3.1. Monitoring
The study period was from August 20th to November
3rd 2021. Climate data (air temperature, precipitation
amount, wind speed and direction, relative humidity, air pressure,
global radiation) were available from the rooftop of IGB
~300 m away. Additionally on site, precipitation
(tipping bucket raingauge, 0.2
mm/tip, precision ±3% of total rainfall; AeroCone® Rain Collector,
Davis Instruments, Hayward, USA) was recorded with a CR800 Datalogger
(Campbell Scientific, Inc. Logan, USA) logging every 15 min. Temperature
was recorded (every 5 min) with
BetaTherm 100K6A1IA Thermistors
T107 (Campbell Scientific, Inc. Logan, USA; tolerance ±0.2°C (over
0°–50°C)), with a CR300 Datalogger (Campbell Scientific, Inc. Logan,
USA). Precipitation and temperature data were verified against available
data from the German Weather Service (DWD) of the “Berlin-Marzahn”
station (Station ID: 420), ~12 km north of the study
site.
Precipitation for stable water isotope analysis was collected using a
HDPE deposition sampler (100 cm2 opening;
Umwelt-Geräte-Technik GmbH, Müncheberg, Germany). Overall, 32 daily and
15 bulk (interval ~weekly) samples with precipitation
>1 mm (to limit evaporation effects) were collected between
July and November 2021. Further, daily precipitation samples were
collected ~350 m away from the study site with an
autosampler (ISCO 3700, Teledyne Isco, Lincoln, USA) at a 24 hours
interval. All autosampler bottles were filled with a paraffin oil layer
> 0.5 cm in thickness (after IAEA/GNIP, 2014) to avoid
evaporative effects. Additionally, groundwater samples were taken weekly
with a submersible pump (COMET-Pumpen Systemtechnik GmbH & Co. KG,
Pfaffschwende, Germany) from a well on IGB grounds ~300
m away from the site.
For isotope analysis of the liquid water samples at the IGB laboratory,
samples were filtered (0.2 um cellulose acetate) and decanted into 1.5
ml glass vials (LLG LABWARE). They were analysed by
cavity ring-down spectroscopy
(CRDS) with a L2130-i Isotopic Water Analyser (PICARRO, INC., Santa
Clara, CA) using four standards for a linear correction function and
which were referenced against three primary standards of the
International Atomic Energy Agency (IAEA) for calibration (VSMOW2
(Vienna Standard Mean Ocean Water 2), GRESP (Greenland Summit
Precipitation) and SLAP2 (Standard Light Antarctic Precipitation 2)).
Liquid samples were injected six times and the first three injections
discarded. To screen for interference from organics, the ChemCorrect
software (Picarro, Inc.) was applied and contaminated samples discarded.
After quality-checking and averaging multiple analyses for each sample,
the results were expressed in δ-notation with Vienna Standard Mean Ocean
Water (VSMOW). Analytical precision was 0.05 ‰ standard deviation (SD)
for δ18O and 0.14 ‰ SD for δD.
Stable isotopes of atmospheric water vapour (δv) were
measured in-situ at the tree-dominated and grassland sites,
respectively at 0.15 m, 2 m and 10 m height to capture the effects of
vegetation heterogeneity and potential turbulence within an urban
surface boundary layer. To monitor the elevation profile above the
grassland, a 10 m flag mast with ~ 100 cm long
perpendicular poles at required sample points was set up (Fig. 2). At
the tree site, we measured directly at the trunk within the canopy of
the maple tree. The measurement campaign started on 20.08.2021 above the
grassland and on 03.09.2021 in the tree canopy.
We performed in-situ real-time sequential measurements of water
vapour via CRDS (Picarro L2130-i, Picarro Inc., Santa Clara, CA, USA)
placed in a box between the sampling sites. Air inlets and CRDS were
connected with polytetrafluoroethylene (PTFE) tubing (1.6 mm x 3.2 mm).
We used PET bottles covered with aluminum foil to prevent the inlets
from rain and sun exposure. Each tube inlet (Fig. 2) was sampled for 20
min in resolution of seconds. Then sampling was switched automatically
to the next one; resulting in a 2-hourly resolution for each inlet. We
only used the data when a measurement showed stable values (i.e. ranges
of 2 ‰ for 2H and 0.3 ‰ for 18O).
The first 5 min of data after switching inlets were always discarded to
avoid memory effects. Prior the vapour entering the CRDS unit, a
preceding sub-micron particulate filter was connected to prevent liquid
water from entering by creating a low dew point by lowering the air
pressure. The sample flow rate was
at 0.04 L min−1. Water vapour concentrations were
always above 6000 ppm (this is where the concentration dependent
deviation becomes low and thus measurement precision is not
compromised).
