Tree Water Uptake Research Articles

Construction of gradient structure C/C–ZrC–SiC composites with good ablation resistance

With the upgrade of aircraft and the increasingly harsh service environment, it is urgent to develop high-performance thermal structural materials. In this study, inspired by foliar and root water uptake of trees, we propose a one-step facile method to achieve an autonomous gradient in C/C–ZrC–SiC sharp leading edge composites. The gradual increase of SiC content from top to bottom is beneficial to improve the heat transfer capacity of the composites. After oxyacetylene ablation for 60 s, the mass and linear ablation rates of the samples with gradient structure are 0.83 mg/s and 0.5 μm/s, 35 % and 83 % lower than conventional samples. The composites with gradient structure of ZrC and SiC are favorable to the formation of a compact and smooth ZrO2 oxide protective layer, which not only protects the composites from further penetration of O2, but also resists the mechanical denudation. The novel gradient structure composites with enhanced ablation resistance show a bright application prospect in next-generation thermal protection systems.

Tree water uptake patterns across the globe.

Plant water uptake from the soil is a crucial element of the global hydrological cycle and essential for vegetation drought resilience. Yet, knowledge of how the distribution of water uptake depth (WUD) varies across species, climates, and seasons is scarce relative to our knowledge of aboveground plant functions. With a global literature review, we found that average WUD varied more among biomes than plant functional types (i.e. deciduous/evergreen broadleaves and conifers), illustrating the importance of the hydroclimate, especially precipitation seasonality, on WUD. By combining records of rooting depth with WUD, we observed a consistently deeper maximum rooting depth than WUD with the largest differences in arid regions - indicating that deep taproots act as lifelines while not contributing to the majority of water uptake. The most ubiquitous observation across the literature was that woody plants switch water sources to soil layers with the highest water availability within short timescales. Hence, seasonal shifts to deep soil layers occur across the globe when shallow soils are drying out, allowing continued transpiration and hydraulic safety. While there are still significant gaps in our understanding of WUD, the consistency across global ecosystems allows integration of existing knowledge into the next generation of vegetation process models.

Tight coupling between leaf δ13 C and N content along leaf ageing in the N2 -fixing legume tree black locust (Robinia pseudoacacia L.).

N2 -fixing legumes can strongly affect ecosystem functions by supplying nitrogen (N) and improving the carbon-fixing capacity of vegetation. Still, the question of how their leaf-level N status and carbon metabolism are coordinated along leaf ageing remains unexplored. Leaf tissue carbon isotopic composition (δ13 C) provides a useful indicator of time-integrated intrinsic water use efficiency (WUEi). Here, we quantified the seasonal changes of leaf δ13 C, N content on a mass and area basis (Nmass , Narea , respectively), Δ18 O (leaf 18 O enrichment above source water, a proxy of time-integrated stomatal conductance) and morphological traits in an emblematic N2 -fixing legume tree, the black locust (Robinia pseudoacacia L.), at a subtropical site in Southwest China. We also measured xylem, soil and rainwater isotopes (δ18 O, δ2 H) to characterize tree water uptake patterns. Xylem water isotopic data reveal that black locust primarily used shallow soil water in this humid habitat. Black locust exhibited a decreasing δ13 C along leaf ageing, which was largely driven by decreasing leaf Nmass , despite roughly constant Narea . In contrast, the decreasing δ13 C along leaf ageing was largely uncoupled from parallel increases in Δ18 O and leaf thickness. Leaf N content is used as a proxy of leaf photosynthetic capacity; thus, it plays a key role in determining the seasonality in δ13 C, whereas the roles of stomatal conductance and leaf morphology are minor. Black locust leaves can effectively adjust to changing environmental conditions along leaf ageing through LMA increases and moderate stomatal conductance reduction while maintaining constant Narea to optimize photosynthesis and carbon assimilation, despite declining leaf Nmass and δ13 C.

