Water Uptake Dynamics Research Articles

Three-dimensional in vivo analysis of water uptake and translocation in maize roots by fast neutron tomography

Root water uptake is an essential process for terrestrial plants that strongly affects the spatiotemporal distribution of water in vegetated soil. Fast neutron tomography is a recently established non-invasive imaging technique capable to capture the 3D architecture of root systems in situ and even allows for tracking of three-dimensional water flow in soil and roots. We present an in vivo analysis of local water uptake and transport by roots of soil-grown maize plants—for the first time measured in a three-dimensional time-resolved manner. Using deuterated water as tracer in infiltration experiments, we visualized soil imbibition, local root uptake, and tracked the transport of deuterated water throughout the fibrous root system for a day and night situation. This revealed significant differences in water transport between different root types. The primary root was the preferred water transport path in the 13-days-old plants while seminal roots of comparable size and length contributed little to plant water supply. The results underline the unique potential of fast neutron tomography to provide time-resolved 3D in vivo information on the water uptake and transport dynamics of plant root systems, thus contributing to a better understanding of the complex interactions of plant, soil and water.

Dual labelling by 2H and 15N revealed differences in uptake potential by deep roots of chicory

AimsDeep-rooted crops have been widely used in agricultural systems to access deep resources such as water and nitrogen (N). However, the potential of deep roots to take water and N at various depths have not been well studied. Here we used chicory (Cichorium intybus L.) to study the potential and dynamics of water and nitrogen uptake in deep soil layers (below 1 m). MethodsChicory plants grown in outdoor rhizotrons were labelled by injecting a 2H2O and Ca(15NO3)2 mixture into the soil column at 1.1, 2.3 and 3.5 m depth. Five, ten and twenty days after injection, 2H and 15N were traced in transpiration water and leaves. ResultsWe found enriched 2H and 15N in water and plant samples, and both water and N uptake were observed down to 3.5 m. The 2H enrichment after injection at 1.1 m depth was 1552‰, almost 10 times higher than after injection at 2.3 m depth, which was 156‰. In contrast, injection at 1.1 and 2.3 m depth resulted in similar 15N enrichment of leaf samples. ConclusionDeep water uptake was found to be more sensitive to increased depth and reduced root intensity than N uptake, and labelled N was used more rapidly than labelled water. We propose several possible explanations for the discrepancies between deep water and N uptake, and further discuss the challenges of using isotopes and models in deep root studies.

Application of a root water uptake model and numerical simulation to walnut trees in arid areas of northwest China

Water shortages restrict the sustainable development of agriculture in arid areas of northwest China. Quantitative studies of root water uptake (RWU) and soil water dynamics can guide the optimal regulation of irrigation. We studied walnut trees under drip irrigation in Hongqopo Farm Experimental Base in Aksu Prefecture, Xinjiang. Our aim was to parameterize the root distribution of walnut trees, establish a flexible and practical macroscopic two-dimensional RWU model, and numerically simulate soil moisture. Multiple regression analysis was applied to establish the root length density (RLD) distribution function and RWU model. The finite difference method was applied to the RWU rate inversion solution, and the alternating direction implicit difference (ADI) method was applied to numerically simulate soil moisture. The RLD distribution function accurately described the location of maximum root density and zero root density using four spatial distribution parameters (R2 = 0.86). The RWU model comprised three parameters: transpiration intensity, RLD distribution, and soil moisture distribution. There was a close agreement between simulated and measured RWU rates. The RWU was concentrated at depths of 10–60 cm. The mean absolute error of soil moisture numerical simulation for 16 days was 0.027 cm3·cm-3, which indicated that the proposed RLD distribution function, RWU model, and numerical simulation method were reliable. These approaches have the advantages of simple parameter measurement and flexible consideration of the spatial variability of the RWU rate and are applicable to other fruit trees. They provide a theoretical basis for water-saving irrigation and scientific management of orchards in arid areas.