To allow for later conversion of δv measurements into
liquid water isotope values, temperature probes were installed at all
heights near the tube inlets at both sites with BetaTherm 100K6A1IA
Thermistors T107 (Campbell Scientific, Inc. Logan, USA; tolerance ±0.2°C
(over 0°–50°C)), with a CR300 Datalogger (Campbell Scientific, Inc.
Logan, USA) logging mean values every 5 min from secondly-resoluted
data. To avoid tube condensation, heating cables (ILLw.CT/Qx, Quintex
GmbH, Lauda-Königshofen, Germany) were installed and wrapped with the
tube in insulation material. The cables were controlled via an automatic
multi socket (Gembird 235 EG-PMS2, Gembird Software Ltd., Almere, The
Netherlands) to prevent overheating in summer. To minimise condensation
effects, the measurements were checked daily. The PTFE-tubes were
flushed weekly or if required for 10 minutes per probe to remove any
water . Data was discarded when condensation inside the system was
identified in the respective tube.
By combining δv and temperature data from each inlet we
derived the values for all heights of temperature dependent equilibrium
fractionation from vapour to liquid with the correction formulated by
Majoube :
\(\alpha=exp\frac{a\left(\frac{10^{6}}{T_{k}^{2}}\right)+b\ \left(\frac{10^{3}}{T_{k}}\right)+c}{1000}\)(1)
where α is the isotopic fractionation factor,Tk is the temperature (in K), and a ,b , and c are empirical parameters that vary depending on
the isotopologue. All values of isotopic compositions are given in
liquid phase and relative to Vienna Standard Mean Ocean Water (VSMOW).
To investigate the local evaporative effects, the line-conditioned
excess (short lc-excess) (see ) was calculated. The lc-excess describes
the deviation of the sample from the local meteoric water line (LMWL):
lc-excess = δ2H − a · δ18O −b (2)
where a is the slope and b the intercept of the weighted
isotopic composition of the local precipitation. The LMWL was calculated
by amount-weighted least square regression from daily precipitation
isotopes measured at IGB from July until November 2021.
In order to assure stable values to offset variability in the field,
stability of the CRDS was tested in the lab before installing the setup
outside. During the sampling campaign, we calibrated once a week (cf.
calibration periods ) with two standards. Stored in sealed glass
containers, the standards were connected to the CRDS for two-point
calibrations (liquid values: light: 2H -109.91 ‰/18O -17.86 ‰; medium: 2H -56.7 ‰/18O -7.68 ‰). We used measured water vapour
concentrations and added linear regressions of temperature dependency
slopes to correct for isotopic offsets and vapour concentration
dependency (resembling the approach by Schmidt et al. ).
We also monitored sap velocities and stem circumference of the maple
tree. Two sap flow sensors (SFM-4, Umwelt-Geräte-Technik GmbH,
Müncheberg, Germany; ±0.1 cm/hr heat velocity precision) were installed
at breast height (1.3 m) at the north and south side of the tree stem.
The sap flow sensors work according to the heat ratio method by Marshall
. Daily reference crop evapotranspiration (ET0) was
estimated using the FAO Penman–Monteith method with “R”-Package
“Evapotranspiration” . To investigate dynamics during the growing
season, both daily mean sap velocity [cm h-1] and
ET0 were then normalized (to
sapvelocitynorm and ETnorm,
respectively) by feature scaling. One dendrometer (DR Radius
Dendrometer, Ecomatik, Dachau, Ger170; accuracy max. ± 4.5% of the
measured value (stable offset)) was also installed to measure stem
diameter dynamics at high temporal resolution. Sap velocity and stem
increments were logged as 15 min intervals using a CR300 Datalogger
(Campbell Scientific, Inc. Logan, USA). Throughfall amount was sampled
manually at a height of 30 cm above ground using four rain gauges (Rain
gauge kit, S. Brannan & Sons, Cleator Moor, UK) which were installed 1
m and 3 m, respectively, north and south of the tree’s stem.
Volumetric soil water content and soil temperature were measured at both
sites (Fig. 2) by soil moisture temperature probes (SMT-100,
Umwelt-Geräte-Technik GmbH, Müncheberg, Germany) in the upper soil at 6
cm depth. Recording took place with a CR800 Datalogger (Campbell
Scientific, Inc. Logan, USA) with a 15 min frequency and a precision of
±3 % for volumetric soil water content and ±0.2 ◦C
for soil temperature. Groundwater level in one well was monitored with
an automatic datalogger (groundwater level probe) at an interval of 15
min (see location in Fig. 1C).