles, depending on the species and strain. These microbes mostly tolerate up to 10% NaCl concentration in the environment. Vegetative cells have low nutritional requirements, so Bacillus cultures can be carried out using soils containing only one organic compound. Thanks to a highly efficient protein secretion and synthesis system and the ability to produce various metabolites of commercial importance, including biosurfactants, which are important due to the elimination of environmental pollutants of SMA, Bacillus strains have wide applications in various industries. The ability of bacteria of the Bacillus group to synthesize compounds with antifungal effects makes these microbes a promising research model used in environmental microbiology research. Still, as in our interdisciplinary field of urban ecology and landscape architecture, they have not yet been used. As mentioned above, Bacillus bacteria are one of the best-known microbiological biosurfactants. One of the key challenges in the application of biological plant protection agents is their interaction with environmental contaminants, such as residues of synthetic pesticides. Environmental contaminants include azo dyes, used primarily in fabric dyeing processes, and PAHs or heavy metals, which are visible in SMA surroundings due to GIOŚ reports and noted pollution [46,47]. Many azo dyes and their breakdown products show high toxicity to animals, plants, and humans. These contaminants can significantly impact soil microbiota collection during research, including beneficial microorganisms and biological plant protection agents introduced to the soil, making the city sustainable thanks to the green microbiology approach.

3.4. Current Condition of the Alley

Remote sensing analysis by a DJI drone operator. The movie shows that the alley is incomplete [33], with several gaps caused by fallen trees and road reconstruction. An inventory conducted in 2018 and updated in 2024, combined with species age tables by Majdecki [2] and a local database [36], estimates the trees’ age at 100–120 years. Their mean basic parameters range and are as follows: 18–20 m of crown width, 242–404 cm of trunk circumference (at 1.3 m height), and 17–23 m of total height. The validity of these measurements and estimates is confirmed by the information about the avenue’s construction in 1918 on the day Poland regained its independence. The protection of SMA species is nature protection, not conservation protection, as we will strongly emphasize in the corrected text. The trees are protected because they are old, and the city authorities decided in the 1990s that they needed to protect them because they have historical natural values. This was influenced by the creation of forms of protection of the Łódź agglomeration, such as the Łódz Landscape Park (Park Krajobrazowy Wzniesień Łódzkich) or two nature reserves in the territory of Łódź. However, in the 30 years before that, the trees were not taken care of properly, which led to their progressive degradation against the background of road transport development, the road infrastructure’s modernization, and the increase in pollution. In the past, the silver maple species was very popular, especially in the 19th century, in urban areas and essential transit routes [27,53,54].

Health assessments indicate that 54% of the trees are in deplorable condition, 42% are in poor condition, and only 4% are in satisfactory condition. Common issues include outgrowths (13%), branch breakage (11%), limb decay due to improper pruning (17%), guide die-off (16%), dry trees (8%), semiparasitic worm infestations (12%), and fungal infections (23%), as shown in Figure 5 and Figure 6A–F.

3.5. Extended Tree Health Research

To better understand the relationship between environmental quality and SMA tree health, additional research was conducted on green microbiology and air quality (AQI). Soil samples (38) were collected near 14 tree species in the western part of SMA, near Łódź’s boundary with Zgierz (Figure 7). The study focused on declining trees exhibiting emaciation and visible pathogens [36].

Table 2. Summary of microbiological analysis results for samples taken from Silver Maples Alley (SMA) in Łódź against locations with healthier environments (W, Kol, KAS) for Bacillus bacteria, AQI, and pH soil [43].

