# High transpirational cooling by urban trees despite extreme summer heatwaves

Christoph Bachofen, Marion Peillon, Naika Meili, Ilann Bourgeois, Charlotte Grossiord

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Christoph Bachofen, Marion Peillon, Naika Meili, Ilann Bourgeois, Charlotte Grossiord. High transpirational cooling by urban trees despite extreme summer heatwaves. Urban Forestry & Urban Greening, 2025, 107, pp.128819. 10.1016/j.ufug.2025.128819. hal-05447237

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Original Article

High transpirational cooling by urban trees despite extreme
summer heatwaves

<figure><img src="image_2.png" /></figure>

Christoph Bachofen<sup>a,b,*</sup> <sup>●</sup>, Marion Peillon<sup>c</sup>, Naika Meili<sup>d,e</sup>, Ilann Bourgeois<sup>f</sup>,
Charlotte Grossiord<sup>a,b</sup>

ARTICLE INFO

Keywords:

Air temperature

Ecohydrological model

Latent heat

Leaf temperature

Stomatal conductance

Stomatal uncoupling

VPD

A B S T R A C T

Urban trees cool their environment by shading and transpiration (latent heat, LE), thereby alleviating urban heat. LE may be critically reduced during heatwaves, when trees reduce stomatal conductance (gs) to prevent hydraulic dysfunctions. Recent observations indicate that gs may still be maintained during heatwaves, but implications for urban heat stress mitigation remain elusive. We recorded sap flow on eight *Platanus x acerifolia* trees in Geneva to assess LE during 2023, which had two record-breaking summer heatwaves. We repeatedly assessed leaf water potentials at pre-dawn and midday (Ψpre, Ψmid), gs, and leaf, canopy, and ground surface temperatures in shaded and sunlit parts (Tleaf, Tcan, Tsurf). Using ecohydrological modelling (UT&C), we determined the energy budget of the urban square and assessed whether LE and gs predictions match measurements. Despite air temperatures (Tair) reaching 39.1 °C, trees continued transpiring up to 37.1 kg h⁻¹ (LE of 25.3 kW). LE was similar during heatwaves (Tair > 30 °C) as during cooler periods and compensated approximately 33.3 % of the urban heating by solar radiation (R$_S$). In contrast, the model predicted a higher decrease of gs, and LE to 22.8 % of R$_S$ during heatwaves, thereby underestimating actual tree cooling. Despite unprecedented heatwaves, *Platanus x acerifolia* trees efficiently cooled the urban environment. Measured LE largely surpassed model estimations during heatwaves, hence actual cooling effects of urban trees during heatwaves might be considerably underestimated by current predictions with common stomatal models. Cities with intermittent heatwaves may thus continue to rely on vegetation cooling by transpiration, but further research is needed to determine the best suited tree species to optimise the cooling effect.

# 1. Introduction

Extreme summer heatwaves have become more frequent in the last
decades (Wang et al., 2023), causing a large number of heat-related
deaths of people (Robine et al., 2008; Barriopedro et al., 2011). In the
coming decades, the number and severity of heatwaves will continue to
increase dramatically due to global warming (Fischer et al., 2021; Wang
and Yan, 2021). At the same time, growing urbanisation increases air
and surface temperatures within cities, which exacerbates thermal
discomfort for people (Arnfield, 2003; Kundu and Pandey, 2020), and
calls for effective mitigation measures to be set in place.

Depending on the vegetation type and climatic conditions, cooling
from urban vegetation plays an essential role in reducing thermal
discomfort (Norton et al., 2015; Konarska et al., 2016) by lowering air
temperatures in the range of 0.5–5 °C (Qiu et al., 2013; Rahman et al.,
2019). Trees cool the urban microclimate mainly through shading and
transpiration (Smithers et al., 2018; Hein et al., 2020). Shading,
particularly from large trees, cools by intercepting solar radiation (R<sub>S</sub>)
and preventing the warming of surfaces (Oke, 1989). This creates locally
cool areas below tree canopies (Armson et al., 2013), which can be
important for human comfort in an otherwise overheated environment
(Bowler et al., 2010; Gillner et al., 2015). Transpiration (*E*), on the other

<footnote><sup>a</sup> Plant Ecology Research Laboratory PERL, School of Architecture, Civil and Environmental Engineering, EPFL, Lausanne, Switzerland</footnote>

<footnote><sup>b</sup> Functional Plant Ecology, Community Ecology Unit, Swiss Federal Institute for Forest, Snow and Landscape WSL, Lausanne, Switzerland</footnote>

<footnote><sup>c</sup> Haute Ecole INSA, Lyon, France</footnote>

<footnote><sup>d</sup> Department of Civil and Environmental Engineering, National University of Singapore, Singapore, Singapore</footnote>

<footnote><sup>e</sup> Future Cities Laboratory Global, Singapore-ETH Centre, Singapore, Singapore</footnote>

<footnote><sup>f</sup> Université Savoie Mont Blanc, INRAE, CARRTEL, Thonon-les-Bains, France</footnote>

<footnote>* Correspondence to: GR B2 405, Station 2, Lausanne 1015, Switzerland.</footnote>

<footnote>E-mail address: christoph.bachofen@epfl.ch (C. Bachofen).</footnote>
hand, cools the leaves and the surrounding air in the local environment as the energy (i.e., latent heat, LE) used to evaporate water during E consumes heat energy (i.e., sensible heat, H; Taha et al., 1988; Grimmond and Oke, 1991). Thus, the energy heating up the city, mostly R<sub>s</sub>, is partly converted into LE by evapotranspiration of plants (Barradas, 2000; Konarska et al., 2016; Rahman et al., 2020). As a result, tree canopies can be cooler by up to 11–30 °C compared to surrounding surfaces (Pauleit and Duhme, 2000; Leuzinger et al., 2010). This effect is largest during sunny and warm weather (Armson et al., 2013; Chen et al., 2019) and may play an important role in the mitigation of thermal stress for trees growing in hot environments (Lin et al., 2017).