Water uptake dynamics in soybean seeds: influence in seeds performance and DNA integrity

ABSTRACT: Relationships among water uptake rate and temperatures were investigated in five soybean seeds lots cv. M 6410 IPRO. Germination, field emergence and electrical conductivity tests were applied to determine seed performance after fast (complete immersion in water), control (moist substrate) and slow (moist atmosphere) imbibitions both at 20 °C and 30 °C. The fresh weight increment over time in soybean seeds during imbibition was recorded. In parallel, analysis of the DNA integrity of seedlings obtained from such seeds was performed in gel electrophoresis. Absorption pattern of soybean seeds were similar among temperatures, but absorption characteristics vary with time progress according to the way of water entrance into the seeds. Upon fast imbibition seeds germinated lower than non-fast-imbibed seeds, besides loss of seed performance. In contrast, slow-imbibed seeds showed high germination, low abnormal seedlings and maintenance of seeds performance. DNA isolation from fast imbibed seeds was highly degraded; although, some considerably degraded samples were reported in controlled imbibed seeds and the best preserved DNA was found in slow-imbibed seeds. The pattern of DNA degradation is typically passive or non programmed cell death. Our results showed it is important to consider the water uptake rate during germination test, since DNA integrity plays a critical role during seed imbibition, preserving soybean seed performance.

Tree water deficit and dynamic source water partitioning

AbstractThe stable isotopes of hydrogen and oxygen (δ2H and δ18O, respectively) have been widely used to investigate tree water source partitioning. These tracers have shed new light on patterns of tree water use in time and space. However, there are several limiting factors to this methodology (e.g., the difficult assessment of isotope fractionation in trees, and the labor‐intensity associated with the collection of significant sample sizes) and the use of isotopes alone has not been enough to provide a mechanistic understanding of source water partitioning. Here, we combine isotope data in xylem and soil water with measurements of tree's physiological information including tree water deficit (TWD), fine root distribution, and soil matric potential, to investigate the mechanism driving tree water source partitioning. We used a 2 m3 lysimeter with willow trees (Salix viminalis) planted within, to conduct a high spatial–temporal resolution experiment. TWD provided an integrated response of plant water status to water supply and demand. The combined isotopic and TWD measurement showed that short‐term variation (within days) in source water partitioning is determined mainly by plant hydraulic response to changes in soil matric potential. We observed changes in the relationship between soil matric potential and TWD that are matched by shifts in source water partitioning. Our results show that tree water use is a dynamic process on the time scale of days. These findings demonstrate tree's plasticity to water supply over days can be identified with high‐resolution measurements of plant water status. Our results further support that root distribution alone is not an indicator of water uptake dynamics. Overall, we show that combining physiological measurements with traditional isotope tracing can reveal mechanistic insights into plant responses to changing environmental conditions.

Spatio-Temporal Variation in Water Uptake in Seminal and Nodal Root Systems of Barley Plants Grown in Soil.

The spatial and temporal dynamics of root water uptake in nodal and seminal roots are poorly understood, especially in relation to root system development and aging. Here we non-destructively quantify 1) root water uptake and 2) root length of nodal and seminal roots of barley in three dimensions during 43 days of growth. We developed a concentric split root system to hydraulically and physically isolate the seminal and nodal root systems. Using magnetic resonance imaging (MRI), roots were visualized, root length was determined, and soil water depletion in both compartments was measured. From 19 days after germination and onwards, the nodal root system had greater water uptake compared to the seminal root system due to both greater root length and greater root conductivity. At 29 days after germination onwards, the average age of the seminal and nodal root systems was similar and no differences were observed in water uptake per root length between seminal and nodal root systems, indicating the importance of embryonic root systems for seedling establishment and nodal root systems in more mature plants. Since nodal roots perform the majority of water uptake at 29 days after germination and onwards, nodal root phenes merit consideration as a selection target to improve water capture in barley and possibly other crops.

