## 2 Methods
### Description of the Sites
#### 2.1.2 Meteorological Conditions  
The meteorological conditions at our study sites can be approximated using (long-term) values of the temperature, the precipitation, the number of rainy days, the relative humidity, the sunshine duration, and the global solar radiation measured at the Royal Meteorological Institute's (KMI) meteorological station in Ukkel (Table 1). These values were averaged for each season and compared against normal values for the reference periods of 1981-2010 and 1991-2020. Daily values of the temperature, the precipitation, the relative humidity, the vapor pressure deficit, and the volumetric water content are also presented here (Figure 1A-E). These were measured by the Flemish Institute for Nature and Forest (INBO) and the Integrated Carbon Observation System (ICOS) at the meteorological station of Brasschaat. When required, the data were gap-filled using measurements of the Royal Netherlands Meteorological Institute's (KNMI) meteorological station of Woensdrecht (The Netherlands). More information on the meteorological stations and the measurements methods can be found in the literature [74; 75; 71]. \begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c} \hline  & \multicolumn{4}{c}{**Normal**} & \multicolumn{4}{c}{**2019**} & \multicolumn{4}{c}{**2020**} & \multicolumn{4}{c}{**Normal**} & \multicolumn{4}{c}{**2021**} \\  & \multicolumn{4}{c}{**OBS4-2010**} & \multicolumn{4}{c}{**OBS4-2010**} & \multicolumn{4}{c}{**} \\ \cline{2-13}  & **Winter** & **Spping** & **Summer** & **Autumnumn{1}{c}{**Winter**} & **Spping** & **Summer** & **Autumn{1}{c}{**Winter**} & **Spping** & **Summer** & **Autumn{1}{c}{**Winter**} & **Winter** & **Winter** \\ \hline
**Average temperature (C)** & 3.6 & 10.1 & 17.6 & 10.9 & 5.2 & 10.5 & 19.1 (+) & 11.3 & 6.3 (+) & 11.3 & 18.8 & 12.3 (+) & 4.1 & 4.7 \\ \hline
**Total precipitation (nm)** & 220.5 & 187.8 & 224.6 & 219.9 & 238.5 & 178.5 & 198.6 & 209.3 & 203.3 & 105/C (\(\gamma\)) & 1682.2 & 219.2 & 228.6 & 264.1 \\ \hline
**Average number of elany days** & 54.8 & 49 & 43.9 & 51 & 48 & 44 & 33 & 53 & 58 & 23(-) & 46 & 43 & 55.2 & 54.8 \\ \hline
**Relative humidity (\%)** & 84 & 74 & 73 & 82 & 84(-) & 72 & 70 & 83 & 85 & 61(-) & 66(-) & 79(-) & 84 & 84 \\ \hline
**Smulating duration (nm)** & 180.18 & 463.58 & 578.20 & 322.00 & 226.13 (+) & 489.42 & 714.85 (+) & 322.23 & 169.58 & 764.66 (++) & 602.50 & 346.35 & 180.17 & 182.22 \\ \hline Global solar radiation (QWR/m*) & 73.9 & 325 & 429.6 & 168.2 & 87.6 & 345.6 & 487.9 (+) & 178.4 & 73.3 & 61(-) & 454.8 & 177 & 75.5 & 83.1 \\ \hline Vapor pressure (MPa) & 6.9 & 9.2 & 145 & 11 & 6.9 & 9 & 15 & 11.2 & 8.2 (+) & 8.1 (-) & 13.9 & 11.2 & 7.3 & 7.4 \\ \hline Air pressure (MPa) & 1017.3 & 1015.2 & 1016.2 & 1015.6 & 1016.4 & 1015.6 & 1015.4 & 1011(-) & 1015.2 & 1007.8 & 1014.2 & 1016.2 & 1007.1 & 1011.3 (+) \\ \hline \end{tabular}
\end{table} 
Table 1: Overview of the meteorological conditions perceived by the mature trees in the study region in 2019, 2020, and the winter of 2021. All data were measured by the meteorological station of the Royal Meteorological Institute (KMI) in Ukkel, Belgium [76–41]. Values within the five highest/lowest values since the reference periods 1981–2010 and 1991–2020 are marked by (+/-), while values within the three highest/lowest values are marked by (++/-). To indicate the drought stress, we also present here the rainfall deficit for the hydrological years 2019 to 2021 (Figure 2). We first computed the potential evapotranspiration using the [PERSON] et al. [86] method and using data from the KMI's meteorological station in Ukkel (see [PERSON] [87] and [PERSON] et al. [88] for details). By accumulating the daily potential evapotranspiration minus the daily sum of the precipitation, we were then able to compute the daily rainfall deficit per hydrological year (i.e., starting from a zero deficit around the first of April) and using continuous computation (i.e., when accounting for the hydrological fraction in the wet period that does not contribute to building up ground water reserves).