Abdoulaye Mahamat Malabad

Fabienne Tatin-Froux

Gilles Gallinet

Jean-Michel Colin

Michel Chalot

Julien Parelle
email: julien.parelle@univ-fcomte.fr

A combined approach utilizing UAV 3D imaging methods, in-situ measurements, and laboratory experiments to assess water evaporation and trace element uptake by tree species growing in a red gypsum landfill

Keywords: UAV Remote Sensing, LAI, Canopy conductance, Transpiration, Ostrya carpinifolia

The extractive industry is increasingly faced with problems of managing contaminated sites. The red gypsum landfill at the Ochsenfeld site is representative of the typology byproduct storage of the Tiextraction activity. The management of the elemental content and the water body are the issues at this site. The aim of this study was to evaluate the canopy conductance (gcmax) of various tree species and the content of elements in the leaves, utilizing the opportunity of a demonstration plantation setup in 2014 with sixteen tree species, combined with a growth chamber experiment. We combined the gas exchange measurements with the data from two multispectral cameras with RGB and NIR bands embarked on an unmanned aerial vehicle (UAV). In the field, Ostrya carpinifolia, Maclura pomifera, and Rhus copallina had the highest gcmax of all planted tree species, and the high transpiration rate in O. carpinifolia was confirmed in a pot-based controlled experiment. Except R. copallina, the species with a high Mn content (O. carpinifolia, Betula pendula, and Salix aquatica grandis) had high stomatal conductance. O. carpinifolia could therefore be a species to exploit in the management of landfill leachates, especially in the context of climate change since this species is well adapted to dry environments.

