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\begin{document}

\title{BIFoR FACE: Water-soil-vegetation-atmosphere research in a temperate
deciduous forest catchment, including under elevated CO2}



\author[1]{Angus Robert MacKenzie}%
\author[2]{Stefan Krause}%
\author[1]{Kris Hart}%
\author[1]{Richard Thomas}%
\author[2]{Philip Blaen}%
\author[1]{R. Hamilton}%
\author[2]{Giulio Curioni}%
\author[1]{Susan Quick}%
\author[1]{Angeliki Kourmouli}%
\author[2]{David Hannah}%
\author[1]{Sophie Comer-Warner}%
\author[1]{Nicolai Brekenfeld}%
\author[1]{Sami Ullah}%
\author[3]{Malcolm Press}%
\affil[1]{University of Birmingham Edgbaston Campus}%
\affil[2]{University of Birmingham}%
\affil[3]{Manchester Metropolitan University}%


\vspace{-1em}



  
  \date{June 18, 2024}


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A. R. MacKenzie\textsuperscript{1,2,*}, S. Krause\textsuperscript{1,2},
K. M. Hart\textsuperscript{1}, R. M. Thomas\textsuperscript{1,3}, P. J.
Blaen\textsuperscript{1,4}, R.L. Hamilton\textsuperscript{1,2}, G.
Curioni\textsuperscript{1,2}, S. E. Quick\textsuperscript{1,2}, A.
Kourmouli\textsuperscript{1,2}, D. M. Hannah\textsuperscript{1,2}, S. A.
Comer-Warner\textsuperscript{1,2}, N. Brekenfeld\textsuperscript{1,2},
S. Ullah\textsuperscript{1,2} and M. C. Press\textsuperscript{1,5}

1. Birmingham Institute of Forest Research, University of Birmingham,
Birmingham B15 2TT, UK

2. School of Geography, Earth and Environmental Science, University of
Birmingham, Birmingham B15 2TT, UK

3. Now at Big Sky Science Ltd, Sutton Coldfield, West Midlands, B72 1SY,
UK

4. Now at Yorkshire Water, Chadwick Street, Leeds, LS10 1LJ, UK

5. Now at Manchester Metropolitan University, Manchester, M15 6BH, UK

* Corresponding author:\emph{a.r.mackenzie@bham.ac.uk}

\subsection*{Keywords}

{\label{keywords}}

Soil moisture; stream metabolism; climate change; long-term monitoring

\subsection*{Summary Paragraph}

{\label{summary-paragraph}}

The ecosystem services provided by forests modulate runoff generation
processes, nutrient cycling and water and energy exchange between soils,
vegetation and atmosphere. Increasing atmospheric
CO\textsubscript{2}affects many linked aspects of forest and catchment
function in ways we do not adequately understand. Most significantly,
global levels of atmospheric CO\textsubscript{2} will be around 40\%
higher in 2050 than current levels, yet estimates of how water and
solute fluxes in forested catchments will respond to increased
CO\textsubscript{2} are highly uncertain. The Free Air Carbon Enrichment
(FACE) facility of the University of Birmingham's Institute of Forest
Research (BIFoR) is an intensively monitored forest site specialising in
fundamental studies of the response of whole ecosystem patches of
mature, deciduous, temperate woodland to elevated CO\textsubscript{2}.
Here, we introduce the facility, situated in a mixed land-use headwater
catchment, with a particular focus on its environmental setting and the
experimental infrastructure. The facility offers a significant
opportunity to advance multi- and interdisciplinary understanding at the
interfaces of soil, vegetation, hydrosphere and atmosphere under changed
atmospheric composition.

\subsection*{Site Description and
Methods}

{\label{site-description-and-methods}}

This summary complements online introductory videos
(\emph{https://tinyurl.com/y3a2hkkx}) and draws on the facility `White
Book', which is a live web-document containing extensive details of all
the projects undertaken at the facility and details of instrument
placement (heights, depths, spatial separation).

