Spatiotemporal contribution of soil water sources to total tree water uptake in a Mediterranean Scots pine forest

Abstract. We investigated plant water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in south-western France using stable water isotopes. It is generally assumed that no isotopic fractionation occurs during root water uptake, so that the isotopic composition of xylem water effectively reflects that of source water. However, this assumption has been called into question by recent studies that found that, at least at some dates during the growing season, plant water did not reflect any mixture of the potential water sources. In this context, highly resolved datasets covering a range of environmental conditions could shed light on possible plant–soil fractionation processes responsible for this phenomenon. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured fortnightly over an entire growing season. Using a Bayesian isotope mixing model (MixSIAR), we then quantified the relative contribution of water sources for F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using deeper soil water than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than the considered water sources. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected relatively depleted δ2H of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant water sources using mixing models was strongly affected by this δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

The isotopic composition of xylem water is frequently measured to identify sources of plant water uptake and evaluate the ecosystem water budget. The most common approach to sample xylem water is cryogenic vacuum distillation (CVD). However, the water recovered by CVD is total xylem water from the complex xylem tissue, including living xylem parenchyma cells, embolized tracheary conduits, and small or disconnected conduits that may have a different isotopic composition from water conducted through conduits of the dominant flow from roots to leaves. The isotopic composition of water in the dominant flow network is likely more representative of the isotopic composition of daily transpiration whereas the total xylem water likely integrates water with a longer residence time that may undergo exchange with organic compounds. An alternative extraction method using a pressure chamber (PC) can capture predominantly the transpiration-stream water through the dominant flow network. We compared the offsets in the isotopic composition of water recovered using CVD and PC from eight conifer species that vary in xylem anatomical and functional traits. The PC method accessed a significantly distinct isotopic domain of stem xylem water compared to the total xylem water accessed by CVD (δ2H, p = 0.012; δ18O, p = 0.028). The difference between δ2H of stem water extracted by PC and CVD methods (Δ2Hstem) was significantly correlated with stem water content (p = 0.048) and the mean Δ2Hstem for each species had a significant relationship with species-specific xylem vulnerability to cavitation (i.e., ψ50) from literature values (p = 0.030). We found a significant positive relationship between Δ2Hstem and Δ18Ostem across all trees sampled (p = <0.001). These results support the existence of isotopically heterogeneous water pools, but we cannot exclude potential CVD artifacts contributing to a portion of the Δ2Hstem offsets. Our data suggest additional mechanisms of incomplete mixing and variable residence time in xylem conduits may contribute to isotopic heterogeneity proposed by previous work. Future work should consider using the PC method for assessing the isotopic composition of daily scale transpiration and determining species-specific xylem anatomical properties that could explain isotopic differences between various xylem water pools.

Ecohydrological and isotopic insight into Mediterranean montane Scots pines water use dynamics under different wetness conditions

Forest trees greatly influence both the routing of water downward into the subsurface and the re‐routing of water upward through water uptake and transpiration. To reveal how the subsurface soil water pools used by trees change across seasons, we analysed 2 years of stable isotope ratios of precipitation, soil water from different depths (using both bulk sampling and suction‐cup lysimeters), and xylem in a mixed beech and spruce forest. Precipitation as well as mobile and bulk soil waters all showed a distinct seasonal signature; the seasonal amplitude decreased with depth, and mobile soil waters fluctuated less than bulk soil waters. Xylem water signatures in both tree species were similar to the bulk soil water signatures and rather different from the mobile soil water signatures. The beech and spruce trees had different isotope ratios, suggesting the use of different water sources, and these differences were larger under dry antecedent conditions than wet antecedent conditions. Despite these differences, both species predominantly transpired waters with a winter‐precipitation isotopic signature throughout the summer, including during wet conditions when more recent precipitation was available. Over most of the sampling dates, the fraction of recent precipitation (i.e. from the preceding 30 days) in xylem water was low, despite both species typically demonstrating the use of both shallow and deeper soil waters. These results provide evidence that the soil water storages used by these trees are largely filled in winter and bypassed by recent precipitation, implying long residence times.

Infiltration of stormwater is a widely used strategy to mitigate the flooding and environmental risks that come from urban runoff and conventional urban drainage. An understanding of the fate of this infiltrated water is required for rigorous design. Principal design objectives are typically to restore more natural hydrology in order to protect receiving waters from pollution and hydrologic change. Without such understanding there is also a risk of unforeseen impacts on nearby infrastructure and urban vegetation. We sought to understand the pathways and fate of water from a stormwater infiltration basin. To trace water, we used a combination of water table monitoring and isotopic composition analysis in the infiltration basin, as well as in rainfall, soil water, the shallow groundwater, and in vegetation upslope and downslope of the basin. We also measured tree water use directly using sap flow sensors. The infiltration basin was shown to increase the availability of water downslope, allowing trees to maintain elevated levels of water use during dry periods with high energy demand. In contrast, water limitation upslope saw substantial seasonal reductions in tree water use. The soil water isotopic composition demonstrated significant differences from upslope to downslope, with downslope water being more reflective of rainfall, while the upslope water used by the trees was more depleted. The results paint a picture of stormwater infiltration being a significant source of lateral flow, while trees are a significant sink of lateral flow emanating from the basin. This finding suggests that stormwater infiltration could be used as a strategy to support the health and growth of urban trees. Urban trees have demonstrated benefits for human health and comfort, particularly in a warming climate. It also suggests that stormwater infiltration may not always recharge groundwater and provide baseflow in receiving waters, being instead taken up by vegetation. These findings should be considered in the siting of stormwater infiltration systems, to ensure that the objectives they were designed for are actually met.

