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Encroachment of vascular plants (VP) in temperate raised bogs, as a consequence of altered hydrological conditions and nutrient input, is widely observed. Effects of such vegetation shift on water and carbon cycles are, however, largely unknown and identification of responsible plant physiological traits is challenging. Process-based modelling offers the opportunity of gaining insights into ecosystem functioning beyond observations, and to infer decisive trait shifts of plant functional groups. We adapted the Soil–Vegetation–Atmosphere Transfer model pyAPES to a temperate raised bog site by calibration against measured peat temperature, water table and surface CO2 fluxes. We identified the most important traits determining CO2 fluxes by conducting Morris sensitivity analysis (MSA) under changing conditions throughout the year and simulated VP encroachment. We further investigated transferability of results to other sites by extending MSA to parameter ranges derived from literature review. We found highly variable intra-annual plant traits importance determining ecosystem CO2 fluxes, but only a partial shift of importance of photosynthetic processes from moss to VP during encroachment. Ecosystem respiration was dominated by peat respiration. Overall, carboxylation rate, base respiration rate and temperature sensitivity (Q10) were most important for determining bog CO2 balance and parameter ranking was robust even under the extended MSA. [ABSTRACT FROM AUTHOR]

The isotopic composition of xylem water (δX) is of considerable interest for plant source water studies. In‐situ monitored isotopic composition of transpired water (δT) could provide a nondestructive proxy for δX‐values. Using flow‐through leaf chambers, we monitored 2‐hourly δT‐dynamics in two tropical plant species, one canopy‐forming tree and one understory herbaceous species. In an enclosed rainforest (Biosphere 2), we observed δT‐dynamics in response to an experimental severe drought, followed by a 2H deep‐water pulse applied belowground before starting regular rain. We also sampled branches to obtain δX‐values from cryogenic vacuum extraction (CVE). Daily flux‐weighted δ18OT‐values were a good proxy for δ18OX‐values under well‐watered and drought conditions that matched the rainforest's water source. Transpiration‐derived δ18OX‐values were mostly lower than CVE‐derived values. Transpiration‐derived δ2HX‐values were relatively high compared to source water and consistently higher than CVE‐derived values during drought. Tracing the 2H deep‐water pulse in real‐time showed distinct water uptake and transport responses: a fast and strong contribution of deep water to canopy tree transpiration contrasting with a slow and limited contribution to understory species transpiration. Thus, the in‐situ transpiration method is a promising tool to capture rapid dynamics in plant water uptake and use by both woody and nonwoody species. Summary Statement: Tracing the isotopic composition of transpiration (δT) in real‐time showed rapid dynamics in plant water uptake and use by woody and nonwoody species. Daily δ18OT‐values weighted by the transpiration flux are a good proxy for δ18O of xylem water. [ABSTRACT FROM AUTHOR]

Summary: The 'two‐water‐worlds' hypothesis is based on stable isotope differences in stream, soil and xylem waters in dual isotope space. It postulates no connectivity between bound and mobile soil waters, and preferential plant water uptake of bound soil water sources. We tested the pool‐weighted impact of isotopically distinct water pools for hydrological cycling, the influence of species‐specific water use and the degree of ecohydrological separation.We combined stable isotope analysis (δ18O and δ2H) of ecosystem water pools of precipitation, groundwater, soil and xylem water of two distinct species (Quercus suber, Cistus ladanifer) with observations of soil water contents and sap flow.Shallow soil water was evaporatively enriched during dry‐down periods, but enrichment faded strongly with depth and upon precipitation events. Despite clearly distinct water sources and water‐use strategies, both species displayed a highly opportunistic pattern of root water uptake.Here we offer an alternative explanation, showing that the isotopic differences between soil and plant water vs groundwater can be fully explained by spatio‐temporal dynamics. Pool weighting the contribution of evaporatively enriched soil water reveals only minor annual impacts of these sources to ecosystem water cycling (c. 11% of bulk soil water and c. 14% of transpired water). [ABSTRACT FROM AUTHOR]

