As this research field has progressed, it has become apparent that extraction of soil and plant waters for isotope analysis is beset with a number of methodological issues . Soil waters held under different tensions may have different isotopic characteristics: for example, freely moving water sampled by suction lysimeters often shows a much less marked evaporative fractionation signal than bulk soil waters dominated by less mobile storage extracted by cryogenic or equilibration methods . Such differences between extraction techniques may be exacerbated by soil characteristics, such as texture and organic content, which may in turn affect the degree to which water held under different tensions can mix . Similarly, sampling xylem and its resulting isotopic composition has been shown to be affected by methodology. It is usually assumed that methods such as cryogenic extraction isolate water held in xylem, when in fact water stored in other cells may be mobilized to “contaminate” the results . Interpretation of plant-soil water relationships can also be complicated by processes in plants and soils that alter isotopic compositions independently. For example, the spatio-temporal isotopic composition of soil water can change dramatically in relation to precipitation inputs, evaporative losses, internal redistribution and phase changes between liquid and gaseous phases . Moreover,10 plastic plant pots there is increasing evidence that plant physiological mechanisms may affect water cycling and the composition of xylem water . These include effects of mychorrizal interactions in plant roots that may result in exchange and fractionation of water entering the xylem stream .
Research also indicates that as flow in xylem slows, diffusion and fractionation can occur , which may involve exchange with phloem cells . Finally, there is increasing evidence that water storage and release from non-xylem cells may sustain transpiration during dry periods or early in the day , also affecting xylem composition. Thus, there is a need to understand the different timescales involved in uptake processes in the rooting zone, residence times and mixing of water in different vegetation covers . There is also evidence of differences between how such factors affect water movement in angiosperms and gymnosperms, as well as species-specific differences . Clearly, these methodological issues will take some time to address; in the interim there is a need for cautious interpretation of emerging data from critical zone studies in order to improve our understanding.A striking feature of isotopic studies of soil-vegetation systems is a bias to lower and temperate latitudes, with northern latitudes and cold environments being under-represented . Yet, northern environments present particular challenges and opportunities to further advance the growing body of knowledge about plant-soil water interactions. For example, the coupled seasonality of precipitation magnitude and vegetative water demand can be complicated by the seasonality of the precipitation phase. Cold season precipitation that accumulates as snow can replenish soil water in the spring and be available to plants months after deposition . Despite the lack of studies, these areas are experiencing some of the most rapid changes in climate and, as a result, vegetation . The effects of climatic warming on patterns of snow pack accumulation and melt can have particularly marked consequences for soil water replenishment and plant water availability, particularly at the start of the growing season .
Despite the importance of northern environments, remoteness and harshness of environmental conditions result in logistical problems that constrain lengthy field studies and data collection . This study seeks to contribute to the growing body of knowledge about plant-soil water interactions by expanding the geographical representation of sites in cold northern environments. We report the findings of a coordinated project on xylem water isotopic data collection in the dominant soil – vegetation systems of five long-term experimental sites. Isotopic characteristics of soil water have previously been reported for all five sites; this used a comparative approach with, as far as possible, common sampling methods across the sites for a 12 month period . Here, we present xylem water isotopic composition data collected using common methods over the same time period encompassing the complete growing season, and then relate findings to soil water isotopic compositions. The study was conducted at five long-term experimental catchments across the boreal or mountainous regions of the northern latitudes . The catchments were part of the VeWa project funded by the European Research Council investigating vegetation effects on water mixing and partitioning in high-latitude ecosystems . Previous inter-comparison work on this project has examined such issues as changing seasonality of vegetation-hydrology interactions , soil water storage and mixing , water ages and modelling the interactions between water storage, fluxes and ages .At Bruntland Burn, study sites were dominated either by Scots pine or Ericacae species . Dominant vegetation at the Dorset sites was either coniferous trees , Eastern white cedar , Eastern white pine at sites He, Ce, Pw, respectively or deciduous red oaks . At Dry Creek, tree-dominated high elevation locations included Douglas fir and Ponderosa pine .