Imaging plant responses to water deficit using electrical resistivity tomography

Background and aimsMonitoring root water uptake dynamics under water deficit (WD) conditions in fields are crucial to assess plant drought tolerance. In this study, we investigate the ability of Electrical Resistivity Tomography (ERT) to capture specific soil water depletion induced by root water uptake.MethodsA combination of surface and depth electrodes with a high spatial resolution (10 cm) was used to map 2-D changes of bulk soil electrical conductivity (EC) in an agronomic trial with different herbaceous species. A synthetic experiment was performed with a mechanistic model to assess the ability of the electrode configuration to discriminate abstraction patterns due to roots. The impact of root segments was incorporated in the forward electrical model using the power-law mixing model.ResultsThe time-lapse analysis of the synthetic ERT experiment shows that different root water uptake patterns can be delineated for measurements collected under WD conditions but not under wet conditions. Three indices were found (depletion amount, maximum depth, and spread), which allow capturing plant-specific water signatures based moisture profile changes derived from EC profiles. When root electrical properties were incorporated in the synthetic experiments, it led to the wrong estimation of the amount of water depletion, but a correct ranking of plants depletion depth. When applied to the filed data, our indices showed that Cocksfoot and Ryegrass had shallower soil water depletion zones than white clover and white clover combined with Ryegrass. However, in terms of water depletion amount, Cocksfoot consumed the largest amount of water, followed by White Clover, Ryegrass+White Clover mixture, and Ryegrass.ConclusionERT is a well-suited method for phenotyping root water uptake ability in field trials under WD conditions.

Intercrops improve surface water availability in rubber-based agroforestry systems

Despite the development of rubber agroforestry systems for ecological and economic benefits in Southeast Asia, knowledge of their water uptake dynamics and interspecific water interactions remains limited. The objective of this study is to reveal the water relations (i.e., competition/complementarity) between rubber (Hevea brasiliensis) trees and different kinds of intercrops. We investigated the stable isotopes (δD and δ18O), fine root length density, and soil water content (SWC) under three agroforestry practices and one rubber monoculture across a year (2017/2018). Our results indicated that rubber trees acquired more than 40.5 ± 21.3 % of their water from shallow (0−20 cm) soil stratum, as do perennial galangal (Alpinia officinarum), tea (Camellia sinensis), and cocoa (Theobroma cacao). The complementarity hypothesis was not supported for rubber trees and the intercrops. In the dry season (November to April), there was strong interspecific competition for shallow water resources where the intercropping was practiced. However, intercropping increased the available soil water, enabling rubber trees to acquire more (9.4–24.3 %) shallow soil water. In the wet season (May to October), interspecific water competition was less pronounced based on the relative difference in soil water content (RDSW). Higher relative water content, in the order rubber-galangal > rubber-tea > rubber-cocoa, further showed that facilitative effects dominated interspecific water competition in all the agroforestry practices. This information regarding water relations between rubber trees and their intercrops will be essential to optimize land and water resource utilization in this region.

Canopy isotopic investigation reveals different water uptake dynamics of maples and oaks

Variations in drought responses exhibited by cohabiting tree species such as Acer sacharrum and Quercus alba have often been attributed to differences in rooting depth or water accessibility. A. sacharrum is thought to be a shallow rooted species, and is assumed to not have access to the deep and stable water resources available to Q. alba. As such, A. sacharrum conserves water by minimizing stomatal conductance under drought conditions whereas Q. alba does not. However, detailed records of sufficient temporal resolution which integrate water accessibility, meteorological drivers, and leaf level parameters (e.g., photosynthesis, stomatal conductance) are lacking, making such assumptions—though plausible— largely untested. In this study, we investigated the water accessibility of both maples (A. sacharrum) and oaks (Q. alba) during the late growing season using novel canopy stable isotope measurements. Our results showed that maples can draw from the same water pool as cohabitating oaks, but can also switch to a shallow water source in response to available moisture in the shallow soil profile. We also found that maples tended to use a deep water source under high vapor pressure deficit even when shallow soil water was available. On the other hand, oaks had consistent deep water access during our study period. It is noted that our measurements do not cover the whole growing season and should be extrapolated with caution. Such findings indicate that differences in leaf functions during drought between maples and oaks may be due to both soil water accessibility and atmospheric water demand.

