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Summary: Stomatal optimization models can improve estimates of water and carbon fluxes with relatively low complexity, yet there is no consensus on which formulations are most appropriate for ecosystem‐scale applications. We implemented three existing analytical equations for stomatal conductance, based on different water penalty functions, in a big‐leaf comparison framework, and determined which optimization principles were most consistent with flux tower observations from different biomes.We used information theory to dissect controls of soil water supply and atmospheric demand on evapotranspiration in wet to dry conditions and to quantify missing or inadequate information in model variants. We ranked stomatal optimization principles based on parameter uncertainty, parsimony, predictive accuracy, and functional accuracy of the interactions between soil moisture, vapor pressure deficit, and evapotranspiration.Performance was high for all model variants. Water penalty functions with explicit representation of plant hydraulics did not substantially improve predictive or functional accuracy of ecosystem‐scale evapotranspiration estimates, and parameterizations were more uncertain, despite having physiological underpinnings at the plant level.Stomatal optimization based on water use efficiency thus provided more information about ecosystem‐scale evapotranspiration compared to those based on xylem vulnerability and proved more useful in improving ecosystem‐scale models with less complexity. [ABSTRACT FROM AUTHOR]

In Northern Europe, poplars (Populus) can provide biomass for energy and material use, but most available clones were developed for lower latitudes and are unlikely to be well adapted to higher latitudes, even under warmer climates. We thus need to understand how clones respond to climatic conditions and photoperiod, and how these responses can be predicted. We answer these questions exploiting leaf phenological data of Populus clones, grown in six sites across the Baltic region, in Northern Europe, for 2 years with contrasting climatic conditions. Regarding the effects of climatic conditions and photoperiod, within each site, higher temperatures advanced the timing and enhanced the speed of spring and autumn phenology, but reduced the effective growing season length. Across sites, latitude affected the timing of spring and autumn phenology, the speed of spring phenology, and the effective growing season length; clone affected only the timing of phenology. Regarding the predictability of clone response to growing conditions, the growing degree day (GDD) model could not predict spring phenology, because the growing degree day threshold for a specific phenological stage was not only clone-, but also latitude- and year-specific. Yet, this GDD threshold allowed a robust ranking of clones across sites and years, thus providing a tool to determine the relative differences across clones, independently of latitude and temperature. A similar, but not as strong, pattern was observed in the timing of spring and autumn phenological stages. Hence, while prediction of spring phenology remains elusive, the ranking of clones based on observations of their phenology in a single location can provide useful indications on the clones' relative performance under different latitudes and climates. [ABSTRACT FROM AUTHOR]

Climate change is predicted to hamper crop production due to precipitation deficits and warmer temperatures inducing both water stress and increasing herbivory due to more abundant insect pests. Consequently, crop yields will be impacted simultaneously by abiotic and biotic stressors. Extensive yield losses due to such climate change stressors might, however, be mitigated by ecosystem services such as insect pollination. We examined the single and combined effects of water stress, insect herbivory and insect pollination on faba bean yield components and above‐ and belowground plant biomass under realistic field conditions. We used rainout shelters to simulate a scenario in line with climate change projections, with adequate water supply at sowing followed by a long period without precipitation. This induced a gradually increasing water stress, culminating around crop flowering and yield formation. We found that gradually increasing water stress combined with insect herbivory by aphids interactively shaped yield in faba beans. Individually, aphid herbivory reduced yield by 79% and water stress reduced yield by 52%. However, the combined effect of water stress and aphid herbivory reduced yield less (84%) than the sum of the individual stressor effects. In contrast, insect pollination increased yield by 68% independently of water availability and insect herbivory. Our results suggest that yield losses can be greatly reduced when both water stress and insect herbivory are reduced simultaneously. In contrast, reducing only one stressor has negligible benefits on yield as long as the crop is suffering from the other stressor. We call for further exploration of interactions among ecosystem services and biotic and abiotic stressors that simulate realistic conditions under climate change. [ABSTRACT FROM AUTHOR]