With the physiological constraints that heatwaves impose on plants, and the predicted rates of global warming, the cooling potential of urban vegetation by E has become uncertain (Konarska et al., 2016). City trees are particularly exposed to extreme environmental conditions, often with little rooting space for water uptake, high irradiation, and air temperatures. Plant E is primarily driven by the vapour pressure deficit of the air (VPD) and the soil water availability (SWC), which both directly affect the plant hydraulic status (i.e., the leaf water potential; Ψ<sub>leaf</sub>; Grossiord et al., 2020; Bachofen et al., 2023). During heatwaves, VPD can easily more than double (De Boeck et al., 2010), leading to a reduction of stomatal conductance (g<sub>s</sub>) and, above a certain threshold, to a reduction in E (Grossiord et al., 2020; Slot et al., 2024). Trees thus usually reduce g<sub>s</sub> during the time of highest evaporative demand, i.e., in the early afternoon hours (Krause et al., 2012; Bachofen et al., 2017; Meili et al., 2021) and during hot and dry periods (Shashua-Bar et al., 2012; Armson et al., 2013; Shashua-bar et al., 2023) when the cooling effect would be most required. When E is limited by low SWC, high VPD, or during the night when stomata are closed, non-transpiring trees may even increase urban temperatures through their effect on wind flow, radiation trapping, and urban aerodynamic roughness (night-time warming; Ziter et al., 2019; Meili et al., 2021). Thus, extreme heatwaves, particularly in combination with drought, may hamper the cooling potential of urban trees during the time of highest cooling demand.

Higher air temperature (T<sub>air</sub>) itself increases g<sub>s</sub>, and thus evapo-
transpiration, in many tree species (Mills et al., 2024). While this can
lead to quicker depletion of available soil water, it might enhance leaf
cooling and, thus, plant survival during hot days (Drake et al., 2018;
Marchin et al., 2023). The temperature-driven E increase results from
multiple interacting mechanisms, including higher fluidity of water at
high temperatures that increase the hydraulic conductivity (Roderick
and Berry, 2001) and biochemical stimulation of stomatal opening (Diao
et al., 2024; Mills et al., 2024). However, our current lack of under-
standing of g<sub>s</sub> responses to high T<sub>air</sub> restricts our predictions of leaf gas
exchange (Marchin et al., 2023). A consequence might be that actual g<sub>s</sub>
is higher than predicted at high T<sub>air</sub> using current physiological models
(Crawford et al., 2012; Diao et al., 2024), which would translate to
higher E. Hence, transpirational cooling of urban trees during heatwaves
might be higher than expected, given sufficient soil water access.

In addition to the limits of our understanding of plant physiological
cooling mechanisms, we also lack insight into the amount of actual
cooling provided by *E* at the level of an urban street or square flanked by
buildings (i.e., urban canyon; Oke *et al.*, 2017). Directly translating *LE* to
measured changes in *T*<sub>air</sub>, and thereby to the heat perceived by people is
difficult, mostly due to radiative effects around the tree canopy, and the
air advection from the urban boundary layer (Oke, 1987; Panagopoulos,
2008). Therefore, many studies use *E* simulations to calculate the cool-
ing effect on the urban canyon with empirical data on tree physiological
processes (Hein *et al.*, 2020). To estimate the combined cooling effect of
trees, including shading, albedo, and *LE*, we need sufficiently detailed
urban climate models that account for plant physiological characteris-
tics. The recently developed urban ecohydrological model Urban
Tethys-Chloris (UT&C; Meili *et al.*, 2020) models *E* of urban vegetation
in response to environmental conditions (e.g., soil moisture, photosyn-
thetic active radiation, VPD) and plant physiological traits. In this
model, g<sub>s</sub> is calculated with the widely used Ball, Berry, Leuning

semi-empirical function (Leuning, 1990; Leuning et al., 1995; Fatichi
et al., 2012) that predicts a decreasing *E* above a certain VPD threshold
(E and tree cooling by *LE* can be modelled by UT&C, whereas poorly
understood T<sub>air</sub> responses of g<sub>s</sub> might not be sufficiently reproduced. By
combining ecohydrological modelling with *in-situ* measurements of *E*,
we may better understand heatwaves effect on tree transpirational
cooling, and the current limitations of scaling from plant physiological
measurements to the urban canyon.

In this study, we assessed the *in-situ* LE cooling of urban trees during heatwaves (i.e., T<sub>air</sub> > 30 °C) and complemented measurements with cooling predictions from the UT&C model. The year 2023 was the third-hottest summer in Switzerland since the beginning of temperature records in 1864, with two pronounced heatwaves in July and August. In Geneva, the August heatwave lasted 15 days and reached the highest August temperatures ever measured in the region. We assessed *E* of eight *Platanus* trees in an urban square in Geneva during the growing season of 2023, and expected that (1) during the heatwaves, *E* decreases due to restriction of g<sub>s</sub> with rising VPD. Consequently, we expected that (2) *LE* is markedly lower during heatwaves compared to non-heatwave periods. Furthermore, we hypothesised that (3) the cooling predicted by UT&C deviates negatively from measured *LE* during heatwaves due to unaccounted T<sub>air</sub> effects on g<sub>s</sub>.

## 2. Methods

2.1. Research site

We selected eight *Platanus* × *acerifolia* trees on a highly urbanised square in Lancy (46.182 °N, 6.120 °E, 420 m asl.), a suburb of Geneva (Switzerland) with a population of 610'000 inhabitants including the agglomeration. The eight *Platanus* trees had been planted in two rows of four trees and are of similar stature with a DBH of 39.5–53.1 cm and a height of 9.2–12.3 m (Table 1). The square is 50 m long and 45 m wide, and the ground surface of the square is entirely sealed with cobbled granite stones except for a small opening of the pits in which the trees had been planted. The trees were not irrigated during the study period (pers. comm. with local city gardeners). On the NW and SE sides, the square is flanked by four-story buildings that slightly surmounted the canopy height (Supporting information S1, S2).

The climate of Geneva is characterised as temperate oceanic (Köppen classification: Cfb) with cool winters and warm summers (MAT: 11.0 °C, MAP 946 mm, mean of 1991–2020, MeteoSwiss station Cointrin, adjacent to Geneva). The city is located where the Rhone river exits the Lake Geneva and is surrounded by mountains of up to 1720 m and 1380 m in height in the north and the south, respectively. This geomorphology results in mostly SW/NE oriented wind directions. In the past, Geneva has had an average of 48 heatwave days per decade, which may increase to 108–362 days per decade under future climate conditions (Vaghefi et al., 2022). Between 1991–2020 mean and maximum air temperatures

<caption>Table 1</caption>

Dimensions of the eight *P. acerifolia* trees on the urban square. Diameter at breast height (DBH, tree height, canopy volume, leaf area index (LAI), and leaf area density (LAD; leaf surface to crown volume ratio)).

