Fig. 2: The re-quantified δ²H offsets between plant water and “possible source” water line (PWL) across 212 globally distributed field sites sourced from 110 published studies.

A The global distribution of past research sites in dryland and non-dryland ecosystems is evaluated here. The aridity index (AI), defined as the ratio of precipitation to potential evapotranspiration, was used to classify climate zones as follows: hyper-arid (AI < 0.03), arid (0.03 ≤ AI < 0.2), semi-arid (0.2 ≤ AI < 0.5), dry sub-humid (0.5 ≤ AI ≤ 0.65), and humid (AI > 0.65). The AI data and base map were derived from the Version 3 product of the global aridity index and potential evapotranspiration (ET0) Database¹¹⁶. Sites overlap due to multiple samples or water extraction methods in a small area. B Schematic diagram of the quantification method of PW-excess, SW-excess, and LC-excess. The line-condition excess (LC-excess) describes the δ²H offset of the water samples from the local meteoric water line (LMWL)¹¹⁷. The SW-excess is defined as the δ²H offset between plant water and the soil water line (SWL)²². The PW-excess is proposed to quantify the δ²H offsets between plant water and the PWL⁴⁸. PW-excess could provide more accurate information on isotope offsets between plants and source waters because PWL considers more possible water sources that plants use than both LMWL and SWL. C Violin plots of the distribution of five-scenario δ²H offsets considering different water pool combinations within soils and plants. Violin widths represent kernel density distributions. Box plots within violins show means (centre lines), medians (two square dots), interquartile ranges (boxes) and whiskers extending to 1.5 times the interquartile range, and outliers (points). Values above each violin indicate sample size (N) and mean ± standard deviation. “**” above each violin represents there is significant difference between δ²H offsets and zero (p > 0.05), while “NS” indicates offsets not statistically different from zero (p > 0.05). δ²H offsets in the “bulk soil vs. bulk xylem” scenario represent δ²H offsets between the PWL based on bulk soil water (i.e., three soil water pools) and bulk woody xylem water (i.e., two plant water pools). δ²H offsets in the “gravitational vs. bulk xylem” scenario represent δ²H offsets between the PWL based on gravitational soil water and bulk woody xylem water. δ²H offsets in the “plant-available + hygroscopic vs. bulk xylem” scenario represent δ²H offsets between the PWL based on the combination of plant-available and hygroscopic soil water (i.e., two water pools including plant-available soil water and hygroscopic soil water) and bulk xylem water. δ²H offsets in the “plant-available + hygroscopic vs. sap flow” scenario represent δ²H offsets between the PWL based on the combination of plant-available and hygroscopic soil water and sap flow water. δ²H offsets in the “plant-available vs. sap flow” scenario represent δ²H offsets between the PWL based on plant-available soil water and sap flow water.