Tree‐ and stand‐scale variability of xylem water stable isotope signatures in mature beech, oak and spruce

AbstractIn ecohydrology, water isotopologues are used to assess potential sources of root water uptake by comparing xylem water signatures with source water signatures. Such comparisons are affected by the variability and uncertainty of the isotope signatures of plant water and water sources. The tree‐scale and stand‐scale variabilities of the isotope signatures in stem xylem water are often unknown but are important for sampling design and uncertainty estimation in assessing the sources of tree water uptake. Here, we quantified tree‐scale and stand‐scale variabilities of xylem water isotope signatures in beech, oak and spruce trees in a mature forest on the Swiss plateau. For stem xylem water, sub‐daily replicates and replicates in different cardinal directions showed no systematic differences, but we found systematic differences with sampling height. The observed variability of isotope signatures at different heights along the stem suggests that water residence times within trees need to be considered, along with their effects on the isotope signatures in different compartments (stem, branches, leaves). Further, concerning the hydrogen signatures, we found height‐ and species‐specific offsets (SW‐excess δ2H). Stem xylem water's tree‐scale variability was similar in magnitude to its stand‐scale variability and smaller than the variabilities in branch xylem and bulk soil water around each tree. Xylem water from stem cores close to the ground, therefore, can give a more precise estimate of the isotopic signal of the most recent root water uptake and facilitate more accurate source water attribution.

Large variation in the radial patterns of sap flow among urban trees

Quantifying sap flow patterns along the sapwood radius is necessary for accurate up-scaling and estimation of tree and stand transpiration based on sap flow measurements. However, little information on radial patterns of sap flow exists in urban environments where precise estimates of tree water uptake and transpiration are important to quantify ecosystem services and plan tree management. Because of the heterogeneous conditions in urban environment, upscaling approaches specific to urban environments and species may be required to estimate whole-tree transpiration.We measured sap flow at four sapwood depths for Acer platanoides, Acer saccharinum and Tilia cordata, in two contrasting urban environments (site types): parks and streets, in Montreal, Canada. We then estimated the potential importance of tree-to-tree and temporal variation of radial patterns by comparing generalized additive models (GAM) with or without tree ID or a temporal effect, and using the models to upscale sap flow measurements conducted at the sapwood surface to tree-level daily water use estimates.The radial pattern of sap flow diverged between park and street trees, with a slightly more monotone pattern in park than street trees in the two Acer species. However, tree-to-tree variation was large even within a given species and site type, and partly associated with tree size. The radial pattern of sap flow varied in time in five of the twelve trees that were measured twice over the growing season. Sap flow in deeper layers relative to the sapwood surface correlated with vapor pressure deficit, solar radiation and the sap flow density at the sapwood surface. When comparing the two sources of uncertainty (tree-to-tree and temporal variation) in upscaling daily water use, a larger uncertainty was associated with tree-to-tree variation than with temporal variation. Thus, understanding the inter-tree variability in radial patterns of sap flow patterns should be prioritized in future studies.

Root water uptake and water transport to above-ground organs compensate for winter water losses and prevent shoot dehydration in apple trees

Plants need to maintain a minimum moisture content in their tissues to preserve their living functions not only during the growing season but also during periods of dormancy. Whereas water uptake of trees during the growing season has been widely investigated, very little attention has been paid to tree water uptake and losses during winter dormancy. A study was therefore conducted aiming at quantifying root water uptake of apple trees in winter, comparing the water content of two groups of field-grown apple trees for two winter seasons. One group of trees was isolated from its roots by a transversal cut at the base of the trunk and the other was left intact. In an additional experiment, deuterium-enriched water was added to the soil of potted apple trees during winter dormancy, to monitor water uptake and translocation within the plant. The field experiment revealed incrementing moisture losses of annual shoots in late winter, driven by the rising evaporative demand of the atmosphere. Trees with unimpaired water transport systems compensated shoot water losses by root water uptake and internal translocation, whereas trees with severed trunks only partially mitigated losses by internal translocation from trunk and larger branches. Root water uptake during winter dormancy was also confirmed in the potted tree experiment with deuterium-enriched water. This work provides evidence that water flow along the soil-plant-atmosphere continuum in apple trees takes place also in winter, with possible impacts on tree survival during prolonged periods of soil or trunk freezing.