Introduction

According to the Joint Research Centre (JRC), since the 1970s, the extraction of metals has increased by more than 75% worldwide (Joint Research Centre, 2017). This significant increase in extractive activities has had a strong impact on the environment. In European Union countries, there are overall between 150,000 and 500,000 mine landfills, the majority of which are no longer active (Vaverková, 2019), which raises the question of the management of these landfills. Red gypsum landfills represent an environmental and ecological model of the typology of byproduct storage of Ti-extraction activity (high content of some trace elements (TEs): notably Fe and Mn and low biodiversity) [START_REF] Edraki | Designing mine tailings for better environmental, social and economic outcomes: a review of alternative approaches[END_REF][START_REF] Huang | Is rhizosphere remediation sufficient for sustainable revegetation of mine tailings?[END_REF][START_REF] Zapata-Carbonell | Improving Silver Birch (Betula pendula) Growth and Mn Accumulation in Residual Red Gypsum Using Organic Amendments[END_REF]. Extraction of Ti from ilmenite using the sulfuric process leads to the production of red gypsum during the neutralization of the company's wastewater with limestone. This red gypsum, accumulated over decades, allows the formation of a technosol, on which the development of vegetation is lessened due to the presence of various mineral oxides and TEs [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF][START_REF] Zapata-Carbonell | Spontaneous ecological recovery of vegetation in a red gypsum landfill: Betula pendula dominates after 10 years of inactivity[END_REF]. The lack of vegetation potentially leads to mineral (including TEs) leaching and dust enriched in TEs. Moreover, this red gypsum is stored on a waterproof structure that requires appropriate management of the incoming and outgoing water masses. The revegetation of landfills is therefore an important issue to better manage water flows via evapotranspiration and to limit the spread of dust. Phytomanagement, which has recently become an alternative for site and soil management, comprises a set of sustainable techniques using tree species in situ, for example, to restore some ecosystem services, to control erosion, and more relevantly here to regulate water flow [START_REF] Cundy | Brownfields to green fields: Realising wider benefits from practical contaminant phytomanagement strategies[END_REF]. Trees indeed contribute to water balance through transpiration. Using a lysimeter-based approach, Populus sp. was found to enhance evapotranspiration and to reduce bore leaching by accumulation in aerial biomass [START_REF] Robinson | Poplar for the phytomanagement of boron contaminated sites[END_REF]. Important mass flow may enhance plant nutrient acquisition by promoting the movement of nutrients from the soil to the root and aerial parts of the plant [START_REF] Huang | Peppermint trees shift their phosphorus-acquisition strategy along a strong gradient of plant-available phosphorus by increasing their transpiration at very low phosphorus availability[END_REF]. Mn moves freely in the xylem sap and is transported to leaves via transpiration stream with little redistribution [START_REF] Ducic | Transport and detoxification of manganese and copper in plants[END_REF]. [START_REF] Barber | Ground-based measurements of leaf area index: a review of methods, instruments and current controversies[END_REF] described and modelled nutrient uptake by crop tree species and concluded that the water flux is a key parameter. Element uptake modelling was used to describe K, P, Zn and Mn uptake by different tree species and to further demonstrate the role of water flux [START_REF] Adhikari | Modelling zinc uptake by rice crop using a Barber-Cushman approach[END_REF][START_REF] Brouder | Evaluation of a Mechanistic Model of Potassium Uptake by Cotton in Vermiculitic Soil[END_REF][START_REF] Chen | Soil pH and Phosphorus and Potassium Uptake by Maize Evaluated with an Uptake Model[END_REF][START_REF] Sadana | Manganese dynamics in the rhizosphere and Mn uptake by different crops evaluated by a mechanistic model[END_REF]. Previous studies have reported negative relationships between foliar TE content and transpiration rate in tree species [START_REF] Kitao | Comparison of photosynthetic responses to manganese toxicity of deciduous broad-leaved trees in northern Japan[END_REF][START_REF] Milan | Response of Salix alba L. to heavy metals and diesel fuel contamination[END_REF][START_REF] Pajevic | Phytoremediation capacity of poplar (Populus spp.) and willow (Salix spp.) clonesin relation to photosynthesis[END_REF][START_REF] Arsenov | Greenhouse Assessment of Citric Acid-Assisted Phytoremediation of Cadmium by Willows (Salix spp.) Effect on Photosynthetic Performances and Metal Tolerance[END_REF]. In plants, metals exert their toxic action, mostly by damaging chloroplast and disturbing photosynthesis and the inhibition of photosynthesis is the consequence of interference of metal ions with photosynthetic enzymes and chloroplast membranes, thus inducing a biochemical limitation of stomatal conductance, as reviewed in [START_REF] Aggarwal | Metal Toxicity and Photosynthesis, in: Photosynthesis: Overviews on Recent Progress and Future Perspectives[END_REF]. Many studies have shown that an excess supply of TEs leads to significant changes in the water relations of tree species [START_REF] Pandey | Chromium interference in iron nutrition and water relations of cabbage[END_REF][START_REF] Vernay | Interaction of bioaccumulation of heavy metal chromium with water relation, mineral nutrition and photosynthesis in developed leaves of Lolium perenne L[END_REF]. It has also been observed in experiments conducted under greenhouse conditions with Populus hybrids that the transpiration rate decreased with an increase in mixed TE (Cd, Cr, Cu, and Zn) content [START_REF] Chandra | Mixed heavy metal stress on photosynthesis, transpiration rate, and chlorophyll content in poplar hybrids[END_REF]. The transpiration rate and stomatal conductance in Phaseolus vulgaris were reduced by a high Zn content [START_REF] Van Assche | Zinc mediated effects on leaf CO2 diffusion conductances and net photosynthesis in Phaseolus vulgaris L[END_REF] or by ZnSO4 at 600 mg/kg in peat moss soil [START_REF] Kasim | Physiological Consequences of Structural and Ultra-structural Changes Induced by Zn Stress in Phaseolus vulgaris. II. Enzymes, Amino Acids and Protein Profile[END_REF]. Mn, which is one of the major components of the red gypsum sediment at the Ochsenfeld site [START_REF] Zapata-Carbonell | Spontaneous ecological recovery of vegetation in a red gypsum landfill: Betula pendula dominates after 10 years of inactivity[END_REF], accumulates predominantly in the plant shoot [START_REF] Page | Heavy metals in white lupin: uptake, root-to-shoot transfer and redistribution within the plant[END_REF]. High Mn content leads to excess TE deposition in photosynthetic cells and might potentially impair cellular metabolism [START_REF] Fernando | Multiple metal accumulation within a manganese-specific genus[END_REF], which may lead to metal toxicity in cells. High Mn content can decrease the net photosynthetic rate, stomatal conductance, internal carbon dioxide concentration and transpiration in several genotypes of cotton [START_REF] Tavanti | Changes in photosynthesis and antioxidant metabolism of cotton ( Gossypium hirsutum L.) plants in response to manganese stress[END_REF]. The transpiration of plants depends not only on tree species physiology, i.e., the ability to regulate the stomatal aperture but also the leaf area. The transpiration capacity is estimated by the transpiring surface of plants and thus by the leaf area index (LAI). This index is the ratio between the sum of the foliar area and the unit of ground area [START_REF] Watson | Comparative physiological studies on the growth of field crops[END_REF]. LAI is traditionally obtained by harvesting all leaves of a plot and by measuring the surface of each leaf. Modern alternative methods have been developed. Flatbed scanners have made this process more efficient, but it is still laborious, time consuming and destructive. It is possible to use instruments based on atmospheric sensors (LI-COR LAI-2000 Plant Canopy Analyser and CI-100 Plant Canopy Analyser) or a linear array of photosynthetically active radiation sensors (AccuPAR Ceptometer) to estimate the LAI [START_REF] Barber | Ground-based measurements of leaf area index: a review of methods, instruments and current controversies[END_REF][START_REF] Peper | Comparison of five methods for estimating leaf area index of opengrown deciduous trees 14[END_REF][START_REF] Wilhelm | Comparison of Three Leaf Area Index Meters in a Corn Canopy[END_REF]. These indirect non-destructive techniques comprise the application of a logarithmic regression and allometric equation, which is faster to set up for homogeneous covers but has not yet been applied for multispecific row plantations [START_REF] Nowak | Notes: Estimating Leaf Area and Leaf Biomass of Open-Grown Deciduous Urban Trees[END_REF]. As the alternative method, the Light Detection and Ranging (LiDAR) can be used as an active sensor which is based on the emission of a highly collimated laser pulse and analysing the reflected signal [START_REF] Lin | LiDAR: An important tool for next-generation phenotyping technology of high potential for plant phenomics?[END_REF] to estimate LAI using volumetric methods such as surface differencing [START_REF] Loudermilk | Ground-based LIDAR: a novel approach to quantify fine-scale fuelbed characteristics[END_REF] where the digital terrain model (DTM) is subtracted to the digital surface model (DSM). In a recent review [START_REF] Wang | Estimation of LAI with the LiDAR Technology: A Review[END_REF], where LiDAR technology, LAI retrieval and validation methods are reviewed, it is concluded that terrestrial laser scanning (TLS), airborne laser scanning (ALS) or spaceborne laser scanning (SLS) data can used for the estimation of the vegetation LAI. While the use of LiDAR for the estimation of LAI have shown promising results especially in forest application [START_REF] Alonzo | Mapping urban forest leaf area index with airborne lidar using penetration metrics and allometry[END_REF][START_REF] Lefsky | Lidar Remote Sensing of the Canopy Structure and Biophysical Properties of Douglas-Fir Western Hemlock Forests[END_REF], other approaches have been developed. Among these new technologies, the use of unmanned aerial vehicles (UAVs) to record low-altitude images is becoming increasingly widespread [START_REF] Zhang | The application of small unmanned aerial systems for precision agriculture: a review[END_REF] and is the only automated alternative for heterogeneous cover. LAI estimation from UAV-based remote sensing multispectral imagery [START_REF] Hunt | REMOTE SENSING OF CROP LEAF AREA INDEX USING UNMANNED AIRBORNE VEHICLES 9[END_REF] is increasingly reported, for instance in agriculture [START_REF] Yao | Estimation of Wheat LAI at Middle to High Levels Using Unmanned Aerial Vehicle Narrowband Multispectral Imagery[END_REF], in mangrove [START_REF] Tian | Comparison of UAV and WorldView-2 imagery for mapping leaf area index of mangrove forest[END_REF] and forest [START_REF] Lin | UAV Based Estimation of Forest Leaf Area Index (LAI) through Oblique Photogrammetry[END_REF] ecosystems. Although UAV have been used in landfill monitoring [START_REF] Rauhala | UAV Remote Sensing Surveillance of a Mine Tailings Impoundment in Sub-Arctic Conditions[END_REF][START_REF] Shahmoradi | A Comprehensive Review of Applications of Drone Technology in the Mining Industry[END_REF], to our knowledge, no studies have been conducted on the use of UAV to estimate LAI at landfills under phytomanagement. Most authors have used vegetation indices based on photogrammetry analysis, such as the normalized difference vegetation index (NDVI), which is used to monitor the degree of vegetation greenness [START_REF] Tucker | Red and photographic infrared linear combinations for monitoring vegetation[END_REF][START_REF] Zarco-Tejada | Fluorescence, temperature and narrow-band indices acquired from a UAV platform for water stress detection using a micro-hyperspectral imager and a thermal camera[END_REF]. NDVI data have proven to be a robust approach for estimating biotic stress (chlorophyll deficiency) and abiotic stress (water drought). NDVI is a rate of operation of reflectance per specific bands (NIR-R/NIR+R) for multispectral signature where NIR and R are respectively the near-infrared and red bands. It is also a measure of reflectance and an estimation of vegetation growth and biomass [START_REF] Gandhi | Ndvi: Vegetation Change Detection Using Remote Sensing and Gis -A Case Study of Vellore District[END_REF]; however, this index is not quantitative and has to be calibrated using field data.

The present study focuses on the Ochsenfeld red gypsum landfill, located in northeastern France, with an area of 80 ha. The main management issues at the Ochsenfeld site are (i) to revegetate the site to stabilize the soil, thus limiting the dispersion of dust and TEs, or for producing a valuable Mn-enriched biomass and (ii) to increase the evapotranspiration potential of the site, thus reducing the volume of water stored in the landfill that remains to be treated. On part of the site, we set up a phytomanagement trial in 2014 with different tree species to select the tree species best suited to the site conditions [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF]. Considering these particular objectives and conditions, our hypothesis was that the selection of tree species for TE uptake capacity would not be antagonist to the selection of species with a high transpiration rate. The opportunity of the 5-year-old demonstration plantation was used to estimate leaf area with a surface integration method using UAV-based remote sensing, to screen transpiration capacity and to determine TE content in leaves. The leaf surface and surface area of the trees were originally developed using a surface integration method to quantitatively estimate the LAI.