Estimated Parameter	Case Study SMA	Control Test No. 1 W	Control Test No. 2 Kol	Control Test No. 3 KAS
Number of soil samples	38	32	5	4
The number of bacteria in 1 g of soil	1 × 104–1 × 108	1 × 105–3.6 × 107	2.4 × 109–4.8 × 1011	3.2 × 109–1.5 × 1012
Soil pH range	6.5–7.5	4.5–6.5	5.7–7.1	5.2–5.7
AQI range 1 (Air Quality Index)	101–150	51–100	0–50	0–50
Number and percentage of Bacillus strains producing biosurfactants	0 [0%]	5 [2.5%]	39 [75%]	22 [71%]
PAHs content range [μg/g dry weight]	0.05–2.7	26.39	0.04–0.19	0.03–0.24

Legend: SMA—Silver Maples Alley; W—an area of the city square in the center of Łódź (Wiedźmin Square); Kol—rural area Kolnica village (Wielkopolskie Voivodeship province); KAS—Kaszubski Park Krajobrazowy (Pomorskie Voivodeship). ¹ The range of pollutants is presented in Table S1.

presents microbiological analysis results, including Bacillus bacteria presence, soil pH, and AQI on the foreground control test of Kolnica in Wielkopolskie Voivodeship in Turek province (Kol), and the area of Kaszubski Park Krajobrazowy in Pomerania—KAS (Pomorskie Voivodeship).

According to the microbiological results (Table 2), no strains (0, 0%) of Bacillus bacteria were isolated for SMA, while bacterial growth was proportional to air cleanliness in areas with better environmental health. The SMA data were compared in a control test with locations featuring better ecological characteristics. These included areas in the Łódź city center (W—Wiedźmin Square), which have better air quality parameters (AQI from 51 to 100) than the analyzed SMA (AQI from 101 to 150) due to restrictions on car traffic and the use of low-emission transportation. The healthiest environments were rural areas, where the most significant presence of Bacillus was observed in Kolnica in Wielkopolskie Voivodeship, Turek province (Kol), and the Kaszubski Park Krajobrazowy in Pomerania (KAS), with their share of 35 [75%] and 22 [71%], respectively. These areas experience minimal pollution emissions (AQI from 0 to 50) and virtually no transit traffic. Correlations illustrating the relationship between air quality (AQI) and the occurrence of Bacillus strains are presented in Figure 8.

Route 71 is characterized by constant transit traffic, with a significant polycyclic aromatic hydrocarbons (PAHs). Laboratory analysis identified the presence of compounds such as benzo(a)pyrene, benzo(a)anthracene, benzo(a)fluoranthene, and fentanyl. The percentage content of these compounds in the samples ranged from 17% to 94%. In four soil samples (Nos. 9, 22, 225, and 298), benzo(a)anthracene accounted for more than 90% of all detected hydrocarbons. The results also indicated that fentanyl was the second most common contaminant after benzo(a)anthracene. Of the 13 tested soil samples, only three (Nos. 23, 267, and 298) did not contain fentanyl. In sample 260, benzo(a)pyrene was the dominant PAH, comprising 60% of the detected compounds. Similarly, benzo(a)fluoranthene was identified as the dominant PAH in other samples, with a concentration of 38%.

The highest concentration in each soil sample from the W area was benzo(a)anthracene. Additionally, fentanyl was found in the majority (69.2%) of the samples. No naphthalene, acenaphthylene, pyrene, or fluoranthene was detected. All soil samples from the Kol area contained benzo(a)fluoranthene and benzo(a)anthracene in 80% of cases. In Kolnica (Kol), the highest concentrations of benzo(a)pyrene and benzo(a)anthracene were detected. However, naphthalene, acenaphthylene, acenaphthene, pyrene, and indeno(1,2,3-c)pyrene were not detected in the Kol samples. Comparing results across locations, benzo(a)anthracene was the most common PAH pollutant.

In the KAS area, the highest PAH levels were found in samples taken along the main road, likely due to poor-quality car engines operating in this part of Poland. However, this did not significantly affect air or soil quality in the region. In soil samples 9 and 22, PAH content ranged from 2.0 to 2.7 μg/g dry weight. The lowest PAH content (0.05–0.16 μg/g dry weight) was observed in samples 23 and 298, taken from young replacement trees (Acer campestre L.). PAH levels in other SMA samples ranged from 0.32 to 0.96 μg/g dry weight. Comparatively, KAS and Kol soil samples exhibited very low PAH content, ranging from 0.03 to 0.24 μg/g and 0.04 to 0.19 μg/g dry weight, respectively, suggesting that Bacillus strains contributed to pollution reduction. The highest PAH content (26.39 μg/g dry weight) was found in a sample from the W area, likely due to the accumulation of these compounds since the 1950s and their limited decomposition by the small number (5, 2.5%) of Bacillus bacteria strains present in anthropogenic soils of Łódź city center. Notably, the KAS samples showed a very high bacterial content, suggesting that the soil in this area is conducive to bacterial growth and beneficial for tree development.