Under future climates, leaf temperature (Tl) will be higher and more variable. This will affect plant carbon (C) balance because photosynthesis and respiration both respond to short‐term (subdaily) fluctuations in Tl and acclimate in the longer term (days to months). This study asks the question: To what extent can the potential and speed of photosynthetic acclimation buffer leaf C gain from rising and increasing variable Tl? We quantified how increases in the mean and variability of growth temperature affect leaf performance (mean net CO2 assimilation rates, Anet; its variability; and time under near‐optimal photosynthetic conditions), as mediated by thermal acclimation. To this aim, the probability distribution of Anet was obtained by combining a probabilistic description of short‐ and long‐term changes in Tl with data on Anet responses to these changes, encompassing 75 genera and 111 species, including both C3 and C4 species. Our results show that (a) expected increases in Tl variability will decrease mean Anet and increase its variability, whereas the effects of higher mean Tl depend on species and initial Tl, and (b) acclimation reduces the effects of leaf warming, maintaining Anet at >80% of its maximum under most thermal regimes. Under future climates, leaf temperatures will be higher and more variable, with effects on the plant carbon balance. This study asks the question: To what extent can leaf net photosynthesis acclimate to buffer leaf carbon gains from rising and increasingly variable leaf temperatures? A probabilistic description of short‐ and long‐term changes in leaf temperature is parameterized by means of a dataset on net photosynthetic responses to these changes, encompassing 75 genera and 111 species. Our results show that (a) the expected increases in leaf temperature variability have always detrimental effects on leaf performance, whereas the effects of higher mean leaf temperatures depend on the species and initial temperature, and (b) acclimation greatly reduces the consequences of leaf warming. [ABSTRACT FROM AUTHOR]

Population growth and changes in climate and diets will likely further increase the pressure on agriculture and water resources globally. Currently, staple crops are obtained from annuals plants. A shift towards perennial crops may enhance many ecosystem services, but at the cost of higher water requirements and lower yields. It is still unclear when the advantages of perennial crops overcome their disadvantages and perennial crops are thus a sustainable solution. Here we combine a probabilistic description of the soil water balance and crop development with an extensive dataset of traits of congeneric annuals and perennials to identify the conditions for which perennial crops are more viable than annual ones with reference to yield, yield stability, and effective use of water. We show that the larger and more developed roots of perennial crops allow a better exploitation of soil water resources and a reduction of yield variability with respect to annual species, but their yields remain lower when considering grain crops. Furthermore, perennial crops have higher and more variable irrigation requirements and lower water productivity. These results are important to understand the potential consequences for yield, its stability, and water resource use of a shift from annual to perennial crops and, more generally, if perennial crops may be more resilient than annual crops in the face of climatic fluctuations. [ABSTRACT FROM AUTHOR]

Farm specialization and associated simplification of the crop rotation raise concern about the ability of cropping systems to deliver high and stable yield in the long-term. Exploiting data from 52 years of a long-term experiment in Southern Sweden, we investigated impacts of three conventional cropping systems (‘crop-livestock’, ‘specialized’, and ‘diverse’) on yield levels and stability of winter and spring wheat. For winter wheat, the ‘diverse’ and ‘crop-livestock’ systems enhanced yields by 15% compared to the ‘specialized’ system. For spring wheat, the ‘crop-livestock’ system tended to show higher yield than the ‘diverse’ one. The stability analysis showed that in winter wheat the three systems lead to equally stable yields considering year-to-year variability. For spring wheat, the ‘crop-livestock’ system tended to perform better in favorable years relative to the other systems. Overall, the results revealed that for winter wheat cultivation, stockless diverse cropping provides an valid alternative to crop-livestock in the context of specialized farming where crop and livestock are separated. [ABSTRACT FROM AUTHOR]

The article quantitatively compares the traits between perennial and annual congeneric plant species, including in regard to the plants' seed yields, plant productivity, plant biomass and lifespans. An overview of the sustainability of grain crops, including in regard to their relationship with the ecosystem, is provided. An overview of variations in plant yields between perennials and annual plants is also provided.

Irrigation represents one of the main strategies to enhance and stabilize agricultural productivity, by mitigating the effects of rainfall vagaries. In the face of the projected growth in population and in biofuel demands, as well as shifts in climate and dietary habits, a more sustainable management of water resources in agroecosystems is needed. The field of ecohydrology, traditionally focusing on natural ecosystems, has the potential to offer the necessary quantitative tools to assess and compare agricultural enterprises across climates, soil types, crops, and irrigation strategies, accounting for the unpredictability of the hydro-climatic forcing. Here, agricultural sustainability and productivity are assessed with reference to water productivity (defined as the ratio between yield and total supplied water), yields, water requirements, and their variability-a crucial element for food security and resource allocation planning. These synthetic indicators are quantified by means of a probabilistic description of the soil water balance and crop development. The model results allow the interpretation of patterns of water productivity observed in Zea mays (maize) and Triticum aestivum (wheat), grown under a variety of soils, climates, and irrigation strategies. Employing the same modeling framework, the impact of rainfall pattern and irrigation strategy on yield and water requirements is further explored. The obtained standard deviations of yield and water requirements suggest the existence of a nonlinear tradeoff between yield stabilization and variability of water requirements, which in turn is strongly impacted by irrigation strategy. Moreover, intermediate rainfall amounts are associated to the highest variability in yields and irrigation requirements, although allowing the maximum water productivity. The existence of these tradeoffs between productivity, reliability, and sustainability poses a problem for water management, in particular in mesic climates. [ABSTRACT FROM AUTHOR]