Water uptake dynamics for adult peach trees in a subtropical humid climate

Low-chill peach [Prunus persica (L.) Batsch] cultivars allow Florida growers to become competitive by offering fruits earlier than northern states for premium prices. Due to the predominance of sandy soils in Florida, irrigation is required to ensure fruit production in peaches. Available irrigation recommendations and peach crop coefficients (Kc) were determined in Mediterranean and temperate climates, and they can overestimate water demand in humid subtropical climates. Thus, a two-year study was conducted in Citra, FL, aiming to determine the water requirements and Kc values for four-year-old peach trees in humid subtropical conditions. A daily soil water balance and the crop evapotranspiration (ETc) was determined using soil volumetric water content recorded every 10 min. Five soil probes, each equipped with soil moisture sensors at 10, 20, 40, and 80 cm of soil depth representing soil depth layers of 0–15, 15–30, 30–60, and 60–90 cm, respectively placed under the tree canopy. The ratios of daily ETc and Penman-Monteith FAO-56 reference evapotranspiration were used to estimate Kc for phenological stages of adult (≥4-year-old) peach trees. A soil water depletion coefficient (Ks) was estimated obtaining a threshold of 25.8 % available soil water depletion before trees undergo water stress. Daily ETc ranged from 0.22 mm d−1 during dormancy to 3.65 mm d−1 during shoot development. Values of Kc for peach trees ranged from 0.30 during dormancy to 0.69 during fruit maturity. The use of peach tree Kc adapted for subtropical humid conditions reduce the estimated crop water requirements by 21 % compared to the Kc values reported in FAO-56 guidelines.

Quantification of uncertainties introduced by data-processing procedures of sap flow measurements using the cut-tree method on a large mature tree

Sap flow sensors are crucial instruments to understand whole-tree water use. The lack of direct calibration of the available methods on large trees and the application of several data-processing procedures may jeopardize our understanding of water uptake dynamics by increasing the uncertainties around sensor-based estimates. We directly compared the heat ratio method (HRM) sap flow measurements to water uptake measured gravimetrically using the cut-tree method on a large mature aspen tree to quantify those uncertainties for ten consecutive days. The influence of the azimuthal position of the sensors and the application of different data-processing procedures on the accuracy of the sap flow sensors’ estimates was assessed using different metrics. Overall, the sap flow measurements showed high temporal precision with the gravimetric data. Azimuthal and radial variability of measured sap flux density showed the most substantial effect on the accuracy of the sensors’ estimates of whole tree sap flow. The zero-flow corrections applied altered the accuracy and linearity of the sensors’ measurements at the hourly scale, while the sapwood area method used had a lesser impact. Across the ensemble of available data-processing procedures, the cumulative whole-tree water uptake estimates for five consecutive days from the sensor diverged from the gravimetric measurements by less than 1% to more than 50% depending on the sensors azimuthal position and data corrections applied. This study illustrates some of the uncertainties associated with the methodological approaches chosen when using sap flow sensors to estimate water uptake in tall and large diameter trees.

Water sources for root water uptake: Using stable isotopes of hydrogen and oxygen as a research tool in agricultural and agroforestry systems

Understanding water sources for crop water uptake in agricultural and agroforestry systems is an essential step to develop more efficient and sustainable water management strategies, which is increasingly important in the light of current world population growth, changing climatic conditions and consequent growing pressures on agricultural- and agroforestry production. Stable isotopes of hydrogen and oxygen in the water molecule are powerful and, nowadays, affordable tracers that can help to define the proportion of water sources accessed by plants. Yet, contrary to natural environments, their application is still relatively limited in agricultural and agroforestry research. In this work, we synthesize the advantages and the current knowledge deriving from the use of the stable isotopes of hydrogen and oxygen, in support of more traditional techniques, to understand root water uptake dynamics in agricultural and agroforestry systems. We also underline the practical implications related to the application of this technique for management purposes, and provide a vision for new challenges and future research opportunities in exploring crop and plant water use based on isotopic data.