The increased pollutant content in SMA compared to Kol and KAS suggests that biosurfactants do not effectively reduce pollution. This aligns with existing literature and research methodologies in green microbiology, indicating that bacteria-rich soils contribute to pollutant reduction. Soil pH analysis showed that in most samples from SMA and Kol, pH values were close to neutral (pH = 7.0). Meanwhile, samples from KAS and W exhibited lower pH values (<7.0), likely due to the presence of natural rather than anthropogenic soils.

The study revealed that SMA has a lower microbial population compared to environmentally healthier locations such as Kaszubski Park Krajobrazowy (KAS), the rural Kolnica area (Kol), and Wiedźmin Square (W) in Łódź after a natural revitalization process. Table 1 shows that microorganisms were significantly higher in these locations. The results further indicate that soil samples from the Łódź agglomeration (SMA) contain fewer bacteria and Bacillus strains than rural soils. This suggests that the soil in urbanized areas has limited ecological value and is not conducive to tree growth. Moreover, poor environmental quality in SMA is evidenced by high PAH concentrations, particularly benzopyrene, which has been found to reach dominant levels. These pollutants further weaken street trees, contributing to their deteriorating health.

3.6. Statistical Analysis of Soil Samples Analyzed for the Presence of Bacillus-Producing Biosurfactants

Considering the results of the presence of biosurfactants of Bacillus strains, it can be seen that in the case of areas with a low degree of anthropopression, the total number of isolated strains is lower than in the case of degraded areas and those subjected to anthropopression, but at the same time, the share of biosurfactant producers in their total number increases. Considering the known properties of biosurfactants produced by Bacillus strains, the next step was to compare which part of them, at each of the studied sites, produces surfactin, iturin, and fengycin. Next, it was checked which part of them produces biosurfactant systems (Table S2).

The results presented in Table S3 show that Bacillus strains isolated from samples taken from SMA do not produce the mentioned biosurfactants. In contrast, strains from the W area produce all of the analyzed biosurfactants. In the case of samples collected from areas with a lower degree of anthropopression, a slightly greater diversity of produced biosurfactants and their systems is visible. Thus, for samples collected from KAS and Kol, the percentage of strains producing surfactin is lower than SMA. All of the analyzed biosurfactants were produced by slightly over 70% of the strains isolated from soil samples from Kolnica (Kol). Still, surprisingly, this percentage is lower for samples from the Kashubian Landscape Park (KAS), which may result from the fact that one of them was collected at a distance of about 10 m from the transit road. At the same time, in samples from the KAS area, the percentage of strains producing the iturin and fengycin biosurfactant system is higher. Regarding the production of certain biosurfactants by Bacillus strains, the only statistically significant association was observed for fengycin (Table S4).

Based on the results, it can be concluded that as pollution decreases, the number of observed surfactant-producing strains increases statistically significantly. Similarly, the frequency of Bacillus strains increases significantly. We also used regression analysis in which the degree of contamination included in the number of surfactant overproducers and the number of Bacillus strains; the results obtained indicate that their occurrence depends on the degree of contamination (r = 0.9855; r2 = 0.94; p = 0.0288).

