Due to soil water fractionation resulting in deviation from the local meteoric water line, the data are not well represented in an ellipsoid shape such as that employed in Amin et al. . Therefore a minimum polygon area was used to encompass the data points.Plant water and soil water data from the five sites are plotted in Figure 2. For both soils and xylem, the sites occupied partially overlapping regions showing a general gradient from highly isotopically depleted at Wolf Creek, the coldest of our sites in Canada, to the more isotopically enriched waters at Bruntland Burn at the temperate/boreal transition in Scotland. For each site there was a substantial range of variability in soil and xylem water isotope composition over the course of the sampling year. Most soil and xylem samples plotted below the GMWL, although xylem waters were generally more 2 H-depleted at each site, which was also evident from the lc-excess data . Samples from Dry Creek and, in particular, Wolf Creek showed the greatest divergence from the GMWL. These two sites slightly obscured an otherwise clear relationship between plotting position along the GMWL and the mean annual temperature gradient through Krycklan, Dorset and Bruntland Burn. Despite this, the isotopic ratios of δ2 H and δ18O in soils and xylem water correlate positively with air temperature, annual precipitation and aridity index, raspberry grow in pots and negatively with elevation and to a lesser extent latitude .

At all sites, substantial isotopic differences were apparent between xylem and soil water isotopes, and between angiosperms and gymnosperms . Soil waters at each site generally tracked precipitation and snowmelt inputs being more 2 H- and 18O-depleted in winter/spring and more enriched in summer; evidence of evaporative fractionation was also most evident in the more 2 H- and 18O-enriched summer soil water samples. The soil water data are shown relative to the sampling dates for each site in Figures S2 to S6 in the Supplementary material; also see Sprenger et al. for more detail. Soil water δ2 H data were significantly different from precipitation at Dry Creek, Dorset and Wolf Creek, while soil water δ18O differed from precipitation at Bruntland Burn and Dorset . Bruntland Burn, Krycklan and Dorset showed the greatest visual deviation of xylem δ2 H samples from soil water, while the most southern site, Dry Creek, and the most northern site, Wolf Creek, showed smaller differences between the xylem and soil water isotopes for δ2 H . However, at all sites the δ2 H characteristics of both angiosperms and gymnosperms were significantly different from soil water . Angiosperm xylem water δ18O at all sites, apart from Krycklan, was significantly different from soil water δ18O; whereas significant differences for gymnosperms were apparent only for Dorset and Bruntland Burn. Xylem water isotopic characteristics differed between angiosperms and gymnosperms at some sites. For δ2 H, they were significantly different for Krycklan, Bruntland Burn and Dry Creek, while for δ18O they were different for Dorset and Dry Creek . Snowmelt plotted on the LMWL and was more depleted for 18O than almost all measured soil and xylem waters, although a substantial number of xylem samples at Dry Creek, Krycklan and Wolf Creek were more depleted in 2 H but plotted off the LMWL .

Similarly, at the four sites where groundwater samples were collected, the mean isotopic composition of groundwater fell on the LMWL but plotted towards the more depleted end of the range of soil water samples. This reflects the generally higher recharge of groundwater by depleted water following the spring melt at Dry Creek, Krycklan, and Wolf Creek ; and during winter rainfall at Bruntland Burn . Isotopic composition of groundwater at all sites showed limited temporal variation, indicating the volume of annual recharge is small relative to groundwater storage. Groundwater was generally more strongly depleted in 18O than xylem waters for. The minimum boundary polygon analysis quantifies the degree to which xylem water for both angiosperms and gymnosperms overlaps bulk soil water sources at different depths. The use of the spatially bulked data for soils and vegetation at each site was necessary to provide a sufficient number of samples for the development of encompassing polygons. This may lead to larger estimated polygon areas and a greater estimated overlap of soil and xylem water, although the effect is much less marked than for the ellipse method of Amin et al. . This may provide insight into the sources of xylem water, although the proportion that cannot be ascribed to soil water sources is equally informative regarding the need to hypothesise and identify other causal reasons. Distinct inter-site differences emerged in terms of the overall overlap of xylem and soil water isotopic composition . For Bruntland Burn, soil water had a 77% overlap for angiosperms, but only 6% for Gymnosperms. At Dorset, like Bruntland Burn, angiosperms showed a much higher degree of overlap than gymnosperms .