<table>
  <thead>
    <tr>
      <th>Tree</th>
      <th>DBH (cm)</th>
      <th>height (m)</th>
      <th>canopy volume (m³)</th>
      <th>canopy projected area (m²)</th>
      <th>LAI (m² m⁻²)</th>
      <th>LAD (m² m⁻³)</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Average</td>
      <td>45.1</td>
      <td>11.0</td>
      <td>288</td>
      <td>64</td>
      <td>2.08</td>
      <td>0.46</td>
    </tr>
    <tr>
      <td>1</td>
      <td>47.2</td>
      <td>11.0</td>
      <td>299</td>
      <td>69</td>
      <td>2.03</td>
      <td>0.47</td>
    </tr>
    <tr>
      <td>2</td>
      <td>39.5</td>
      <td>9.2</td>
      <td>167</td>
      <td>45</td>
      <td>1.57</td>
      <td>0.42</td>
    </tr>
    <tr>
      <td>3</td>
      <td>42.5</td>
      <td>10.7</td>
      <td>250</td>
      <td>56</td>
      <td>1.85</td>
      <td>0.41</td>
    </tr>
    <tr>
      <td>4</td>
      <td>53.1</td>
      <td>10.7</td>
      <td>365</td>
      <td>80</td>
      <td>2.05</td>
      <td>0.45</td>
    </tr>
    <tr>
      <td>5</td>
      <td>46.3</td>
      <td>12.3</td>
      <td>385</td>
      <td>77</td>
      <td>2.69</td>
      <td>0.54</td>
    </tr>
    <tr>
      <td>6</td>
      <td>43.1</td>
      <td>11.8</td>
      <td>291</td>
      <td>62</td>
      <td>2.43</td>
      <td>0.52</td>
    </tr>
    <tr>
      <td>7</td>
      <td>44.2</td>
      <td>11.9</td>
      <td>297</td>
      <td>63</td>
      <td>2.16</td>
      <td>0.46</td>
    </tr>
    <tr>
      <td>8</td>
      <td>44.7</td>
      <td>10.6</td>
      <td>248</td>
      <td>57</td>
      <td>1.83</td>
      <td>0.42</td>
    </tr>
  </tbody>
</table>
(T<sub>air</sub>) of the hottest month (July) reached 20.6 and 26.7 °C, respectively (MeteoSwiss station Cointrin, adjacent to Geneva). In contrast, the growing season of 2023 was characterised by two record-breaking heatwaves that occurred in July and August (Fig. 1). T<sub>air</sub> in the urban square reached a maximum of 38.6 °C on August 24th (39.1 °C at MeteoSwiss station Cointrin) when the VPD reached its maximum of 6.8 kPa. Due to a long dry period from May to the end of August (max. 24 days without rain), precipitation sum only amounted to 100.2 mm, which is markedly lower than the long-term average precipitation sum of 321 mm during these four months (mean of 1991–2020, Cointrin MeteoSwiss station).

## 2.2. Sap flow measurements and latent heat flux calculations

In spring 2023, we installed sap flow sensors that use the heat pulse technique (SFM1x sap flow meter, ICT international, Australia) on each of the eight trees at a height of 2 m on the north side of the trunk. The sap flow sensors were covered with an insulated aluminium sheet to prevent heating from irradiation. Sap flux density (SFD; kg m<sup>-2</sup> s<sup>-1</sup>) was recorded at 10 minute intervals from the beginning of May until the end of October 2023. SFD was converted using the Granier equation (Granier, 1987), which we corrected with species-specific wood properties. We estimated sapwood depth using the DBH and species-specific allometric equations, to calculate sapwood area and obtain sap flow (SF; kg hr<sup>-1</sup>). Two of the trees with the largest canopies had repeated record gaps of 1–8 h during times of high E due to insufficient heat pulse energy (20 J). We gap-filled the data by predicting E of these trees with linear regression models using E data from adjacent trees. Whole-tree canopy conductance per unit sapwood area (G<sub>Asw</sub>) was calculated from SFD following Flo et al. (2020) and Hu et al. (2024) using:

$$G_{Asw} = \frac{(115.8 + 0.4236 \times T_{air}) \times SFD}{VPD} + 44.6 \times \frac{273.18}{273.18 + T_{air}} \times \frac{101.325 \times e^{-0.00012 \times h}}{101.325} \quad (1)$$

where T<sub>air</sub> is the air temperature (°C) in the urban square, SFD is the sap flux density (kg m<sup>-2</sup> s<sup>-1</sup>), and h is the elevation of the site (m asl). Latent heat flux (LE; kJ s<sup>-1</sup>) was calculated from SF using the enthalpy of the

state change of liquid to gaseous water at the current T<sub>air</sub>:

$$LE = SF \times (h_g(T_{air}) - h_f(T_{air})) \quad (2)$$

where SF is the sap flow (kg s<sup>-1</sup>), h<sub>g</sub> (T<sub>air</sub>) the enthalpy of saturated steam as a function of the current T<sub>air</sub> (kJ kg<sup>-1</sup>), and h<sub>f</sub> (T<sub>air</sub>) the enthalpy of saturated water as a function of the current T<sub>air</sub> (kJ kg<sup>-1</sup>). To compare measured LE with LE predicted by the UT&C model (see below), we converted it using the projected canopy area to LE (W m<sup>-2</sup>).

## 2.3. Microclimate of the urban square

We installed meteorological stations on two streetlamps in the square at approximately 3 m height, just outside of reach by passers-by. Oke (2004) recommends relaxing the requirements of standard observational heights to avoid sensor damages in urban settings. The stations measured T<sub>air</sub> and RH (ATMOS 14, METER Group, Pullman, WA, USA), photosynthetic active radiation (PAR; MQ-200, Apogee Instruments, Logan, UT, USA) and precipitation (ECRN-100, METER Group, Pullman, WA, USA) at 10 min intervals. T<sub>air</sub> and RH sensors were placed on the north side of the streetlamps, whereas the PAR sensors and the rain gauge were installed on the south side. Wind speed and incoming shortwave and longwave radiation were retrieved from nearby MeteoSwiss stations located in the same urban agglomeration as the study site and thus representative (Perly, 3.5 km from the study site and Cointrin, 7.6 km from the study site). In addition, we put air temperature sensors (HOBO MX2300, Onset Computers, Bourne, MA, USA) in the canopy of each tree at about 4 m above the ground (T<sub>air can</sub>). The sensors were protected by homemade solar shields from direct radiation. In each tree, one sensor was installed close to the trunk and two others at the end of a branch facing orthogonal directions.