Summary: Plants mediate water fluxes within the soil–vegetation–atmosphere continuum. This water transfer in soils, through plants, into the atmosphere can be effectively traced by stable isotopologues of water. However, rapid dynamic processes have only recently gained attention, such as adaptations in root water uptake depths (within hours to days) or the imprint of transpirational fluxes on atmospheric moisture, particularly promoted by the development of real‐time in‐situ water vapour stable isotope observation techniques. We focus on open questions and emerging insights at the soil–plant and plant–atmosphere interfaces, as we believe that these are the controlling factors for ecosystem water cycling. At both interfaces, complex pictures of interacting ecophysiological and hydrological processes emerge: root water uptake dynamics depend on both spatiotemporal variations in water availability and species‐specific regulation of adaptive root conductivity within the rooting system by, for example, modulating soil–root conductivity in response to water and nutrient demands. Similarly, plant water transport and losses are a fine‐tuned interplay between species‐specific structural and functional strategies of water use and atmospheric processes. We propose that only by explicitly merging insights from distinct disciplines – for example, hydrology, plant physiology and atmospheric sciences – will we gain a holistic picture of the impact of vegetation on processes governing the soil–plant–atmosphere continuum. [ABSTRACT FROM AUTHOR]

• The oxygen isotope signature of water is a powerful tracer of water movement from plants to the global scale. However, little is known about the short-term variability of oxygen isotopes leaving the ecosystem via transpiration, as high-frequency measurements are lacking. • A laser spectrometer was coupled to a gas-exchange chamber directly estimating branch-level fluxes in order to evaluate the short-term variability of the isotopic composition of transpiration (δE) and to investigate the role of isotopic non-steady-state transpiration under natural conditions in cork-oak trees (Quercus suber) during distinct Mediterranean seasons. • The measured δ18O of transpiration (δE) deviated from isotopic steady state throughout most of the day even when leaf water at the evaporating sites was near isotopic steady state. High agreement was found between estimated and modeled δE values assuming non-steady-state enrichment of leaf water. • Isoforcing, that is, the influence of the transpirational δ18O flux on atmospheric values, deviated from steady-state calculations but daily means were similar between steady state and non-steady state. However, strong daytime isoforcing on the atmosphere implies that short-term variations in δE are likely to have consequences for large-scale applications, for example, partitioning of ecosystem fluxes or satellite-based applications. [ABSTRACT FROM AUTHOR]

Facilitation and competition between different vegetation layers may have a large impact on small-scale vegetation development. We propose that this should not only influence overall herbaceous layer yield but also species distribution and understory longevity, and hence the ecosystems carbon uptake capacity especially during spring. We analyzed the effects of trees on microclimate and soil properties (water and nitrate content) as well as the development of an herbaceous community layer regarding species composition, aboveground biomass and net water and carbon fluxes in a cork-oak woodland in Portugal, between April and November 2011. The presence of trees caused a significant reduction in photosynthetic active radiation of 35mol m−2 d−1 and in soil temperature of 5°C from April to October. At the same time differences in species composition between experimental plots located in open areas and directly below trees could be observed: species composition and abundance of functional groups became increasingly different between locations from mid April onwards. During late spring drought adapted native forbs had significantly higher cover and biomass in the open area while cover and biomass of grasses and nitrogen fixing forbs was highest under the trees. Further, evapotranspiration and net carbon exchange decreased significantly stronger under the tree crowns compared to the open during late spring and the die back of herbaceous plants occurred earlier and faster under trees. This was most likely caused by interspecific competition for water between trees and herbaceous plants, despite the more favorable microclimate conditions under the trees during the onset of summer drought. [ABSTRACT FROM AUTHOR]

Highlights: [•] First evaluation of modeled evaporative δ18O (δ18OE ) based on Craig & Gordon against field measurements. [•] Modeled δ18OE is highly sensitive to changes in T and δ18O of the evaporating front. [•] The formulation of kinetic fractionation clearly affects the agreement of measured and modeled δ18OE . [•] With a precise characterization of the sensitive inputs Craig & Gordon reflects measured δ18OE well. [•] A precise characterization of sensitive inputs to Craig & Gordon clearly improves the partition of ET. [Copyright &y& Elsevier]

This article is a commentary on Volkmann et al., 210: 839–849. [ABSTRACT FROM AUTHOR]