At Dry Creek, almost all xylem water in both angiosperms and gymnosperms overlapped soil water at almost all profile depths. Of all sites, Krycklan had the lowest degree of overlap with only 27% for angiosperms and 0% of gymnosperms . Finally, while Wolf Creek had only angiosperms present as willow and birch shrubs, a 99% overlap between xylem water and soil water was evident. The depth dependent overlap of xylem and soil water isotopic composition showed differences between depths, with higher overlap tending to be in shallow soil depths for most sites. For Bruntland Burn, there was 72% and 55% overlap between angiosperms and soil water at 0-10 cm and 10-20 cm depths, respectively, but only 9% and 3% for gymnosperms. Dorset was the only site with the greatest overlap occurring in deeper soil, with overlaps of 34%, 28%, 24%, 59% and 31% for 0-10, 10-20, 20-30, 30-40, and >40cm, respectively. The gymnosperms at Dorset had a similar deviation to that of angiosperms, with much smaller overlaps of 4%, 7%, 8%, 18%, and 7%, for 0-10, 10-20, 20-30, 30-40, and >40cm, respectively. Depth-dependent overlap of soil and angiosperms at Dry Creek was high through all soil layers with the greatest overlap in the near-surface soils . Gymnosperms at Dry Creek had a similarly high overlap of 78%, 55%, 86%, 72% and 86 % for 0-10, 10-20, 20-30, 30-40, and >40cm, respectively. At Krycklan, the upper two soil depths had approximately the same overlap for angiosperms , 30 planter pot with a moderate decrease to 12% in the 20-30 cm soils. None of the soil water at any depth overlapped the gymnosperm samples. Wolf Creek angiosperms showed a high overlap in the upper two soil depths with a more substantial decrease in deeper soils .The general patterns of the pooled data sets for the entire study year mask differences in the degree to which seasonal variations in the isotopic composition of xylem water can be ascribed to soil water data collected on the same day or integrated over increasing monthly time windows to capture antecedent conditions. However, as described in section 2.2, soil water boundary polygons for increased averaging periods can also be calculated to estimate the overlap relative to xylem. The bulk soil overlaps are summarised for angiosperms and gymnosperms . Depth dependentover laps are shown in Figure S7 and Figure S8, respectively. At Bruntland Burn, a longer time window of soil water isotopes explained a greater degree of variation in xylem water isotopic composition for angiosperms . Bulked soil water samples collected on the same day provided 80% and 87% of overlap in spring and autumn, respectively, but only 4% in summer. Increasing this window to 3 months increased overlap to 90%, 38% and 87% in spring, summer, and autumn, respectively. The spring and summer bulked soil and xylem water overlap increased to 100% and 58%, respectively, with a 6 month window. For gymnosperms, same day sampling provided no overlap in spring and summer, and only 7% in autumn . For a 3 month window, overlap increased to 20% in spring, but only 3% in summer and 7% in autumn. For a 6 month window, the autumn overlap increased to 13%. There were marked seasonal differences between angiosperms and gymnosperms at Dorset. For angiosperms, bulked soil and xylem water overlapped for same day sampling 100% in spring, 0% in summer and 20% in autumn . This increased to 20% in summer for a 3 month averaging window and 47% in summer for a 6 month average. The overlaps were much lower for gymnosperms; same day sampling showed bulked soil and xylem water overlaps of only 13% in spring, 2% in summer and 7% in autumn .

The respective increases were to 13%, 4% and 15% using a 3 month window; and 13%, 9%, and 15% using a 6 month window. For Dry Creek angiosperms, same day bulked soil water sampling provided 34% overlap with xylem water in spring, 78% in summer and 30% in autumn. For a 3 month sampling window, overlaps increased to 34% in spring, 81% in summer, 73% in autumn; and for a 6 month window respective overlaps were 80%, 81% and 86% . This implies xylem water in angiosperms, especially inspring , is reflecting bulked soil water integrated over longer periods, including the previous growing seasons. Similar patterns were evident for gymnosperms at Dry Creek, with same day samples overlapping with xylem water by 35% in spring, 92% in summer, 40% in autumn. Overlaps using a 6 month window were 78%, 93%, and 80%, respectively. The 3 month window values were intermediate . Of all sites, the vegetation at Krycklan showed the least overlap with bulked soil water, and this changed little with sampling period . Same day sampling for angiosperms showed only 27%, 3% and 0% overlap for spring, summer and autumn. Values increased slightly for bulked soil sampling over the preceding 3 months to 27%, 13%, and 0% for the three seasons, but remained constant for the 6 month window . There was no overlap with any time window for gymnosperms . Only angiosperms were sampled at Wolf Creek, and the severe winter conditions allowed analysis only for summer and autumn. A 52% overlap was evident in summer and 89% in autumn for same day sampling. This increased to 64% and 97%for 3 month wand 6 month windows, respectively .Unsurprisingly, sw-excess values of individual soil water samples plotted around 0 ‰ throughout the year . This gave confidence that the sw-excess is an appropriate metric to describe the potential water source, since individual soil water samples deviated relatively little from the regression through all soil water samples. Plant sw-excess was usually <0 ‰, indicating that xylem water was generally more depleted in 2 H compared to soil water. At Bruntland Burn, Dorset and Krycklan, swexcess was more negative for gymnosperms than for angiosperms. The deviation from sw-excess of 0 ‰ occurred generally under lower soil moisture conditions. At Bruntland Burn, angiosperms had a similar sw-excess to soils in most sampling periods, apart from the start of the study period in October 2015 and the following summer, when it dropped in July, August and September before recovering in September 2016. Differences for gymnosperms were more pronounced and only close to the soils in winter and early spring. Similar patterns were evident for Dorset, although the differences from sw-excess were greater for both plant groups. In general, summer saw the greatest isotopic difference between xylem and soil waters. For Krycklan, angiosperms occasionally showed sw-excess closer to 0 ‰, although the timing was generally limited to early summer after snowmelt. Gymnosperms were closer to soils at this time too, although both plant groups deviated from the soils with the approach of autumn. The S4 site at Krycklan also had the wettest soil conditions. At Dry Creek, differences between angiosperms and gymnosperms were less pronounced and gymnosperm values were usually less negative than for angiosperms. Both gymnosperms and angiosperms periodically reached sw-excess of 0 ‰, although the timing was not as consistent as at the other sites. It was striking that the Dry Creek site with the greatest similarity between soil and xylem waters was also the driest, with the lowest soil water content. At Wolf Creek, angiosperm sw-excess was usually close to the soil water sw-excess with the exception of May 2016, which was at the end of winter when shrubs were not active.