Many environmental challenges are exacerbated within the urban landscape, such as stormwater runoff and flood risk, chemical and particulate pollution of urban air, soil and water, the urban heat ...island, and summer heat waves. Urban trees, and the urban forest as a whole, can be managed to have an impact on the urban water, heat, carbon and pollution cycles. However, there is an increasing need for empirical evidence as to the magnitude of the impacts, both beneficial and adverse, that urban trees can provide and the role that climatic region and built landscape circumstance play in modifying those impacts. This special section presents new research that advances our knowledge of the ecological and environmental services provided by the urban forest. The 14 studies included provide a global perspective on the role of trees in towns and cities from five continents. Some studies provide evidence for the cooling benefit of the local microclimate in urban green space with and without trees. Other studies focus solely on the cooling benefit of urban tree transpiration at a mesoscale or on cooling from canopy shade at a street and pedestrian scale. Other studies are concerned with tree species differences in canopy interception of rainfall, water uptake from biofilter systems, and water quality improvements through nutrient uptake from stormwater runoff. Research reported here also considers both the positive and the negative impacts of trees on air quality, through the role of trees in removing air pollutants such as ozone as well as in releasing potentially harmful volatile organic compounds and allergenic particulates. A transdisciplinary framework to support future urban forest research is proposed to better understand and communicate the role of urban trees in urban biogeochemical cycles that are highly disturbed, highly managed, and of paramount importance to human health and well‐being.
Core Ideas
The urban forest can be managed to impact the urban water, heat, carbon, and pollution cycles.
An evidence base is needed for the ecosystem service benefit urban trees can provide.
This special section presents 14 studies from five continents on the ecosystem service impact of urban trees.
Biogenic Volatile Organic Compounds (BVOC) play a critical role in biosphere–atmosphere interactions and are key factors of the physical and chemical properties of the atmosphere and climate. ...However, few studies have been carried out at urban level to investigate the interactions between BVOC emissions and ozone (O3) concentration. The contribution of urban vegetation to the load of BVOCs in the air and the interactions between biogenic emissions and urban pollution, including the likely formation of O3, needs to be investigated, but also the effects of O3 on the biochemical reactions and physiological conditions leading to BVOC emissions are largely unknown. The effect of BVOC emission on the O3 uptake by the trees is further complicating the interactions BVOC–O3, thus making challenging the estimation of the calculation of BVOC effect on O3 concentration at urban level.
•We examine the role of BVOC emitted from urban trees for O3 formation in our cities.•We state that the high BVOC emitter trees are dangerous especially in VOC limited conditions for ozone formation.•We conclude that the choice of the tree species can be very important for the quality of the air in our cities.
BVOC emission from urban trees can be very important for ozone concentration.
Air pollution in the urban environment is widely recognized as one of the most harmful threats for human health. International organizations such as the United Nations and the European Commission are ...highlighting the potential role of nature in mitigating air pollution and are now funding the implementation of Nature-Based Solutions, especially at the city level. Over the past few decades, the attention of the scientific community has grown around the role of urban forest in air pollution mitigation. Nevertheless, the understanding on Particulate Matter (PM) retention mechanisms by tree leaves is still limited. In this study, twelve tree species were sampled within an urban park of an industrial city. Two techniques were used for leaf analysis: Vacuum/Filtration and Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy, in order to obtain a quali-quantitative analysis of the different PM size fractions. Results showed that deposited PM loads vary significantly among species. Different leaf traits, including micro and macromorphological characteristics, were observed, measured and ranked, with the final aim to relate them with PM load. Even if no significant correlation between each single leaf characteristic and PM deposition was observed (p > 0.05), multivariate analysis revealed relationships between clusters of leaf traits and deposited PM. Thus, by assigning a score to each trait, an Accumulation index (Ai) was calculated, which was significantly related to the leaf deposited PM load (p ≤ 0.05).
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•Nature-based Solutions efficiency in air quality mitigation depends on plant species.•PM load on twelve tree species is quantified by SEM/EDX technique and V/F method.•PM loads are related to leaf micro and macro morphological characteristics.•A species-specific morphological-based index for PM removal efficiency is presented.
