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More and more uses and applications are being given to local climate zone (LCZ) maps, which describe the structure of the urban and semi-urban areas. Among others, it is worth highlighting its use in studies of urban heat islands (UHI), sustainability and urban energy balance. Even if the classes are well described in the literature, it is difficult to estimate the general precision of these classification maps because the classification is highly dependent of the urban typology of the city under study. However, LCZ maps represent a reference in the field of urban climatology. This research work aims to make use of these maps to explain the strong influence of LCZ classes on land surface temperature (LST) and, consequently, on air temperature (AT). This kind of investigations will help us to explain the outliers observed in previous work between LST and AT at specific locations in the city of Strasbourg for the period 2012–2019. The LST data were obtained from the thermal infrared data of both ASTER (with 90-m spatial resolution and 16-days temporal resolution) and MODIS satellite (with 1-km spatial resolution and daily revisit period). The reference ATs were obtained from different field measurement provided by a huge network of meteorological stations distributed in the city of Strasbourg. The comparison of measured ATs and remote LSTs provide the opportunity to thoroughly evaluate the relationship between these two parameters both during the day and night, for different land covers and for different times of the year. Finally, UHI maps of Strasbourg for every season are presented.

The increase in urbanization of cities coupled with some effects of climate change is leading to the emergence of urban microclimates. The rising temperatures in cities create the phenomenon known as Urban Heat Island (UHI). This is a difference between the temperature of urban and rural areas that intensifies more during the night and varies according to the season of the year. This paper focuses on the estimation of the UHI and its variations in and around the city of Strasbourg. To this end, thermal remote sensing data from different satellite has been used to isolate and analyse this phenomenon, specifically, Land Surface Temperatures (LSTs) provided by ASTER and MODIS. The LST provided by these sensors has been compared to air temperatures of the last 15 years from meteorological stations distributed throughout the city that have served as ground truth. Forty-seven meteorological stations spread throughout the area of interest provided measurements of air humidity and temperature. This valuable data base has allowed the assessment of the correlations between LST from satellite and ground truth air temperature. Based on satellite data resampled at different spatial units, this work led to the creation of the first UHI map of Strasbourg.

This study presents the coupling of TEB (Town Energy Balance) and Surfatm models developed for energy exchange estimates for urban impervious and vegetation surfaces, respectively. Once coupled, the TEB-Surfatm model allows the estimate of radiative, sensible (H), and latent heat (LE) fluxes in urban areas accounting for urban vegetation. The modelled fluxes were compared with measurements performed in an urban garden. The model was able to reproduce the energy fluxes, but its performance varied. The variability of the model accuracy depended on the measurement footprint in link with the heterogeneity of the site characteristics: while the measurement footprint fitted with the area characteristics considered by the TEB-Surfatm model, the modelled H and LE fluxes presented a good agreement with the measurements. In the other cases, some overestimation and underestimation occurred, in link with different fractions of impervious surfaces or green spaces. The validation of the TEB-Surfatm model for energy fluxes is a first step, the second will be to include the pollutant exchanges since Surfatm is able to quantify the atmosphere-biosphere fluxes for numerous pollutants. It will allow the TEB-Surfatm model to quantify the impact of urban greening on the assessment of air quality in urban areas.

3D models of tree geometry are important for numerous studies, such as for urban planning or agricultural studies. In climatology, tree models can be necessary for simulating the cooling effect of trees by estimating their evapotranspiration. The literature shows that the more accurate the 3D structure of a tree is, the more accurate microclimate models are. This is the reason why, since 2013, we have been developing an algorithm for the reconstruction of trees from terrestrial laser scanner (TLS) data, which we call TreeArchitecture. Meanwhile, new promising algorithms dedicated to tree reconstruction have emerged in the literature. In this paper, we assess the capacity of our algorithm and of two others -PlantScan3D and SimpleTree- to reconstruct the 3D structure of trees. The aim of this reconstruction is to be able to characterize the geometric complexity of trees, with different heights, sizes and shapes of branches. Based on a specific surveying workflow with a TLS, we have acquired dense point clouds of six different urban trees, with specific architectures, before reconstructing them with each algorithm. Finally, qualitative and quantitative assessments of the models are performed using reference tree reconstructions and field measurements. Based on this assessment, the advantages and the limits of every reconstruction algorithm are highlighted. Anyway, very satisfying results can be reached for 3D reconstructions of tree topology as well as of tree volume.

