The study, led by Daniel Niepsch of Manchester Metropolitan University and UBU Environmental Ltd, is the first to track tree performance over a full year in growing media derived from road sweeping waste. The researchers planted sixty silver birch (Betula pendula) whips into four waste-derived soil formulations and four commercially available comparator soils, including BS 3882 topsoil, BS 8601 subsoil, Amsterdam tree sand and an enhanced tree sand containing organic fines. By the end of the trial, all sixty trees were alive, and none showed visual signs of nutrient deficiency. Trees in the waste-derived blends were marginally taller on average than those in the commercial soils after eight and twelve months, with statistically significant differences recorded particularly against the subsoil comparator.

The raw material for these experimental soils comes from an unusual industrial source. A fleet of roughly 120 road sweepers collects street cleansing residues within a fifty-mile radius around Greater Manchester. At a physico-chemical treatment plant, the collected material is separated into wet and dry fractions and then washed, sieved and separated by density and attrition. Coarse aggregates larger than four millimetres and sand fractions between 0.075 and 4 millimetres can be reclaimed for construction and landscaping. What remains, a fine residue of silt and clay known as filter cake, currently has no commercial outlet and is typically landfilled. It is this fine fraction, rich in organic matter and plant nutrients, that the researchers identified as a potential soil ingredient.

From this waste-derived material, the team manufactured four test soils. The first was the pure filter cake used on its own. The second mimicked a Stockholm structural soil, blending reclaimed aggregates with 7.5 percent waste-derived material and 7.5 percent PAS100 certified compost. The third and fourth mimicked urban tree sands, one combining equal parts waste material and reclaimed sand, the other adding compost and a small proportion of biochar. Biochar, a carbon-rich product of thermal decomposition of organic matter, was included because it can improve nutrient and moisture retention, sequester carbon and adsorb potentially harmful elements, offering some of the benefits of clay without compromising drainage.

The physico-chemical analysis revealed both promise and caveats. The waste-derived soils showed higher pH, elevated carbon-to-nitrogen ratios and greater organic matter contents than the commercial comparators. Water holding capacity was notably high, and the pure waste material required the least supplementary irrigation of any soil in the trial, needing only 15 litres of additional water over twelve months compared with up to 55 litres for some control soils. However, concentrations of copper, zinc and nickel were approximately ten times higher in the waste-derived soils than in the controls, and zinc exceeded the BS 3882 phytotoxicity threshold of 300 milligrams per kilogram for alkaline soils. The researchers note that the alkaline pH of these materials limits the solubility and plant availability of such elements, and that soil respiration gradually lowers pH over time, so monitoring remains essential.

Tree health was assessed with a combination of destructive and non-destructive techniques. Height and trunk diameter were measured at planting and again after eight and twelve months. Chlorophyll was extracted from leaves in the laboratory and quantified by ultraviolet-visible spectroscopy, yielding total chlorophyll concentrations between 14.7 and 97.1 micrograms per square centimetre, ranges consistent with healthy silver birch reported in earlier studies. Leaf nitrogen contents, measured with an elemental analyser, ranged from 0.95 to 2.98 percent by weight and correlated significantly with chlorophyll concentrations, further indicating that the trees were well nourished regardless of soil type.

Particularly noteworthy is the study’s use of smartphone-based monitoring. By pressing a phone lens directly against a leaf with the flashlight illuminating it from behind, the researchers captured contact images free of background interference, focus variation and distance effects. From the red, green and blue pixel values they calculated chlorophyll estimates and a dark green colour index, a metric originally developed for turfgrass that reflects nitrogen status. These inexpensive, high-throughput measurements tracked the expected seasonal pattern, dipping during winter dormancy and recovering during spring growth, and proved suitable for assessing overall tree health without harming the plants. The approach could make long-term urban tree monitoring far more accessible to researchers and practitioners alike.