## 2.4. Stomatal conductance, water potential, leaf temperature, and tree dimensions

Four times during the growing season (May 23rd, June 19th, July 17th, August 14th), we measured the leaf water potentials at pre-dawn (Ψ<sub>pre</sub>) and midday (Ψ<sub>mid</sub>) of three leaves from the top, middle and lower part of the canopy of each tree with a Scholander-type pressure chamber

<figure><img src="image_3.png" /><figcaption>Fig. 1. Mean daily air temperature (T<sub>air</sub>), mean daily air vapour pressure deficit (VPD), and daily precipitation sums (PP) during the growing season. Shaded areas show daily minima and maxima for T<sub>air</sub> and VPD. Arrows indicate dates of leaf physiological measurements (May 23rd, June 19th, July 17th, August 14th). The yellow rectangles show the two summer heatwaves from July 9–11th and August 18–24th 2023 when T<sub>air</sub> exceeded 35 °C in the urban square.</figcaption></figure>
(M-600; PMS Instruments Inc., Corvallis, OR, USA). Immediately after the cut, leaves were put in plastic bags, sealed, and kept in the dark until processing to avoid further leaf transpiration. On the same dates, we measured the diurnal variation in stomatal conductance (gS) and leaf temperature (Tleaf) four times during the day from approximately 07:30–16:30 local time with an open-path porometer (Li-600, Li-Cor Inc., Lincoln, NE, USA) on five leaves per tree. We selected three leaves from the shaded bottom of the canopy and two leaves from the sunlit top of the canopy facing the urban square. For each tree, we measured the tree canopy's height and radius in four orthogonal directions with a laser meter (Vertex Laser Geo 360°, Haglöf, Sweden). Canopy volume and projected area were calculated assuming canopies with an ellipsoid shape, which was approximately true for the eight *Platanus x acerifolia* trees (Fig. S1). We measured the leaf area index (LAI) of each tree with a plant canopy analyser (LAI-2200C, Li-Cor Inc., Lincoln, NE, USA) under regular overcast sky and with a 180° cap to avoid the influence of the nearby buildings. LAI and the projected canopy were used to calculate the leaf area density of each tree (LAD).

## 2.5. Thermal images of shaded and sunlit canopy and ground surfaces

During the same dates as the leaf measurements, we recorded the surface temperatures of the cobbled granite ground (Tsurf) below each tree in canopy-shaded and sunlit parts three times during the day (09:00, 11:30, 15:30) with a hand-held infrared camera (FLIR E8 Pro, Teledyne FLIR, Germany). At the same time, we determined for all trees the canopy surface temperatures (Tcan) of the side facing the urban square (Fig. S2). Distance from the object (11 m), emissivity of granite (0.96) and foliage (0.97), current Tair, and RH were considered to calculate Tsurf and Tcan from the infrared images with the raw2temp function of the R package *Thermimage* (Tattersall, 2021). We expected that sunlit top and shaded bottom of the canopy would differ in surface temperatures and, therefore, analysed them separately. For this, we used the RGB images of the infrared camera and separated the shaded and sunlit parts of the canopy by the colour (green: "hue" 55°–160° in HSV colour space) and brightness of the pixels ("value" threshold at 20 % in HSV colour space). Transforming from RGB to HSV colour space also allowed to filter out lighting effects (e.g. glare from sunlight reflection with "saturation" threshold at 50 % in HSV colour space) and also removed branches and other surfaces (Fig. S2). We used R for all image analyses (R Core Team, 2023).

## 2.6. Model predictions of latent heat flux in the urban square

We modelled the latent heat flux (LEmod) and stomatal conductance (gSmod) of trees in the urban square with the urban ecohydrological model Urban Tethys-Chloris (UT&C; Meili et al., 2020), which accounts for detailed plant physiological characteristics and environmental conditions. UT&C is a fully coupled water and energy balance model based on a two-dimensional infinite urban canyon. The layout of the urban canyon was parametrised with the measured properties of the urban square (e.g., building height and width, width of the square, tree height, canopy cover, LAI, albedo; Supporting Information S2). While the parametrisation allowed capturing the general properties of the square, spatial variability (e.g., differences in building heights and albedo) were not included for simplicity. Tree LEmod and gSmod are a function of photosynthesis, which is driven by plant traits and environmental forcing (Meili et al., 2020). Predictions for the stomatal response to environmental parameters are based on the widely used Ball-Berry-Leuning function (Fatichi et al., 2012):

$$g_s \cdot co_2 = g_0 \cdot co_2 + a \frac{A_{nC}}{(C_i - \Gamma^*)} f(\text{VPD}) P_{\text{atm}} \quad (3)$$

where gS,CO2 [mmol CO2 m⁻² s⁻¹] is the stomatal conductance, a is an empirical parameter, AnC [µmol CO2 m⁻² s⁻¹] is the net assimilation rate, Ci [ppm] the intercellular CO2 concentration, Γ* [Pa] is the CO2

compensation point, f(VPD) is a sensitivity function to the vapour pressure deficit, Patm [Pa] is the atmospheric pressure, and g0,CO2 [mmol CO2 m⁻² s⁻¹] is the minimum stomatal conductance when AnC = 0. Plant traits were derived from measurements (LAI, leaf dimensions) and environmental conditions retrieved from the local meteo station (Tair, RH, precipitation) or the nearby MeteoSwiss station in Cointrin (short-wave and longwave incoming radiation, wind speed, air pressure) for the growing season 2023 (Supporting Information S2). UT&C performs well compared to flux measurements of eddy-covariance towers, and has been used repeatedly to analyse vegetation impacts of the urban microclimate (Meili et al., 2021; Cheung et al., 2024). The canopy scale fluxes are modelled with a "two-leaf" approach and an exponentially decaying leaf nitrogen profile (Fatichi et al., 2012). The LEmod and gSmod predictions from UT&C were then compared to LE and GAsw calculated from sap flow measurements and gS measured during the field campaigns (see above).