Identification of Source‐Water Oxygen Isotopes in Trees Toolkit (ISO‐Tool) for Deciphering Historical Water Use by Forest Trees

AbstractHydrological regimes are being perturbed under climate change due to the regional expression of the water cycle across the globe, leading to alterations in the spatial and temporal distribution of water near the Earth's surface. Water is a critical resource for plant ecosystems, and hydrological limitations on vegetative health are particularly complex. To anticipate how subsurface water availability may evolve in the future and affect the dynamics of plant water source usage, as well as the health and functioning of vegetation in various biomes, we need a robust, quantitative framework for linking water availability to past plant water use, which is constrained by historical data. Here, we outline the Identification of Source‐water Oxygen isotopes in trees Toolkit (ISO‐Tool), designed to retrospectively investigate the dynamics of tree water uptake. ISO‐Tool utilizes tree‐ring isotopes (δ18O) combined with a biomechanistic fractionation model to retrodict the δ18O of water utilized during any period of growth. Through comparisons with measured δ18O in local water sources, climatic, and hydrological variables, ISO‐Tool can reconstruct and inform on past ecohydrological interactions. We provide an overview of the modeling components and data requirements necessary to constrain the retrodictions of source‐water δ18O. We demonstrate the utility and efficacy of ISO‐Tool for three riparian field sites characterized by differences in climatic, geomorphic, and hydrologic complexity. We also state that ISO‐Tool can be applied to a range of vegetated environments where distinct isotopic endmembers exist. We present a set of tool groups, which can be applied adaptively, ensuring that scientific progress in understanding retrospective ecohydrology can be made, even under varying degrees of data availability.

Multifunctional BSA Scaffolds Prepared with a Novel Combination of UV‐Crosslinking Systems

AbstractThis study presents the hydrogelation of bovine serum albumin by using a natural‐derived photo‐initiator system based on riboflavin and l‐arginine that ensures the preparation of biocompatible hydrogels. Fourier transform infrared spectroscopy reveals that the synthesis conditions do not induce protein denaturation and confirms the potential UV‐crosslinking mechanism for albumin hydrogels realization. Thermal and morphological characterization indicates the formation of thermodynamically stable hydrogels with interconnected inner structure which presents a riboflavin concentration‐dependent porosity. The evolution of the hydrogel's decomposition products is analyzed by thermogravimetric analysis coupled with Fourier transform infrared spectroscopy and mass spectrometry and reveals a degradation pathway characterized by deamination of unreacted amino moieties and peptide NH groups, condensation, decomposition of the condensation products, and oxidation of N‐containing residues. The kinetics and pH‐responsive behavior of hydrogels are investigated by water uptake dynamics revealing a swelling mechanism which can be switched from a less Fickian to a non‐Fickian process by varying the crosslinker concentration. In vivo tests prove the high degree of biocompatibility of the new hydrogels, after oral administration in mice.

