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, 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, plants in pots ideas 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 snowpack 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. This inter-site comparison provides a meta-analysis aimed at answering the following research questions: 1. What is the temporal trajectory of xylem water isotopic composition during the growing season for common plant species across northern environments? 2. Does the relationship between the isotopic composition of xylem water and soil water differ between plant species and environments? 3. Can any differences between the isotopic compositions of xylem and soil water be explained in terms of current process knowledge and methodological issues?
Following on from question 3, we discuss the open research questions that need to be addressed to gain a more comprehensive understanding of the isotope systematics of plant-water interactions in northern/cold environments.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 .The sites cover a broad hydro-meteorological gradient. Bruntland Burn in the Scottish Highlands, UK has a temperate/boreal humid climate with cool summers. At Dorset in south central Ontario, Canada , the climate is cold and humid with warm summers. Dry Creek , Idaho, USA represents a cold arid montane climate with dry summers. Krycklan in northern Sweden is characterised by a cold and humid climate with relatively cool summers. At Wolf Creek in Yukon Territory, Canada the climate is cold with dry and warm summers . At each site, two to four representative landscape units with characteristic soil-vegetation types were investigated with regard to the isotopic composition of precipitation, soil water, and plant xylem water. Dominant plant cover and soil characteristics of the sites are listed in Table 1 and shown schematically in Figure 1. Angiosperms and gymnosperms were sampled at all sites with the exception of WC, where only angiosperms were sampled. 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 Bitterbrush , 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, container size for blueberries 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, 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 . 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.Water samples were analyzed for their stable isotopic compositions using Los Gatos DLT- 100 laser isotope analysers for Dorset and Wolf Creek, a Los Gatos Liquid Water Isotope Analyzer for Bruntland Burn and Dry Creek, and a Picarro L2130-I for Krycklan. The precision of the liquid water stable isotope analysis is reported to be better than ±0.1 ‰ for δ18O and ±0.4 ‰ for δ2 H. All isotope data are given in delta-notation in reference to the VSMOW. At all sites – apart from Dry Creek – direct water-vapor equilibration analysis was used to sample the bulk soil water isotopic composition from the soil . The accuracy of the direct water-vapor equilibration method was ±0.3 ‰ for δ18O and ±1.1 ‰ for δ2 H. For a detailed description of the procedure, we refer to Sprenger et al. . Bulk soil water isotopic compositions at DC were sampled using cryogenic extraction at 100°C under vacuum of < 30 millitorr over 40 minutes, as described by McCutcheon et al. . We are aware that different methods of soil water extraction have been a major focus of research in the past few years, with no definitive agreement on a standard method . While differences between cryogenic extraction and the direct water-vapor method have been reported in laboratory experiments, previous work by the authors has found the direct equilibrium method to be a reliable method for extracting bulk soil water from sandy soils giving similar results to cryogenic extraction .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.