The analysis of the stable isotopic composition of hydrogen and oxygen in water samples from soils and plants can help to identify sources of vegetation water uptake. This approach requires that the heterogeneous nature of plant and soil matrices is carefully accounted for during experimental design, sample collection, water extraction and analyses. The comparability and shortcomings of the different methods for extracting water and analyzing isotopic composition have been discussed in specialized literature. Yet, despite insightful comparisons of extraction methods and benchmarking methodologies of laboratories worldwide, the community still lacks a roadmap to guide sample collection, extraction, and isotopic analyses, and many practical issues for potential users remain unresolved: for example, which (soil or plant) water pool(s) does the extracted water represent? These constitute a hurdle for the implementation of the approach by newcomers. Here, we summarize discussions led in the framework of the COST Action WATSON ("WATer isotopeS in the critical zONe: from groundwater recharge to plant transpiration"—CA19120). We provide guidelines for (1) sampling soil and plant material for isotopic analysis, (2) methods for laboratory or in situ water extraction, and (3) measurements of isotopic composition. We highlight the importance of considering the process chain as a whole, from experimental design to isotopic analysis to minimize biased estimates of the relative contribution of different water sources to plant water uptake. We conclude by acknowledging some of the limitations of this methodology and advice on the collection of key environmental parameters prior to sample collection for isotopic analyses. This article is categorized under:Science of Water > Hydrological ProcessesScience of Water > Water and Environmental ChangeScience of Water > Water Extremes [ABSTRACT FROM AUTHOR]

Mitigating the global climate crisis and its consequences, such as more frequent and severe droughts, is one of the major challenges for future agriculture. Therefore, identifying land use systems and management practices that reduce greenhouse gas (GHG) emissions and promote water use efficiency (WUE) is crucial. This, however, requires accurate and precise measurements of carbon dioxide (CO 2) fluxes and evapotranspiration (ET). Despite that, commercial systems to measure CO 2 and ET fluxes are expensive and thus often exclude research in ecosystems within the Global South. This is especially true for research and data of agroecosystems in these areas, which are to date still widely underrepresented. Here, we present a newly developed low-cost, non-dispersive infrared (NDIR)-based CO 2 and ET flux measurement device (∼ EUR 200) that provides reliable, accurate and precise CO 2 and ET flux measurements in conjunction with manually operated closed chambers. To validate the system, laboratory and field validation experiments were performed, testing multiple different low-cost sensors. We demonstrate that the system delivers accurate and precise CO 2 and ET flux measurements using the K30 FR NDIR (CO 2) and SHT31 (RH, relative humidity) sensor. An additional field trial application demonstrated its longer-term stability (> 3 months) and ability to obtain valid net ecosystem C balances (NECBs) and WUE. This was the case, even though environmental conditions at the field trial application site in sub-Saharan Africa were rather challenging (e.g., extremely high temperatures, humidity and rainfall). Consequently, the developed low-cost CO 2 and ET flux measurement device not only provides reasonable results but also might help with democratizing science and closing current data gaps. [ABSTRACT FROM AUTHOR]

Beneficial and negative effects of species interactions can strongly influence water fluxes in forest ecosystems. However, little is known about how trees dynamically adjust their water use when growing with interspecific neighbours. Therefore, we investigated the interaction effects between Fagus sylvatica (European beech) and Picea abies (Norway spruce) on water-use strategies and aboveground structural characteristics. We used continuous in situ isotope spectroscopy of xylem and soil water to investigate source water dynamics and root water uptake depths. Picea abies exhibited a reduced sun-exposed crown area in equally mixed compared with spruce-dominated sites, which was further correlated to a reduction in sap flow of −14.5 ± 8.2%. Contrarily, F. sylvatica trees showed +13.3 ± 33.3% higher water fluxes in equally mixed compared with beech-dominated forest sites. Although a significantly higher crown interference by neighbouring trees was observed, no correlation of water fluxes and crown structure was found. High time-resolved xylem δ2H values showed a large plasticity of tree water use (−74.1 to −28.5‰), reflecting the δ2H dynamics of soil and especially precipitation water sources. Fagus sylvatica in equally mixed sites shifted water uptake to deeper soil layers, while uptake of fresh precipitation was faster in beech-dominated sites. Our continuous in situ water stable isotope measurements traced root water uptake dynamics at unprecedented temporal resolution, indicating highly dynamic use of water sources in response to precipitation and to neighbouring species competition. Understanding this plasticity may be highly relevant in the context of increasing water scarcity and precipitation variability under climate change. [ABSTRACT FROM AUTHOR]