The trees were deployed across three environments to test real-world resilience. Twenty-four grew in one-thousand-litre intermediate bulk containers designed to mimic urban tree pits, placed at the treatment plant itself amid heavy traffic and machinery. Another twenty-four grew in thirty-litre bags at a Manchester tree nursery representing an urban background site, and twelve served as greenhouse controls with automated drip irrigation. Trees in the container-based urban setting grew significantly more than greenhouse trees, likely because of the far greater root space, while greenhouse conditions still conferred advantages over the nursery site. These results underline how container volume and environment, not just soil chemistry, shape seedling vigour.

Beyond tree growth, the researchers highlight the carbon implications. UK soils hold an estimated 9.8 billion tonnes of carbon, and soil degradation costs the country around 1.2 billion pounds each year. Digging up and transporting virgin topsoil disturbs soil organic carbon and can stimulate carbon dioxide emissions; one study on the Chinese Loess Plateau recorded a 33 percent increase in CO2 emissions from fields receiving imported topsoil. The waste-derived soils, with their elevated organic matter and carbon-to-nitrogen ratios above 25 to 1, suggest more stable soil organic carbon that could sequester carbon in urban landscapes while avoiding the emissions associated with soil displacement. The authors calculate that technosols of this kind have substantial per-hectare carbon and CO2 storage potential, although they caution that long-term measurements of soil CO2 fluxes and biomass gains are needed to complete the carbon budget.

The path from pilot trial to city streets still requires regulatory clearance. Under the UK’s Publicly Available Specification PAS 115, road sweeping and gully waste can be processed for use in soils, but any commercial product must also satisfy the End-of-Waste Criteria of the Waste Framework Directive, demonstrating that recycled materials meet quality and safety standards before being reclassified as products. The authors also stress that silver birch is relatively tolerant of metal contamination, immobilising metals through deep roots and partnerships with ectomycorrhizal fungi, so longer trials exceeding three years and tests with other common street trees such as lime, plane and sycamore are needed. If those hurdles are cleared, the implications are considerable: a waste stream currently burdening landfills could be transformed into tailored growing media for the urban tree-planting schemes that governments are increasingly mandating, closing a loop that benefits waste management, soil conservation, carbon storage and the canopies that cool and green our cities.

Subject of Research: Use of treated road sweeping residues as manufactured soils for urban tree growth

Article Title: Using recovered road sweepings in manufactured soils: a Betula pendula growth trial

Article References: Niepsch, D., Randviir, E., Murphy-Peers, R., Megson, D., Hackett, D., Stringer, P., & Coulthard, E. (2026). Using recovered road sweepings in manufactured soils: a Betula pendula growth trial. Plant and Soil. https://doi.org/10.1007/s11104-026-09175-2

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09175-2

Keywords: road sweepings, manufactured soils, technosols, Betula pendula, silver birch, urban trees, circular economy, soil degradation, carbon sequestration, chlorophyll monitoring, waste-derived material, urban greening

The research team, led by Jianhua Guo of the International Research Center of Big Data for Sustainable Development Goals and the Aerospace Information Research Institute of the Chinese Academy of Sciences, together with colleagues including Xiao Xiang Zhu of the Technical University of Munich, set out to address a stubborn gap in urban mental health research. Lab studies and small surveys have long suggested that exposure to greenery improves mood, but evidence linking tree cover to real-time emotional expression at the scale of entire nations has remained scarce. Social media, for all its noise, offers a rare window into how millions of people feel as they move through their daily environments.

Negative emotions are particularly important to track, the authors argue, because they spread rapidly through online networks. Previous work has shown that negative online news is shared more readily than positive content, and that emotional contagion dynamics amplified distress during the pandemic. If the built environment can dampen negativity at its source, urban forestry becomes not just an aesthetic or climate policy, but a public mental health intervention. The new study provides some of the most granular evidence yet for that proposition, quantifying the sentiment effects of canopy cover with a fixed-effects statistical framework that controls for meteorological, environmental and socioeconomic confounders.