Autumn-sown field crops have important agronomic advantages (e.g., reduction of soil erosion and nutrient leaching, maximizing the use of spring moisture) and have the potential to be highly productive even though adverse winter conditions can negatively affect crop viability and yield. In the face of the unpredictable weather patterns and the expected shifts in climate in the near future, there is an imperative to develop methods to quantify both the risk of winter damage and how it is affected by altered climatic conditions and crop variety. We propose a set of indices to characterize synthetically the risk of crop damage stemming from cold spells, extended periods at low temperature, frequent occurrence of freeze-thaw cycles, and prolonged snow cover. An existing model of crop hardening and dehardening is further developed to account in full for the variability of lethal threshold temperature among individual plants. This model is coupled to a simple yet realistic description of crop-sensed temperature, so that required inputs are limited to crop-specific responses to low temperature and standard meteorogical data (average daily temperature and snow depth). This framework is applied to winter wheat under the current climatic conditions for central and southern Sweden. The roles of variety-specific hardening ability, temperature, and snow are assessed separately, thus obtaining indications of the potential impacts of variety selection and future predicted changes in temperature and snow cover in the region. Variety-specific hardening ability and response to exposure to low temperature may drastically alter the extent of winter damage. The most prevalent damaging mechanism depends on the climatic regime, with crops in colder areas benefiting from extended snow cover. A tradeoff between temperature (and hence latitude) and snow emerges, with locations at intermediate latitudes subjected to the highest risk of crop damage from exposure to low temperature and frequent freeze-thaw cycles. The same locations are also characterized by the highest inter-annual variability in the extent of winter damage - a fact that has potential implications for yield reliability. [ABSTRACT FROM AUTHOR]

Background and Aims: Due to their high water use efficiency, Crassulacean acid metabolism (CAM) plants are of environmental and economic importance in the arid and semiarid regions of the world. Moreover, many CAM plants (e.g., Agave tequilana) have attractive qualities for biofuel production such as a relatively low lignin content and high amount of soluble carbohydrates. However, the current estimates of CAM productivity are based on empirical stress indices that create large uncertainties. As a first step towards a more accurate quantification of CAM productivity, this paper introduces a new model that couples both soil and atmosphere conditions to CAM photosynthesis. Methods: The new CAM model is based upon well established C photosynthesis models coupled to a nonlinear circadian rhythm oscillator for the control of the photosynthesis carbon fluxes. The leaf-level dynamics are coupled to a simple, yet realistic description of the soil-plant-atmosphere continuum, including a plant water capacitance module. Results: The resulting model reproduces the four phases of CAM photosynthes is and the evolution of their dynamics during a soil moisture drydown, as a function of soil type, plant features, and climatic conditions. Conclusion: The results help quantify the impact of soil water availability on CAM carbon assimilation and transpiration flux. [ABSTRACT FROM AUTHOR]

Highlights: [•] Optimal stomatal conductance under CO2-light co-limitation is analytically obtained. [•] It reconciles existing formulations based on single limitations to photosynthesis. [•] Model predictions are compared to known stomatal responses to micro-environment. [•] Marginal water use efficiency response to short- and long-term CO2 shifts are discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Stomatal optimization theories have neglected the effect of dynamic soil moisture. [•] Here soil moisture is considered to provide a closure for the optimization problem. [•] The new solutions link marginal water use efficiency to air [CO2] and soil water. [•] Previously inconsistent theoretical results and data are generalized and explained. [Copyright &y& Elsevier]

Soil and plant hydraulics constrain ecosystem productivity by setting physical limits to water transport and hence carbon uptake by leaves. While more negative xylem water potentials provide a larger driving force for water transport, they also cause cavitation that limits hydraulic conductivity. An optimum balance between driving force and cavitation occurs at intermediate water potentials, thus defining the maximum transpiration rate the xylem can sustain (denoted as Emax). The presence of this maximum raises the question as to whether plants regulate transpiration through stomata to function near Emax., To address this question, we calculated Emax across plant functional types and climates using a hydraulic model and a global database of plant hydraulic traits., The predicted Emax compared well with measured peak transpiration across plant sizes and growth conditions ( R = 0.86, P < 0.001) and was relatively conserved among plant types (for a given plant size), while increasing across climates following the atmospheric evaporative demand. The fact that Emax was roughly conserved across plant types and scales with the product of xylem saturated conductivity and water potential at 50% cavitation was used here to explain the safety-efficiency trade-off in plant xylem., Stomatal conductance allows maximum transpiration rates despite partial cavitation in the xylem thereby suggesting coordination between stomatal regulation and xylem hydraulic characteristics. [ABSTRACT FROM AUTHOR]