Conversion to organic and biodynamic viticultural practices: impact on soil, grapevine development and grape quality

Aim: The effects of integrated, organic and biodynamic management on soil quality and the growth and morphological development of Riesling grapevines were assessed during the first 4 years of a long-term field trial in Geisenheim, Germany. The overall aim was to understand the effects of these different viticultural practices on soil quality and plant morphology as the basis for product quality.Methods and results: As indicators of soil quality, earthworm abundance and the activity of selected enzymes were assessed. The vegetative and reproductive development of the grapevines, as well as their susceptibility to fungal diseases in the field, wood and grape composition, and grape yield, were investigated. Individual variables were subjected to analysis of variance. Additionally, all variables were subjected to multivariate principal component analysis. Compared with plots under integrated management, plots under the two biological treatments were characterized by higher soil quality and lower vegetative growth and grape yield, and therefore higher exposure of grapes and lower grape cluster compactness, and, probably as a result of these morphological differences, lower incidence of acetic acid rot. Principal component analysis clearly differentiated the three treatments, and showed that biodynamic management had more pronounced effects than organic management in terms of enhanced soil fertility and reduction of vegetative growth.Conclusions: In the present study, organic and especially biodynamic management resulted in a morphology favouring production of high-quality grapes. The treatments differed in terms of fertilization and plant protection methods as well as choice of cover crops. Therefore, further research is necessary with respect to root growth and the nitrogen and water uptake dynamics of vines and cover crops. The differences between grapes produced under organic and biodynamic management emphasize the need for more research on the mode of action of biodynamic preparations.Significance and impact of the study: In recent years, both winegrowers and consumers have expressed steadily growing interest in organic and especially biodynamic wine production. The present study contributes to a better understanding of the effects on grapevine growth and morphological development of shifting to these methods as a way to increase product quality.

Morphological changes of hydrophobic matrix and hydrophilic ionomers in water-swollen perfluorinated sulfonic acid membranes detected using small-angle X-ray scattering

Time-resolved small-angle X-ray scattering (SAXS) was employed to examine the hydration dynamics of perfluorinated sulfonic acid membranes in contact with liquid water. Following thermal pretreatment of the membranes, a peak corresponding to the fluorocarbon polymer backbones was clearly observed in the SAXS profiles, in addition to a peak corresponding to the ionomer clusters consisting of hydrophilic side chains and water molecules. The variation of these two peaks was monitored with a frame time as low as 21 ms, which permitted the successful capture of the subsecond dynamics of water uptake by 25 and 50 μm thick membranes. The dynamics of the initial rapid water uptake were well reproduced using Fick's equation, although the thinner sample exhibited a smaller diffusion coefficient. This result suggests that the surface resistance influences the diffusion of water into the membrane. The initial rapid water absorption was followed by the slow and continuous reconstruction of the polymer structure. The measured dynamics of the two peaks indicated that, during this long-term reconstruction, the increase in the ionomer volume became slower and the fusion of clusters was the dominant factor responsible for increasing the ionomer spacing.

Estimating evapotranspiration parameters by inverse modelling and non-linear optimization

The use of models like HYDRUS in planning irrigation and fertigation scheduling is increasingly researched. Simulation of the root zone processes using HYDRUS largely depends on characterizing parameters related to unsaturated hydraulic conductivity, root growth and water uptake dynamics. A methodology for estimating evapotranspiration parameters for drip irrigated paddy is developed in the present work. The methodology involves soil moisture logging of root zone followed by modelling the dynamics of soil water movement in the root zone by inversely solving the Richards’ equation and evapotranspiration parameters estimation by non-linear programming. All the computer programming was done with MS-EXCEL and it is found that this tool is versatile for this purpose. The results of this work are very useful for using HYDRUS in developing optimal irrigation and fertigation schedule. A threshold level of soil water suction pressure for irrigation scheduling of paddy was also obtained through this work. Temporal changes in the location of the average volumetric water content and average evapotranspiration in the root zone were evaluated. These data are useful for developing guidelines for soil water sensor placement in automatic irrigation triggers.