Mid-elevation sites had a mixture of similar trees plus shrubs including Sagebrush . Low elevation sites had no trees, but a variety of additional shrubs including Bitter brush , Chokecherry , Yellow willow and Water birch . At Krycklan, Norway spruce and Blueberry were present at site S04 about 4 m away from a stream, while Scots pine and Blueberry were the dominant species at the upslope site S22 about 22 m from the stream. The Wolf Creek sites, RP in the riparian zone and PL located on a relatively dry plateau, were vegetated by birch and willow shrubs . Prevailing soil textures at the sites varied from loam to silty sands . Soil characteristics are described in detail by Sprenger et al. . Briefly, these are podzolic soils at Bruntland Burn, Dorset and Krycklan, loamy sand at Dry Creek, and Wolf Creek had considerable amounts of organic matter in the upper soil layers. At Dry Creek, shrub and tree roots extend through the soil column, which ranges from ~10 cm to ~120 cm thick. Ponderosa pine roots may extend into fractured bedrock. The rooting depths are limited to the upper 15 cm for the heather sites at Bruntland Burn and to 50 cm depth for trees at Krycklan and Dorset. Rooting depths at Wolf Creek and Bruntland Burn are largely within the top 30 cm with smaller fractions to 50 cm. At each site, plants and surrounding soils were sampled concurrently for isotope analysis following a common sampling protocol . Depending on the nature of the soil cover,plastic pots large the maximum depth of sampling varied from -20 cm at BB to -70 cm at Dry Creek . Sampling took place at 5 cm intervals for Bruntlad Burn, Dorset, and Krycklan with two to five replicates for each depth. At Dry Creek, sampling was done at -10, -25, -45, and -70 cm with two to four replicates. Sampling depths at Wolf Creek varied between -2 and -40 cm with one to three replicates. Daily soil moisture data based on continuous soil moisture measurements at 10 or 15 cm soil depth were available for each soil water sampling location at Bruntland Burn, Dry CReek, Krycklan, and Wolf Creek. Only weekly manual soil moisture measurements were available for Dorset, and daily soil moisture data were derived from soil physical modelling . The volumetric soil moisture data were used to assess the hydrologic state on the sampling days. Plant samples from trees with a diameter > 30 cm were taken horizontally with increment borers at breast height . Retrieved plant xylem cores were directly placed in vials without bark and phloem. Shrub vegetation was sampled by clipping branches. These were immediately placed in vials after the bark was chipped off or left on . All vials were directly sealed with parafilm and immediately frozen until extraction was conducted at Boise State University, Boise, Idaho, USA. There were five replicates for each species and day at the sites in Bruntland Burn, Krycklan, Dorset. At Wolf Creek, the number of replicates varied between two and five and there were always four replicates for each sampling campaign at the Dry Creek sites. In total, 1160 xylem water samples were collected; 831 for angiosperms and 329 for gymnosperms . Dates of sample events varied at each site, but included the end of the growing season/senescence, pre-leaf out the following year, post leaf out, peak growing season and senescence . Precipitation was sampled daily or on an event basis at Bruntland Burn and Krycklan. Daily to fortnightly precipitation sampling was conducted at Dorset, Dry Creek, and Wolf Creek. Melt water was sampled from lysimeters at Krycklan, Dorset, Dry Creek and Wolf Creek during several snow melt events, while snowfall seldom occurred over the study year at Bruntland Burn . Various measures were taken to prevent evaporation of collected precipitation, including paraffin oil and water locks prior to transfer to the laboratory. The long-term groundwater signal was assessed at all sites, apart from Dorset, using several sampling campaigns of springs and wells tapping the saturated zone over the last few years .
There were no nearby wells from which to sample the regional groundwater at Dorset, which is found well below the surface in the granitic gneiss and amphibolite bedrock.Source water apportionment of plant xylem: To quantify the potential source of vegetation water from different soil depths and over a range of time periods, a modification of the ellipsoid method was utilized for the gymnosperms and angiosperms at soil depths in 10 cm increments up to 40 cm. All soil samples deeper than 40 cm were lumped together. The 40 cm cut off was chosen due to fewer sites sampling below 40 cm and a large decrease in the temporal resolution of sampling which could otherwise skew results. 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, 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.