## 2.7. Statistical analyses

We analysed the differences in Ψpre and Ψmid across measurement dates with mixed effects models using lmer (Bates et al., 2014) with the date as the fixed effect, and the tree individual as random effect using R (R Core Team, 2023). Similarly, diurnal changes in gS of shaded and sunlit leaves were analysed with mixed effects models with the time-point during the day as the fixed effect, and the date and tree individual as random effects. Differences in Tleaf, Tcan, Tsurf of shaded and sunlit parts were analysed at each timepoint during the day with mixed effects models. The shading and the object type (canopy, ground surface, sunlit, shaded) were fixed effects and the date was a random effect. For all mixed effects models, the assumptions of homoscedasticity were visually assessed, and met without transformation (Tleaf, Tcan, Tsurf) or with square root transformation (Ψpre, Ψmid, gS). Breakpoints where the linear relation of gS, GAsw, modelled gS and modelled LE to VPD and Tair change were identified by analysing segmented linear relationships with the function segmented (Fasola et al., 2018). The residuals were visually inspected and no outliers with high leverage were observed in the data.

# 3. Results

## 3.1. Leaf water potential, stomatal conductance, and surface temperatures

Despite the high Tair and low precipitation throughout the growing season (Fig. 1), Ψpre never dropped below −1 MPa. While Ψmid also stayed above critical water stress thresholds throughout most of the growing season, it reached −1.7 MPa in late summer before the second heatwave. It thereby exceeded the species-specific water potential that induces leaf turgor loss (ΨTLP; −1.58 MPa; Petruzzellis et al., 2022) and approached the species-specific water potential at which the xylem loses hydraulic conductivity by 50 % (P50; −1.75 MPa; Nardini et al., 2021; Fig. 2a). Across the growing season, canopy temperatures (Tcan) were low in the morning in both the shaded and sunlit parts (20.4 and 20.9 °C, respectively) and increased to 27.6 and 28.3 °C in the afternoon during the hottest time of the day. Still, Tcan remained 0.86 and 0.36 °C below Tair for the shaded and sunlit parts, respectively (Fig. 2b, Fig. S3). Tcan did not differ from the shaded cobblestone ground (Tsurf), except during early morning hours when shaded Tcan was slightly cooler than shaded Tsurf. In contrast, Tcan was, on average, 9.0 °C below the sunlit Tsurf. Differences were most pronounced in the afternoon when sunlit Tcan were up to 14.8 °C lower compared to the sunlit Tsurf (Fig. 2b). Tair can showed a distinct diel cycle and was, on average, 0.35 °C warmer than Tair during the night, and 0.16 °C warmer than Tair during the day (Fig. S4). gS of shaded and sunlit leaves were high throughout the day (up to 0.13 mmol m⁻² s⁻¹) and both showed a diurnal increase and decrease (Fig. 2c). Contrary to our expectations, the sunlit leaves showed no distinct midday depression, similarly to shaded leaves
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** a) Leaf water potential measured at pre-dawn (Ψ<sub>pre</sub>, blue circles) and midday (Ψ<sub>mid</sub>, red circles) from May to August 2023 (means ± s.e. of eight trees). The dashed horizontal lines indicate the leaf turgor loss point (Ψ<sub>TLP</sub>, Petruzzellis et al., 2022), and the xylem pressure inducing 50 % loss of hydraulic conductivity (P<sub>50</sub>, Nardini et al., 2021) of *Platanus x acerifolia*. b) Diurnal surface temperatures of the cobbled ground (T<sub>surf</sub>) in sunlit and shaded parts of the urban square (means ± s.e. of 24 measurements), shaded and sunlit parts of the canopy (T<sub>can</sub>; means ± s.e. of 24 measurements), and T<sub>air</sub> at the time of the measurements. c) Diurnal stomatal conductance (g<sub>s</sub>) from early morning to late afternoon across the four measurement campaigns for the shaded (dark green) and sunlit (light green) leaves (means ± s.e. of eight trees). Letters indicate significant differences (p < 0.05).</figcaption></figure>

(Fig. 2c).

3.2. Stomatal and canopy conductance during heatwaves

Canopy conductance (G<sub>Asw</sub>) and the UT&C modelled stomatal conductance (g<sub>s</sub> mod) both increased with T<sub>air</sub> and VPD to a peak at 19.3 and 18.7 °C, and at 2.0 and 1.9 kPa, respectively, with a subsequent linear decrease at higher T<sub>air</sub> and VPD (Fig. 3). In contrast, measured gs did not show an initial increase, as readings were done at a smaller range of environmental conditions during four campaigns during daytime

(Fig. 1), covering T<sub>air</sub> of 19.3–35.9 °C and VPD of 1.9–5.0 kPa (Fig. 3c, d). gs measurements thus covered the entire range of the linear decrease observed for G<sub>Asw</sub> and g<sub>s</sub> mod. From T<sub>air</sub> of 20–35 °C and VPD of 2.0–5.0 kPa, the measured conductances (G<sub>Asw</sub> and g<sub>s</sub>) decreased at a much lesser rate than g<sub>s</sub> mod. While G<sub>Asw</sub> decreased by only 48 % and 36 % in these T<sub>air</sub> and VPD ranges, respectively, gs decreased by 71 % and 48 %. By contrast, gs was reduced by 80 % and 67 % at these T<sub>air</sub> and VPD ranges, respectively. Hence, UT&C predicted much stronger decrease of gs under high T<sub>air</sub> and VPD compared to the measurements.

3.3. Tree sap flow and latent heat flux sensitivity to air temperature and VPD

Sap flow (SF) was strongly related to the volume of the tree canopy (R² = 0.89; Fig. S5), therefore SF was normalised to an averaged crown volume for subsequent analyses. G<sub>Asw</sub> was markedly higher when T<sub>air</sub> and VPD were relatively low in May, compared to the rest of the growing season, and showed slight midday depression during the two heatwaves (Fig. S6). Nevertheless, trees showed consistently high SF and LE with daily maximum of up to 37.1 kg hr⁻¹ of SF and 25.3 kW LE per tree throughout the growing season (Fig. S6). Even during the heatwaves in July and August, where T<sub>air</sub> reached > 35 °C and VPD > 6 kPa, trees showed no E and LE decline (Fig. S6). SF and LE increased with T<sub>air</sub> and VPD without any reduction up to extremely high maximum T<sub>air</sub> of 38.6 °C and VPD of 6.8 kPa (Fig. 4a–d).