The depth distribution of soil water contributions to plant water uptake is poorly known. Here we evaluate the main water sources used by plants at the global scale and the effect of climate and plant groups on water uptake variability and depth distribution. The global meta‐analysis is based on isotope data (δ2H and δ18O) extracted from 65 peer‐reviewed papers published between 1990 and 2017.We applied a new direct inference method to quantify the overlap between xylem water and soil water sources used by plants. The median overlap between xylem water and soil water at different depths varied between 28% and 100%, but they were generally >50%. The shallow (0‐10 cm) soil water overlap with xylem water was largest in cold regions (100% ± 0%) and lowest at tropical sites (about 28%). Conversely, the median overlap between xylem water and deep soil water was largest in the arid and the tropical zones (>75%) and much smaller in the temperate and cold zones. Our results suggest that the isotopic composition of xylem water reflects mostly the signature of shallow soil water (<30 cm) in the cold and the temperate zones, whereas in the arid and the tropical zones, plants appear to exploit water in deeper soil layers. Our novel, simple statistically‐based direct inference method performed well in determining these differences in water sources, and can be applied more widely to isotope‐based plant water uptake studies.

Ecohydrological studies aiming to understand patterns in root water uptake by trees based on plant and soil water isotope data are often confined to one or a few nearby locations. In this study, we took advantage of recently established pan-European hydrogen (δ2H) and oxygen (δ18O) isotope datasets (10.16904/envidat.542) to assess root water uptake depth for beech and spruce trees across Europe. For a subset of sites, δ17O data were available as well.Our analysis revealed consistent isotopic enrichment in xylem water of spruce trees compared to beech trees across all mixed-species sites (N=13), suggesting that spruce predominantly used shallower soil water regardless of environmental conditions. Additionally, we observed isotopic enrichment in stem xylem water from spring to summer at most beech and spruce sites (N=32), suggesting both species relied on isotopically enriched summer precipitation. Interestingly, for a subset of sites (N=8), there was an inverse pattern, with isotopic depletion in summer, implying shifts to deeper soil water sources or uptake of shallow soil water that was isotopically depleted in summer compared to spring conditions.To further explore these findings, we will visually and statistically examine them using isotope data from the soil (10–90 cm depth). We will analyze the role of climate (using gridded data), alongside site-, soil-, and tree-specific metadata to better understand the factors influencing the variation in root water uptake at the continental scale. Additionally, we will explore the potential of oxygen-17 excess to provide further insights into root water uptake dynamics.

A growing number of studies show a discrepancy between the isotopic composition of xylem water and plant water sources. We tested the effect of arbuscular mycorrhizal fungi (AMF) on the isotopic composition of Acacia caven xylem water. As the most common plant-fungal association, AMF might explain this isotopic mismatch. Seedlings were grown with and without AMF and irrigated with the same water. After 120 days, stem and soil samples were collected and following cryogenic distillation, H and O isotopic composition of xylem and soil water, as well as irrigation water, was measured. Xylem water of non-mycorrhizal seedlings was significantly depleted in 2H compared to soil water (differences up to −15.6‰). When AMF were present, the depletion was significantly higher and appeared for both H and O (differences up to −24.6‰ for δ2H and − 2.9‰ for δ18O between soil and xylem water). Results suggest that isotopic fractionation occurred during water uptake in this xerophytic species. To explain this, we propose an aquaporin-driven mechanism mediating water transport via transmembrane passage. Furthermore, we show for the first time, that AMF enhance the observed discrimination against heavy isotopes, probably by enforcing water passage through aquaporins. Given their ubiquity, AMF could question the fractionation-free assumption during root water uptake.

Larger water use efficiency associated with greater dependence on deep water sources in hot-dry valleys with reservoirs

Riparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ18O and δ2H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ13C values of leaf tissue.