In the present study, we further used low-altitude photos obtained with UAVs that allow threedimensional integration to obtain developed surfaces. The transpiration rate of some selected tree species growing on anthroposol from Ochsenfeld site and its relationship with the Mn content were further explored under controlled conditions with a more accurate method (by utilizing the weight loss method) to potentially confirm the field results. The objectives of these experiments were (i) to test the method of LAI estimation by using UAV 3D-imaging-based remote sensing on landfill, (ii) to combine LAI estimation by using UAV 3D-imaging and gas exchange measurements for estimating the maximum canopy cover conductance as an estimator of tree performance for their capacities of management of water flow, and (iii) to test a potential concomitance between transpiration performance and leaf element uptake performance in a phytomanagement context.

2.

Materials and methods

Experimental design

Field experiment

The pilot site of a 1-ha area was implemented in April 2014 on a storage landfill of mining byproducts located in the municipality of Thann in Northeast France (Fig. 1) and was fully described by [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF]. The soil has silty and alkaline characteristics and is enriched mainly in Mn, Fe, and S [START_REF] Assad | Accumulation of trace elements in edible crops and poplar grown on a titanium ore landfill[END_REF]. A 30 cm layer of backfill was added on top of the red gypsum before planting. According to [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF], only 16 species of the 18 planted species with a significant growth performance were selected for this study (Table 1).

Growth chamber experiment

A pot experiment was performed in a controlled growth chamber on five tree species showing contrasting growth and elemental contents in the leaves (according to [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF]

Ecophysiological analysis

For the field experiment, the maximum stomatal conductance (gsmax expressed in mol H2O/leaf m 2 /s) was monitored between June and July 2019 on one fully sun-exposed leaf (n=9 randomly chosen by species) using a portable gas-exchange system (Licor-6400 Lincoln, Nebraska) at 400 ppm CO2, 30 °C block temperature, photosynthetically active radiation at 1400 µmol/m 2 /s and ambient air humidity (min = 50% and max = 80%). Each measurement was recorded when the stabilization of gsmax was observed (approximately 3 min). Minimum leaf water potentials (Ѱmin expressed in MPa) were measured immediately after on the same twig with a pressure chamber [START_REF] Scholander | Sap Pressure in Vascular Plants: Negative hydrostatic pressure can be measured in plants[END_REF]. These measurements were collected between 10 a.m. and 4 p.m. local time to ensure light conditions supported maximum photosynthetic rates and minimum Ѱmin.

For the growth chamber experiment, the plant transpiration stream was measured through water loss method by weighing pots packed in a plastic bag for 4 h to limit the evaporation of the soil. To express the transpiration per unit leaf area, an estimation of the total leaf area was performed by measuring the leaf length of all leaves of each measured plant. The etalon curve leaf area -leaf length and leaf areaand leaf dry mass, realized for intermediate time and final time measurements, respectively, were measured for a subset of leaves from each species with a WinRHIZO scanner (Regent Instrument, Quebec, Canada). Details of the equations used are provided in the supplemental information Fig. S1.

Remote sensing

UAV-based remote sensing was used to calculate the height of tree species and vegetation indices (LAI and NDVI). Two high-resolution digital cameras, Sony (RX100 M3; Tokyo, Japan) for red (R: 620 to 670 nm), green (G: 490 to 570 nm), and blue (B: 450 to 490 nm) bands and Canon (S110; Tokyo, Japan, modified by LDP LLC Max, Carlstadt, USA) for near-infrared (NIR: 680 to 800 nm) bands with a resolution of 8 megapixels, were embarked on the drone TurtleQuad® designed by Gilles Gallinet. On The source images had the following resolution: 9.89 mm/pix for RGB and 1.46 cm/pix for NIR. The two RGB and NIR orthomosaics were first calculated using Metashape, each separately with their native resolution. They were then resampled when exported by Metashape to provide images with a GSD (Ground Spatial Distance) of 10 cm/pix. Processing in QGIS was then performed with this resampled resolution of 10 cm/pix for all bands used. The sampling algorithm during orthomosaïc generation process and the resampling algorithm during export were the same i.e. bilinear interpolation. The geometric calibration of the optics was carried out using the Metashape software, which has built-in tools for this. The overlay between the captured images was chosen to be greater than 80% in both length and width. Furthermore, the flight lines overlapped. Therefore, each pixel of the orthomosaïc was calculated from at least nine photographs (see supplemental information Fig. S2 andS3). The mapping was deliberately done beyond the perimeter of the study area to avoid any edge effect. The maps extended at least 25 m beyond the lateral limits of the actual study area.

The tree height (TH) was calculated in QGIS by calculating the difference in altitude between the DSMs and the DTM (Eq. 1). The DSM was the 3D surface of the cover canopy of vegetation. The precision of the DSM was x = 3 cm, y = 3 cm and z = 5 cm. The DTM was a 3-D area of the soil without vegetation.

The DTM was recorded by traditional terrestrial acquisition using a high precision RTK-GPS. This DTM integrated the reading of the 11 ground targets used for the 3-dimensional calibration of the photogrammetric model. The accuracy of the RTK GPS reading on the ground was x, y = 1 cm and z = 2 cm. The soil DTM and vegetation canopy DSM were in the same projection system and the same altimeter setting.

𝑇𝐻 = 𝐷𝑆𝑀 -𝐷𝑇𝑀

(1)

To obtain the LAI, we calculated the ratio between the leaf area of canopy cover (LA) and the area projected at the ground of the canopy cover (GC) of the tree species (Eq. 2). The leaf area of the canopy cover was measured on Agisoft Metashape by manual delineation of the tree crown (Fig. 2). The area of the canopy cover projected on the ground was calculated on QGIS also by manual delineation of the tree crown.

𝐿𝐴𝐼 = 𝐿𝐴 𝐺𝐶

(2)

The LAI multiplied by the mean of the gsmax allowed us to obtain the canopy conductance (gcmax expressed in mol H2O/ground m 2 /s), which is controlled mainly by these two variables [START_REF] Granier | A generic model of forest canopy conductance dependent on climate, soil water availability and leaf area index[END_REF].

The NDVI values were calculated according to the following equation (Eq. 3) using QGIS, where NIR and R are reflectance values in the near-infrared and red wavelengths, respectively, given by Tucker

𝑁𝐷𝑉𝐼 = 𝑁𝐼𝑅 -𝑅 𝑁𝐼𝑅 + 𝑅

(3)

The experimental site was divided into eight different plots as described by [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF].

Each plot consisted of a replicate of the 18 different tree species planted in rows (one row by species).