The technical backbone of the analysis is a pair of deep-learning pipelines. First, the team mapped urban tree canopy across Brazil using high-resolution satellite images, building on their earlier nationwide canopy mapping work published in 2023 and their continent-wide South American assessment in 2024. Second, they harvested geotagged posts from Twitter, Instagram and Foursquare, applying multilingual sentiment analysis to translate each post into a sentiment score. The sentiment data drew on established approaches, including the multilingual universal sentence encoder for semantic retrieval and distant-supervision classifiers originally developed for Twitter sentiment classification. Because the posts carry location and time stamps, they could be aggregated to neighborhoods and days, then matched against tree canopy measured within 500-meter buffers around posting locations.

The headline result is strikingly precise: each 1-percentage-point increase in tree coverage was associated with a reduction of 0.028 in negative sentiment scores before the pandemic, and 0.042 during COVID-19, both statistically significant at P < 0.001. In other words, the association was nearly 1.5 times stronger during the pandemic than in the pre-pandemic period. The pattern aligns with prior findings that expressed sentiment shifted globally during COVID-19, and with studies showing that contact with blue-green spaces during lockdowns benefited mental health. But the new analysis goes further by measuring the specific ingredient of urban greenery that matters: trees themselves, mapped pixel by pixel from orbit.

Crucially, the relationship is not linear. The benefits of tree cover increased with canopy extent up to roughly 40 to 45 percent coverage, after which additional gains diminished. This saturating curve has immediate planning implications. A city moving from sparse canopy to a substantial urban forest stands to gain the most sentiment benefit, while pushing coverage beyond the saturation threshold yields progressively smaller emotional returns. The authors present these thresholds as evidence-based tree planning targets for urban mental health, framing the work as a contribution to Sustainable Development Goal 3.4, which concerns promoting mental health and well-being, and SDG 11.7, which calls for universal access to safe, inclusive and accessible green public spaces.

The team also probed how the sentiment benefits of trees vary across urban and climatic contexts, and the heterogeneity is instructive. The protective association was stronger in wealthier regions, echoing long-standing concerns about environmental inequity: affluent neighborhoods already tend to enjoy more canopy, and the analysis suggests they also extract more psychological value from it. Conversely, the benefits weakened under high temperature, high precipitation and high population density. Extreme heat and heavy rain may suppress the outdoor activity through which people actually experience trees, while dense crowds can dilute the restorative, stress-reducing qualities that quieter green settings provide. The findings resonate with earlier research linking green space to lower stress in deprived communities and with meta-analyses showing that tree traits cool urban heat islands, suggesting that trees’ mood benefits may be entangled with the thermal and social conditions in which they are encountered.

The fixed-effects modeling design deserves emphasis because it strengthens the causal interpretation. By comparing the same cities over time and across locations within them, while controlling for weather variables drawn from the ERA5 global reanalysis, impervious surface extent, nighttime lights as a proxy for economic activity, population density and municipal GDP per capita, the models strip away many alternative explanations for why greener places might appear happier. The team further bolstered the analysis with nonlinear curve-fitting techniques to characterize the dose-response relationship and heterogeneity analyses spanning climate zones and urban environments. All code is publicly available on GitHub, and the processed sentiment and control-variable datasets are archived on Figshare, allowing independent researchers to replicate and extend the results.

Still, the authors and observers of this literature caution about limits. Sentiment analysis of social media captures expressed emotion among platform users, not clinical diagnoses, and posting populations skew toward certain demographics. The study identifies an association, and while fixed effects and rich controls reduce confounding, they cannot fully rule out reverse causation or unobserved factors. Nonetheless, the consistency of the signal, its dose-response structure and its amplification during a documented mental health crisis make a compelling case that urban trees are doing measurable emotional work. The study joins a growing body of remote-sensing-based health research from the same group, including assessments of how residential greenness relates to long-term depression and anxiety risk, and how biodiversity-rich recreational areas near cities can serve as nature-based mental health solutions.