Extensive variation in gross primary production (GPP) among growing seasons in rice ecosystems, which has previously been reported, may be caused by a number of biophysical factors. Two field-based studies, one on Oryza sativa L. cv. Odae in the Haean Basin and the other on O. sativa L. cv. Unkwang in Gwangju, South Korea, were conducted to evaluate physiological regulators of the ecosystem. A literature survey across climatic zones was additionally undertaken to unravel the roles of relevant climatic factors. The results from the two field studies showed that the correlation between the fraction of canopy light interception (fPAR) and leaf area index (LAI) across all data sets exhibited a common trajectory. Analogous values of canopy light use efficiency (LUE c ) at similar growth stages of the two rice crops, but, clearly varietal and nutritional differences in LAI development were present, which implies that temporal differences in carbon gain capacity are related to modifications in LAI, thereby influencing fPAR. The literature survey revealed that most variations in growing season GPP can be explained by changes in daily solar radiation (SR) which is negatively correlated to the proportion of rainy days; this is supported by the two field studies. In the context of temperature warming, a reduction in SR noticeably exacerbated impacts on photosynthetic productivity. Moreover, global values of maximum carbon gain capacity over the growing season (GPP day-max ) in field-cultivated rice were significantly affected by SR. We suggest that: 1) temporal differences in carbon gain capacity and variations in GPP day-max are jointly driven by LAI development, which varies according to nutrient availability and rice genotypes, and SR, especially at the reproductive stage; 2) although photosynthetic productivity of rice ecosystems is vulnerable to temperature increasing and prolonged growth duration contributes to larger photosynthetic productivity, its fluctuations across growing seasons are profoundly mediated by SR, which statistically correlates with the amount of precipitation; and 3) the important regulating implication of SR changes is attributed to the structure rather than physiology dependent light sensitivity of canopy photosynthesis at the reproductive stage. Further researches regarding interactions among climate change, phenology and canopy carbon dynamics are discussed. [ABSTRACT FROM AUTHOR]

Aims: We aimed at assessing the influence of above- and below-ground environmental conditions over the performance of Centaurea jacea L., a drought-resistant grassland forb species. Methods: Transpiration rate, CO2 assimilation rate, leaf water potential, instantaneous and intrinsic water use efficiency, temperature, relative humidity, vapor pressure deficit and soil water content in one plant and root length density in four plants, all grown in custom-made columns, were monitored daily for 87 days in the lab. The soil water isotopic composition in eleven depths was recorded daily in a non-destructive manner. The isotopic composition of plant transpiration was inferred from gas chamber measurements. Vertical isotopic gradients in the soil column were created by adding labeled water. Daily root water uptake (RWU) profiles were computed using the multi-source mixing model Stable Isotope Analysis in R (Parnell et al. PLoS ONE 5(3):1–5, 2010). Results: RWU occurred mainly in soil layer 0–15 cm, ranging from 79 to 44%, even when water was more easily available in deeper layers. In wet soil, the transpiration rate was driven mainly by vapor pressure deficit and light intensity. Once soil water content was less than 0.12 cm3 cm− 3, the computed canopy conductance declined, which restricted leaf gas exchange. Leaf water potential dropped steeply to around − 3 MPa after soil water content was below 0.10 cm3 cm− 3. Conclusion: Our comprehensive data set contributes to a better understanding of the effects of drought on a grassland species and the limits of its acclimation in dry conditions. [ABSTRACT FROM AUTHOR]

Understanding of plant hydraulic strategies (i.e. the degree of iso‐/anisohydricity) is crucial to predict the response of plants to changing environmental conditions such as climate‐change induced extreme drought. Several abiotic factors, including evaporative demand, have been shown to seasonally modify the isohydricity of plants. However, the impact of biotic factors such as plant–plant interactions on hydraulic strategies has seldom been explored.Here, we investigated adaptations and changes in hydraulic strategies of two woody species in response to seasonal abiotic conditions, experimental drought and plant invasion in a Mediterranean cork oak (Quercus suber) ecosystem with a combined shrub invasion (Cistus ladanifer) and rain exclusion experiment.From the dry to wet season, Q. suber shifted from a partial isohydric to an anisohydric behaviour while C. ladanifer shifted from strict anisohydric to partial isohydric. During drought, water competition by plant invasion significantly modified the hydraulic strategy of invaded Q. suber, which was accompanied by lower pre‐dawn leaf water potentials, sap flow density, leaf area index and trunk increment rates.This altered isohydricity of invaded Q. suber trees was most likely caused by interspecific competition for water resources by water spending C. ladanifer shrubs. Both species do have the highest proportion of fine roots in the topsoil and thus, an additional water consumer, such as C. ladanifer can lead to more stressful conditions for Q. suber during times of water scarcity. Further underlying mechanisms of the altered isohydricity of Q. suber, such as potential allelopathic effects of C. ladanifer exudates on root growth of Q. suber, have to be investigated in the future.In conclusion, we demonstrate that the degree of isohydricity of two woody Mediterranean plant species is dynamically determined by the interplay of species‐specific hydraulic traits and their abiotic and biotic environment. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Droughts can strongly affect grassland productivity and biodiversity, but responses differ widely. Nutrient availability may be a critical factor explaining this variation, but is often ignored in analyses of drought responses. Here, we used a standardized nutrient addition experiment covering 10 European grasslands to test if full‐factorial nitrogen, phosphorus, and potassium addition affected plant community responses to inter‐annual variation in drought stress and to the extreme summer drought of 2018 in Europe. We found that nutrient addition amplified detrimental drought effects on community aboveground biomass production. Drought effects also differed between functional groups, with a negative effect on graminoid but not forb biomass production. Our results imply that eutrophication in grasslands, which promotes dominance of drought‐sensitive graminoids over forbs, amplifies detrimental drought effects. In terms of climate change adaptation, agricultural management would benefit from taking into account differential drought impacts on fertilized versus unfertilized grasslands, which differ in ecosystem services they provide to society. [ABSTRACT FROM AUTHOR]

