BIFoR FACE: Water–soil–vegetation–atmosphere data from a temperate deciduous forest catchment, including under elevated CO2

The ecosystem services provided by forests modulate runoff generation processes, nutrient cycling and water and energy exchange between soils, vegetation and atmosphere. Increasing atmospheric CO2 affects many linked aspects of forest and catchment function in ways we do not adequately understand. Global levels of atmospheric CO2 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 CO2 are highly uncertain. The Free Air CO2 Enrichment (FACE) facility of the University of Birmingham's Institute of Forest Research (BIFoR) is the only FACE in mature deciduous forest. The site specializes in fundamental studies of the response of whole ecosystem patches of mature, deciduous, temperate woodland to elevated CO2 (eCO2). Here, we describe a dataset of hydrological parameters – seven weather parameters at each of three heights and four locations, shallow soil moisture and temperature, stream hydrology and CO2 enrichment – retrieved at high frequency from the BIFoR FACE catchment.


| The Wood Brook catchment and FACE facility
The BIFoR FACE facility (52 48 0 3.6 00 N, 2 18 0 0 00 W, 106 m above mean sea level [amsl]) is situated in a mainly agricultural headwater catchment in the UK drained by the Wood Brook, and consists of the main elevated CO 2 (eCO 2 ) facility and a number of other study sites including tree planting of different age and management (Figure 1).
Wood Brook is a second-order stream with a 3.1 km 2 catchment ranging in elevation from 90 to 150 m amsl (Blaen, Brekenfeld, et al., 2017) and subsequently draining into the River Severn catchment (the most voluminous river in England and Wales). The entire catchment is undergoing substantial land-use change, having been converted to organic farming since 2019. Herbal leys (i.e., mixed grass, legumes, and deep-rooting herbs) and tree planting now replace what was previously grass monoculture or arable.
The BIFoR FACE forest (Mill Haft, Figure 1), at the bottom of the Wood Brook catchment, is a deciduous woodland, with dominant (25-m tall) English oak (Quercus robur) planted around 1850 (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.  Norby et al., 2015). Three undisturbed patches have no CO 2 -dosing infrastructure. Three 'control' patches are exposed to ambient CO 2 (aCO 2 ) concentrations, delivered via the same infrastructure used in     (Gamage et al., 2018).
Each treatment and control experimental patch is ringed by 16 free-standing, climbable, lattice towers that reach 2-3 m above the local oak canopy; a 17th tower is sited in the centre of each patch.
The lattice towers are secured by helical 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 can-

| DATA PROTOCOL AND OWNERSHIP
BIFoR FACE is highly collaborative in nature and welcomes partners 1 who wish to contribute as part of a multidisciplinary Community of Practice. 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 (see the online White Book described above). The BIFoR FACE science community believe and advocate transparency in science, assured through open data, after an agreed period of use by the data originators typically equal to the duration of the project (i.e., 1-5 years). Data ownership and data originators are included in the dataset metadata. The dataset is F I G U R E 4 (a) Time series of daily top-of-forest precipitation and field scale mean soil moisture from distributed temperature sensing (DTS) by fibre-optic cable embedded at 10 cm depth between rows on young broadleaf tree planting (6 years) immediately south of the FACE forest (see Figure 1). (b) Time series of shallow volumetric soil moisture from an array of CS655 sensors in the elevated (eCO 2 ), ambient (aCO 2 ) and undisturbed (i.e., with no infrastructure) CO 2 patches of the FACE forest. The numbers of sensors was increased in December 2018 and February 2019, as reported in the top of the panel. (c) Discharge (in blue) and dissolved oxygen (in green) measured on the Wood Brook stream (see Figures 1 and 2) supported by a full-time data manager (author GC), responsible for tracking all data and tissue samples (e.g., leaf and insect collections) and is the first point of contact for specific data requests, including the raw data. The data manager and the researchers are responsible for the quality control of the data which usually consists of constraining values within physical limits, removing outliers after inspection and filling gaps with alternative sources (if available) or via interpolation methods.

| CONTRIBUTORS
In addition to the authors of the present work, technical and scientific assistance and advice has contributed to the BIFoR FACE dataset.
The soil moisture FO-DTS system installation was led by Francesco