For city planners, the message is concrete. Trees are not merely decoration; they are infrastructure for psychological resilience, and their value becomes most apparent precisely when societies are under the greatest stress. The Brazilian dataset, spanning tropical metropolises and mid-sized cities over a five-year window that includes the pandemic, suggests a quantifiable planning rule: target canopy coverage in the 40 to 45 percent range where feasible, prioritize greening in lower-income districts that currently enjoy neither the canopy nor its benefits, and recognize that in hot, wet or densely built environments, trees may need to be paired with other design choices to deliver their full emotional dividend. As urban populations grow and climate change intensifies both heat and hardship, the case for treating every planted street tree as a small investment in collective mood has never been better quantified.

Subject of Research: Association between urban tree canopy coverage and reduced negative sentiment in social media posts across Brazilian cities, with stronger effects during COVID-19

Article Title: Urban trees associated with a reduction in negative sentiment with stronger links during COVID-19

Article References: Guo, J., Guo, H., Wang, J., Hong, D., Kruspe, A., & Zhu, X. X. (2026). Urban trees associated with a reduction in negative sentiment with stronger links during COVID-19. Nature Cities. https://doi.org/10.1038/s44284-026-00519-8

Image Credits: AI Generated

DOI: 10.1038/s44284-026-00519-8

Keywords: urban trees, tree canopy, negative sentiment, social media, mental health, COVID-19, remote sensing, Brazil, urban green space, sentiment analysis, Nature Cities, sustainable development goals

The study, published in Air Quality, Atmosphere & Health, examined five tree species growing under roadside conditions in Nagpur, a rapidly developing city in Maharashtra where traffic, construction and urban expansion are contributing to deteriorating air quality. Roadside vegetation is exposed to a complex mixture of stressors. Exhaust gases can enter leaves through stomata, the microscopic pores that regulate gas exchange, while particles settle on leaf surfaces and may block light or interfere with stomatal function. Heat, limited soil moisture and repeated physical disturbance add to the burden. The researchers therefore assessed not only how much particulate matter leaves retained, but also how their internal physiology responded.

To estimate pollution tolerance, the team measured several biochemical and physiological traits used to calculate APTI. These included total chlorophyll, leaf-extract pH, relative water content and ascorbic acid concentration. Chlorophyll is essential for photosynthesis, and a decline in its concentration can indicate damage to the photosynthetic apparatus or reduced pigment production under stress. Relative water content reflects the hydration state of leaf tissue and its ability to maintain cellular function. Leaf pH can influence enzyme activity and the plant’s response to acidic pollutants, while ascorbic acid acts as an antioxidant that helps neutralize reactive oxygen species generated by pollution. Together, these variables provide a composite picture of how well a plant may withstand a contaminated environment.

The measurements revealed that roadside exposure altered the trees’ physiology. A two-way analysis of variance found significant effects of both site and species on total chlorophyll, leaf pH and relative water content, with probabilities below 0.001. The significant site-by-species interactions for chlorophyll and pH were especially important: they indicate that species did not respond identically to the same roadside conditions. Pollution and associated environmental stress could therefore not be represented by a universal effect across the urban forest. Some trees maintained more stable physiological characteristics, whereas others showed changes consistent with greater stress.

Overall, APTI values fell under roadside conditions, suggesting that pollution reduced physiological resilience across the trees examined. Yet the decline was not uniform. Pongamia pinnata recorded the highest polluted-site APTI, with a value of 8.0, while Lagerstroemia speciosa had the lowest value, 6.4. These numbers should not be interpreted as a direct measure of pollutant removal or as a universal ranking for every city. APTI is an index derived from plant traits, and its meaning depends on local climate, pollution composition, soil conditions and the particular species being tested. In Nagpur, however, the results identify Pongamia as the strongest physiological candidate among the trees studied.