3.4. Model predictions vs. measured LE during heatwaves

In contrast to measured *LE*, modelled *LE* with UT&C (*LE*<sub>mod</sub>) showed a sharp increase to a maximum of 352 kW at 28.1 °C T<sub>air</sub> and 3.4 kPa VPD, above which it quickly decreased to 207 and 218 kW at the maximum T<sub>air</sub> and VPD, respectively (Fig. 4e, f). As a consequence, UT&C underestimated *LE* at heatwave temperatures above 30 °C compared to the *LE* measured with the sap flow sensors (Fig. 5a). As high T<sub>air</sub> above 30 °C were mostly measured during afternoons, the under-estimation of *LE* cooling particularly concerns afternoon hours (Fig. 5b). Hence, UT&C predictions lead to an inaccurate representation of the diel course of *LE* during heatwaves.

The differences in *LE* between measurements and model predictions have important consequences on the energy budget of the urban square. *LE* was on average 33.3 % of the incoming solar radiation (R<sub>S</sub>), irrespective of T<sub>air</sub>, whereas *LE*<sub>mod</sub> showed a strong temperature decrease and was only 22.8 % of R<sub>S</sub> at T<sub>air</sub> above 35 °C (Fig. 6).

4. Discussion

4.1. Heat and drought effects on tree water stress

By studying *Platanus x acerifolia* during an exceptionally hot and dry summer in an urban square in Geneva, we observed that the trees were able to maintain relatively high *E* throughout two heatwaves despite VPD of up to 6.8 kPa, T<sub>air</sub> reaching extremes of 39.1 °C, and markedly lower precipitation (Fig. 1). During early summer, tree physiological drought stress was low, as indicated by leaf water potentials at pre-dawn (Ψ<sub>pre</sub>) and midday (Ψ<sub>mid</sub>) that remained above critical thresholds of leaf turgor loss point (Ψ<sub>TLP</sub> of -1.58 MPa; Petruzzellis et al., 2022) and P<sub>50</sub> (-1.75 MPa; Nardini et al., 2021; Fig. 2). In mid-August, however, Ψ<sub>mid</sub> surpassed Ψ<sub>TLP</sub> and approached P<sub>50</sub>, suggesting significant hydraulic stress during the second heatwave. Nevertheless, we did not observe heat- or drought-induced damage on the trees during that time. Changes in water availability, for instance through irrigation, can influence urban tree functioning to an even larger extent than trees growing in natural ecosystems (Ibsen et al., 2023). Unfortunately, the sealed surface at the study site (Fig. S1) prevented soil water content measurements. According to local city gardeners, the trees were not irrigated during the study period, but at the study site the groundwater table is relatively
<figure><img src="image_5.png" /><figcaption>Fig. 3. a), b) Canopy conductance (G<sub>Asw</sub>) normalised to the average canopy volume (288 m<sup>3</sup>) in relation to the air temperature (T<sub>air</sub>) and vapour pressure deficit (VPD) measured from May to October 2023. c), d) stomatal conductance (g<sub>s</sub>) measured with the leaf porometer during four campaigns (see Fig. 1) in relation to T<sub>air</sub> and VPD. e), f) g<sub>s</sub> predicted with the UT&C model (g<sub>s mod</sub>) from May to October 2023 in relation to T<sub>air</sub> and VPD. Lines show LOESS regressions of the median values and shaded parts are the 20 % and 80 % quantiles. Night-time data (PPFD < 50 µmol m<sup>-2</sup> s<sup>-1</sup>) was omitted. Vertical dashed lines show breakpoint estimates for linear responses of G<sub>Asw</sub> and g<sub>s mod</sub> to T<sub>air</sub> and VPD. Arrows and corresponding numbers show the decrease of G<sub>Asw</sub> and g<sub>s</sub> between 20–35 °C and 2–5 kPa.</figcaption></figure>

shallow (https://map.sitg.ge.ch), which may have allowed the trees to access soil water and maintain less negative Ψ<sub>pre</sub> throughout the growing season. However, the large decrease in Ψ<sub>mid</sub> and potential loss of hydraulic conductance despite absence of soil water stress may have been induced by the particularly high VPD and T<sub>air</sub> during the second heatwave, which is consistent with experimental observations of trees under well-watered conditions (Schönbeck et al., 2022).

## 4.2. Canopy conductance, stomatal conductance, and canopy cooling

Platanus *x* *acerifolia* is considered a relatively isohydric tree species (Caplan et al., 2019), thus g<sub>s</sub> is expected to be tightly restricted by low leaf water potentials. The degree of isohydricity may, however, not be a characteristic that is invariable, but can shift with changing environmental conditions in woody plants (Guo et al., 2020). Indeed, while the trees in the urban square showed an expected reduction of stomatal conductance (g<sub>s</sub>) and canopy conductance (G<sub>Asw</sub>) in response to high T<sub>air</sub> and VPD, the decrease was much less pronounced than the UT&C model predicted (Fig. 3). The monitored *Platanus x acerifolia* thus showed a less isohydric response despite reaching critical hydraulic stress levels (Fig. 2). G<sub>Asw</sub> is controlled by trees to limit *E* during times of hydraulic stress (Chen et al., 2011). In our study, G<sub>Asw</sub> increased to a VPD threshold of 2 kPa and a T<sub>air</sub> threshold of 19.3 °C above which it steadily decreased to its minimum at the highest VPD and T<sub>air</sub>. The extent of the decrease by 48 % between 20 °C and 35 °C slightly exceeded observations from eddy covariance estimates of deciduous broadleaved forests

G<sub>Asw</sub> that showed a 40 % decline during heatwaves (Wang et al., 2023). Hence, the single trees on the urban square appear to be more sensitive to VPD than forest trees. This generally corresponds to observations across European forests, where trees in less dense forests, i.e. lower basal area, show a higher *E* sensitivity to VPD (Bachofen et al., 2023). The higher sensitivity of less densely standing tree individuals compared to dense forest stands can be explained by higher light availability and reduced aerodynamic resistance through increased airflow within the canopy (Moore et al., 2004; Gebhardt et al., 2014), conditions that urban street trees regularly face.

