Research directions from the 27th conference for Specialists in Air Pollution and Climate Change Effects on Forest Ecosystems (2015) reflect knowledge advancements about (i) Mechanistic bases of tree responses to multiple climate and pollution stressors, in particular the interaction of ozone (O3) with nitrogen (N) deposition and drought; (ii) Linking genetic control with physiological whole-tree activity; (iii) Epigenetic responses to climate change and air pollution; (iv) Embedding individual tree performance into the multi-factorial stand-level interaction network; (v) Interactions of biogenic and anthropogenic volatile compounds (molecular, functional and ecological bases); (vi) Estimating the potential for carbon/pollution mitigation and cost effectiveness of urban and peri-urban forests; (vii) Selection of trees adapted to the urban environment; (viii) Trophic, competitive and host/parasite relationships under changing pollution and climate; (ix) Atmosphere–biosphere–pedosphere interactions as affected by anthropospheric changes; (x) Statistical analyses for epidemiological investigations; (xi) Use of monitoring for the validation of models; (xii) Holistic view for linking the climate, carbon, N and O3 modelling; (xiii) Inclusion of multiple environmental stresses (biotic and abiotic) in critical load determinations; (xiv) Ecological impacts of N deposition in the under-investigated areas; (xv) Empirical models for mechanistic effects at the local scale; (xvi) Broad-scale N and sulphur deposition input and their effects on forest ecosystem services; (xvii) Measurements of dry deposition of N; (xviii) Assessment of evapotranspiration; (xix) Remote sensing assessment of hydrological parameters; and (xx) Forest management for maximizing water provision and overall forest ecosystem services. Ground-level O3 is still the phytotoxic air pollutant of major concern to forest health. Specific issues about O3 are: (xxi) Developing dose–response relationships and stomatal O3 flux parameterizations for risk assessment, especially, in under-investigated regions; (xxii) Defining biologically based O3 standards for protection thresholds and critical levels; (xxiii) Use of free-air exposure facilities; (xxiv) Assessing O3 impacts on forest ecosystem services.

Trees remove air pollution by the interception of particulate matter on plant surfaces and the absorption of gaseous pollutants through the leaf stomata. However, the magnitude and value of the effects of trees and forests on air quality and human health across the United States remains unknown. Computer simulations with local environmental data reveal that trees and forests in the conterminous United States removed 17.4 million tonnes (t) of air pollution in 2010 (range: 9.0–23.2 million t), with human health effects valued at 6.8 billion U.S. dollars (range: $1.5–13.0 billion). This pollution removal equated to an average air quality improvement of less than one percent. Most of the pollution removal occurred in rural areas, while most of the health impacts and values were within urban areas. Health impacts included the avoidance of more than 850 incidences of human mortality and 670,000 incidences of acute respiratory symptoms.

Urban particulate air pollution is a serious health issue. Trees within cities can remove fine particles from the atmosphere and consequently improve air quality and human health. Tree effects on PM2.5 concentrations and human health are modeled for 10 U.S. cities. The total amount of PM2.5 removed annually by trees varied from 4.7 tonnes in Syracuse to 64.5 tonnes in Atlanta, with annual values varying from $1.1 million in Syracuse to $60.1 million in New York City. Most of these values were from the effects of reducing human mortality. Mortality reductions were typically around 1 person yr−1 per city, but were as high as 7.6 people yr−1 in New York City. Average annual percent air quality improvement ranged between 0.05% in San Francisco and 0.24% in Atlanta. Understanding the impact of urban trees on air quality can lead to improved urban forest management strategies to sustain human health in cities.

Urban trees have been assumed to effectively clean air particulate matter (PM), while the inter-species differences are not yet well defined, especially the PM chemical composition. In this study, PM from leaf surface and wax layer of 3 evergreen tree species (Juniper: Juniperus rigida; Black pine: Pinus tabuliformis var. mukdeais; Spruce: Picea koraiensis) were used for finding differences in PM adsorption and its compositional traits (characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrum and Inductively coupled plasma-optical emission spectrometry). Possible improvement in PM removal was also evaluated by a detail whole city tree census and different scenarios of species adjustment data. We found that: 1) the amount of PM on juniper leaves was 5.73 g m−2, 2–2.5-fold higher than black pine and spruce (p < 0.05). Of them, 38.73%, 38.22%, and 23.11% were in the wax layer. 2) Compared with the explicit interspecies differences in PM quantity, more complex interspecies difference showed different patterns for different compositional traits. In general, leaf surface PM had higher O, Si, Al, Fe, N, Pb, Cu, Ni, Cr, and Cd, while the wax PM had higher C and Na contents (p < 0.05). 3) Association ordination found that the smaller leaf size, lower leaf water content, higher leaf area per unit mass, higher wax content, and larger stomatal openness aligned with the more PM adsorption by leaf, together with the higher amounts of CO stretching, O, Si, Al, N, heavy metals of Pb, Cu, Ni, Cr, and Cd in PM. 4) Compared with the other 2 species, increase of juniper percentage in urban forests is more effective for maximizing PM removal from air, accompanying more heavy metal removal but less crystalized minerals in PM. Our findings highlight that proper species configuration in urban afforestation could maximize the air PM removal capacity.