Summary: Root water uptake is a key ecohydrological process for which a physically based understanding has been developed in the past decades. However, due to methodological constraints, knowledge gaps remain about the plastic response of whole plant root systems to a rapidly changing environment.We designed a laboratory system for nondestructive monitoring of stable isotopic composition in plant transpiration of a herbaceous species (Centaurea jacea) and of soil water across depths, taking advantage of newly developed in situ methods. Daily root water uptake profiles were obtained using a statistical Bayesian multisource mixing model.Fast shifts in the isotopic composition of both soil and transpiration water could be observed with the setup and translated into dynamic and pronounced shifts of the root water uptake profile, even in well watered conditions.The incorporation of plant physiological and soil physical information into statistical modelling improved the model output. A simple exercise of water balance closure underlined the nonunique relationship between root water uptake profile on the one hand, and water content and root distribution profiles on the other, illustrating the continuous adaption of the plant water uptake as a function of its root hydraulic architecture and soil water availability during the experiment. [ABSTRACT FROM AUTHOR]

The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil‐atmosphere or soil‐groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone. Plain language Summary: Investigating how long it takes for a drop of rainwater until it is either evaporated back to the atmosphere, taken up by plants, or infiltrated into groundwater or discharged in streams provides new understanding of how water flows through the water cycle. Knowledge about the time water travels further helps assessing groundwater recharge, transport of contaminants, and weathering rates. Such water age studies typically focus either on water in individual compartments of the water cycle such as soils, groundwater, or stream runoff. But we argue that the interfaces between these compartments can have an influence on the water age. Here, we present methods how water ages can be estimated using tracers and hydrological models. We further discuss the "demographics of water" (water age distribution) in the critical zone that spans from the tree canopy to the bottom of the groundwater. Our review highlights how water flows and mixes between plants, soils, groundwater, and streams and how this interaction affects the water ages. This way, our work contributes toward a better understanding of vital resource water sustaining the life in the Earth's living skin. Key Points: New tracer techniques now allow tracking water at high spatiotemporal resolution across the vastly varying water ages in the water cycleExchanges of water between hydrological compartments at key interfaces influence the water age distribution more than previously assumedVariation from complete to nearly absent mixing of water at the interfaces in the critical zone affects the water ages in compartments [ABSTRACT FROM AUTHOR]

Reduced precipitation treatments often are used in field experiments to explore the effects of drought on plant productivity and species composition. However, in seasonally snow-covered regions reduced precipitation also reduces snow cover, which can increase soil frost depth, decrease minimum soil temperatures and increase soil freeze-thaw cycles. Therefore, in addition to the effects of reduced precipitation on plants via drought, freezing damage to overwintering plant tissues at or below the soil surface could further affect plant productivity and relative species abundances during the growing season. We examined the effects of both reduced rainfall (via rain-out shelters) and reduced snow cover (via snow removal) at 13 sites globally (primarily grasslands) within the framework of the International Drought Experiment, a coordinated distributed experiment. Plant cover was estimated at the species level, and aboveground biomass was quantified at the functional group level. Among sites, we observed a negative correlation between the snow removal effect on minimum soil temperature and plant biomass production the next growing season. Three sites exhibited significant rain-out shelter effects on plant productivity, but there was no correlation among sites between the rain-out shelter effect on minimum soil moisture and plant biomass. There was no interaction between snow removal and rain-out shelters for plant biomass, although these two factors only exhibited significant effects simultaneously for a single site. Overall, our results reveal that reduced snowfall, when it decreases minimum soil temperatures, can be an important component of the total effect of reduced precipitation on plant productivity. [ABSTRACT FROM AUTHOR]