The Seasonal Variability and Environmental Factors Influencing the Transpiration of Western Juniper (Juniperus occidentalis) Saplings

There is scarce information regarding the interactions between young tree water uptake and the environment in water-limited ecosystems. This study was conducted in a semiarid rangeland ecosystem in central Oregon, Pacific Northwest Region, USA. We measured the tree transpiration of western juniper (Juniperus occidentalis) saplings using the stem heat balance (SHB) method. We analyzed the correlation between transpiration and environmental factors affecting the saplings’ water use from May to October for 2017, 2018, 2019, 2021, and 2022. The study results showed that total annual precipitation for all but one year was below the long-term (2005 to 2022) mean precipitation value of 307 mm for the study site. Significantly higher transpiration rates were observed in the wet vs. dry years. The highest monthly averaged transpiration rates (2.95 L d−1) were obtained in August during the above-average precipitation year (2017). Peak transpiration rates for the below-average precipitation years were generally reached in June or July, ranging from 0.91 to 1.65 L d−1. The seasonal response of transpiration to different environmental factors varied. For all years, vapor pressure deficit (VPD), solar radiation (SR), and air temperature (AT) showed a positive correlation with transpiration, whereas precipitation (Pr) and relative humidity (RH) indicated a negative correlation with transpiration. Soil moisture (SM) and soil temperature (ST) positively correlated with transpiration for most years. A strong association between VPD and transpiration was observed during the wettest (2017; 327 mm) and driest (2021; 198 mm) years. Results from this study add to the limited literature on sapling transpiration and can contribute to the improved management of cool-climate rangeland ecosystems through an enhanced understanding of water use by young-stage trees and its potential impacts on the water balance of restored juniper landscapes.

Optimization of a Tree Pit as a Blue–Green Infrastructure Object

Trees in dense urban environments are often planted in bioretention cells with an underlying trench (BC-T) providing both stormwater pretreatment and storage. The BC-T design is based on a water balance; however, some input data (tree water uptake and water-holding capacities of soil filter and trench substrate) are difficult to obtain. The goals of this paper were (i) to study the sensitivity of such data in the BC-T design (i.e., their effect on the size of the drained area which may be connected to the tree pit), and (ii) to recommend a possible simplification of the water balance for engineering practice. Global sensitivity analysis was performed for the setup of a BC-T used in Prague, Czech Republic, assuming three different trench exfiltration rates. The most sensitive variable affecting the size of the drained area is the available water-holding capacity in the trench. The simplification of the water balance is highly dependent on exfiltration conditions. At high exfiltration rates (18 mm·h−1 and more) or for a trench with an underdrain, the water-holding capacity in the soil filter and the tree water uptake can be omitted; whereas, at low trench exfiltration rates (1.8 mm·h−1, without an underdrain), both the water-holding capacity of the trench substrate and the potential tree water uptake have a significant influence and cannot be omitted.

Estimating uptake and internal transport dynamics of irrigation water in apple trees using deuterium-enriched water

With future climatic scenarios foreseeing increased crop water demands and reduced irrigation water availability, a deeper knowledge on the dynamics of water uptake and translocation inside plants is needed. Little is known about the time interval existing between the irrigation water supply and the presence of irrigation water inside trees, and whether the dripper localization can affect water uptake and translocation dynamics. Another research gap concerns the redistribution of irrigation water in the canopy following irrigation localized to one side of the tree.A field and a pot experiment were designed to gain more insight into this context. In the field experiment, we tested the effect of different drip irrigation layouts on the extent and velocity of water uptake by apple trees. Trees were irrigated using deuterium-enriched water using one, two, or four drippers per tree. Samples were collected from different heights in the canopy at regular intervals following the irrigation event. In the pot experiment, the soil was saturated with labelled water and samples were collected at different time intervals and heights along the tree stem. Labelled water was detected in the lowest stem section of potted trees after 1 h from irrigation. In field-grown trees, labelled water appeared in the shoots after 4 h and 6 h in the bottom and top part of the canopy, respectively. By increasing the number of drippers per tree, the fraction of irrigation water in the shoots increased accordingly. However, uptake and transport velocity were unaffected by the number of drippers, averaging 0.60–0.65 m h-1. In trees that were irrigated from one side only, irrigation water could be found on the opposite side in the top part of the canopy. Our results suggest that the localization and amount of irrigation water can significantly influence root water uptake in apple trees.