\subsubsection*{The Wood Brook catchment and FACE
facility}

{\label{the-wood-brook-catchment-and-face-facility}}

The BIFoR FACE facility is situated in a mainly agricultural headwater
catchment in the UK drained by the Wood Brook, and consists of the main
elevated CO\textsubscript{2} (eCO\textsubscript{2}) facility and a
number of spatially nested satellite study sites including various
forest plantations of different age and management (Figure 1). The
facility is in lowland, rural, central England (52\textsuperscript{o}48'
3.6'' N, 2\textsuperscript{o} 18' 0'' W, 106 m above mean sea level
(amsl)), within a patchy landscape typical of most temperate forest
settings (Haddad et al., 2015). Wood Brook is a second-order stream with
a 3.1 km\selectlanguage{ngerman}² catchment ranging in elevation from 90 to 150 m amsl (Blaen et
al., 2017) and subsequently draining into the River Severn catchment
(the most voluminous river in England and Wales). The entire catchment
is experiencing drastic land-use changes, having been converted to
organic farming since 2019 and herbal lays in replacement of what was
previously grass monoculture or arable, in addition to the new forest
plantations.

{[}Figure 1 here{]}

The BIFoR FACE forest at the bottom of the Wood Brook catchment is a
mature deciduous woodland, with dominant (25-m tall) English oak
(\emph{Quercus robur} ) planted around 1850. Sub-dominant (ca. 10 m
tall) species consist of common hazel (\emph{Corylus avellana} ), common
hawthorn (\emph{Crataegus monogyna} ), sycamore maple (\emph{Acer
pseudoplatanus} ) and other native species (Hart et al., 2019). Each
stem with diameter-at-breast-height greater than 10 cm has been
geolocated and tagged. Centimetre-scale forest structure was measured by
a lidar overflight in August 2014 and by terrestrial laser scanning
(private communication, Eric Casella, Forest Research, Surrey, UK); this
structure establishes the basis for penetration of air, light, and water
into the forest canopy.

The central, eCO\textsubscript{2} component of BIFoR FACE consists of
nine experimental patches of 15 m radius (Hart et al., 2019). Three
`undisturbed' (or `ghost') patches have no CO\textsubscript{2}-dosing
infrastructure; three `control' patches are exposed to ambient
CO\textsubscript{2} concentrations delivered via the same infrastructure
used in the three `treatment' patches to maintain +150 ppmv above
ambient CO\textsubscript{2} at all levels of the canopy. Elevated
CO\textsubscript{2} is maintained during daylight hours from oak bud
burst (ca. 1\textsuperscript{st} April) to last leaf fall (ca.
31\textsuperscript{st} October). The CO\textsubscript{2}-dosing system
works well; one-minute running means are within 15\% of target in the
treatment plots, with less than 1\% of the time showing deviation above
10\% of the baseline value in the control plots (Hart et al., 2019). The
first season with eCO\textsubscript{2} was 2017 and the treatment will
continue until at least 2026. A parallel study of the effect of nitrogen
and phosphorus addition began in 2020 in the forest away from the FACE
patches.

Surrounding the BIFoR FACE, the Wood Brook catchment hosts several
long-term forest hydrological observatories. In partnership with the
estate owners, young mixed-deciduous plantations are subjected to
different manipulation treatment including irrigation and fertilisation
experiments.

\subsubsection*{The environmental
context}

{\label{the-environmental-context}}

The climate at the Wood Brook catchment is that of the temperate
maritime zone of north-west Europe (Barry and Chorley, 2010). The
site-mean annual temperature (MAT) measured between 2016 and 2019 was
10.6 ± 0.8 \textsuperscript{o}C and its mean annual precipitation (MAP)
was 676 ± 66 mm. This situates BIFoR FACE well inside the MAT-MAP
climate space for temperate forests (Sommerfeld et al., 2018). The
catchment is within the area covered by the Central England Temperature
record, which provides a time series back to 1772 (Parker et al.,
1992).The Wood Brook catchment is situated in a Nitrate Vulnerable Zone
(European Union Directive 91/676/EEC) with mean nitrate concentrations
in the Wood Brook ranging from 5 to 7 mg N l\textsuperscript{-1} (Blaen
et al., 2017). The contemporary reactive nitrogen deposition from the
atmosphere in the catchment is \textasciitilde{}22 kg N
ha\textsuperscript{-1} y\textsuperscript{-1} with an ammonium to nitrate
deposition ratio of 7:3 (private communication, S. Tomlinson, UK Centre
for Ecology \& Hydrology). Deposition of this scale represents less than
about 15\% of the total nitrogen nutrition of temperate deciduous forest
trees (Rennenberg and Dannenmann, 2015).