4. Discussion

4.1. Green Microbiology Research and Its Impact on Tree Protection

The microbiological studies conducted on the soil and moss around the Silver Maples Alley (SMA) in Łódź reveal a noticeable decline in microbial health, particularly with a lack of Bacillus species, which are key indicators of soil vitality and play a crucial role in nutrient absorption and pollutant breakdown [43,53,54,56]. When compared to data from cleaner rural environments, such as the Kaszubski Park Krajobrazowy (KAS), which exhibited a richer microbial flora, it becomes evident that urban trees like those in SMA face significant environmental stress. This aligns with findings in the literature, suggesting that urbanization leads to a decline in microbial diversity, negatively affecting soil fertility and tree health [28,29]. Furthermore, field observations and literature indicate that silver maple (Acer saccharinum L.) is unsuitable for planting along high-speed urban routes. However, due to its aesthetic appeal and historical significance—having been widely planted on urban roads in the 19th and 20th centuries and commemorating Poland’s independence anniversary of 1918—the decision was made to preserve existing specimens. Unfortunately, this has led to health and maintenance issues, as these trees pose safety risks. This raises concerns about whether such preservation efforts align with sound urban planning practices, as seen in cities like Leipzig, Berlin, or those in the UK [14,15,21,55].

Their health and safety risks compound the challenges of keeping trees old in urbanized landscapes. Trees in SMA, affected by environmental stress and pollution, have been implicated in traffic accidents, power line disruptions, and property damage due to uprooting during storms (Motorway No. 71). Continuous pruning of tree roots due to road infrastructure improvements, such as pavement construction, further weakens the trees, leading to breakages and eventual removal. This necessitates replacing fallen specimens with more resilient species and cultivars such as field maple (Acer campestre L.) [43]. The newly planted field maples are already visible in the reproduction of SMA (Figure 9). Given the poor wood properties, short lifespan, and susceptibility to pollution of silver maples, their continued use in urban streetscapes is not recommended, particularly in areas with high PM2.5 and PM10 dust levels, which contribute to respiratory diseases and hinder sustainable urban development [15] (Table S1). Microbiological research conducted by the Department of Microbiology at the University of Łódź in 2020 and 2021 confirmed this [43,53,54,56]. Additionally, statistical air quality studies from the local database [36] characterized this region with an AQI index of 50–101 from 2020 to 2023 [16]. In Polish urban contexts, where winter conditions, traditional heating methods, and aging vehicle fleets contribute to pollution, an AQI above 100 harms health, particularly for vulnerable populations. The highest AQI values were observed in windless autumn–winter periods, with improvements in spring–summer when vegetation was more active. Poor air quality is closely linked to soil degradation, emphasizing the need for innovative remediation strategies to improve urban environmental conditions, including biosurfactants produced by Bacillus bacteria.

4.2. Reconstruction of the Avenue—Problem Analysis

The gradual reduction in tree numbers at SMA has slowed in recent years, potentially due to improved maintenance and reduced interference with road infrastructure since 2021. The last significant reductions occurred during the final round of infrastructure upgrades in 2021, with previous improvements completed between 2014 and 2018 (Figure 4). The most damaged trees were removed from the natural monument registry during this period. Historical care practices, including incorrect pruning methods, contributed to tree degradation, leading to fractures and decay [13,27,53]. Despite their historical and decorative value, many aging trees now interfere with local inhabitants’ daily lives, obstructing properties adjacent to the avenue and public transport routes (Figure 9). Given that further road widening or pavement construction would conflict with green infrastructure, a critical question remains: Should old trees be preserved at all costs, or should they be gradually replaced with species better suited to urban conditions? This replacement process began a decade ago, with new plantings of field maple (Acer campestre L.) proving more resilient in urban environments (Figure 10A,B).

The SMA case study illustrates the impact of anthropogenic pressures, including road development, on tree longevity and overall environmental degradation. Granting the alley legal protection does not guarantee its preservation, as proper care practices, such as sanitary pruning and continuous health monitoring, are essential. Without these measures, trees become vulnerable to breakage, decay, and infestations of semi-parasitic species like mistletoe. Once trees reach 100 years, it is recommended to gradually reduce their numbers and supplement them with younger, more resilient varieties (Figure 10). Given their resistance to drought and pollution, maples remain a suitable choice for street plantings, ensuring SMA’s future aesthetic and ecological value. However, newly planted specimens should be carefully selected for resilience against harsh urban conditions.