Sap flux and stable isotopes of water show contrasting tree water uptake strategies in two co‐occurring tropical rainforest tree species

AbstractThe short‐term dynamics of tree water use strategies for neighbouring co‐occurring species are poorly understood. Here, we quantify the high frequency changes in water sources and sap flux patterns of two commonly co‐occurring tropical rainforest tree species: Dendrocnide photinophylla (Kunth; Chew) and Argyrodendron peralatum (F.M. Bailey; Edlin ex J.H. Boas). A combination of continuous sap flux measurements and hourly sampling of xylem water stable isotope composition (δD and δ18O) were used to observe water use strategies during a 24‐h transpiration cycle. Sap flux ranged between 2.82 and 28.50 L day−1, with A. peralatum recording a 67% higher rate than D. photinophylla. For both tree species, sap flux increased with tree size and diurnal sap flux increase resulted in more isotopically enriched xylem water. A Bayesian Mixing Model analysis, which used sampled soil water isotopic composition from five soil depths ranging from of 0 to 1 m, revealed that D. photinophylla primarily used water from very shallow depth or soil surface layer (2–60 cm), while A. peralatum sourced its water mostly from deeper layers (60–100 cm). We propose that these differences in species' water consumption patterns are related to plant water storage capacity and wood anatomical features. This research demonstrates that combining xylem isotope composition and sap flux measurements can help reveal species‐specific water use strategies, which can be beneficial for improved process understanding in ecohydrological modelling.

Progressive enrichment in 18O and 2H in xylem water along sap flow paths in Fagus sylvatica trees

AbstractIsotopic signatures of xylem water in different tree compartments such as roots, boles and branches, differ due to physiological and physical processes occurring inside trees. Accordingly, we hypothesised that the extent of such differences among the isotopic signatures of tree compartments is coherent with the distance travelled by the water inside trees and to its residence time. To test this, we compared the O–H isotopic composition of xylem water collected using an in‐situ water extraction method from roots, boles and branches of Fagus sylvatica trees growing on three geomorphological units of the Weierbach experimental catchment, Luxembourg. There was progressive 18O and 2H enrichment in xylem waters along the root–branch flow path for all the studied trees. Three explanations could be considered for this progressive enrichment: internal fractionation by xylem–phloem water exchange, chemical reactions of metabolic pathways and variable ages of water retained in the xylem, reflecting historical variation in isotopic composition of uptake water. Support for the hypothesis of isotopic fractionation linked to xylem–phloem water exchange and chemical reactions is that enrichment was generally consistent with the distance travelled by the water and to its residence time inside the trees. However, the relative enrichment of 2H and 18O was not consistent along the flow path, with Δ2H/Δ18O ~7.5 from the soil into the roots and bole and ~4.7–6.5 for pathways that included smaller branches. This contrast suggests different processes controlling above‐ground isotopic enrichment. In particular, the slope of ~7.5 in the lower tree is also consistent with variation in tree water uptake varying along the local meteoric water line, with water in the roots being closer to the composition of rainfall close to the time of sampling and water in the bole being closer to the composition of rainfall from the previous summer. The timing of root and bole sampling in early spring, just before leaf‐out, means sap flow was very slow and makes it plausible that varying‐age water was present in the tree at that time. We also compared the O–H isotopic composition of those samples with the ones of the potential water sources to identify the origin of the water uptaken. The latter varied during the 3 years of sampling, with a preferential uptake from near‐surface waters. Our results suggest multiple biochemical, physiological and physical processes may play fundamental roles in the isotopic composition of xylem water within trees.

Links of apple tree water uptake strategies with precipitation and soil water dynamics in the deep loess deposits

Understanding how trees absorb water is important for the sustainable management of water resources and vegetation restoration. However, the water uptake strategy of apple trees and its link with precipitation and soil water dynamics (i.e., contents, recharge proportion and residence time) remains poorly understood. In this study, we determined the source water of apple trees, the recharge proportion and mean residence time (MRT) of soil water through collecting precipitation, soil water and xylem water samples with a high frequency. Results indicated that apple trees mainly absorbed water from shallow soil layers of 0–2 m (>64%) in the early growing season (May to July), but deep soil water below 2 m (∼60%) in the late growing season (August to October). As such, apple trees had seasonal variations in water uptake strategies by shifting water sources from shallow to deep soils. In addition, the proportions of water uptake from the same soil layer varied with tree ages. The proportions of water uptake from different soil layers were not significantly correlated to the corresponding soil water contents (p > 0.05), which may be attributed to the long residence time of soil water (14–99 days). Therefore, the water uptake strategies of apple trees were likely to be influenced by precipitation and MRT of soil water, as well as stand ages. The indicated relationship between water uptake strategies and precipitation and soil water dynamics can improve insights into the interaction between soil and vegetation.