For the present UAV-based study, the experimental site was partitioned row-by-row, constituting eight replicates per species (Fig. 1C). The results for each species were the median and the quartiles of these eight rows. obtained by manual delineation of the tree crown. Each row of a tree species was cut, isolated and trimmed, which further allowed us to estimate the developed leaf area using a surface integration method.

Elemental analysis

For each tree species, the leaves used for gas exchange measurements were washed with deionized water and dried at 70 °C for 72 h. These dried leaf samples were ground using a ball mill (Retsch MM300, Eragny, France). Mineralization was realized by acid digestion with concentrated HNO3 using a VELP DK6 heating digester (VELP Scientifica, Usmate Velate, Italy). As described by [START_REF] Assad | Accumulation of trace elements in edible crops and poplar grown on a titanium ore landfill[END_REF], the total elemental content (Al, B, Ca, Cd, Cu, Fe, K, Mg, Mn, Na, P, S, Si, Sr, Ti, and Zn) in the leaves from the field samples was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES, Radial ICAP 6500 Model, Thermo Fischer Scientific, Inc., Pittsburgh, USA). For the pot experiment, Mn content was determined by atomic absorption spectroscopy (AAS, Agilent Technologies, Inc., Santa Clara, USA).

Statistical Analysis

All statistical analyses were performed using R software version 4.0.4, the R Foundation for Statistical Computing (R Core Team, 2021), at a significance level when P < 0.05. To verify the homoscedasticity and the normality of the all dataset, we had used a Levene test [START_REF] Levene | Essays in Honor of Harold Hotelling[END_REF] and Shapiro-Wilk test [START_REF] Shapiro | An Analysis of Variance Test for Normality (Complete Samples)[END_REF] respectively. As dataset did not fit the assumption of the normal distribution, they were analysed with non-parametrical tests. For all considered variables, the statistically significant difference between tree species within a variable was analysed using a Kruskal-Wallis test by ranks.

Spearman's rank-order correlation was performed to compare the different variables assayed and to compare the rank of different tree species for each variable. A confusion matrix with a 10-fold crossvalidation was performed to estimate the generalized accuracy average and to evaluate confusion between the different tree species, using caret package (version 6.0-90).

Results

In-situ measurements

Transpiration proxies

We observed significant differences in gsmax and Ѱmin values among the 16 tree species (Fig. 3). O. carpinifolia, R. copallina and M. pomifera exhibited significantly higher gsmax to water vapour than a group of 11 other species (Fig. 3A). The two species, S. aquatica grandis and B. pendula, had intermediate gsmax values. The Ѱmin ranged from -2.27 to -0.09 MPa, with Ѱmin values of -0.28 ± 0.13 MPa and -2.03 ± 0.08 MPa for M. pomifera and A. ginnala, respectively (Fig. 3B). The rank of the different tree species for their Ѱmin value was significantly similar to the rank obtained for the gsmax values (Spearman rank correlation with P = 0.003), suggesting that the two proxies could be used for estimating differences in the transpiration stream. As exceptions, M. pomifera and R. copallina combined a high gsmax with a high Ѱmin. The analysis of the Ci (concentration of CO2 in the intercellular spaces of the leaf) versus gsmax graph underlined that for the two species, C. sinensis and A. buergerianum, the lowest conductance might be attributed to a biochemical limitation (Fig. 4). These two species were clearly limited in their CO2 assimilation among all tested tree samples, with a negative correlation between Ci and gsmax. The other species, e.g., O. carpinifolia, R. copallina and M. pomifera, seemed to cope better with the soil conditions.

Remote sensing

UAV-based remote sensing allowed us to obtain the tree height (expressed in m) (Fig. 5A), LAI (expressed as the ratio of m 2 of leaf area to m 2 of ground area, Fig. 5B), and NDVI value (Fig. 5C) of the tree species present at the Ochsenfeld site. For these three parameters obtained using UAV-based remote sensing, the rank order of the tree species was highly similar, with P < 0.001 (Spearman's rankorder correlation). We further combined the LAI data (Fig. 5A) and gsmax data (Fig. 3A) to obtain the maximal canopy conductance (gcmax expressed as mol H2O/ground m 2 /s, Fig. 5D).

U

Foliar elemental content and leaf water status

The elemental content detected in leaves was significantly different among species (Fig. 6). Mn, the major element to be considered for the management of red gypsum sediment, was found in greater amounts in species belonging to the Betulaceae (B. pendula, B papyrifera, and O. carpinifolia) and Salicaceae (S. alba and S. aquatica grandis) families. B. pendula and O. carpinifolia had the highest Mn contents, with values 3 times greater than those of C. sinensis, for example. Similarly, Zn was found at significantly higher levels in Salicaceae and Betulaceae species, except for O. carpinifolia (Fig. 6). O. carpinifolia, Z. serrata, and S. aquatica grandis had on average 4 -5 times higher Fe content than Q. acutissima and R. copallina (Fig. 6C). The remaining 11 species had intermediate values. Mg was the element for which the largest variations in leaf content among tree species were observed. A. ginnala and R. copallina had Mg leaf contents 4 to 5 times lower than those of B. pendula, B papyrifera, U. pumila, S. alba and M. pomifera. Leaf contents and associated statistical data for the entire ICP set of 16 elements are provided in supplemental Table S1. To highlight the potential concomitance between the foliar elemental content and transpiration stream proxies (gsmax and Ѱmin), the rank of the different tree species was obtained using the Spearman rank coefficient (Table 2). The species were ranked for gsmax significantly similarly to their foliar elemental contents in K, Mn (Fig. 6) and Sr. The relationship between leaf Mn content and gsmax showed that tree species that had a high Mn content also had high gsmax (O. carpinifola, B. pendula, and S. alba). Four of the tree species with the lowest gsmax also had low Mn contents (A. ginnala, U. pumila, A. buergerianum, and C. sinensis). R. copallina and M. pomifera combined a low foliar Mn content with high gsmax values.

Similarly, the Ѱmin values were also positively correlated with several elements, namely, Ca, Cd, Mg and Sr, and negatively correlated with Cu, Fe and Si. The other elements, i.e., Al, B, Na, P, S, Ti and Zn, were only related to the tree species and did not correlate to any of the two transpiration proxies.

Table 2. Spearman rank-order correlation between gsmax, Ѱmin, and the foliar elemental content *** P < 0.001, ** P < 0.01, * P < 0.05, ns = not significant

We further performed a 10-fold cross-validation confusion matrix on all tree species that includes all studied parameters (supplemental Table S2). A 80% generalized accuracy was determined, with 6 species showing 100% accuracy (Csi, Qac, Rco, Sal, Upu and Zse). Most of confusion occurred between species of the same plant genera (e.g. Bpa versus Bpe, Sal versus Sag), that showed similar gcmax values (Fig. 5) and elemental content (Fig. 6). The gcmax values contributed in increasing confusion among species of different genera (e.g. Qac versus Pst).