\subsubsection*{Site infrastructure}

{\label{site-infrastructure}}

The Wood Brook is equipped with two continuous water quality monitoring
stations comprising in-stream sensors measuring stage, water
temperature, and electrical conductivity continuously (up to every 10
seconds). Sensors to measure further parameters (UV-VIS absorbance, DO,
pH, and turbidity) are housed in an insulated kiosk located on the
streambank (Blaen et al., 2017a). An ISCO peristaltic pump (Lincoln, NE,
USA) passes 1 L of stream water through these sensors every hour.
Continuous stream monitoring is supplemented with campaign-based
sampling facilitated by networks of surface water ISCO autosamplers, for
instance during tracer tests (Blaen et al., 2017a,b), as well as
spatially nested multi-level mini-piezometers installed in the streambed
to investigate streambed biogeochemical processes and
groundwater-surface water interactions (Comer-Warner et al., 2019,
Comer-Warner et al., 2020).

Soil moisture in the main BIFoR FACE facility is monitored by 12 cm long
frequency domain sensors (CS655 by Campbell Scientific, claimed accuracy
± 3 \% v/v for `typical' soils) installed diagonally from the surface in
groups of three spaced 1 m apart, with two groups in the 'control' and
'treatment' patches and one group in the 'ghost' patches, and monitoring
at 15 to 30 min resolution.

In addition, one of the juvenile plantations close to the catchment
outlet has been instrumented since 2016 with active fibre-optic
distributed temperature sensing (FO-DTS) for measuring soil temperature
and soil moisture at a submeter spatial resolution, resulting in 1850
soil temperature and soil moisture sampling locations across the site,
ranging from 10-40 cm depths (Ciocca et al., 2020). The retrieval from
the FO-DTS has a maximum at 38\%v/v, a value empirically determined from
a soil-specific field calibration against point soil moisture sensors
installed adjacent to the fibre-optic cable. The variability shown for
the FO-DTS is that for 4 quasi-independent measurements per day at 25 cm
intervals along the fibre-optic cable. Uncertainties of ca. 3-5\% v/v
have been reported for soil moisture measurements with the DTS technique
(Gamage et al. 2018).

Each treatment (eCO\textsubscript{2}) and control experimental patch is
ringed by 16 free\selectlanguage{english}-standing, climbable, lattice towers that reach 2-3 m
above the local oak canopy; a 17\textsuperscript{th} tower is sited in
the centre of each patch. The lattice towers are secured by screw piles;
the experimental site contains no concrete foundations or guy wires.
Access to the experimental patches is via low-level walkways raised
approximately 30 cm above ground level to prevent compaction. Canopy
access above 5 m is contracted to climbing arborists or achieved using a
bespoke canopy access system (CAS) installed from the
17\textsuperscript{th} central tower of each infrastructure array. The
CAS is operated by trained staff so that rope access training is not
required for researchers. Welfare and simple laboratory facilities are
provided. Elevated CO\textsubscript{2} dosing, canopy access, and
routine monitoring is operated by a team of six technical staff
permanently stationed at BIFoR FACE.

Four meteorological masts are located at the periphery of the woodland
and a 40 m `flux tower' stands towards the downwind end of the wooded
area so that its flux `footprint' is within the forest for the
prevailing south-westerly winds. During dosing, true biogeochemical
CO\textsubscript{2} fluxes are, of course, obscured by the gas released
to provide the eCO\textsubscript{2} treatment effect but sensible and
latent heat fluxes are recorded.

Figure 2 illustrates the flow of data and tissue samples into their
permanent repositories. Other equipment (not shown) is deployed \emph{ad
hoc} within specific projects.

{[}Figure 2 here.{]}

To complement the experimental infrastructure in the Wood Brook
catchment and BIFoR FACE facility, an integrated groundwater-surface
water model has been developed and validated by a combination of flow
signatures and applied to investigate stream and subsurface water and
energy balance in response to forest shading (Qiu et al., 2019).

\subsection*{Example Results}

{\label{example-results}}

\subsubsection*{Soil moisture dynamics, stream discharge, and water
quality in mature forest and young
plantation}

{\label{soil-moisture-dynamics-stream-discharge-and-water-quality-in-mature-forest-and-young-plantation}}

Example core data (precipitation and FACE soil water content, discharge
and DO) and project-specific data (field-scale soil moisture measured by
FO-DTS at 10 cm) demonstrate the value of long-term integrated
monitoring in ecohydrological observatories such as the Wood Brook
catchment (Figure 3).