Influence of Alstonia Angustiloba tree water uptake on slope stability: A case study at the unsaturated slope, Pahang, Malaysia

Although previous research has significantly enhanced our understanding of problematic and unsaturated soil behaviour and interaction with structures, there is still an urgent need to address the difficult scenarios that are met with problematic and unsaturated soils. This study examines the effects of tree water uptake at different depths and distances on the improvement of induced water uptakes caused by transpiration via mature Alstonia Angustiloba tree. This study is performed to examine the tree water uptake profile in a vegetated slope with the existence of mature Alstonia Angustiloba tree at the top and the stability of the slope during various precipitation penetration events by which the data of the tree water uptake produced within this section of the slope is recorded and implemented to evaluate the factor of safety (FOS). Slope stability analysis is further conducted to explore how plant transpiration affects slope stability. The results indicate that higher tree water uptake lead to the greatest increase of FOS of the slope up to 53% (from 2.17 to 4.57). The highest tree water uptake recorded was at the slope station with Alstonia Angustiloba tree with a depth of 0.25 m and a distance of 1.1 m from the tree. The tree water uptake utilized in this study can contribute to a carbon-free and eco-friendly approach which can be implemented globally to prevent slope failures.

Instructive Surprises in the Hydrological Functioning of Landscapes

Landscapes receive water from precipitation and then transport, store, mix, and release it, both downward to streams and upward to vegetation. How they do this shapes floods, droughts, biogeochemical cycles, contaminant transport, and the health of terrestrial and aquatic ecosystems. Because many of the key processes occur invisibly in the subsurface, our conceptualization of them has often relied heavily on physical intuition. In recent decades, however, much of this intuition has been overthrown by field observations and emerging measurement methods, particularly involving isotopic tracers. Here we summarize key surprises that have transformed our understanding of hydrological processes at the scale of hillslopes and drainage basins. These surprises have forced a shift in perspective from process conceptualizations that are relatively static, homogeneous, linear, and stationary to ones that are predominantly dynamic, heterogeneous, nonlinear, and nonstationary. ▪ Surprising observations and novel measurements are transforming our understanding of the hydrological functioning of landscapes. ▪ Even during storm peaks, streamflow is composed mostly of water that has been stored in the landscape for weeks, months, or years. ▪ Streamflow and tree water uptake often originate from different subsurface storages and from different seasons’ precipitation. ▪ Stream networks dynamically extend and retract as the landscape wets and dries, and many stream reaches lose flow into underlying aquifers.

Sap flow of sweet cherry reveals distinct effects of humidity and wind under rain covered and netted protected cropping systems

Protected cropping systems (PCS) alter the plant growing environment, though understanding of this in ventilated systems and how the new climate affects tree water uptake is limited. Sap flow sensors and weather stations were deployed in 16-year-old ‘Lapins’ on ‘Colt’ rootstock cherry trees under a ventilated Voen PCS and in an adjacent bird netted PCS. Average and maximum temperatures were consistently higher (14.7 °C and 22.9 °C) while total daily solar radiation and average wind were consistently lower (12.9 MJ/m2 and 0.2 m/s) in rain covered, in contrast to netted, PCS (13.9 °C, 21.3 °C, 13.7 MJ/m2 and 0.9 m/s). Over the season, a threefold lower daily sap flow rate was observed under rain covered PCS. Using generalised additive modelling (GAM), the influence of individual climate parameters on sap flow were predicted. Whilst sap flow was only slightly affected by relative humidity (RH) less than 60%, above this threshold sap flow rapidly declined under rain covered PCS whereas sap flow more gradually declined above 20% RH under netted PCS. Overall, our novel modelling approach led to the discovery of the 60% RH critical threshold on predicted sap flow and the indirect effect that wind speeds have on sap flow under PCS.

Orchard microclimate, tree water uptake and sweet cherry fruit quality under protected cropping.