Growth chamber experiments

Five of the 16 tree species at the Ochsenfeld site were further chosen to analyse the transpiration rate and compare with field proxy data (measured after 90 and 152 days of growth). This will allow us to obtain quantitative transpiration rate data at the scale of the whole plant for each of the species studied, which is not easy to obtain at the field scale. O. carpinifolia had the highest transpiration rate, with a value greater than 1.5 mmol H2O/leaf m 2 /, which was higher than that for the other 4 tree species at day 90 (Fig. 7A). The transpiration rates were not significantly different at the intermediate time (90 days)

and at the end time (152 days), except for O. carpinifolia, for which a significant decrease was observed, but its transpiration rate remained twice that of all other species. The rank order of these five tree species according to their transpiration rate was the same as that obtained with gsmax in the field experiment (O. stenoptera and S. aquatica grandis (Fig. 7B). There was no significant difference between the two measurement times despite a slight increase (1.2 on average) in leaf Mn content for B. pendula, which is a Mn-accumulating species. No significant rank-order correlation was observed between foliar Mn content and transpiration rate.

4.

Discussion

O. carpinifolia exhibited the highest transpiration capacity at the leaf level

According to their ecophysiological traits, the field experiment allowed us to identify three groups of tree species. The first group comprised R. copallina and M. pomifera, which had the highest gsmax, combined with the highest Ѱmin values, close to 0 MPa. We hypothesized that this performance of these two species could be due to their highly developed root system (Ciadamidaro, unpublished data), which provides them access to deep water, allowing them to maintain their Ѱmin close to zero despite a high maximum stomatal conductance to water vapour. The second group included A. buergerianum and C.

sinensis, with the lowest gsmax and Ѱmin; the negative relationship among gsmax and CO2 intercellular concentration underlines a biochemical limitation of the stomatal aperture, which is in agreement with their low growth performance. This group of tree species would not be able to grow optimally on this site and could not be retained for revegetation. The third group was composed of all other tree species, including O. carpinifolia, characterized by a stomatal limitation of Ci concentration and a classical negative correlation between the stomatal aperture and leaf water potential [START_REF] Comstock | Control of stomatal conductance by leaf water potential in Hymenoclea salsola (T. & G.), a desert subshrub[END_REF][START_REF] Cowan | Transport of Water in the Soil-Plant-Atmosphere System[END_REF]. Stomata regulate water loss from the leaves to maintain xylem pressure in a range that will protect xylem from embolism. Ψ50 is the xylem water potential provoking a 50% loss of hydraulic conductivity. At this potential, which is species-dependent, water transport in the xylem is highly impaired and can lead to drastic damage in the plant. In the current study, considering the Ѱmin between -2.27 and -0.09 MPa compared with Ψ50 recorded in the literature for the main temperate forest tree species [START_REF] Choat | Global convergence in the vulnerability of forests to drought[END_REF], the water constraint was not sufficiently severe for all tree species to cause water transport damage. For example, we recorded a Ѱmin of -1.39 ± 0.1 MPa for B. pendula compared with a Ψ50 of -2.4 MPa that is known for this species [START_REF] Herbette | Calcium Is a Major Determinant of Xylem Vulnerability to Cavitation[END_REF]. Ѱmin for O. carpinifolia was -1.81 ± 0.14 MPa in this study compared with a Ѱ50 equal of -4.31 MPa in the study conducted by [START_REF] Kiorapostolou | Vulnerability to xylem embolism correlates to wood parenchyma fraction in angiosperms but not in gymnosperms[END_REF]. In contrast, the Ѱmin of S. aquatic grandis was -1.22 ± 0.2

MPa and was close to the Ѱ50 commonly described for this genus (-0.91 to -2.32 MPa) [START_REF] Cochard | Use of Positive Pressures to Establish Vulnerability Curves: Further Support for the Air-Seeding Hypothesis and Implications for Pressure-Volume Analysis[END_REF], while no data are available in the literature for this species to date to the knowledge of the authors.

Therefore, with a Ѱmin greater than Ѱ50, there was no risk for creating trapped gas emboli in the water transport system [START_REF] Choat | Global convergence in the vulnerability of forests to drought[END_REF], and stomata should be fully or partially opened. Indeed, Salix species are well known to exhibit a high transpiration rate and would be interesting candidates to improve the evapotranspiration of landfills in phytomanagement solutions [START_REF] Frédette | Willows for environmental projects: A literature review of results on evapotranspiration rate and its driving factors across the genus Salix[END_REF].

However, based on our site conditions, the stomatal limitation clearly moderated its performance, and the maximum stomatal conductance recorded for B. pendula or O. carpinifolia remained higher.

Regarding the gsmax for O. carpinifolia, [START_REF] Tomasella | Nonstructural carbohydrate and hydraulic dynamics during drought and recovery in Fraxinus ornus and Ostrya carpinifolia saplings[END_REF] obtained values more than twice as high as ours, but O. carpinifolia was grown in a greenhouse under controlled conditions in pots containing lightweight substrate and potting compost irrigated at field capacity. Indeed, a measurement of gsmax over a few days would have been influenced by meteorological conditions, while we chose a sunny week and recorded a moderate decrease in Ѱmin. In our pot study under controlled conditions, a ranking similar to that obtained with the field experiment was found for the transpiration rate in well-irrigated conditions after three and five months of growth (Fig. 7), based on their gsmax values on a subset of species with different performances (O. carpinifolia > B. pendula > S. aquatica grandis > P. stenoptera > A. buergerianum). Indeed, we should consider that gsmax measured during a few days of summer is a good proxy of transpiration performance at the leaf level under these field conditions.

4.2.