The dust measurements told a different story. The researchers quantified DCC using a gravimetric method, weighing the particulate material retained on leaf surfaces after collection and processing. Ficus hispida captured the greatest amount, holding 1.26 milligrams of dust per square centimetre of leaf area. Leaf texture, hairiness, waxiness, surface roughness, orientation and canopy structure can all affect this capacity. Rough or complex surfaces create more opportunities for particles to adhere, while dense foliage can slow air movement and encourage deposition. The result is a form of passive filtration: leaves intercept some airborne material, although wind, rain and leaf movement can later redistribute or remove it.

The contrast between Pongamia pinnata and Ficus hispida exposed a central complication in designing pollution-fighting vegetation. A weak negative correlation was observed between APTI and DCC, with a correlation coefficient of −0.242 and a probability value of 0.090. In practical terms, trees that were more tolerant according to their internal biochemical traits did not necessarily retain more dust, and the relationship was not statistically strong. Survival and particle capture are distinct biological functions. A tree may protect its photosynthetic machinery effectively while presenting a relatively smooth leaf surface, whereas another may collect substantial dust but experience greater physiological stress as a result.

To combine these different qualities, the researchers used the Anticipated Performance Index. API extends the interpretation of APTI by incorporating additional characteristics relevant to urban planting and greenbelt performance. Such composite assessments can account for traits including growth form, canopy structure, evergreen or deciduous behavior and other practical features, rather than relying on a single biochemical score. In this analysis, Pongamia pinnata achieved the highest API score, followed by Ficus hispida. Both were categorized as good performers, while Tabebuia rosea recorded the lowest API performance. The ranking reinforces the idea that urban tree selection should be based on multiple functions rather than visual appeal or availability alone.

The findings do not suggest that planting trees can replace emission controls. Leaves can intercept particles, but they do not eliminate the sources of nitrogen oxides, sulfur dioxide, carbon monoxide or fine particulate matter, and vegetation may have complex effects on airflow in narrow streets. Dense canopies can sometimes reduce pollutant dispersion if poorly designed, while maintenance activities, resuspension of deposited dust and seasonal changes can alter the net benefit. The study also measured dust retained on leaves rather than directly demonstrating reductions in human exposure or ambient PM2.5 concentrations. A tree covered in particles is not automatically proof that nearby residents are breathing cleaner air.

Instead, the research offers city planners a more precise way to build layered green infrastructure. Species with high physiological tolerance could be positioned where pollution and heat are most intense, reducing the likelihood that a roadside planting will rapidly decline. Species with strong dust-capture characteristics could be incorporated into buffer zones, traffic dividers or vegetation barriers, provided they can remain healthy under local conditions. Mixing species may also reduce vulnerability to pests, drought and disease while combining complementary functions. The authors emphasize that such decisions should be adapted to pollution-affected areas, rather than copied unchanged from one city to another.

Nagpur’s roadside trees could also serve as living monitors of environmental stress. Changes in chlorophyll, leaf pH, water status and antioxidant chemistry can reveal that plants are being affected before visible injury becomes obvious. Repeated measurements across seasons and traffic environments could help distinguish the effects of exhaust emissions from those of drought, heat or poor soil. The authors acknowledge that the study’s data are available on reasonable request and that further work will be needed to connect leaf-level measurements with pollutant concentrations, canopy-scale removal and public-health outcomes.

The broader message is that a city’s greenbelt is a biological system, not a decorative strip of uniform foliage. Urban forests work through a combination of chemistry, physiology and physical structure, and their performance changes with species and location. In Nagpur, Pongamia pinnata emerged as the most promising all-round candidate according to the study’s combined index, while Ficus hispida distinguished itself as a highly effective dust collector. Using both kinds of evidence could help planners avoid a common mistake: choosing trees that look resilient or attractive without testing whether they can tolerate pollution, capture particles and continue growing. The most effective urban canopy may ultimately be one designed as carefully as an engineered air-filtration system.