<p>Recent studies applying stable isotopes of hydrogen and oxygen showed that different methods for extracting water from plant tissues can return different isotopic composition. One of the most used methods to extract plant water is the cryogenic vacuum distillation (CVD), which tends to extract total plant water. Conversely, the Scholander pressure chamber (SPC), which is commonly used by tree physiologists to measure water potential in plant tissues, likely accesses only the mobile plant water (i.e., xylem and inter-cellular water). However, only few studies reported the application of SPC to extract plant water for isotopic analyses, and therefore, an inter-method comparison between SPC and CVD is needed.</p><p>In this study, we analyzed the variability in the isotopic signature of plant water extracted by SPC and CVD. Furthermore, we considered the potential variability in the isotopic composition of the plant water extracted from various tissues by CVD (i.e., leaves, twig without bark, twig with bark, twig close to the trunk of the tree, and wood core), and from different tree species (i.e., alder, apple, chestnut and beech) located in three different study areas in northern Italy.</p><p>Results indicate that plant waters extracted by SPC and CVD were significantly different, likely due to the extraction of different plant water domains. The difference in the isotopic composition obtained by the two extraction methods was smaller in the beech samples compared to alder, apple and chestnut samples. The signature of alder, apple and chestnut plant water extracted by SPC was more enriched in heavy isotopes than the samples obtained by CVD (except for the leaf water obtained by CVD, which also had a marked evaporative signature). We conclude that plant water extraction by SPC does not represent an alternative for CVD, as SPC likely extracts mostly the mobile plant water, whereas CVD tends to retrieve all water stored in the sampled tissues. However, studies aiming to quantify the relative contribution of the water sources to transpiration should rely more on the isotopic composition of xylem water transpiring during the sampling day (theoretically sampled by SPC), than the isotopic composition of total plant water (sampled by CVD), which also contains a fraction of water that could be stored in plant tissues for a long time.</p><p> </p><p> </p><p>Keywords: stable isotopes of hydrogen and oxygen; cryogenic vacuum distillation; Scholander pressure chamber; plant water; xylem water.</p>

Assessing the ecohydrological separation hypothesis and seasonal variations in water use by Ginkgo biloba L. in a subtropical riparian area

Regulation of tree water use by biophysical factors: Insights based on multi-layer hierarchical residual analysis

Salinization is a major threat to the sustainability of land and water resources, especially in arid and semiarid regions. Understanding the water uptake from different soil depths for desert plants is useful for exploring salinity-tolerance mechanism in desert plants in extremely-arid and salinity-affected area. To understand water uptake from different soil depths for desert plants in Dunhuang, NW China, we used oxygen isotope composition in plant xylem water and soil water to determine the water sources in three different saline sites differing in their degree of soil electrical conductance (site 2 < site 1 < site 3). The co-existing desert plants in each saline site extracted different depth of soil water respectively: K. foliatum mainly used shallow soil water (0–20 cm); H. caspica and N. tangutorum mainly used deep soil water (40–200 cm); A. sparsifolia used water from the 120–200 cm soil layers, while T. ramosissima and E. angustifolia mainly extracted deeper soil water (>200 cm). Compared to that in saline site 2, Tamarix ramosissima and Alhagi sparsifolia can switch their water sources to deeper soil water when enduring more salt stress. Also, a significant and positive correlation between soil EC and soil water δ18O values was observed, indicating the evaporation would cause increase in salt concentration and isotopic enrichment in the upper soil profile. Overall, our results suggest that plants may explore deeper soil water to adapt to salt stress under severe salinity. This work may contribute to selecting salt-tolerant plants species which is vital to saline soil rehabilitation and utilization.

As high latitudes warm, there is limited knowledge of how rapidly changing species composition and density, combined with shifting precipitation and thawing permafrost, will affect critical zone water fluxes across the subarctic. Here, we use stable isotopes of hydrogen and oxygen to assess the role of soil moisture, precipitation dynamics and plant species on the timing, magnitude and sources of plant water uptake at three sites along an elevational gradient in a subarctic, alpine catchment in southern Yukon, Canada. The sites ranged from a low‐elevation boreal forest to higher elevation shrub taiga with variable shrub cover. We sampled soil and xylem water approximately every 3 weeks from pre‐leaf out to post‐senescence over two hydrologically distinct years. We answer the questions: (1) What are the seasonal and interannual changes in the isotopic composition of soil and xylem water across this range of subarctic vegetation covers?, (2) How does the seasonal origin of xylem water vary in wet and dry conditions? and (3) Do different shrub species at the same location rely on different sources of water? Results showed that while δ 2 H and δ 18 O of volume weighted precipitation became more negative with elevation, the opposite was true of xylem water. Despite less snowfall at lower elevations, plant water uptake was more reflective of snow water at the forest than at the high elevation shrub sites. Near‐surface bulk soil water had lower line‐conditioned excess at the forest than at the shrub sites throughout the season and with depth, highlighting increased contributions from soil evaporation at the forest. Differences in annual precipitation and climate had a strong influence on stable isotopes of hydrogen and oxygen in the soil. These results demonstrate that vegetation type and elevation strongly mediate plant water sourcing and evaporative partitioning in subarctic catchments, underscoring the need to account for species‐specific and landscape‐scale variability when predicting blue/green water fluxes in a changing climate.