Ostrya carpinifolia had the best growth performance, as estimated by UAV-based remote sensing Among the 16 tree species that were planted in April 2014 at the Ochsenfeld site, U. pumila, O. carpinifolia, M. pomifera and S. aquatica showed the highest growth (estimated by height) compared with the other tree species, such as B. pendula or Q. acutissima. The species that had the greatest height in our soil condition, U. pumila, was one of the dominant tree species at the Pingshuo mining landfill located in northern Shanxi Province in China [START_REF] Yuan | Intra-and Interspecific Interactions among Pioneer Trees Affect Forest-Biomass Carbon Accumulation in a Nutrient-Deficient Reclaimed Coal Mine Spoil[END_REF]; U. pumila is resistant to drought and fast growing. U. pumila is a species with high adaptability to different climates: glacial temperate, temperate and subtropical zones [START_REF] Zhao | Development of population structure and spatial distribution patterns of a restored forest during 17-year succession (1993-2010) in Pingshuo opencast mine spoil, China[END_REF]. The O. carpinifolia species was one of the intermediate-growing species in 2015 (direct height measurements, [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF] but became the best-performing tree species during our last measurement in 2018, after 4 years of growth at the Ochsenfeld site. It also performed very well at two other phytomanagement sites (Ciadamidaro, unpublished data). O. carpinifolia needs a low amount of resources for growth [START_REF] Tardella | Changes in species and functional composition in the herb layer of sub-Mediterranean Ostrya carpinifolia abandoned coppices[END_REF] and plays an important function in the forest vegetation in sub-Mediterranean areas in Italy at altitudes lower than 1200 m. It has been described as a species capable of growing on shallow and poorly developed soils, mainly on limestone and gypsum [START_REF] Pasta | Ostrya carpinifolia in Europe: distribution, habitat, usage and threats 2[END_REF]. M. pomifera intrinsically has a strong adaptive capacity in nutrient-poor soils and a high resistance to severe drought [START_REF] Khaleghi | Morphological, physiochemical and antioxidant responses of Maclura pomifera to drought stress[END_REF]. The species belonging to the Salix genus is known to be a pioneer species suitable for cultivation in difficult sites with suboptimal soil conditions [START_REF] Kuzovkina | The characterization of willow (Salix L.) varieties for use in ecological engineering applications: Co-ordination of structure, function and autecology[END_REF]. The authors of this review explain that Salix species have a good adaptation to soil conditions, rapid growth, and the highest biomass production obtained in short rotations, combined with high transpiration rates; therefore, Salix species would be good candidates for phytomanagement in landfills. In contrast, B. pendula, which spontaneously colonized the red gypsum landfill, [START_REF] Zapata-Carbonell | Spontaneous ecological recovery of vegetation in a red gypsum landfill: Betula pendula dominates after 10 years of inactivity[END_REF] did not grow well in the experimental plot, probably because of the presence of other competing herbaceous species. Betula species are known to be affected by planting density, which significantly reduces their crown and branch sizes [START_REF] Mäkinen | Effect of stand density on the branch development of silver birch (Betula pendula Roth) in central Finland[END_REF][START_REF] Wang | Crown and branch attributes of mid-aged Betula alnoides plantations in response to planting density[END_REF]. The poor growth of Quercus species observed at the Ochsenfeld site could be explained by the intrinsic low-growth character of this genus and/or the vulnerability of the species to poor soil nutrient status and moisture deficiency [START_REF] Demchik | The effect of soil nutrition, soil acidity and drought on northern red oak (Quercus rubra L.) growth and nutrition on Pennsylvania sites with high and low red a mis en forme : Anglais (E.U.) oak mortality[END_REF].

UAV-based surface integration method was innovatively applied to a landfill phytomanagement to estimate tree LAI and canopy conductance

The LAI values obtained in the present study by using the UAV-based surface integration method were within the same order as those found in previous studies. For instance, the LAI for S. alba and S. aquatica grandis were in the range of 2.1-3.4 leaf m²/ground m² in our study, comparable to the 1-5.5 leaf m²/ground m² range found for other Salix species [START_REF] Mirck | Seasonal Sap Flow of Four Salix Varieties Growing on the Solvay Wastebeds in Syracuse, NY, USA[END_REF]. In that study, the LAI values measured with a LI-COR LAI 2000 for Salix, one of the best shrub willow varieties growing at an industrial landfill, peaked during the beginning of July and August and were minimal in November. The LAI development course of poplar and willow clones was obtained, and LAI values also greatly varied during the season [START_REF] Rüth | Water regime of mechanical-biological pretreated waste materials under fast-growing trees[END_REF]. LAI values obtained during May in that study (which correspond to our sampling date) were in the 2-3 leaf m 2 /ground m 2 range, similar to the values obtained for our Salix clones. U. pumila and O. carpinifolia had the highest LAI values in our study, which were also well correlated with tree heights. Similar relationships between vegetation height and LAI have been found in other studies [START_REF] Luo | Estimation of wetland vegetation height and leaf area index using airborne laser scanning data[END_REF][START_REF] Rüth | Water regime of mechanical-biological pretreated waste materials under fast-growing trees[END_REF]. The LAI values that varied on average from 0.23 to 0.40 leaf m²/ground m² for the different tree species studied at the Ochsenfeld site showed that the vegetation was dense because the NDVI of a densely vegetated plot or area tends towards positive values [START_REF] Viana | Land Use/Land Cover Change Detection and Urban Sprawl Analysis[END_REF]. Therefore, high NDVI values would indicate high NIR reflectance and low red reflectance related to high leaf density and/or high chlorophyll content, while low NDVI values would indicate a diminution of NIR reflectance indicating leaf damage or high red reflectance due to low leaf density. Indeed, the NDVI is intrinsically correlated to LAI by using the calculation mode but integrates chlorophyll content and leaf health [START_REF] Tucker | Red and photographic infrared linear combinations for monitoring vegetation[END_REF]. In our study, except for P. stenoptera, the five tree species that had significantly higher NDVI values were the species with the greatest gsmax (O.

carpinifolia, R. copallina and M. pomifera) and the highest LAI (O. carpinifolia, U. pumila and S. aquatica grandis). Despite a relatively low height, P. stenoptera had a high density of long and pinnately leaves, thus could have led to an underestimation of LAI with our method. Regardless, the low gcmax integration method combined with gsmax allows us to obtain an estimation of the maximal canopy conductance gcmax. Although this proxy is quantitative, it could not replace the methods cited above; however, it allowed obtaining a rapid comparison indicator of the difference in performance for the species to evaporate water at the landfill. We found that the best performing species should be O. carpinifolia, M. pomifera, R. copallina, S. aquatic grandis, U. pumila and P. orientalis. However, R.

copallina behaved as an invasive species in the experimental plot at the Ochsenfeld site, which casts doubt on its potential use in phytomanagement scenarios. Although they performed very well in terms of growth, S. aquatica grandis and U. pumila with a Ѱmin close to Ψ50 and limited gsmax would be sensitive tree species in the event of drought. Therefore, we recommended only three species, i.e., O. carpinifolia, M. pomifera and P. orientalis, to be utilized for phytomanagement at the Ochsenfeld site.

Mn uptake and transpiration rate were correlated

In our study, the concentrations of the different elements, including TEs (Table S1), were lower than those generally observed at anthropized sites (Kabata-Pendias, 2011). This might be because tree sampling was conducted in early summer, when the elemental concentrations were not yet at their optimal levels [START_REF] Migeon | Metal Accumulation by Woody Species on Contaminated Sites in the North of France[END_REF]. These authors demonstrated that the elemental content in the leaves collected in October was significantly higher than that collected in June, suggesting seasonal variability in the leaf elemental content. The other elemental contents found in our study were lower than the basal physiological compositions recorded in tree species (Fe=150 mg/kg DM, Mn=200 mg/kg DM and Zn=50 mg/kg DM) [START_REF] Assad | Accumulation of trace elements in edible crops and poplar grown on a titanium ore landfill[END_REF][START_REF] Greger | Metal Availability, Uptake, Transport and Accumulation in Plants[END_REF]. Excess of these elements in plant tissues may cause toxicity by damaging the cell structure [START_REF] Zhao | The effects of copper, manganese and zinc on plant growth and elemental accumulation in the manganese-hyperaccumulator Phytolacca americana[END_REF] and therefore may affect plant growth and limit transpiration. With elemental contents slightly lower than usual physiological values, our tree species are therefore unlikely to have problems in their physiological processes, such as transpiration.