Protected cropping systems (PCS) de-risk adverse climatic effects in intensive horticultural production but alter the growing environment. The objectives of this study were to investigate the effects of modern, commercial-scale PCS on sweet cherry orchard microclimate, tree water uptake and fruit quality. Sap flow sensors and weather stations were positioned at four locations under a 21 ha PCS at varying elevations (125, 114, 111, 102 m above sea level) and distances from the block boundary (105, 75, 60 or 50 m, referred to hereafter as Locations 1 to 4, respectively). Generalised additive models (GAMs) were used to predict the effect of individual climate parameters (temperature, relative humidity, solar radiation and wind speed) on tree sap flow at each of the four locations. Average and maximum temperatures and average minimum relative humidity (RH) were higher (15.9°C, 26.1°C and 49.0%) at locations with higher elevations and located further from the PCS boundary (locations 1 and 2) in contrast to locations at lower elevations and closer proximity to the PCS boundary (locations 3 and 4) (15.4°C, 24.6°C and 48.1%). Predicted sap flow was strongly correlated (r2 = 0.92) with time across the four locations under the PCS. GAMS modelling indicated that the hourly water uptake by trees within close proximity to the block boundary (locations 3 and 4) responded with greater intensity to increases in temperature and reductions in relative humidity, taking up on average 0.15 L h-1 (at temperatures >30°C) and 0.08 L h-1 (at RH<50%), respectively, in contrast to trees further under the PCS (locations 1 and 2) where average tree water uptake was 0.08 and 0.04 L h-1 at temperatures >30°C and RH<50%, respectively. Highest average predicted hourly tree sap flow was associated with high wind speeds (0.67 L h-1) and low relative humidity levels (0.61 L h-1). Fruit harvested from locations further from the PCS boundary had significantly higher dry matter content (18.2%), total soluble solids (17.8%) and compression firmness (311.3 g mm-1) in contrast to fruit closer to the PCS boundaries (16.1%, 15.7% and 258.3 g mm-1). This study provides greater understanding of the effects of PCS on microclimate and consequences for tree water uptake and fruit quality.

Vapour pressure deficit and solar radiation are the major drivers of transpiration in montane tropical secondary forests in eastern Madagascar

Young secondary tropical forests occupy a larger area than mature forests nowadays but our understanding of their ecohydrological functioning, particularly with respect to tree water uptake, remains poor. Deep soil water uptake may make mature forests resilient to periods of water stress, but little is known in this regard for young forests with possibly less extensive root networks. We, therefore, studied sap flow dynamics for one year in two 50 m x 50 m forest plots: a young secondary forest (YSF, 5–7 years) and a semi-mature forest (SMF; 20 years) in montane eastern Madagascar. Temporal variations in the depth of water uptake were inferred from the stable isotope compositions of soil- and xylem water. Transpiration rates were low for both forest sites (265 and 462 mm y−1 for the YSF and SMF, respectively). Vapour pressure deficit and global radiation explained most of the variation in transpiration rates at both sites. There was little evidence of transpiration limitation by soil water, despite an extended dry season. Trees in the YSF extracted water mostly from the intermediate soil depth (30–70 cm) during the dry season. In the SMF, the depth of uptake increased as the dry season progressed for some species (Abrahamia, Brachylaena and Cryptocaria), but not for others (Ocotea and Eugenia). Although the transpiration rates are low for both forests, they are comparable to results reported for other tropical montane sites after normalising for net energy input and leaf area. Estimated evapotranspiration totals (including interception loss, understorey and litter evaporation) were 679 mm and 1063 mm y−1 for the YSF and SMF, respectively (42% and 61% of precipitation, respectively). These results suggest that the stage of forest regrowth affects water uptake, and thus the water balance during forest succession.

Seasonal variations in soil–plant interactions in contrasting urban green spaces: Insights from water stable isotopes