4.3. Environmental Research Using Green Microbiology

Environmental monitoring of SMA indicated poor air quality (AQI) and a low presence of Bacillus bacteria, which serve as bioindicators of soil health (Table S1). Road dust, a significant carrier of harmful pollutants, originates from natural and anthropogenic sources, including fuel combustion, vehicle emissions, tire wear, and brake pad friction. High-traffic areas, such as city centers, exhibit elevated carcinogenic polycyclic aromatic hydrocarbons (PAHs) concentrations.

The potential of Bacillus microbes in mitigating urban pollution is significant. The statistical analyses of collected samples (Tables S3 and S4) show that these bacteria produce biosurfactants (surfactin, iturin, and fengycin) capable of reducing harmful compounds in highly polluted soils (PM, NOx, O3, PAHs). Biosurfactants facilitate soil decontamination by breaking down pollutants, making them crucial for sustainable urban environments. Although microbial remediation has traditionally been considered costly, recent studies indicate that Bacillus strains from clean environments, such as Kolnica (Kol) or KAS, can be introduced into urban soils, offering a cost-effective and sustainable solution [43,54,56].

Despite the promise of bioremediation, challenges remain, particularly regarding the high costs associated with PAH removal [57,58,59,60]. Ongoing research in green microbiology aims to develop more affordable cultivation substrates, optimize biosurfactant separation methods, and identify new high-efficiency bacterial strains [27,61,62,63]. Cost reductions can also be achieved through repeated biosurfactant applications, leveraging their physicochemical stability across varying pH and temperature conditions. Biosurfactants derived from renewable sources, including coal and industrial waste, offer an environmentally friendly alternative for urban soil remediation. This method, although innovative, is not yet used, and its many advantages could be listed. It is not cost-intensive because it allows a ready-made soil collection from clean environments to be selected and transported to urban areas. Similar practices are currently carried out with garden soil sold in gardening stores and with the addition of fertilizers and microorganisms. It is certainly easier to develop than artificially producing strains of bacteria to improve soil properties using strictly laboratory methods, which is an economically unprofitable procedure.

Introducing Bacillus-enriched soil amendments into urban tree maintenance practices may enhance the survival rates of newly planted field maples (Acer campestre L.) specimens, particularly during the critical first year of development. Until recently, the economic viability of microbial remediation using Bacillus was questioned due to high strain acquisition costs. However, our microbiological studies confirm that naturally occurring Bacillus strains in rural rhizospheres can be transplanted into urban environments to restore soil health. This aligns with EU green zone standards and offers a sustainable model for preserving historic urban tree alleys in highly polluted European cities.

Applying Bacillus microbes in urban landscapes bridges multiple disciplines, including green microbiology, landscape architecture, and environmental protection. Our findings [13,43,54,56] demonstrate the feasibility of using biosurfactants for bioremediation, contributing to sustainable urban development and improved environmental quality.

4.4. Sustainable Maintenance and Treatment of Old Alley Trees

The Silver Maples Alley (SMA) in Łódź serves as a historical testimony to the spatial development from the late 19th century to modern times. It is closely linked to the expansion of surrounding estates and the territorial growth of Łódź, Zgierz, and Stryków (Łódź agglomeration). The Silver Maple tree is often considered a long-lived species, living around 100–150 years old. In contrast, Acer campestre L. is a native species of the Eurasian continent that can live up to 250–350 years. However, Acer saccharinum (L.) is a less resistant species to harsh urban conditions, and its health is further worsened by pollution and poor sanitation, as we have observed in the studies. In addition, with the age of the individuals, their health is worse than those living in a healthy environment, e.g., in the countryside—Kolnica (Kol) or Kashubian Landscape Park (KAS). Acer campestre is a recommended replacement species for this first species (Acer saccharinum L.) for cities in replacement alleys due to its better adaptive properties [11,15,23,48].

