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 2 affects many linked aspects of forest and catchment function in ways we do not adequately understand. Most significantly, global levels of atmospheric CO 2 will be around 40% higher in 2050 than current levels, yet estimates of how water and solute fluxes in forested catchments will respond

more »

... to increased CO 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 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-1 Posted on Authorea 1 Oct 2020 -The copyright holder is the author/funder. All rights reserved. No reuse without permission. -https://doi.org/10.22541/au.160157598.86879557 -This a preprint and has not been peer reviewed. Data may be preliminary. and interdisciplinary understanding at the interfaces of soil, vegetation, hydrosphere and atmosphere under changed atmospheric composition. Site Description and Methods This summary complements online introductory videos (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). 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 2 (eCO 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 o 48' 3.6" N, 2 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 km² 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 (Quercus robur ) planted around 1850. Sub-dominant (ca. 10 m tall) species consist of common hazel (Corylus avellana ), common hawthorn (Crataegus monogyna ), sycamore maple (Acer pseudoplatanus ) and other native species (Hart et al., 2019). Each stem with diameter-atbreast-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 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 2 -dosing infrastructure; three 'control' patches are exposed to ambient CO 2 concentrations delivered via the same infrastructure used in the three 'treatment' patches to maintain +150 ppmv above ambient CO 2 at all levels of the canopy. Elevated CO 2 is maintained during daylight hours from oak bud burst (ca. 1 st April) to last leaf fall (ca. 31 st October). The CO 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 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.