• Stable water isotope monitoring in soil and vegetation in green spaces to understand partitioning. • Soil evaporation higher with grassland cover. • High turnover of water particular in shallow soil under grassland. • Depending on moisture conditions strong seasonal root water uptake of trees. We used stable isotopes to help assess ecohydrological partitioning in different types of urban green space in the city of Berlin. We focused particularly on the role of the near-surface of soils as a crucial interface that determines shallow subsurface (“non-Hortonian”) flows and water cycling in cities. Grassland soils tended to be wetter than soils under continuous urban tree cover, and areas around individual trees in parks or on streets. This is consistent with greater interception losses and transpiration from trees. Soil water isotopes showed distinct seasonality in response to precipitation inputs, mixing with resident soil water and the effects of evaporative fractionation. Effects of fractionation were most marked under individual trees and grasslands, where the effects of evaporation on the isotopic composition of percolating water from the upper 0.1 m of the soil was also most pronounced. Isotope dynamics showed that under all land covers, the upper soils had rapid water turnover and were dominated by younger water (<2 months old). At depths of 0.4 m, more damping was evident with water being ∼ 6 months old. Isotopes in plant water also show seasonal variations which were more marked in grasses than trees. Mixing models revealed that grass most likely recycled shallow, younger soil water in transpiration, whilst trees were more dependent on deeper, older sub-soil and groundwater sources. These preliminary results highlight the urgent need for a much greater understanding of water movement and cycling in the shallow critical zone of urban green spaces. This is essential to manage future resilience to climate change and understand the trade-offs between partitioning urban water between evapotranspiration and groundwater recharge.

Effect of intercrops complexity on water uptake patterns in rubber plantations: Evidence from stable isotopes (C-H-O) analysis

Rubber monoculture plantations are expanding but may cause environmental problems, and rubber-based agroforestry can provide ways of more environmentally friendly rubber cultivation. The general objective of this study was to investigate plant soil water uptake in different rubber agroforestry systems with water stable isotopes (δ2H and δ18O), and water use efficiency with leaf δ13C. We studied rubber trees in a monocultural plantation, and in three agroforestry systems which were mixed with (a) orange trees, (b) tea trees, and (c) both orange and tea trees, respectively. The results indicated that soil water content was enhanced in the agroforestry systems, especially for rubber-tea and rubber-orange-tea agroforesty systems. The soil water uptake of rubber trees varied seasonally, and exhibited significant different among the cultivation systems. Rubber trees in agroforestry systems seemingly absorb water from deeper soil horizons more than monocultural plantations. We also found that rubber trees adjusted their water source according to their growing stage: they increased their uptake of water from deeper soil horizons to meet water requirements during their leaf flushing stage. For the soil water uptake of intercrops, we found that they use a shallow soil horizon as their primary source of water uptake, with little seasonal variation in vertical water extraction patterns. Changes in leaf δ13C values indicate that rubber trees adjusted their water use efficiency seasonally, which helps to cope with seasonal drought and growth requirements. However, neither intercrops nor rubber trees’ δ13C values differ with sites, indicating that intercropping has little influence on intercrops and rubber trees’ water supply. In conclusion, rubber trees have a flexible water use strategy, as intercropping shaped the soil water uptake patterns of rubber trees without apparent consequences on water use efficiency. These findings provide valuable insights into agroforestry systems’ interspecific water use patterns that can help intercrop selection for rubber agroforestry systems in this region.

How do geomorphic characteristics affect the source of tree water uptake in restored river floodplains?

AbstractAlpine rivers and their floodplains have been highly modified by human activities during the last decades. River restoration projects aim to counteract these negative impacts and to restore ecosystem services provided by riparian habitats. We studied two recently restored river sites in the Ahr/Aurino and Mareit/Mareta Rivers (Italian Alps) to investigate how geomorphic conditions, soil moisture, and groundwater level affect the source of water used by grey alder (Alnus incana (L.) Moench). We compared the isotopic composition (δ2H) of tree sap at different locations (low terraces formed during bed incision and recent floodplains formed after restoration) with that of potential water sources, that is, groundwater, soil water, and rainfall. The monthly variation in the isotopic composition of rainfall was reflected in both shallow and deeper soil water, as well as in the isotopic composition of sap. The redistribution of precipitation and groundwater in the soil differed between the post‐restoration floodplain sites and the post‐incision terraces, leading to a different relation between the sap water, soil water, and groundwater isotopic composition. The results show that transpiration of A. incana trees growing on recent floodplains is mostly supported by stream‐fed soil water, whereas trees growing on terraces mainly use precipitation‐fed soil water. These marked, morphology‐related differences in the source of transpiration water of grey alder highlight how channel degradation still affects the ecohydrological processes in Alpine fluvial corridors. Nonetheless, large restoration interventions—in terms of channel widening—can enable the self‐formation of new floodplain areas characterized by stream water‐fed riparian ecosystems.