Agricultural crops play an important role in the global carbon and water cycle. Global climate change scenarios predict enhanced water scarcity and altered precipitation pattern in many parts of the world. Hence, a mechanistic understanding of water fluxes, productivity and water use efficiency of cultivated crops is of major importance, i.e. to adapt management practices. We compared water and carbon fluxes of paddy and rainfed rice by canopy scale gas exchange measurements, crop growth, daily evapotranspiration, transpiration and carbon flux modeling. Throughout a monsoon rice growing season, soil evaporation in paddy rice contributed strongly to evapotranspiration (96.6% to 43.3% from initial growth to fully developed canopy and amounted to 57.9% of total water losses over the growing seasons. Evaporation of rainfed rice was significantly lower (by 65% on average) particularly before canopy closure. Water use efficiency (WUE) was significantly higher in rainfed rice both from an agronomic (WUEagro, i.e. grain yield per evapotranspiration) and ecosystem (WUEeco, i.e. gross primary production per evapotranspiration) perspective. However, our results also show that higher WUE in rainfed rice comes at the expense of higher respiration losses compared to paddy rice (26% higher on average). Hence, suggestions on water management depend on the regional water availability (i.e. Mediterranean vs. Monsoon climate) and the balance between higher respiratory losses versus a potential reduction in CH4 and other greenhouse gas emissions. Our results suggest that a shift from rainfed/unsaturated soil to waterlogged paddy conditions after closure of the rice canopy might be a good compromise towards a sustainable use of water while preserving grain yield, particularly for water-limited production areas. [ABSTRACT FROM AUTHOR]

Light use efficiency (LUE) plays a vital role in determination of crop biomass and yield. Important components of LUE, i.e. canopy structure, nitrogen distribution, photosynthetic capacity and CO 2 diffusion conductance were investigated in paddy rice grown under low, normal and high supplemental nitrogen (0, 115, and 180 kg N ha −1 ). Photosynthetic characteristics varied linearly with leaf nitrogen content (N a ), independent of treatment and canopy position, so differences in photosynthesis were due to differences in N allocation. CO 2 diffusion resistances were significant and constrained LUE (more during late season), but there were no differences between treatments. Early in the season (tillering stage) leaves in the fertilized treatments had higher photosynthetic rates due to higher leaf N content leading to larger amounts of rate-limiting photosynthetic proteins, which gave them an early head start and boost in productivity and leaf area index (LAI), bringing increases in canopy light interception. Later during the growth season, differences in leaf N a and photosynthetic characteristics between treatments were slight. Enhanced LAI in fertilized plots throughout the growing season was related to greater leaf number and leaf area per planted bundle and a larger leaf area in the upper canopy (LAUC). Fertilized treatments had a higher LAUC with high leaf nitrogen concentration and reduced mesophyll diffusion limitation but greater exposure to full sunlight that led to improved nitrogen use efficiency and efficient carbon gain. In conclusion, differences in carbon gain and biomass accumulation under differing N fertilization were associated primarily with resource allocation associated with canopy leaf area development rather than leaf morphological or physiological properties. The results provide new insights with respect to the multidimensional coordinated structural and physiological adjustments governing LUE over the course of rice crop development. [ABSTRACT FROM AUTHOR]

Leaf intrinsic water use efficiency (WUE i ) coupling maximum assimilation rate ( A max ) and transpirable water lost via stomatal conductance ( g sc ) has been gaining increasing concern in sustainable crop production. Factors that influence leaf A max and WUE i in rice ( Oryza sativa L. cv Unkang) at flooding and rainfed conditions were evaluated. Positive correlations for leaf nitrogen content ( N m ) and maximum carboxylation rate ( V cmax ), for nitrogen allocation in Rubisco enzymes and mesophyll conductance ( g m ) were evident independent of cropping cultures. Rainfed rice exhibited enriched canopy leaf average N m resulting in higher A max , partially supporting improved leaf WUE i . Maximum WUE i (up to 0.14 μmol mmol −1 ) recorded in rainfed rice under drought conditions resulted from increasing g m / g sc ratio while at cost of significant decline in A max due to hydraulically constrained g sc . A max sensitivity related to g sc which was regulated by plant hydraulic conductance. WUE i was tightly correlated to V cmax / g sc and g m / g sc ratios across the paddy and rainfed not to light environment, morphological and physiological traits, highlighting enhance capacity of N m accumulation in rainfed rice with g sc at moderately high level similar to paddy rice facilitate optimization in A max and WUE i while, is challenged by drought-vulnerable plant hydraulic conductance. [ABSTRACT FROM AUTHOR]

