Water usage of old growth oak at elevated CO2 in the FACE of climate change
Authors: Quick Susan Elizabeth , Curioni Giulio , Harper Nicholas J. , Krause Stefan , MacKenzie Angus Rob 
Published: July 2023

Abstract. Predicting how increased atmospheric carbon dioxide levels will affect water usage by whole mature trees remains a challenge. The present study focuses on diurnal (i.e. daylight) water usage of old growth oaks within an experimental treatment season from April to October inclusive. Over five years, from 2017 to 2022, we collected 12,259 days of individual tree data (770,667 diurnal sap flux measurements across all treatment months) from eighteen oaks (Quercus robur L.) within a large-scale manipulative experiment at the Birmingham Institute of Forest Research (BIFoR) Free-Air CO2 Enrichment (FACE) temperate forest in central England, UK. Sap flux data were measured using the compensation heat pulse (HPC) method and used to calculate diurnal tree water usage per day (TWU) across the leaf-on seasons. Six trees were monitored in each of three treatments: FACE infrastructure arrays of elevated (+150 mmol mol-1) CO2 (eCO2); FACE infrastructure control ambient CO2 (aCO2) arrays; and control Ghost (no-treatment-no-infrastructure) arrays. For each tree, sap flux demonstrated a circumferential imbalance across two orientations of the stem. Median and peak (95 %ile) diurnal sap flux increased in the spring from first leaf to achieve peak daily values in summer months (July, August) for all trees in the study. TWU increased similarly, declining more slowly towards full leaf senescence (Oct/ Nov). Water usage varied between individual oaks in July of each year. TWU was linearly proportional to tree bark radius, Rb, at the point of probeset insertion ca. 1.1–1.3 m above ground level (ca. 3.1 litres d-1 mm-1 radius; 274 mm ≤ radius ≤ 465 mm). We also found that bark radius is a very good proxy for canopy area, Ac. Ac was linearly proportional to Rb (ca. 616.5 m2 mm-1 radius), which implies a mean July water usage of almost 5 litres m-2 of projected canopy area in the BIFoR FACE forest. In comparing seasonal responses, TWU was seen to vary by treatment season precipitation amounts and in response to cloudy days, also seen from the diurnal sap flux data. We normalised TWU by individual tree bark radius Rb, which we call TWUn. TWUn treatment comparisons differed year on year. Trees treated with eCO2 compared to the aCO2 controls exhibited different median TWUn results both within and between treatment years, but with no consistency in this difference. Infrastructure control trees exhibited higher TWUn than Ghost, no-infrastructure, trees, especially for the larger trees. The greater TWUn may be due to one or more of several factors: the installation or operation of FACE infrastructure; or to array-specific differences in soil moisture, slope, soil respiration; or sub-dominant tree species presence. The results indicate the importance of infrastructure controls in forest FACE experiments. This first set of plant water usage results encourages the conclusion that old growth oak forests cope well with eCO2 conditions in the FACE(sic) of climate change. From our tree-centred viewpoint, the results reported improve our understanding of future-forest water dynamics of old growth forest and could contribute to the development of more realistic dynamic vegetation models.