The variability of the temperate maritime climate is evident: prolonged
wetting and drying events with occasional, shorter, high-intensity
rainfall events. Signals can take a long time to emerge within such
variability, which is a key argument for a long-term experimental
platform such as BIFoR FACE. The time series at this relatively early
stage suggests that: (i) the plantation is systematically wetter than
the neighbouring FACE forest even though the plantation slopes downwards
towards FACE; (ii) there is significant spatial variability in the
plantation and FACE forest; and (iii) the eCO\textsubscript{2} patches
are drier than the aCO\textsubscript{2} and undisturbed patches. Point
(iii) is a result of spatial variability in the forest; the strength of
soil moistening due to eCO\textsubscript{2}, if any is present (cf.
Ellsworth, 1999; Drake et al., 2016), remains to be quantified.

Corresponding water levels at the Wood Brook catchment outlet highlight
the general ``flashiness'' of the flow regime with relatively fast
responses to precipitation events for a permeable catchment as well as
fast recession of flow (Figure 3 bottom). This example time series of
one of our monitoring stations also indicates some of the challenges in
maintaining consistent quality control throughout long-term observation
networks. In addition to data losses induced by power supply failures in
Spring 2019, observed values up to early 2019 were an order of magnitude
lower than from summer 2019 onwards due to repeated sedimentation of the
water level sensor and recurring changes to the channel cross sectional
profile that finally led to a relocation of the sensor as indicated in
Figure 3. The additional value of continuously monitored water level and
water quality data (as highlighted by the example of dissolved oxygen in
Fig 3 bottom) extends beyond the ability to observe long-term trends in
catchment behaviour in response to land-use changes but also provides
opportunity to enhance mechanistic process understanding of in stream
metabolism and biogeochemical processing as well as event-based
activation of pollution sources (Blaen et al., 2017a).

{[}Figure 3 here.{]}

\subsection*{Data protocol and
availability}

{\label{data-protocol-and-availability}}

All projects form part of the overall collaborative effort to understand
catchment behaviour and forest form and function, and all facility users
sign a data protocol to that effect. The BIFoR FACE science community
believe and advocate transparency in science, assured through open data
after an agreed period of privileged use.

The facility is supported by a full-time data manager (author GC),
responsible for tracking all data and tissue samples. The data is
available upon request; an open data repository for a subset of core
data is under construction.

The continuous streams of data are handled by a suite of dataloggers and
a local LAN network which allows data to be saved on the BIFoR FACE
facility server (Figure 2). A back-up server located on site in a
separate building stores a daily image of the primary server. Data is
transferred daily to the University of Birmingham servers and the raw
and processed data (i.e. organised in a consistent format and cleared of
evident issues) are stored separately to improve resilience.
Non-continuous data collected by researchers is stored in the University
of Birmingham servers and handled directly between researchers and the
data manager.

All tissue is recorded when sampled and a chain-of-custody initiated
using Pro-curo. Quenching of biological samples to
-70\textsuperscript{o}C is accomplished on-site using a dry shipper
(BioTrex-10, Statebourne Cryogenics, Tyne \& Wear, UK), avoiding the
need for transporting liquid nitrogen. Short-term tissue storage at
5\textsuperscript{o}C and -20 \textsuperscript{o}C can be accommodated
on-site, but the permanent tissue bank resides at the University of
Birmingham Edgbaston campus.

In summary, BIFoR FACE is an ambitious field facility designed primarily
to measure the whole-system response of mature temperate forest to
elevated CO\textsubscript{2}, but suitable for a wide range of
complementary catchment studies. The facility is highly collaborative in
nature and welcomes
partners11https://www.birmingham.ac.uk/research/bifor/get-involved/index.aspx
who wish to contribute as part of a multidisciplinary Community of
Practice.

\subsection*{Acknowledgments}

{\label{acknowledgments}}

We very gratefully acknowledge support from the JABBS Trust, Norbury
Park Estate, The John Horseman Trust, Ecological Continuity Trust, NERC
(grants NE/S015833/1, NE/P003486/1, NE/N020502/1; NE/T000449/1;
NE/T012323/1), and the University of Birmingham. The soil moisture
FO-DTS system installation was led by Francesco Ciocca while holding
joint positions at the University of Birmingham and at Silixa Ltd.
(London, UK).