Furthermore, g<sub>s</sub> showed a similar decrease as G<sub>Asw</sub>, but due to a different measurement approach with hand-held instruments (Fig. 1), the T<sub>air</sub> and VPD range was restricted to 19.3–35.9 °C and 1.9–5 kPa (Fig. 3c, d). The same T<sub>air</sub> and VPD thresholds that were observed for G<sub>Asw</sub> could therefore not be assessed for g<sub>s</sub>. A comparison with the g<sub>s</sub> predicted by UT&C was possible in the overlapping T<sub>air</sub> and VPD range, and showed that the model severely underestimated conductance at high T<sub>air</sub> and VPD (Fig. 3). The stomatal responses to heatwaves underlie two principally independent processes: the response to higher T<sub>air</sub> and to high VPD (Mills et al., 2024; Slot et al., 2024). While a high VPD induces stomatal closure (Monteith, 1995; Cochard et al., 2002), higher T<sub>air</sub> at a constant VPD and sufficient soil moisture typically increases g<sub>s</sub> (Mills et al., 2024). In the UT&C model, g<sub>s mod</sub> is a function of the minimum leaf conductance g<sub>0</sub>,CO<sub>2</sub>, and the sensitivity of stomata to VPD (f(VPD): Eq.3). While g<sub>0</sub>,CO<sub>2</sub> and f(VPD) seem to be relatively well described for lower temperatures, we lack a full understanding of their responses to
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** a, b) Sap flow (SF) normalised to the average canopy volume (288 m³), and c, d) latent heat flux (LE) of the averaged projected crown area of the *P. acerifolia* trees in relation to the air temperature (Tair) and vapour pressure deficit (VPD) measured from May to October 2023. e, f) latent heat flux (LEmod) calculated with the UT&C model in relation to Tair and VPD. Lines show LOESS regressions of the median values and shaded parts are the 20 % and 80 % quantiles. Night-time data (PPFD < 50 µmol m⁻² s⁻¹) was omitted. Vertical dashed lines show breakpoint estimates for linear responses of LEmod to Tair and VPD.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** a) Modelled latent heat (LEmod) in comparison to measured LE. At high air temperatures (Tair) > 30 °C the UT&C model underestimated LE. The dashed line shows the 1:1 ratio. b) Diel LEmod and LE at different Tair ranges averaged for the entire growing season. While UT&C predict a strong midday depression of LE under Tair > 30°C, measurements show almost no decline even during hottest times of the day.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Proportion of measured (blue) and predicted (red) latent heat flux (LE) to total incoming solar energy $R_s$ at different air temperatures measured at the study site (medians and 20–80 % quantile ranges). At temperatures above 35 °C, the predicted proportion of LE decreased significantly to 22.8 %, whereas the measured proportion of LE remained relatively constant at 33.3 %.</figcaption></figure>

high $T_{\text{air}}$ (Mills et al., 2024). Prolonged exposure to chronic warming can modify f(VPD), leading to a less steep decrease in $g_s$ with rising VPD (Grossiord et al., 2017), with potential implications for leaf temperature. Similarly, $g_{0,\text{CO}_2}$ may increase with higher temperatures (Duursma et al., 2019), and plants may acclimate their $\Psi_{\text{TLP}}$ (Tordoni et al., 2022), leading to higher than expected transpiration during heatwaves. As these processes are not included in currently widely-used stomatal functions, they fail to capture high $T_{\text{air}}$ responses of $g_s$. The physiological mechanisms by which $g_s$ increases under higher $T_{\text{air}}$ are not fully resolved, but range from increased hydraulic conductivity, changes in guard cell responses, responses of abscisic acid (ABA) metabolism and photosynthesis-related responses that affect $g_s$ (Mills et al., 2024). Higher $g_s$ during heatwaves might be an adaptive mechanisms to increase transpirational cooling of leaves (Crawford et al., 2012; Drake et al., 2018; Diao et al., 2024). It has indeed been observed that on longer time scales, trees can acclimate to higher $T_{\text{air}}$ (and higher VPD) by increasing $E$, and thus reduce leaf temperatures (Marchin et al., 2016), showing an adaptive link between $g_s$ and $T_{\text{leaf}}$.

Hence, in addition to the environmental effects on $G_{\text{Asw}}$ and $g_s$ (e.g., stand density, see above), the observed relatively high $g_s$ at $T_{\text{air}}$ above 30 °C (Fig. 3b) might partly reflect a cooling strategy of the *Platanus* x *acerifolia* trees during heatwaves. Surprisingly, the air temperature measured in the tree canopy ($T_{\text{air can}}$) was higher than $T_{\text{air}}$ most of the time during the growing season (Fig. S4). We assume that longwave radiation trapping below the tree canopies led to the higher $T_{\text{air can}}$ (Ziter et al., 2019; Meili et al., 2021). As we did not measure longwave radiation in the tree canopy, we refrained from using $T_{\text{air can}}$ to infer $E$ cooling effects. Tree leaf temperature can frequently exceed $T_{\text{air}}$, particularly when they are sunlit, and thereby reach 5–15 °C higher temperatures (Still et al., 2019; Gauthey et al., 2024). Excessive heat can induce a wide range of physiological damages to leaves, including photosynthesis (O’sullivan et al., 2017), therefore $E$ cooling might prevent severe damages during heatwaves. In our study, we observed that while the cobblestone ground was up to 15.7 °C hotter in the sunlit than the shaded part of the urban square, the transpiring canopy was constantly cool, and maintained high $g_s$ irrespective of sun exposure (Fig. 2). While the lower temperature of the shaded cobblestone ground was due to the tree shading (dry surfaces, no LE cooling), the sunlit canopy was effectively cooled by LE. The canopy cooling was on average between 0.68 and 0.38 °C below $T_{\text{air}}$ and 0.26 and 9.0 °C below ground temperatures during the day for the shaded and sunlit part, respectively

(Fig. 2b, Fig. S3). Most importantly, canopy cooling increased from early morning to afternoon hours (Fig. 2b), and thus differences were largest during the hottest times of the day (Armson et al., 2013; Chen et al., 2019) when cooling is most desired (Lin et al., 2017). These differences are less pronounced than Pauleit and Duhme (2000) who reported differences in up to 20 °C, but correspond relatively well to reports of Leuzinger et al. (2010) who found differences within a few °C for urban *Platanus* x *acerifolia* and other species. Moreover, Marchin et al. (2023) observed on several urban tree species, one of them also *Platanus* x *acerifolia*, on average a 23 % higher $E$ than predicted by the unified stomatal optimization (USO) model (Medlyn et al., 2011). This effectively allowed cooling the canopy of well-watered trees to 0.86 °C below $T_{\text{air}}$ in an experimental setting, but showed no consistent effect in-situ during heatwaves. *Platanus* x *acerifolia* has relatively long petioles (5–10 cm) that reduce the boundary layer resistance and allow for convective cooling (Bridge et al., 2013), and reflective trichomes that may cool leaves by increasing reflection (Ehleringer and Mooney, 1978). Thus, while many processes of leaf and canopy thermal regulation, including the coupling of canopy and $T_{\text{air}}$, are not yet well understood (Gauthey et al., 2023), $E$ might significantly contribute to avoiding leaf damage during heat waves.