Urban trees provide many ecosystem services, including carbon sequestration, air quality improvement, storm water attenuation and energy conservation, to people living in cities. Provisioning of ecosystem services by urban trees, however, may be jeopardized by the typically poor quality of the soils in urban areas. Given their well-known multifunctional role in forest ecosystems, ectomycorrhizal fungi (EcM) may also contribute to urban tree health and thus ecosystem service provisioning. Yet, no studies so far have directly related in situ EcM community composition to urban tree health indicators. Here, two previously collected datasets were combined: i) tree health data of 175 Tilia tomentosa trees from three European cities (Leuven, Strasbourg and Porto) estimated using a range of reflectance, chlorophyll fluorescence and physical leaf indicators, and ii) ectomycorrhizal diversity of these trees as characterized by next-generation sequencing. Tree health indicators were related to soil characteristics and EcM diversity using canonical redundancy analysis. Soil organic matter significantly explained variation in tree health indicators whereas no significant relation between mycorrhizal diversity variables and the tree health indicators was found. We conclude that mainly soil organic matter, through promoting soil aggregate formation and porosity, and thus indirectly tree water availability, positively affects the health of trees in urban areas. Our results suggest that urban planners should not overlook the importance of soil quality and its water holding capacity for the health of urban trees and potentially also for the ecosystem services they deliver. Further research should also study other soil microbiota which may independently, or in interaction with ectomycorrhiza, mediate tree performance in urban settings.

In the 21st century, urban areas undergo specific climatic conditions like urban heat islands which frequency and intensity increase over the years. Towards the understanding and the monitoring of these conditions, vegetation effects on urban climate are studied. It appears that a natural phenomenon, the evapotranspiration of trees, generates a cooling effect in urban environment. In this work, a 3D microclimate model is used to quantify the evapotranspiration of trees in relation with their architecture, their physiology and the climate. These three characteristics are determined with field measurements and data processing. Based on point clouds acquired with terrestrial laser scanner (TLS), the 3D reconstruction of the tree wood architecture is performed. Then the 3D reconstruction of leaves is carried out from the 3D skeleton of vegetative shoots and allometric statistics. With the aim of extending the simulation on several trees simultaneously, it is necessary to apply the 3D reconstruction process on each tree individually. However, as well for the acquisition as for the processing, the 3D reconstruction approach is time consuming. Mobile laser scanners could provide point clouds in a faster way than static TLS, but this implies a lower point density. Also the processing time could be shortened, but under the assumption that a coarser 3D model is sufficient for the simulation. In this context, the criterion of level of details and accuracy of the tree 3D reconstructed model must be studied. In this paper first tests to assess their impact on the determination of the evapotranspiration are presented.

The MILAGRO field campaign was a multi-agency international collaborative project to evaluate the regional impacts of the Mexico City air pollution plume as a means of understanding urban impacts on the global climate. Mexico City lies on an elevated plateau with mountains on three sides and has complex mountain and surface-driven wind flows. This paper asks what the wind transport was in the basin during the field campaign and how representative it was of the climatology. Surface meteorology and air quality data, radiosondes and radar wind profiler data were collected at sites in the basin and its vicinity. Cluster analysis was used to identify the dominant wind patterns both during the campaign and within the past 10 years of operational data from the warm dry season. Our analysis shows that March 2006 was representative of typical flow patterns experienced in the basin. Six episode types were identified for the basin-scale circulation providing a way of interpreting atmospheric chemistry and particulate data collected during the campaign. Decoupling between surface winds and those aloft had a strong influence in leading to convection and poor air quality episodes. Hourly characterisation of wind circulation during the MILAGRO, MCMA-2003 and IMADA field campaigns enables the comparisons of similar air pollution episodes and the evaluation of the impact of wind transport on measurements of the atmospheric chemistry taking place in the basin.

