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 in spring , 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. 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,large plastic garden pots 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. The xylem waters sampled in this study provided a series of snap-shots of plant water over the course of the growing season at five northern experimental catchments. This resulted in an unusually rich comparative data set allowing a meta-analysis of inter- and intra-site similarities. Some clear findings emerged from this inter-comparison, though there remain many unanswered questions.
The close link to soil water at each site was apparent from the similar positions of xylem water when plotted in dual isotope space . However, for most sites, much of the xylem water tracked towards lower δ2 H and δ18O plotting below the meteoric water line and below the soil water samples. The swexcess was shown to be a helpful metric to describe the dynamics of the deuterium offset of xylem waters compared to soil water. For some sites, there was much less or no overlap for gymnosperms or some angiosperms . The results also showed seasonal variations in xylem composition at most sites, although this differed . The plotting positions of xylem water from angiosperms and gymnosperms were quite distinct at some sites, despite some overlap. Apart from Dry Creek, gymnosperms at most sites were more offset from both the LMWL and soil waters compared to the angiosperms. The operationally-defined boundary polygon analysis provided an objective way of comparing the distribution of the soil and xylem data from the five sites . It is notable that the sites with greatest general overlap between all sampled angiosperm xylem waters and soil waters are characterised by smaller shrubs and trees . That said, larger trees at Dorset also showed quite a high degree of overlap, especially for more depleted, potentially snowmelt-recharged water sources earlier in the growing season. In contrast, Vaccinium at Krycklan showed little overlap. However, the physiology of smaller plants, with shorter rooting systems, lower internal storage and more rapid water throughput rates may at least partly explain the greater coherence between xylem water and soil water. Indeed, previous ecohydrological modelling experiments at Bruntland Burn by Kuppel et al. and calibrated only on hydrometric data, found quite good agreement between simulated and observed soil water and xylem δ2 H values in angiosperm using the spatial distributed EcH2O-iso model. Conversely, the same model failed to simulate the xylem isotopes in gymnosperms . The polygon analysis at most sites also seemed to indicate that overlaps between soil and xylem waters reflected integrating effects of water sources across the rooting zone, which at most sites was relatively shallow .
This is consistent with the conclusions of Amin et al. for northern sites in their global meta-analysis that found isotopic evidence that cold region plant water was sourced from shallower depths compared to more temperate and arid regions. Given the groundwater isotope data available at all sites apart from Dorset, there is little evidence that deeper water sources can help explain the xylem samples not potentially related to soil water sources . Furthermore, at Dorset the thin soil cover overlies what seems to be relatively unfractured bedrock. It is possible that some trees have roots that are tapping water held in fractures,raspberry plant pot but given the geology it is unlikely that there is sufficient storage to sustain a significant fraction of evapotranspiration.It is clear that some of the observed changes in xylem water throughout the growing season are related to phenological changes . This temporal correspondence partly reflects the “switching on” of plants in the spring as photosynthesis and transpiration increase as well as the availability and isotopic composition of soil water. Previous work by Sprenger et al. showed that variations in soil water isotopic composition at the study sites were mainly driven by precipitation and snowmelt over the preceding weeks, although there was also an effect of evaporation on kinetic fractionation of isotope ratios during summer. These dependencies highlight the importance of precipitation frequency and intensity, infiltration, soil wetness and the mixing interactions that govern soil water residence time distributions . The way in which processes and interactions relate to plant demand highlights the importance of the temporal integration of root uptake and water transport into the main plant stems. The non-stationary travel times from uptake to transpiration may average many months , with tailing in the travel time distribution potentially a result of plant-stored water contributing to transpiration under dry conditions and possible mixing of xylem water with other plant water . The temporal trajectory of the xylem waters varied relative to soil water through the growing season, but this differed between angiosperms and gymnosperms. Also, inter-site contrasts between the angiosperm and gymnosperm differences were apparent: For Bruntland Burn, soil and xylem water signals were most similar in spring, deviated more strongly in summer and then returned to greater overlap in autumn for angiosperms. However, this was not the case for gymnosperms which showed dissimilarity throughout the year. For angiosperms at Dorset, there was a degree of overlap to start with, but depletion increased through summer and then closed again in autumn. In contrast, gymnosperm xylem waters became more 2 H- and 18O-enriched. At Dry Creek, there was a large difference through the autumn and winter for both angiosperms and gymnosperms until spring, but compositions became increasingly similar in summer. At Krycklan, angiosperms were most similar in the spring and early summer, but became increasingly different as the summer progressed. At Wolf Creek, there was an offset at the beginning of spring but samples then increasingly converged. This post-winter offset, also evident at Dry Creek, may relate to desiccation and/or diffusion within the plant during the biologically inactive period . Inclusion of longer antecedent periods for soil isotope data generally improved overlaps within the boundary polygons for most sites, especially for angiosperms.
The “sampling window” over which soil water may have been a source for plant uptake and contributed to xylem water in the trunk at breast height is unknown, and is likely to be non-stationary given seasonal variations in soil moisture and plant physiology. However, the greater overlaps for the longer antecedent period would support the hypothesis that xylem water at any point in time represents an integrated sample of soil water accumulating over preceding months, rather than soil water on the sampling day which will be most influenced by the most recent rainfall. In this sense, the results are similar to those of Allen et al. who demonstrated that trees throughout Switzerland predominantly use soil water derived from winter precipitation for summer transpiration. In our study, however, findings across sites and plant species were not consistent. Regardless, results from both studies suggest that caution should be used when constructing conceptual models of how plants access soil water based on synoptic, space-based sampling. Our phenologically-timed sampling strategy, particularly at such high latitude sites, is novel. However, more frequent sampling would likely be advantageous providing more nuanced insights into the phenological controls and short-term dynamics of xylem isotopes, particularly in relation to short term soil moisture dynamics and periods of higher atmospheric moisture demand . Nevertheless, higher-frequency sampling will still likely show that the xylem samples indicate stronger fractionation which has been widely shown for many vegetation types around the world . This focuses attention on potentially fractionating processes linked to small-scale interactions at the root-soil pore interface, especially close to the soil surface where most fine roots are present and where labile nutrients are also highest in acidic, organic soils. However, methodological issues may at least partly explain some of the difference. These are discussed in the following section. Dry Creek stands out as an anomalous site in many results, most of which can be explained by its warm, dry conditions and high seasonality. Wolf Creek, however, the coldest site, shares similar results. The two sites obscure an otherwise clear relationship between plotting position along the GMWL and the mean annual temperature , they show the most overlap between xylem and soil water isotopes in bulk and at various depths , and they have the highest negative lc-excess values for both xylem and soil water . They also have the lowest May-August relative humidity at 38% and 63%, as well as precipitation at 19mm and 44mm, for Dry Creek and Wolf Creek, respectively . The relatively dry conditions shared by both sites expose soil waters to sustained evaporative environments, which may cause hydro-patterning of roots . Roots grow where water is available, which tends to be in less conductive pores where water has longer residence times and likely more isotopic fractionation due to evaporation. This evaporatively-enriched soil water also has limited potential for mixing with isotopically-different incoming precipitation that would alter its isotopic composition, partly because the growing-season precipitation at these sites is low.