Linking Quercus robur tree-water usage to soil-water dynamics within a forest FACE experiment.

Monitoring soil-water dynamics adjacent to mature trees within high forest provides a water balance dataset enabling derivation of ecosystem water cycles and microclimatic effects. Compartmentalising this data to determine gross inputs and outputs is relatively easy, whereas separating the internal components of the complex dynamic matrix presents multiple challenges. Soil-water contributors and users include for example: surface vegetation (through evapotranspiration (ET)), trees (via root water uptake and exudation) differentiated by species and soil microbes. The soil-water balance is affected by both soil and air moisture parameters (i.e. volumetric soil-water content (VWC), vapour pressure deficit (VPD)), by temperature, abiotic soil characteristics (e.g. soil texture) and is fed by infiltration of canopy throughfall (gross precipitation minus interception) providing challenges to attempted partitioning. Here we report results from concurrent measurements of VWC measured at multiple depths down to 1 metre at positions adjacent to mature dominant oak (Quercus robur L.) at the Birmingham Institute of Forest Research (BIFoR) Free-Air CO₂ Enrichment (FACE) experimental site in Staffordshire UK. We calculate relative extractable soil-water (REW) and look for short-timescale correlation with previously reported daylight whole tree water usages derived from sap flow probesets in 18 mature oak (Quercus robur L.) stems. The study was conducted in nine experimental arrays in three-replicate groups (3 arrays with elevated CO₂ infrastructure (eCO₂); 3 with infrastructure but ambient-control CO₂ (aCO₂) and 3 no-infrastructure ambient-control arrays (Ghosts)). Here we present leaf-on season (May to October) differences between trees’ daily soil-water usage under the three experimental CO₂ conditions and consider relative rates of VWC decrease. We derived the VWC values for field capacity (maxima) across autumn and winter (no-leaf season) and permanent wilting point results from VWC minima within the summer treatment season. Environmental measurements were sampled at 30-minute intervals enabling determination of these seasonal maxima and minima. Thus we compile simple models of relative soil-water usage by mature trees under current and future elevated CO₂ levels, which complement concurrent ongoing studies of soil-water-respiration, tree-root-water and nutrient dynamics at BIFoR FACE. The study aims to fill gaps in data deficiency within global vegetation models and to clarify tree-water versus soil-water interactions.