### 4.3. Latent heat cooling during heatwaves

Under given environmental conditions, $E$ increases with higher $g_s$, therefore the relatively high measured $g_s$ at high $T_{\text{air}}$ led to high $E$ and LE under these environmental conditions (Fig. 4, S5). We observed repeated record gaps of 1–8 h of the trees with the largest canopies during times of high $E$, which we gapfilled using $E$ data from adjacent trees. While this might have introduced some uncertainty in the LE estimations, comparisons of sap flow and canopy size across all trees indicate that no bias was introduced by this (Fig. S5). The disparity of $LE_{\text{mod}}$ and LE at high $T_{\text{air}}$ can thus be explained by differences of modelled $g_s$ ($g_s$ mod) with measured $g_s$ and $G_{\text{Asw}}$ in relation to $T_{\text{air}}$ (Figs. 3 and 4). Hence, despite high $T_{\text{air}}$ of up to 39.1 °C during the summer heatwaves, LE cooling persisted in the urban square in Geneva. In particular, the cooling by LE with the heating by the incoming solar energy ($R_s$) remained consistently high, around 33.3 % at all $T_{\text{air}}$, even above 35 °C (Fig. 6). This corresponds to findings in the significantly cooler city of Gothenburg, where 30 % of $R_s$ was converted into LE at $T_{\text{air}}$ of up to 24.4 °C (Konarska et al., 2016). The large contribution of LE observed in our study was surprising, as predictions based on the widely used Ball-Berry-Leuning function of stomatal response (Fatichi et al., 2012) were estimating a decline to 22.8 % at $T_{\text{air}}$ above 35 °C (Fig. 6). Moreover, previous reports of urban tree transpiration cooling have reported declining LE cooling both during hot days ($T_{\text{air}}$ > 30 °C; Rahman et al., 2018) as well in hot ($T_{\text{air}}$ of 36.2 °C) compared to temperate cities (Shashua-bar et al., 2023). As the trees in these two studies experienced similarly hot $T_{\text{air}}$ as the trees in the urban square of Geneva, $T_{\text{air}}$ alone cannot explain the reduced LE cooling in these studies. Rather, lower soil moisture might have reduced $E$ (Rahman et al., 2018; Shashua-bar et al., 2023). For models of climate predictions, this indicates that the assumed responses of $g_s$ to VPD and $T_{\text{air}}$ (Eq. 3) might not be sufficient for describing $E$ and LE at higher air temperatures. In process-based models, $g_s$ (and by extension $E$) is often based on schemes balancing the trade-off between water loss and $CO_2$ uptake through the stomata (Flo et al., 2024), not accounting for the beneficial effect of $E$ for the plant. However, our findings support the notion that future stomatal optimisation models might need to account for additional optimisation parameters that account for the cooling effect of $E$. More refined models would need to account for photosynthetic acclimation processes, as proposed by (Marchin et al., 2016; Flo et al., 2024), include environmental gradients within tree canopies (Bachofen et al., 2022) and leaf cooling among other evolutionary fitness proxies, as proposed by (Potkay et al., 2025). Understanding tree transpiration under heatwave temperatures is relevant on a larger scale to estimate feedback responses
to regional climate. In particular, the exchange of evapotranspiration with the boundary layer can favour the formation of "mega heatwaves", such as the 2003 and 2010 heatwaves in Europe and Russia, respectively (Seneviratne et al., 2006; Fischer et al., 2007; Kala et al., 2016). Thus, a better estimation of the contribution of E and thus LE to the energy exchange of the land surface with the atmosphere is needed to better simulate such events (Seneviratne et al., 2006).

## 5. Conclusions

The amount of LE cooling providing thermal comfort during more frequently expected summer heatwaves will depend on the physiological responses of the urban vegetation to heat stress. By studying LE of *Platanus x acerifolia* during two summer heatwaves in an urban square, we observed that despite unprecedented Tair of up to 39.1 °C, trees continued cooling at 33.3 % of Rs, and that this cooling effect was severely underestimated by current model predictions of gs. While this can provide some confidence into the continued provisioning of important ecosystem services, several beneficial circumstances might have contributed to the large cooling effect. For instance, the studied *Platanus x acerifolia* trees have a diffuse-porous wood anatomy, which allows for 2–3 times higher transpiration rates compared to ring-porous species (Bush et al., 2008; Peters et al., 2010). Thus, our findings cannot be extrapolated, and species-specific LE cooling needs to be assessed to estimate general urban tree effects. Furthermore, the trees in our study maintained relatively high transpiration despite approaching the risk of hydraulic failure due to a combination of high Tair and VPD under sufficient soil water access. At drier locations, summer heatwaves might co-occur with droughts, imposing additional stress and potentially limiting E more severely. Despite these limitations, we were able to show that current models may severely underestimate urban cooling, because they fail to accurately represent stomatal responses to high temperatures. To optimise tree planting strategies in cities, further research on a wider range of tree species is needed to understand their cooling provided by transpiration during heatwaves.

### CRediT authorship contribution statement

**Grossiord Charlotte:** Writing – review & editing, Funding acquisition, Conceptualization. **Bourgeois Ilann:** Writing – review & editing, Conceptualization. **Meili Naika:** Writing – review & editing, Investigation, Formal analysis. **Peillon Marion:** Writing – review & editing, Investigation, Formal analysis, Data curation. **Bachofen Christoph:** Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization.

### Declaration of Generative AI and AI-assisted technologies in the writing process

No generative AI or AI-assisted technology was used in the preparation of this work.

### Declaration of Competing Interest

The authors declare that they have no conflict of interest or personal relationships that could have influenced this work.

### Acknowledgements

The authors acknowledge funding from the ENAC interdisciplinary cluster grant of EPFL Lausanne. We are also grateful to the city of Lancy for allowing sensor installation at the study site.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ufug.2025.128819.

## Data availability

All data will be made available on a public repository upon publication.

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