Water acquisition, sharing and redistribution by roots: applications to agroforestry systems

AimsIn the face of problems caused by ‘intensive agriculture’ dominated by large areas of monocultures, mixed intercropping mimicking natural ecosystems has been reported to constitute a viable solution to increase and stabilize productivity. When designing such systems, root niche separation was thought to be a prerequisite to optimize production.MethodsThis paper reviews the beneficial and adverse effects of trees and crops on water acquisition and redistribution in agroforestry ecosystems using the concepts of competition and facilitation between plants in link with root functional traits.ResultsThe results of the review showed that the reality was more complex leading agroforestry practitioners to adopt management practices to induce a separation in root activities thus avoid competition, particularly for water. Water uptake by plant roots is triggered by the water potential difference between the soil and the atmosphere when leaf stomata are open and depends largely on the root exploration capacity of the plant. Thus, root water uptake dynamics are strongly related to root-length densities and root surface areas. In addition, plants with deep roots are able to lift up or redistribute water to the upper layers through a process known as hydraulic lift, potentially acting as “bioirrigators” to adjacent plants. The redistributed water could be of importance not only in regulating plant water status, e.g. by enhancing transpiration, but also in increasing the survival and growth of associated crops in mixed systems.ConclusionsEven though some more work is still needed to assess the volume of water transferred to neighbors, hydraulic lift could constitute an ecological viable mechanism to buffer against droughts and ensure productivity in regions with erratic rainfall. Giving the difficulty in measuring the above-mentioned aspects in the field, modeling of some of the most relevant parameters to quantify them might inform the design of future empirical studies.

The <sup>18</sup>O ecohydrology of a grassland ecosystem – predictions and observations

Abstract. The oxygen isotope composition (δ18O) of leaf water (δ18Oleaf) is an important determinant of environmental and physiological information found in biological archives, but the system-scale understanding of the propagation of the δ18O of rain through soil and xylem water to δ18Oleaf has not been verified for grassland. Here we report a unique and comprehensive dataset of fortnightly δ18O observations in soil, stem and leaf waters made over seven growing seasons in a temperate, drought-prone, mixed-species grassland. Using the ecohydrology part of a physically based, 18O-enabled soil–plant–atmosphere transfer model (MuSICA), we evaluated our ability to predict the dynamics of δ18O in soil water, the depth of water uptake, and the effects of soil and atmospheric moisture on 18O enrichment of leaf water (Δ18Oleaf) in this ecosystem. The model accurately predicted the δ18O dynamics of the different ecosystem water pools, suggesting that the model generated realistic predictions of the vertical distribution of soil water and root water uptake dynamics. Observations and model predictions indicated that water uptake occurred predominantly from shallow (<20 cm) soil depths throughout dry and wet periods in all years, presumably due (at least in part) to the effects of high grazing pressure on root system turnover and placement. Δ18Oleaf responded to both soil and atmospheric moisture contents and was best described in terms of constant proportions of unenriched and evaporatively enriched water (two-pool model). The good agreement between model predictions and observations is remarkable as model parameters describing the relevant physical features or functional relationships of soil and vegetation were held constant with one single value for the entire mixed-species ecosystem.

Seasonal patterns of water uptake in Populus tremuloides and Picea glauca on a boreal reclamation site is species specific and modulated by capping soil depth and slope position

Soil water availability is important for tree growth and varies with topographic position and soil depth. We aim to understand how two co-occurring tree species with distinct rooting and physiological characteristics respond to those two variables during two climatically distinct growing seasons. Growing season (May to September) sap and transpiration fluxes were monitored using heat ratio method sap flow sensors on Populus tremuloides and Picea glauca in 2014 and 2015 growing along a hillslope with two different soil cover depths providing different rooting spaces. Across the two growing seasons, a shallow rooting space was the main factor limiting aspen’s leaf area and cumulative sap flux, whereas responses of white spruce were more limited by topographical position. Generally, sap and transpiration fluxes decreased with the season; however, a large precipitation event during the 2015 summer triggered a significant recovery in sap and transpiration fluxes in white spruce, while in aspen this response was more muted. The two species distinct rooting and physiological characteristics produced contrasting water uptake and water use dynamics in response to rooting space, soil water availability and climate, inviting a more detailed exploration of sap flux and its interactions with climatic and edaphic variables.