Highlights • 3D reconstruction of trees can be performed using terrestrial laser scanner data. • The ideal level of detail can be validated thanks to a reference mock-up. • The RATP model can simulate tree transpiration rates from 3D tree mock-ups. • Promising results can be obtained with two different tree mock-ups. • Three main characteristics appear essential for accurate transpiration simulation. Abstract The aim of this paper is to investigate the minimal level of detail of the tree geometry reconstruction required to enable an accurate estimate of the evaporative cooling effect of an individual tree. The Functional Structural Plant Modeling approach, which links the 3D tree structure to its functioning, is used to investigate the impact of the level of detail of a tree reconstruction on its simulated transpiration. Based on terrestrial laser scanning point cloud data of a nine-meter-high silver linden tree (Tilia tomentosa Moench), several methods of reconstruction of its crown were considered. They can be divided into three groups: (i) tree branching structure reconstructions where leafy shoots are reconstructed; (ii) envelope reconstructions such as 3D convex/concave envelopes; and (iii) voxel reconstructions where leaves are uniformly distributed within the given volume. Based on these methods, several mock-ups of resolved tree crowns from low to high level of details were created. The cooling performance of each tree mock-up was compared by simulating its transpiration rate with a validated 3D ecophysiological model based on the turbid medium approach. The resulting mock-ups differ in several characteristics related to tree metrics and light interception: volume, projected leaf area, leaf area index, leaf area density, leaf clumping and silhouette-to-total area ratio. An inter-comparison of the transpiration rates provided by these mock-ups shows that tree branching structure reconstruction methods perform better than envelope computation methods and that these differences are related to the light interception property of the reconstructed trees. This study provides guidelines to determine which structural characteristics of tree crowns must be measured and taken into account in order to carry out accurate estimates of the transpiration rates of individual trees. [ABSTRACT FROM AUTHOR]

Abstract Heavy metals in urban soils may impose a threat to public health and may negatively affect urban tree viability. Vegetation spectroscopy techniques applied to bio-indicators bring new opportunities to characterize heavy metal contamination, without being constrained by laborious soil sampling and lab-based sample processing. Here we used Tilia tomentosa trees, sampled across three European cities, as bio-indicators i) to investigate the impacts of elevated concentrations of cadmium (Cd) and lead (Pb) on leaf mass per area (LMA), total chlorophyll content (Chl), chlorophyll a to b ratio (Chla:Chlb) and the maximal PSII photochemical efficiency (Fv/Fm); and ii) to evaluate the feasibility of detecting Cd and Pb contamination using leaf reflectance spectra. For the latter, we used a partial-least-squares discriminant analysis (PLS-DA) to train spectral-based models for the classification of Cd and/or Pb contamination. We show that elevated soil Pb concentrations induced a significant decrease in the LMA and Chla:Chlb, with no decrease in Chl. We did not observe pronounced reductions of Fv/Fm due to Cd and Pb contamination. Elevated Cd and Pb concentrations induced contrasting spectral changes in the red-edge (690–740 nm) region, which might be associated with the proportional changes in leaf pigments. PLS-DA models allowed for the classifications of Cd and Pb contamination, with a classification accuracy of 86% (Kappa = 0.48) and 83% (Kappa = 0.66), respectively. PLS-DA models also allowed for the detection of a collective elevation of soil Cd and Pb, with an accuracy of 66% (Kappa = 0.49). This study demonstrates the potential of using reflectance spectroscopy for biomonitoring of heavy metal contamination in urban soils. Graphical abstract Image 1 Highlights • Cd and Pb pollution in urban soils reduces the leaf mass per area of Tilia trees. • Soil Cd and Pb pollution reduces the leaf chlorophyll a to b ratio of Tilia trees. • Soil Cd and Pb pollution alters leaf spectral properties in the red-edge region. • PLS-DA models based on leaf spectra allow for the detection of Cd and Pb pollution. Applying leaf reflectance spectroscopy to urban trees allows for biomonitoring of heavy metal pollution and the classification of pollutants in urban soils. [ABSTRACT FROM AUTHOR]

The thermo-radiative model LASER/F aims at computing the energy balance of a district. The components of the urban fabric are represented in vectorial 3D and the scene is fragmented into small elementary triangular facets of a few square meters. The physical processes related to the energy balance are solved for each facet. The operational framework of the model is briefly presented in this paper, as well as the numerical procedures and the physical concepts used for the different types of urban elements. The energy balance of a neighborhood of Strasbourg (France) is then simulated. The results are compared with experimental field measurements and the model accuracy is evaluated with a complete set of statistical indices. The model obtains globally a good level of performance for almost all variables, except the latent heat flux. [ABSTRACT FROM AUTHOR]