Airborne particulate matter can represent a serious issue for human health, especially in densely populated urban areas. Moreover, the inhalation of particulate can be more harmful with decreasing ...particles diameter. Vegetation can provide many ecosystem services to the citizens, including the removal of many different pollutants in the air, but while the effect on many gaseous compounds has already been widely proved, the capability of particulate matter (PM) sequestration driven by vegetation and its resulting benefit on air quality has not been deeply investigated yet at larger spatial scale, especially in Mediterranean environment. This study was conducted in the Real Bosco di Capodimonte, a green area of about 125 ha located inside the urban area of Naples (Italy) containing different species typical of the Mediterranean forest ecosystem. To better understand the interaction between PM and the park area, we measured fluxes of PM10, PM2.5 and PM1 with a fast acquisition analyser, according to the Eddy Covariance technique. We found that the particle deposition was higher during the central hours of the day and it was more evident for smaller size particles. Furthermore, the daily PM fluxes found accorded with evapotranspiration and carbon sequestration operated by plants, suggesting a possible active role of vegetation on the particulate deposition.
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•We measured particulate matter concentrations within a large green area of Naples.•PM and CO2 fluxes were calculated by mean of eddy covariance technique.•Downward fluxes of particulate were detected during the middle of the day.•Correlations between PM deposition, carbon uptake and evapotranspiration were shown.•Data presented are crucial to understand the role of green areas in PM removal.
We showed the downward flux of particulate and the consequent reduction of its concentration over a green area inside a city, although further investigation is needed to better understand vegetation activity role on particulate deposition and formation.
A number of studies have focused on urban trees to understand their mitigation capacity of air pollution. In this study particulate matter (PM) deposition on Quercus ilex leaves was quantitatively ...analyzed in four districts of the City of Terni (Italy) for three periods of the year. Fine (between 0.2 and 2.5 μm) and Large (between 2.5 and 10 μm) PM fractions were analyzed. Mean PM deposition value on Quercus ilex leaves was 20.6 μg cm−2. Variations in PM deposition correlated with distance to main roads and downwind position relatively to industrial area. Epicuticular waxes were measured and related to accumulated PM. For Fine PM deposited in waxes we observed a higher value (40% of total Fine PM) than Large PM (4% of total Large PM). Results from this study allow to increase our understanding about air pollution interactions with urban vegetation and could be hopefully taken into account when guidelines for local urban green management are realized.
•A quantitative analysis of Particulate Matter deposition on urban Quercus ilex leaves was implemented.•Deposition data were correlated with pollutants sources such as roads and local steel factory, and with epicuticular waxes.•Results provide new insight about the capacity of trees in removing pollutants in urban environment.
This paper is providing useful information on PM deposition on urban vegetation.
This study reports application of monitoring and characterization protocol for particulate matter (PM) deposited on tree leaves, using Quercus ilex as a case study species. The study area is located ...in the industrial city of Terni in central Italy, with high PM concentrations. Four trees were selected as representative of distinct pollution environments based on their proximity to a steel factory and a street. Wash off from leaves onto cellulose filters were characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy, inferring the associations between particle sizes, chemical composition, and sampling location. Modeling of particle size distributions showed a tri-modal fingerprint, with the three modes centered at 0.6 (factory related), 1.2 (urban background), and 2.6μm (traffic related). Chemical detection identified 23 elements abundant in the PM samples. Principal component analysis recognized iron and copper as source-specific PM markers, attributed mainly to industrial and heavy traffic pollution respectively. Upscaling these results on leaf area basis provided a useful indicator for strategic evaluation of harmful PM pollutants using tree leaves.
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•Urban tree leaves were used for passive PM10 monitoring combining SEM and EDX analyses.•Particle mean diameter distributions were modeled by multimode lognormal functions.•Site-specific PM10 elemental composition was evaluated by principal component analysis.•PM profile (size and composition) was used to distinguish site-specific urban sources.•PM size and relative content of Cu and Fe emerged as possible pollutant source markers.
Although tree nutrition has not been the primary focus of large climate change experiments on trees, we are beginning to understand its links to elevated atmospheric CO₂ and temperature changes. This ...review focuses on the major nutrients, namely N and P, and deals with the effects of climate change on the processes that alter their cycling and availability. Current knowledge regarding biotic and abiotic agents of weathering, mobilization and immobilization of these elements will be discussed. To date, controlled environment studies have identified possible effects of climate change on tree nutrition. Only some of these findings, however, were verified in ecosystem scale experiments. Moreover, to be able to predict future effects of climate change on tree nutrition at this scale, we need to progress from studying effects of single factors to analysing interactions between factors such as elevated CO₂, temperature or water availability.