However, Zn concentrations measured in Betula and Salix genera in the present field experiments were slightly higher than the physiological levels (Kabata-Pendias, 2011), which is consistent with our previous studies [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF][START_REF] Zapata-Carbonell | Improving Silver Birch (Betula pendula) Growth and Mn Accumulation in Residual Red Gypsum Using Organic Amendments[END_REF]. Mn was also found to accumulate in Betula species in the present field experiment, which also confirms our previous data [START_REF] Ciadamidaro | Early screening of new accumulating versus nonaccumulating tree species for the phytomanagement of marginal lands[END_REF][START_REF] Zapata-Carbonell | Improving Silver Birch (Betula pendula) Growth and Mn Accumulation in Residual Red Gypsum Using Organic Amendments[END_REF]. In the pot experiment, high Mn content measured in the tree species in particular in B. pendula, could be explained by the conditions of the experiment conducted in growth chamber, favorable for the accumulation of ET. However, the use of only five tree species in the laboratory experiment failed to show a rank-order correlation between Mn content and transpiration rate.

In addition to other factors, such as mobility of the element, tissue age, and biomass of the tree, water flow in the tree plays an important role in element uptake and translocation to the shoot, and water is necessary for circulation in the transpiration stream [START_REF] Brown | Boron mobility in plants[END_REF]. The foliar Mn content in some tree species grown in the red gypsum sediment at the Ochsenfeld site was correlated with the transpiration rate of the tree. Although the objective at the Ochsenfeld site is not to decontaminate the sediment, the choice of Mn-accumulating tree species with a high transpiration rate may be relevant to phytomining (Ent et al., 2018) or ecocatalysis processes [START_REF] Escande | Phytoextracted mining wastes for ecocatalysis: Eco-Mn®, an efficient and eco-friendly plant-based catalyst for reductive amination of ketones[END_REF]. [START_REF] Xu | Distribution and mobility of manganese in the hyperaccumulator plant Phytolacca acinosa Roxb. (Phytolaccaceae)[END_REF] nicely reported that the Mn content in a leaf decreased by approximately 90% when it was wrapped in a transparent plastic bag to suppress the transpiration rate. In contrast, other elemental contents were not related to transpiration, e.g., Zn uptake was not affected by the transpiration capacity of the tree species.

Managing both metal uptake and water flow at landfills is therefore highly dependent on tree species and the elements of concern.

Conclusions

Here, we combined a UAV-based remote sensing approach and a surface integration method using 3D imagery to determine the LAI values of different species of trees planted in rows. This approach is an alternative to the long and tedious methods of dendrometric and ecophysiological measurements. We utilized the gcmax (gsmax x LAI) proxy, which was found to be a relevant tool for evaluating the performance of selected trees at the Ochsenfeld site, and in particular for estimating the capacities of trees to manage water flows, compared with traditional proxies, such as NDVI, LAI or gsmax when considered separately. With the highest growth and gcmax values compared with other planted woody species, O. carpinifolia had the greatest potential to maximize transpiration flows in these harsh conditions. These field measurements were confirmed by measurements performed under controlled conditions on the same substrate. O. carpinifolia also retained Mn in its leaves, similar to the other tree species in the Betulaceae family, B. pendula, even though Mn concentrations were highest under controlled conditions. This opens new perspectives in phytomining and ecocatalysis research domains that combine phytomanagement with added values in the enriched biomass. High transpiration of the woody cover obtained with this woody species could drastically reduce the volumes of water infiltrating through the landfill and therefore limit the associated environmental risks and treatment costs. Species more classically used in phytomanagement, such as those belonging to the Salicaceae (genus Salix), are undoubtedly less relevant at this site because they have a much more limited potential transpiration rate.

In the event of multiple choices of tree species to manage these landfills, M. pomifera and P. orientalis would be preferred tree species, in addition to O. carpinifolia. Their potential would now need to be confirmed by, e.g., a lysimeter installation for the accurate and representative measurements of the evapotranspiration rate and water balance at the site. In the future, an atmospheric correction will be provided to manipulate spectral reflectance at ground level to remove potential atmospheric effects. As advanced sensors with large spectral diversity are being developed, we foresee the use of remote sensing as a method to evaluate both plant and soil metal concentrations, and thus correlate metal concentration and UAV-based ecophysiological parameters for a better assessment of phytomanagement strategies in landfill restoration. Regarding the Ochsenfeld study site, which is representative of landfills from the extractive industry, the key issue is the implementation of a plant cover that maximizes water evapotranspiration after deposits have ceased and that limits the aerial dissemination of red gypsum dust, which is imposed by the French regulator. The presence of trees with high evapotranspiration capacities would allow to maximize these impacts. Additionally, we are foreseeing the use of phytomining in future projects to recover metals from the plant biomasses. However, given the huge amounts of metals in the substrate, it would be inappropriate to expect metal depollution.

CRediT authorship contribution statement

Abdoulaye Mahamat Malabad: Investigation, Writing -original draft; Fabienne Tatin-Froux and Julien Parelle: Investigation, Writing -original draft; Gilles Gallinet: UAV acquisition, Writing -original draft;

Jean-Michel Colin: Writing -original draft, site maintenance; and Michel Chalot: Writing -review & editing, Funding acquisition, Supervision.

Fig. 1 .

 1 Fig. 1. The experimental Ochsenfeld site at Thann (France). (A) Geographic location of the red gypsum landfill. (B) Aerial view of the site showing the location of the experimental plot (red rectangle). (C) Partition of the experimental area: each partition corresponds to the plantation of one row of one species, and 8 rows were planted per species.

24

  May 2019, on a clear and sunny day a flight of the UAV was conducted aloft at 30 to 35 metres from the ground between 11:30 a.m. and 12:40 a.m. local time to collect spectral radiance at ground from aerial photos. A total of 547 photos were taken to generate the first 3-D model (dense cloud and shaded model) using Metashape-AGISOFT photogrammetric software (version 1.6.1, Agisoft LLC, Saint Petersburg, Russia). This 3-D model used 11 ground targets (GCPs) for the 3-dimensional calibration of the photogrammetric model, recorded by traditional terrestrial acquisition using a high-precision realtime kinematic -global positioning system (RTK-GPS). From this 3-D model, still using Metashape-AGISOFT, an orthomosaic projection generated two multiband orthophotographs: one with red (band1), green (band2), and blue (band3) bands, and the second with near-infrared (band1), green (band2), and blue (band3) bands. The data from these multiband orthophotographs were then processed using QGIS version 3.10.1-A Coruña(QGIS Development Team, 2018).

(

  1979). The NDVI values range from 0.2 to 0.8 for vegetation and are unitless. High NDVI values indicate healthy and dense green vegetation, and low NDVI values indicate moisture-stressed and sparse vegetation[START_REF] Drisya | Chapter 27 -Spatiotemporal Variability of Soil Moisture and Drought Estimation Using a Distributed Hydrological Model[END_REF][START_REF] Gessesse | Temporal relationships between time series CHIRPS-rainfall estimation and eMODIS-NDVI satellite images in Amhara Region, Ethiopia[END_REF].

Fig. 2 .

 2 Fig. 2. Three-dimensional UAV view of the canopy cover at the experimental Ochsenfeld site. (A) General view of the demonstration site generated at-sensor reflectance orthomosaics using Metashape-AGISOFT photogrammetric software. (B) Detailed view of one plot corresponding to one tree species

Fig. 3 .