Most visitors of a sauna appreciate the heat pulse that is perceived when water is poured on the stones of a sauna stove. However, probably only few bathers are aware that this pleasant heat pulse is caused by latent heat being released onto our skin due to condensation of water vapour. In order to quantify the proportion of condensation water versus sweat to dripping water of test persons we conducted sauna experiments using isotopically labelled (δ18O andδ2H) thrown water as tracer. This allows differentiating between ‘pure sweat’ and ‘condensation water’. Two ways of isotope mass balance calculations were applied and yielded similar results for both water isotopes. Accordingly, condensation contributed considerably to dripping water with mean proportions of 52 ± 12 and 54 ± 7 % in a sauna experiment in winter semester 2011/12 and 30 ± 13 and 33 ± 6 % in a sauna experiment in winter semester 2012/13, respectively, depending on the way of calculating the isotope mass balance. It can be concluded from the results of our dual isotope labelling sauna experiment that it is not all about sweat in the sauna. [ABSTRACT FROM AUTHOR]

• Beech and fir are able to use water from depths >0.5 m during drought. • Beech was unable to support transpiration by water contributions of this soil depth. • Firs transpiration remained unaffected due to isohydry and a taproot system. • In mixture, fir outcompeted beech for water in shallow soil after drought. In the present study we investigated water use strategies of mature stands of Fagus sylvatica (European Beech) with and without co-cultivation of Abies alba (Silver fir) during a summer drought and subsequent rewetting through simulated heavy precipitation in a temperate forest in south-west Germany. We used a combination of sap flow analysis, water stable isotope labeling and a simple linear mixing model with physiological restrictions to determine whether both species were able to switch water uptake depths during drought and reverse this switch after restoration of water supply. We showed that both species utilized water from relatively deep soil layers (>0.5 m) during the drought phase, though for beech trees these contributions were not able to support the high transpiration demands. In contrast, the transpiration of firs remained unaffected by drought probably due to a mixture of its isohydric water use strategy and taproot system. The reactions towards high intensity irrigation differed among species but also between beeches of the pure and mixed stands. While beech trees in the pure stand, once water uptake from the topsoil was re-established, used ~70% of water from more shallow soil depths and were able to restore transpiration, beech trees in the mixed stand obtained only 50% and transpiration decreased further. Firs used around 70% of the water in shallow soil layers and transpiration rates again remained unaffected. These differences indicate competition between beech trees and firs in the mixed stand upon drought. We conclude that drought is able to disrupt water relations in mixed temperate forested ecosystems in such a way that facilitation can turn into competition for resources like water. [ABSTRACT FROM AUTHOR]

\newcommand{\dO}{\textdelta$^{18}\mathrm{O}$}\newcommand {\dH} {\textdelta$^{2}\mathrm{H}$ }\newcommand {\insitu} {\textit{in situ} }Analysis of stable isotopes of water (\dH and \dO) has shown to be useful for tracing processes at the soil-plant-atmosphere interface.Since the development of laser based spectroscopy the spatial and temporal resolutions and representativeness of isotopic measurements have significantly improved. But sample collection still remains challenging. As yet, plant and soil material is most commonly sampled destructively and water is extracted cryogenically under vacuum. However, recent results indicate that cryogenically extracted water is influenced by soil properties and might not be representative for mixing processes in the soil at different temporal scales. In this study, we installed a prior tested sampling technique based on polypropylene tubing that enables continuous \insitu monitoring of the isotopic composition of soil pore water vapor in a grassland in Southern Germany. We set up rainout shelters to induce water limited conditions and applied two artificial isotopically labeled rain pulses, each followed by a period of drought.Spatial and temporal dynamics of the isotopic composition of soil pore water vapor within the soil profiles were measured \insitu on three replicates. Additionally, isotopic composition of the transpiration of two grassland species was monitored \insitu with leaf cuvette measurements. Our observations were cross-validated with isotopic composition of destructively sampled soil and leaf water, which was extracted via cryogenic vacuum extraction. The comparison of the different methods illustrates the usefulness of \insitu non-destructive sampling to investigate spatio-temporal processes at the soil-vegetation-atmosphere interface. The possibility to measure repeatedly at identical soil locations and at daily resolution with high accuracy, allowed us to discern temporal dynamics of the water isotopic composition from lateral soil heterogeneity. The isotope labeling approach facilitated spatio-temporal differentiation between different soil layers. Comparing transpiration isotopic composition revealed destructive sampling to be afflicted with high uncertainties of input-parameter estimates when deriving transpiration isotopic composition from bulk leaf water. Our study contributes to recent developments of more accurate sampling techniques in ecohydrological research. It highlights the necessity to enhance our understanding of the relevance of hydrological processes on temporal scales and calls for wider application of coupled soil and transpiration measurements at high temporal resolution. [ABSTRACT FROM AUTHOR]