Mediterranean forests close to urban areas are exposed to polluted plumes loaded with tropospheric ozone. This is the case of Castelporziano Estate, a 6000 ha Mediterranean forest 25 km from Rome ...downtown on the coast of the Mediterranean Sea. In September 2011 we started an intensive field campaign aimed at investigating ozone deposition from a mixed Mediterranean forest, mainly composed by Quercus suber, Quercus ilex, Pinus pinea. Measurements at canopy level with the eddy covariance technique were supported by a vegetation survey and the measurement of all environmental parameters which allowed to calculate stomatal ozone fluxes. Leaf-level measurements were used to parameterize models to calculate stomatal conductance based on a Jarvis-type and Ball–Berry approach. We show changes in magnitude of ozone fluxes from a warm (September) to a cold period (October–December). Stomatal component explained almost the totality of ozone fluxes during the cold days, but contributed only up to 50% to total ozone deposition during warm days, suggesting that other sinks (e.g. chemistry in the gas-phase) play a major role. Modeled stomatal ozone fluxes based on a Jarvis-type approach (DO3SE) correlated with measured fluxes better than using a Ball–Berry approach. A third model based on a modified Ball–Berry equation was proposed to account for the non-linear dependency of stomatal conductance on relative humidity. This research will help the development of metrics for ozone-risk assessment and advance our understanding of mixed Mediterranean forests in biosphere–atmosphere exchange.
► Flux of H2O, CO2, O3 was measured with Eddy Covariance in a Mediterranean Forest. ► Stomata explained almost the totality of ozone fluxes during the cold days. ► Non-stomatal ozone deposition was the main ozone sink during warm days. ► Modeled Gs based on a Jarvis-type approach correlated well measured Gs. ► A model based on a modified Ball–Berry equation was proposed.
The delay in autumnal senescence that has occurred in recent decades has been linked to rising temperatures. Here, we suggest that increasing atmospheric CO₂ may partly account for delayed autumnal ...senescence and for the first time, through transcriptome analysis, identify gene expression changes associated with this delay. Using a plantation of Populus x euramericana grown in elevated CO₂ (eCO₂) with free-air CO₂ enrichment (FACE) technology, we investigated the molecular and biochemical basis of this response. A Populus cDNA microarray was used to identify genes representing multiple biochemical pathways influenced by eCO₂ during senescence. Gene expression changes were confirmed through real-time quantitative PCR, and leaf biochemical assays. Pathways for secondary metabolism and glycolysis were significantly up-regulated by eCO₂ during senescence, in particular, those related to anthocyanin biosynthesis. Expressed sequence tags (ESTs) representing the two most significantly up-regulated transcripts in eCO₂, LDOX (leucoanthocyanidin dioxgenase) and DFR (dihydroflavonol reductase), gave (eCO₂/ambient CO₂ (aCO₂)) expression ratios of 39.6 and 19.3, respectively. We showed that in eCO₂ there was increased autumnal leaf sugar accumulation and up-regulation of genes determining anthocyanin biosynthesis which, we propose, prolongs leaf longevity during natural autumnal senescence.
Biomass burning is an important ecological factor in the Mediterranean ecosystem and a significant source of several atmospheric gases and particles. This paper demonstrates the performance of a ...recently developed combustion chamber, showing its capability in estimating the emission from wildland fire through a case study with dried leaf litter of Quercus robur. The combustion chamber was equipped with a thermocouple, a high resolution balance, an epiradiometer, two different sampling lines to collect volatile organic compounds (VOCs) and particles, and a portable analyzer to measure carbon monoxide (CO) and carbon dioxide (CO2) emission. VOCs were determined by gas chromatography–mass spectrometry (GC–MS) after enrichment on adsorption traps, but also monitored on-line with a proton-transfer-reaction mass spectrometer (PTR-MS). Preliminary qualitative analyses of emissions from burning dried leaf litter of Q. robur found CO and CO2 as the main gaseous species emitted during the flaming and smoldering stages. Aromatic VOCs, such as benzene and toluene, were detected together with several oxygenated VOCs, like acetaldehyde and methanol. Moreover, a clear picture of the carbon balance during the biomass combustion was obtained with the chamber used. The combustion chamber will allow to distinguish the contribution of different plant tissues to the emissions occurring during different combustion phases.
•We describe a new combustion chamber for studying emission from biomass burning.•We examine the emissions of gases and particulate matter during litter combustion.•We highlight the potentiality of this facility to investigate emissions from fire.
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