The MILAGRO field campaign was a multi-agency international collaborative project to evaluate the regional impacts of the Mexico City air pollution plume as a means of understanding urban impacts on the global climate. Mexico City lies on an elevated plateau with mountains on three sides and has complex mountain and surface-driven wind flows. This paper asks what the wind transport was in the basin during the field campaign and how representative it was of the climatology. Surface meteorology and air quality data, radiosoundings and radar wind profiler data were collected at sites in the basin and its vicinity. Cluster analysis is used to identify the dominant wind patterns both during the campaign and within the past 10 years of operational data from the warm dry season. Our analysis shows that March 2006 was representative of typical flow patterns experienced in the basin. Six episode types were identified for the basin scale circulation providing a way of interpreting atmospheric chemistry and particulate data collected during the campaign. Decoupling between surface winds and those aloft had a strong influence in leading to convection and poor air quality episodes. Hourly characterisation of wind circulation during the MILAGRO, MCMA2003 and IMADA field campaigns will enable the comparisons of similar air pollution episodes and the evaluation of the impact of wind transport on measurements of the atmospheric chemistry taking place in the basin. [ABSTRACT FROM AUTHOR]

In order to identify the representative flow patterns occurring in the Fecht Valley (region of Alsace, France) in summer, an automated wind-field classification is applied to wind observations collected during a field experiment. Fourteen meteorological stations placed at various topographic sites provided the necessary data on two scales. Onthe mesoscale of the Fecht Valley (230km2), the groups obtained by the classification reflect the diurnal and seasonal cycles of winds. On the microscale of the Ringelbach catchment (0.36 km2) , no diurnal cycle is observed in the sequence of the groups. Radiation balance and thermal forcing are the main factors governing the local flows during anticyclonic periods in the Fecht Valley, whereas external factors dominate the flow in the Ringelbach catchment area. A possible explanation for the different behaviour can be foundin differences in scale, topography, and exposure between the two hydrological basins. On the basis of the classifications, the principalcharacteristics of the wind fields are studied and matrices of transition probabilities between wind fields are estimated. For climatic research in complex terrain, with its strong channeling of wind, an automated classification is helpful in the sense that it gives a realistic overview of the representative wind flows occurring in an area without the need of any a priori knowledge. [ABSTRACT FROM AUTHOR]

Urban trees provide many ecosystem services, including carbon sequestration, air quality improvement, storm water attenuation and energy conservation, to people living in cities. Provisioning of ecosystem services by urban trees, however, may be jeopardized by the typically poor quality of the soils in urban areas. Given their well-known multifunctional role in forest ecosystems, ectomycorrhizal fungi (EcM) may also contribute to urban tree health and thus ecosystem service provisioning. Yet, no studies so far have directly related in situ EcM community composition to urban tree health indicators. Here, two previously collected datasets were combined: i) tree health data of 175 Tilia tomentosa trees from three European cities (Leuven, Strasbourg and Porto) estimated using a range of reflectance, chlorophyll fluorescence and physical leaf indicators, and ii) ectomycorrhizal diversity of these trees as characterized by next-generation sequencing. Tree health indicators were related to soil characteristics and EcM diversity using canonical redundancy analysis. Soil organic matter significantly explained variation in tree health indicators whereas no significant relation between mycorrhizal diversity variables and the tree health indicators was found. We conclude that mainly soil organic matter, through promoting soil aggregate formation and porosity, and thus indirectly tree water availability, positively affects the health of trees in urban areas. Our results suggest that urban planners should not overlook the importance of soil quality and its water holding capacity for the health of urban trees and potentially also for the ecosystem services they deliver. Further research should also study other soil microbiota which may independently, or in interaction with ectomycorrhiza, mediate tree performance in urban settings., Competing Interests: The authors have declared that no competing interests exist.

Trees in urban areas face harsh environmental conditions. Ectomycorrhizal fungi (EcM) form a symbiosis with many tree species and provide a range of benefits to their host through their extraradical hyphal network. Although our understanding of the environmental drivers and large scale geographical variation of EcM communities in natural ecosystems is growing, our knowledge of EcM communities within and across urban areas is still limited. Here, we characterized EcM communities using Illumina miseq sequencing on 175 root samples of the urban tree Tilia tomentosa from three European cities, namely Leuven (Belgium), Strasbourg (France) and Porto (Portugal). We found strong differences in EcM richness and community composition between cities. Soil acidity, organic matter and moisture content were significantly associated with EcM community composition. In agreement, the explained variability in EcM communities was mostly attributed to general soil characteristics, whereas very little variation was explained by city and heavy metal pollution. Overall, our results suggest that EcM communities in urban areas are significantly associated with soil characteristics, while heavy metal pollution and biogeography had little or no impact. These findings deliver new insights into EcM distribution patterns in urban areas and contribute to specific inoculation strategies to improve urban tree vitality.