Outdoor air pollution is considered as the most serious environmental problem for human health, associated with some million deaths worldwide per year. Cities have to cope with the challenges due to poor air quality impacting human health and citizen well-being. According to an analysis in the framework of this study, the annual mean concentrations of tropospheric ozone (O₃) have been increasing by on average 0.16 ppb year⁻¹ in cities across the globe over the time period 1995–2014. Green urban infrastructure can improve air quality by removing O₃. To efficiently reduce O₃ in cities, it is important to define suitable urban forest management, including proper species selection, with focus on the removal ability of O₃ and other air pollutants, biogenic emission rates, allergenic effects and maintenance requirements. This study reanalyzes the literature to i) quantify O₃ removal by urban vegetation categorized into trees/shrubs and green roofs; ii) rank 95 urban plant species based on the ability to maximize air quality and minimize disservices, and iii) provide novel insights on the management of urban green spaces to maximize urban air quality. Trees showed higher O₃ removal capacity (3.4 g m⁻² year⁻¹ on average) than green roofs (2.9 g m⁻² year⁻¹ as average removal rate), with lower installation and maintenance costs (around 10 times). To overcome present gaps and uncertainties, a novel Species-specific Air Quality Index (S-AQI) of suitability to air quality improvement is proposed for tree/shrub species. We recommend city planners to select species with an S-AQI>8, i.e. with high O₃ removal capacity, O₃-tolerant, resistant to pests and diseases, tolerant to drought and non-allergenic (e.g. Acer sp., Carpinus sp., Larix decidua, Prunus sp.). Green roofs can be used to supplement urban trees in improving air quality in cities. Urban vegetation, as a cost-effective and nature-based approach, aids in meeting clean air standards and should be taken into account by policy-makers.

Salts used for de-icing roads and sidewalks in northern climates can have a significant impact on water quality and vegetation. Sub-surface engineering systems, such as structural soil cells, can regulate water runoff and pollutants, and provide the necessary soil volume and irrigation to grow trees. However, the ability of such systems to manage de-icing salt contamination, and the impact of this contamination on the trees growing in them, have not been evaluated. We report on an field investigation of de-icing salt contamination in structural cells in two street-revitalization projects in Toronto, Canada, and the impact of this contamination on tree performance. We analyzed soil chemistry and collected tree attributes; these data were examined together to understand the effect of salinity on tree mortality rates and foliar condition. Data collected from continuous soil salinity loggers from April to June for one of the two sites were used to determine whether there was a long-term accumulation of salts in the soils. Results for both sites indicate that both sites displayed high salinity and alkalinity, with levels elevated beyond those suggested before those reported to cause negative tree effects. For one site, trees that were alive and trees that had a better foliar condition had significantly lower levels of soil salinity and alkalinity than other trees. High salinity and alkalinity in the soil were also associated with lower nutrient levels for both sites. Although tests for salinity accumulation in the soils of one site were negative, a longer monitoring of the soil conditions within the soil cells is warranted. Despite structural cells being increasingly utilized for their dual role in storm-water management and tree establishment, there may be a considerable trade-off between storm-water management and urban-forest function in northern climates where de-icing salt application continues to be commonplace.

While the potential of plants to uptake polycyclic aromatic hydrocarbons (PAHs) is widely acknowledged, empirical evidence of the effects of this process on local atmospheric PAH concentrations and human health is tenuous. We measured gaseous PAH concentrations using passive samplers in urban tree-covered areas and adjacent open, treeless areas in a near-road environment in Finland to gain information on the ability of urban vegetation to improve air quality. The ability of urban, mostly deciduous, vegetation to affect PAHs was season dependent: during summer, concentrations were significantly higher in tree-covered areas, while in the fall, concentrations in open areas exceeded those in tree-covered areas. During winter, concentrations in tree-covered areas were either lower or did not differ from those in open areas. Results of this study imply that the commonly believed notion that trees unequivocally improve air quality does not apply to PAHs studied here.