The BIFoR FACE facility cannot run without the dedicated support of its
operations team (currently; Nicholas Harper, Peter Miles, Thomas Downes,
Gael Denny and Robert Grzesik, formerly; Gary McClean and Anna Gardner).
Foundational contributions to the design and implementation of the
facility were made by Michael Tausz and Sabine Tausz-Posch. The FACE
facility eCO\textsubscript{2} treatment uses the system designed by John
Nagy and installed by Keith Lewin, both of Brookhaven National Lab, USA.
We acknowledge the considerable scientific input of visiting fellows
(David Ellsworth, Kristine Crous, Debbie Hemming, Rich Norby, Theresa
Blume and Mantha Phanikumar) and former researchers (Will Allwood, Alex
Poynter, Elizabeth Hamilton). We gratefully acknowledge strategic
guidance from BIFoR Directors (Rob Jackson, Jerry Pritchard, and Nicola
Spence) and the Science Committee (Christine Foyer, Vincent Gauci,
Francis Pope, and Estrella Luna Diez).

\subsection*{References}

{\label{references}}

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9\textsuperscript{th} ed., Routledge, London.

Blaen, P., K. Khamis, C. Lloyd, S. Comer-Warner, F. Ciocca, R. M.
Thomas, A. R. MacKenzie, Stefan Krause (2017a), High-frequency
monitoring of catchment nutrient exports reveals highly variable
storm-event responses and dynamic source zone activation, J. Geophys.
Res-Biogeosciences, 10.1002/2017JG003904

Blaen P., Brekenfeld N., Comer-Warner S., Krause S. (2017b). Multitracer
Field Fluorometry: Accounting for Temperature and Turbidity Variability
during Stream Tracer Tests. Water Resources Research,
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Ciocca F., Abesser C., Findlay J., Chalari A., Mondanos M., Hannah D.M.,
Blaen P., Krause S. 2020. A Distributed Heat Pulse Sensor Network for
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Comer-Warner S., Ullah S., Kettridge N., Gooddy D., Krause S. (2019).
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Comer-Warner, S.A., Gooddy, D.C., Ullah, S., Glover L., Kettridge N.,
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Drake, J.E., Macdonald, C.A., Tjoelker, M.G., Crous, K.Y., Gimeno, T.E.,
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Qiu, H., Blaen, P., Comer\selectlanguage{english}-Warner, S., Hannah, D. M., Krause, S., \&
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\subsection*{Figure Captions}

{\label{figure-captions}}

Figure 1. (a) BIFoR FACE is located in Mill Haft (white dashed line;
lighter patches show locations of the FACE arrays and control patches)
in a patchwork of old-growth forest, new forest plantation on arable
land, and arable fields. (b) Wood Brook catchment (white dashed line)
with the stream running along the northern edge of Mill Haft. (c)
Central England location of Mill Haft.

Figure 2. A schematic view of the sensor deployment and tissue and data
flow through BIFoR FACE and Wood Brook. The main experimental
infrastructure elements are shown (left); replicates are indicated by
``n = ``. Data from electronic sensors are recorded in networked field
dataloggers and relayed to the facility server. Back-up is carried out
on-site and by daily data download to the main University of Birmingham
servers with Retrospect software (Retrospect Inc. USA). Initial quality
assurance is under the control of the BIFoR Data Manager (author GC)
before data is released to the BIFoR FACE community. A parallel system
operates for physical samples, the metadata from which enters the BIFoR
FACE database via chain-of-custody software (Pro-curo Software Ltd, West
Sussex, UK).

Figure 3. a) time series of daily top-of-forest precipitation and soil
moisture from distributed temperature sensing (DTS) by fibre-optic cable
embedded at 10 cm depth between rows on new broadleaf forest plantation
immediately south of the FACE forest (see Figure 1). b) Time series of
shallow soil water content from an array of sensors in the FACE forest.
The numbers of sensors at each part of the time series are reported in
the top of the panel. c) Water level (in blue) and dissolved oxygen (in
green) measured on the Wood Brook stream (see Figure 1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig1-MillHaftMap-Amended2205/Fig1-MillHaftMap-Amended2205}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
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