Extreme drought events and plant invasion are concurrently threatening water-limitedecosystems. Especially in the Mediterranean Basin, extreme drought events have increased inthe past years with detrimental effects on many ecosystems. Such events rapidly increase thevulnerability of forest ecosystems by decreasing soil water resources, reducingfitness and growth of trees and finally leading to increased mortality rates. Shrubinvasion can further exaggerate these effects and dangerously decrease tree resilience,eventually turning ecosystems into an alternate state of degraded shrublands. However,experimental evidence on the interaction of recurrent extreme drought and plant invasion isstill scarce and inconclusive, as synergistic effects of different stressors are oftenneglected. Thus, we aim to gain a comprehensive knowledge of the underlying mechanisms ofrecurrent extreme droughts, shrub invasion and their cumulative effects in a Mediterraneanecosystem. To this end, an open savannah-type cork oak (Quercus suber) ecosystem invadedby the native shrub Cistus ladanifer was chosen in South-Eastern Portugal. In threeexperimental blocks, a rainout shelter (30 %) was installed in an uninvaded and invadedcork-oak stand, supplemented by corresponding control plots, in December 2017.Measurements of water and carbon fluxes of trees (n = 36), shrubs (n = 18) and the ecosystemin total will allow to unravel species-specific responses and competition effects underrecurrent extreme droughts. We aim to determine the stress tolerance and critical thresholdsof physiological functioning for both species as well as for the entire ecosystem. Hence, thesedata will help to predict the vulnerability of cork oak ecosystems to progressing climatechange and support future management and conservation decisions for these valuableecosystems. The first year of measurements was dominated by a wet and cold spring, which wasfollowed by a drought period from July to the middle of October. Due to the heavy springrainfall, no significant effects could be observed between treatments. However, aspecies-specific behaviour was evident. Pre-dawn leaf water potentials (ΨPD) of C. ladaniferdecreased from -0.57±0.03 MPa to -3.46±0.05 MPa in the drought period. On the otherhand, ΨPD of Q. suber did not fall below -1.48±0.06 MPa in any treatment. In addition,ecosystem transpiration was clearly dominated by shrubs with maximum sap flow rates (8.8± 0.8 m3 m−2 day−1) over three-fold higher than those of trees. These differences clearlypoint towards two distinctly different drought adaptation strategies of the investigated species. [ABSTRACT FROM AUTHOR]

Two major threats to semi-natural grasslands in temperate zones are 1) the increase of extreme drought events, and 2) nutrient loading due to agricultural fertilization and pollution. These threats may cause shifts in species diversity and abundance and also affect water and carbon ecosystem budgets. Therefore, it is pivotal to understand the impact of drought stress and nutrient addition on ecosystem carbon and water fluxes. Here, we experimentally investigated the combined effect of nitrogen addition (N) and simulated extreme drought (D) on the water and carbon fluxes of a semi-natural temperate grassland, located in Freiburg (South Germany). The semi-natural grassland was exposed to four different treatments: I) control C, II) nitrogen addition N, III) simulated extreme drought CD, IV) and the combination of nitrogen addition and simulated extreme drought ND. In order to study the response of the grassland, we combined eddy covariance techniques with open gas exchange systems and laser-based spectroscopy of water isotopologues, which are powerful tools to partition evapotranspiration and to trace water movements in plants, communities and ecosystems. Vegetation parameters were described by species richness, species abundance and leaf area index. Our results suggest that grassland communities, strongly changed in their species composition by nitrogen addition, can substantially lose their function as carbon sink during dry periods, while simulated extreme drought even increased this effect. This effect was linked to a loss in forb species (–25%) and to a strong dominance of grass species due to nitrogen loading. The grass dominated community suffered from a strong above-ground dieback in the dry summer months caused by a lower water use efficiency and weaker adaptations to drought of the community. Over the vegetation period (April-September) the carbon sequestration in our system was relative to the control group reduced by more than 60 % as a consequence to nitrogen addition, in combination with simulated extreme drought even by 73 %. Eutrophication can substantially decrease the carbon sink function of grassland communities, in particular when drought periods will increase under future climate conditions. Future management plans should stress on reducing human caused nitrogen input into grassland systems and tackle other threats to species richness. [ABSTRACT FROM AUTHOR]