Emission characteristics of biogenic volatile organic compounds (BVOCs) (such as isoprene and monoterpenes) emitted from major species of street trees and urban forests by variations in temperature and photosynthetically active radiation (PAR) were investigated. The isoprene and monoterpene emission rates from Prunus sargentii, Ginkgo biloba, Zelkova serrata and Taxus cuspidata were meager. In contrast, the isoprene and monoterpene emission rates from Metasequoia glyptostroboides were very high. In particular, the emission of α-pinene from Metasequoia glyptostroboides was over 10,000 times higher than that from Ginkgo biloba. The patterns of isoprene emissions from the four tree species except for Metasequoia glyptostroboides concerning temperature and PAR were difficult to determine because the emission rate is very low. However, monoterpene emissions from all five tree species were clearly affected by temperature and PAR. These results showed that the characteristics of isoprene and monoterpene emissions differed depending on the tree species. Regarding monoterpene composition, α-pinene (81.3%) accounts for the majority of total monoterpenes from Metasequoia glyptostroboides. This could be a critical consideration when planting trees in an urban forest because BVOC emissions (e.g., α-pinene) have been well known to affect ground-level ozone formation rate via photochemical reaction with NOX. Since Metasequoia glyptostroboides showed relatively high BVOC emissions, the use of this type of tree in an urban forest with relatively high nitrogen oxide emissions should be carefully considered.

Chronic atmospheric nitrogen (N) deposition could influence the functioning of ecosystems as well as their biodiversity. However, N deposition in urban forest ecosystems, especially natural evergreen broad-leaved forests, is not well known. In this study, the concentrations and fluxes of dissolved inorganic N (DIN) and dissolved organic N (DON) in bulk deposition, throughfall, and stemflow were assessed in an urban evergreen broad-leaved forest site over three years, in order to clarify the characteristics of N deposition. At the study site, bulk DIN deposition was 3.7 kg N ha⁻¹ year⁻¹ (1.5 kg N ha⁻¹ year⁻¹ for NH₄–N and 2.2 kg N ha⁻¹ year⁻¹ for NO₃ + NO₂–N), which is the same level as that found in rural areas. In contrast, 6.5 kg N ha⁻¹ year⁻¹ for bulk DON deposition contributed to 66% of the bulk N deposition, which suggests the importance of bulk DON deposition in Japanese forest ecosystems. Passing through the tree canopy, DIN was enriched by 8.8 kg N ha⁻¹ year⁻¹ (3.7 kg N ha⁻¹ year⁻¹ for NH₄–N and 5.1 kg N ha⁻¹ year⁻¹ for NO₃ + NO₂–N) and DON was enriched by 1.5 kg N ha⁻¹ year⁻¹ as net throughfall in the evergreen broad-leaved forest. This reveals that dry deposition of DIN dominates the total DIN deposition onto the urban forest floor, compared to that found in the rural areas, due to the non-negligible N emissions from outside and possibly because of the evergreen broad-leaved forest's greater ability to capture N.

It is important to investigate the airborne bacterial air quality in urban forest parks as tree bacteriostasis practices are being increasingly advocated as measures to improve the air quality and public health in urban green spaces around the world. The aim of the study was to quantitatively investigate airborne culturable bacteria (ACB) concentration levels based on field measurements in every season in five selected forest communities and the uncovered space in an urban forest park, as well as the effects of several factors on the culturability of airborne bacteria. Results suggested that the airborne bacterial levels of all the forest communities reached the clean air quality standard with regard to the airborne bacteria content, with the highest concentration of ACB showing in the uncovered space (1658 ± 1298 CFU/m³) and the lowest showing in the mixed community (907 ± 567 CFU/m³). The temporal distribution analysis showed that the airborne bacteria were mostly concentrated in summer, as well as in the morning and afternoon. The bacteriostatic rates of the mixed community were significantly different with seasonal variation (p < 0.05). Spearman’s correlations revealed that the concentration of ACB was significantly positively correlated with the season, wind speed (WS), temperature (T), ultraviolet light (UV), negative air ion (NAI), and total suspended particles (TSP) (p<0.05) but significantly negatively correlated with the forest community type (p < 0.05). Overall, the selection of tree species plays a key role in shaping the forest structure and improving air quality, and the urban forest highlights key priorities for future efforts toward a cleaner, healthier, and more diverse regional forest environment.