The preserved tree specimens make a significant impression; however, their maintenance leaves much to be desired regarding professionalism, scope of work, frequency of maintenance, and monitoring. Additionally, the trees are exposed to external factors such as traffic, pedestrian safety concerns, and the environmental conditions of the protected area.

SMA represents a characteristic green composition of transport corridors, offering aesthetic, spatial, and landscape value and natural benefits, including biological activity and oxygen production [13]. The alley falls into composite greenery, a monument integrated into the vegetation cycle and plant life. Regarding visual appeal, the alley is most striking in summer and autumn when the trees are covered with leaves. Conversely, winter and spring are more suitable for dendrological measurements, including assessments of tree trunk parameters, crown structure, health condition, and microbiological observations.

According to the microbiological studies presented in this paper, the soil surrounding these monument trees is deficient in organic compounds necessary for the healthy condition of mature maple trees. Therefore, it is recommended to supplement the soil mixture with nutrient-rich organic soil, such as horticultural soil or soil from healthy green areas (e.g., Kaszubski Park Krajobrazowy—KAS or Kolnica—Kol). Microbiological studies in these areas revealed a high content of Bacillus bacteria in the soil, correlating with good or very good tree health. Furthermore, air quality in these locations was also satisfactory, indicating favorable environmental conditions.

5. Conclusions

Preserving old roadside trees in urbanized areas presents challenges and opportunities for sustainable city planning. Our pilot study highlights the need for advanced environmental and microbiological research to assess tree health, improve soil conditions, and mitigate pollution. The findings emphasize that maintaining these trees requires an interdisciplinary approach integrating green microbiology, landscape architecture, and environmental protection strategies. Finally, the following conclusions are to be obtained:

• The importance of advanced environmental and microbiological research
  Long-term preservation of old roadside trees in urbanized areas depends on detailed environmental and microbiological studies. Monitoring soil conditions, microbial diversity, and pollution levels can provide essential data for targeted conservation strategies. Microbiological indicators, such as Bacillus strains, can help assess soil health and the potential for bioremediation in polluted areas.
• The role of Bacillus strains in soil health and tree growth
  Soil enriched with Bacillus strains can significantly support the growth of newly planted trees, especially during their early development stages. These bacteria produce biosurfactants that aid in breaking down harmful soil contaminants, such as polycyclic aromatic hydrocarbons (PAHs), reducing environmental stress and enhancing root system development.
• Ensuring the availability of healthy soil sources
  Using uncontaminated soil from naturally healthy areas should accompany new plantings to provide trees with the necessary microbial support. This approach improves survival rates and long-term adaptation in challenging urban conditions.
• The importance of interdisciplinary collaboration
  Protecting urban tree populations requires cooperation across various disciplines, including microbiology, environmental science, landscape architecture, and urban planning. Collaborative efforts between researchers, policymakers, and local communities are crucial for developing effective tree preservation and urban greening strategies. It is essential to monitor the environment and tree health of the SMA long term, supported by an interdisciplinary approach that started during this research as a pilot study.
• Future research directions and community involvement
  Further research should explore cost-effective ways of using Bacillus-based bioremediation techniques to improve soil quality and extend the lifespan of trees in urban environments. Engaging local communities in tree care and environmental monitoring will be essential for the success of future sustainability initiatives.
• Replacement of aging trees for sustainable urban development
  While old trees contribute to biodiversity and cultural heritage, their deteriorating condition and associated risks require careful management. Our study confirms that replacing declining Silver Maple (Acer saccharinum L.) trees with more resilient but similar species, such as field maple (Acer campestre L.), is a viable strategy for ensuring the long-term sustainability of urban greenery.
• Policy recommendations for sustainable urban planning
  The study underscores the need for urban policies across Poland and Europe in various locations, integrating microbiological advancements with tree management and environmental protection. Aligning scientific research with community needs will foster more sustainable and resilient urban landscapes.

By prioritizing environmental and microbiological research, cities can develop innovative strategies to preserve old roadside trees while ensuring urban sustainability, improved air quality, and long-term ecological balance.