Georges Najjar

Mesoscale simulations of thermodynamic fluxes over complex terrain.

Due to photochemical processes, most of the air quality problems arise during anticyclonic weather conditions in summer. Therefore, the problem is how thermodynamic fields and atmospheric processes (and especially wind fields) interact with the transport of pollutants in a valley. To answer such questions, knowledge about typical wind flow patterns occurring in the investigation area, leading to a classification of characteristic situations is useful. In particular, the most important of these flow fields and their generating processes can be analysed and understood by numerical simulations. Fourteen meteorological stations located at various topographic sites in the Fecht Valley (≃ 230 km 2 , Vosges Mountains, Alsace, France) provided the data necessary for processing an automatic wind-field classification. This classification identifies the characteristic summer flow patterns occurring in the valley. Two broad types of wind regimes can be schematically identified: during anticyclonic periods, thermally driven wind regimes are dominant; in contrast during cyclonic periods advective wind regimes are channelled by the orography. On the basis of this classification the authors were able to choose a typical summer day with anticyclonic conditions. A 3D simulation was performed for this day with the new Meso-Nh atmospheric mesoscale model to describe the thermodynamic fluxes and to understand the underlying physical processes. To obtain the most realistic results in this small area, high resolution surface information was introduced into the model. The data collected during a measurement campaign are used to validate the simulation results. The diurnal cycle of wind fields, the thermal stratification, and the heat fluxes are compared with observations.

Variation in ectomycorrhizal fungal communities associated with Silver linden (Tilia tomentosa) within and across urban areas

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.

Vegetation reflectance spectroscopy for biomonitoring of heavy metal pollution in urban soils

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.