 3 Fig. 3. Transpiration proxies (mean (red dot), median, quartiles, range, and outliers) (n=9). (A) gsmax (mol H2O/leaf m 2 /s) and (B) Ѱmin (MPa), measured on the 16 tree species growing at the experimental Ochsenfeld site. Boxes with the same letter did not differ significantly from each other using a pairwise Kruskal-Wallis test (P < 0.05).

Fig. 4 .

 4 Fig. 4. Relationship between intercellular CO2 concentration (Ci expressed in µmol CO2/mol air) and maximal stomatal conductance to water vapour (gsmax expressed in mol H2O/leaf m 2 /s) for the 16 tree species growing at the experimental Ochsenfeld site. Each colour represents one of the 16 trees studied, with 9 measurements per tree, and ellipses correspond to the 0.95 confidence interval.

Fig. 5 .

 5 Fig. 5. UAV-based remotely sensed parameters of the 16 tree species growing at the experimental Ochsenfeld site (mean (red dot), median, quartiles, range, and outliers) (n=8). (A) Tree height (m), (B) LAI values (leaf m 2 /ground m 2 ), (C) NDVI values and (D) gcmax (mol H2O/ground m 2 /s). Boxes with the same letter did not differ significantly from each other based on a pairwise Kruskal-Wallis test (P < 0.05).

Fig. 6 .

 6 Fig. 6. Elemental content (mg/kg DM) (mean (red dot), median, quartiles, range, and outliers) (n=9) measured in leaves of the 16 tree species at the experimental Ochsenfeld site with (A) Mn, (B) Zn, (C) Fe and (D) Mg. Boxes with the same letter did not differ significantly from each other using a pairwise Kruskal-Wallis test (P < 0.05).

Fig. 7 .

 7 Fig. 7. (A) Transpiration rate (mmol H2O/leaf m 2 /s) and (B) leaf Mn content (mg/kg DM) (mean (red dot), median, quartiles, range, and outliers) (n = 24) measured on the 5 tree species grown in Ochsenfeld soil in chamber experiments. For each species, the first boxplot was measured at 90 days, and the second was measured at 152 days. Boxes with the same letter did not differ significantly from each other based on a pairwise Kruskal-Wallis test (P < 0.05).

Table 1 .

 1 List of the tree species used in the present study and their abbreviations

	Abu Acer buergerianum	Por	Platanus orientalis
	Agi	Acer ginnala	Pst	Pterocarya stenoptera

  . pumila, O. carpinifolia, M. pomifera, S. aquatica grandis and P. orientalis had the greatest heights among the tree species planted at the Ochsenfeld site in 2014, whereas A. ginnala, C. speciosa, Q. acutissima, B. pendula and B. papyrifera had the lowest heights. The other tree species had an intermediate height (Fig.5A). The LAI data followed the same trend, with U. pumila, O. carpinifolia,

P. orientalis, S. aquatica grandis and M. pomifera having the greatest LAI values, while A. ginnala and B. pendula had the lowest LAI values (Fig.

5B

). The NDVI values for the study area plotted in the 0.23 (A. ginnala) -0.40 (O. carpinifolia) range (Fig.

5C

), which reflects the difference in tree growth and cover as discussed above. As an exception, the P. stenoptera species, despite a relatively low height, had one of the highest NDVI values. With the highest LAI and gsmax values, O. carpinifolia had the greatest gcmax (Fig.

5D

), while A. ginnala had values 10 times lower than those of O. carpinifolia. Among the tree species studied, only O. carpinifolia, M. pomifera, R. copallina, S. aquatica grandis, U. pumila, P. orientalis and C. speciosa had on average a gcmax greater than 0.1 mol H2O/ground m 2 /s.

Acknowledgement

We acknowledge Dr. Nadia Morin-Crini, Marie-Laure Toussaint and Caroline Amiot of the PEAT² platform for the metal analyses. This work was supported by the French National Research Agency [PHYTOCHEM ANR-13-CDII-0005-01] and the French Environment and Energy Management Agency [PROLIPHYT ADEME-1172C0053]. A.M.M. received a PhD grant from the Ministry of Higher Education, Research and Innovation of the Republic of Chad [561 PR/MESRI/SG/DGCNOU/DEPOB/SEPOB/2018].

recorded for this species could be mainly attributed to its relatively low stomatal conductance compared to other species with similar NDVI values. The species with the highest NDVI values, i.e., O. carpinifolia, also had the highest LAI value. These values reflect the acclimation of this tree species to site conditions, as discussed above. Correlation coefficients of 0.77 and 0.99 between NDVI and LAI were obtained at two sites for vegetation grown in a semiarid grassland [START_REF] Fan | Investigating the relationship between NDVI and LAI in semi-arid grassland in Inner Mongolia using in-situ measurements[END_REF]. At these sites, NDVI and LAI both reached peak values in August and became the maximum values. The present study underlined that our LAI calculation was robust, as it correlated with the NDVI, the widely used spectral reflectance index. The NDVI remains a qualitative proxy, i.e., an index that needs to be calibrated with LAI field measurements [START_REF] Fan | Investigating the relationship between NDVI and LAI in semi-arid grassland in Inner Mongolia using in-situ measurements[END_REF]. The gcmax proxy used in the present study was innovatively measured by combining the surface integration method using UAV-based remote sensing and 3-D imaging with traditional gsmax measurements and provided direct quantitative values allowing a better comparison of different tree species performances. The negative point of this surface integration method was the manual delineation of leaf area, which was tedious. With the rapid development of UAV technology, UAV-based remote sensing has been widely used in forestry, agriculture, and vegetation monitoring [START_REF] Lu | Experimental Evaluation and Consistency Comparison of UAV Multispectral Minisensors[END_REF]. UAV-based remote sensing has many advantages, such as instant information acquisition, high image spatial resolution, convenient operation, high manoeuvrability, freedom from cloud interference, and low cost. Our present study was in line with this, as it combined remote sensing and gas exchange measurements to determine the canopy conductance of different tree species. In view of the diversity of tree species used, other multispectral indices would have been interesting to consider. These indices could take into account spectral ranges between 0.4-2.5 µm (Yang et al., 2017). Remote sensing approaches based on thermal signature as a proxy of transpiration streams [START_REF] Xue | Significant Remote Sensing Vegetation Indices: A Review of Developments and Applications[END_REF] through indices such as the Crop Water Stress Index (CWSI) could be used.

Evapotranspiration estimation at the field level requires water balance measurements using lysimetric methods to collect sap flow measurements, as detailed in a review on classical methods [START_REF] Rana | Measurement and estimation of actual evapotranspiration in the field under Mediterranean climate: a review[END_REF], or modelling methods based on stomatal conductance measurements, meteorological parameters and extractable soil water measurements [START_REF] Granier | A generic model of forest canopy conductance dependent on climate, soil water availability and leaf area index[END_REF]. All of these methods are quantitatively precise and have been improved well but are time-consuming and cannot be applied for rapid screening of plant performance in phytomanagement contexts. However, a key parameter was the estimation of a LAI that conditioned the evaporative surface. This LAI obtained by using the surface

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.