Abstract: An effective description of water transport in the soil–plant–atmosphere continuum (SPAC) is needed for wide-ranging applications in hydrology and climate-vegetation interactions. In this contribution, the theory of water movement within the SPAC is reviewed with emphasis on the eco-physiological and evolutionary constraints to water transport. The description of the SPAC can be framed at two widely separated time scales: (i) sub-hourly to growing season scales, relevant for hydro-climatic effects on ecosystem fluxes (given a set of plant hydraulic traits), and (ii) inter-annual to centennial scales during which either hydraulic traits may change, as individuals grow and acclimate, or species composition may change. At the shorter time scales, water transport can be described by water balance equations where fluxes depend on the hydraulic features of the different compartments, encoded in the form of conductances that nonlinearly depend on water availability. Over longer time scales, ontogeny, acclimation, and shifts in species composition in response to environmental changes can impose constraints on these equations in the form of tradeoffs and coordinated changes in the hydraulic (and biochemical) parameters. Quantification of this evolutionary coordination and the related tradeoffs offers novel theoretical tactics to constrain hydrologic and biogeochemical models. [Copyright &y& Elsevier]

Summary [ABSTRACT FROM AUTHOR]

Quantification of stomatal responses to environmental variables, in particular to soil water status, is needed to model carbon and water exchange rates between plants and the atmosphere. Models based on stomatal optimality theory successfully describe leaf gas exchange under different environmental conditions, but the effects of water availability on the key optimization parameter [the marginal water use efficiency (WUE), λ = ∂ A/∂ E] has resisted complete theoretical treatment. Building on previous optimal leaf gas exchange models, we developed an analytical equation to estimate λ from gas exchange observations along gradients of soil water availability. This expression was then used in a meta-analysis of about 50 species to investigate patterns of variation in λ. Assuming that cuticular water losses are negligible λ increases under mild water stress but decreases when severe water stress limits photosynthesis. When cuticular conductance is considered, however, λ increases monotonically with increasing water stress, in agreement with previous theoretical predictions. Moreover, the shape of these response curves to soil water availability changes with plant functional type and climatic conditions. In general, λ is lower in species grown in dry climates, indicating lower marginal WUE. The proposed parameterization provides a framework to assess the responses of leaf gas exchange to changes in water availability. Moreover, the model can be extended to scale leaf-level fluxes to the canopy and ecosystem level. [ABSTRACT FROM AUTHOR]

Abstract: With vast regions already experiencing water shortages, it is becoming imperative to manage sustainably the available water resources. As agriculture is by far the most important user of freshwater and the role of irrigation is projected to increase in face of climate change and increased food requirements, it is particularly important to develop simple, widely applicable models of irrigation water needs for short- and long-term water resource management. Such models should synthetically provide the key irrigation quantities (volumes, frequencies, etc.) for different irrigation schemes as a function of the main soil, crop, and climatic features, including rainfall unpredictability. Here we consider often-employed irrigation methods (e.g., surface and sprinkler irrigation systems, as well as modern micro-irrigation techniques) and describe them under a unified conceptual and theoretical framework, which includes rainfed agriculture and stress-avoidance irrigation as extreme cases. We obtain mostly analytical solutions for the stochastic steady state of soil moisture probability density function with random rainfall timing and amount, and compute water requirements as a function of climate, crop, and soil parameters. These results provide the necessary starting point for a full assessment of irrigation strategies, with reference to sustainability, productivity, and profitability, developed in a companion paper [Vico G, Porporato A. From rainfed agriculture to stress-avoidance irrigation: II. Sustainability, crop yield, and net profit. Adv Water Resour 2011;34(2):272–81]. [ABSTRACT FROM AUTHOR]

Abstract: The optimality of irrigation strategies may be sought with respect to a number of criteria, including water requirements, crop yield, and profitability. To explore the suitability of different demand-based irrigation strategies, we link the probabilistic description of irrigation requirements under stochastic hydro-climatic conditions, provided in a companion paper [Vico G, Porporato A. From rainfed agriculture to stress-avoidance irrigation: I. A generalized irrigation scheme with stochastic soil moisture. Adv Water Resour 2011;34(2):263–71], to crop-yield and economic analyses. Water requirements, application efficiency, and investment costs of different irrigation methods, such as surface, sprinkler and drip irrigation systems, are described via a unified conceptual and theoretical approach, which includes rainfed agriculture and stress-avoidance irrigation as extreme cases. This allows us to analyze irrigation strategies with respect to sustainability, productivity, and economic return, using the same framework, and quantify them as a function of climate, crop, and soil parameters. We apply our results to corn (Zea mays), a food staple and biofuel source, which is currently mainly irrigated through surface systems. As our analysis shows, micro-irrigation maximizes water productivity, but more traditional solutions may be more profitable at least in some contexts. [ABSTRACT FROM AUTHOR]

Despite the observed impact of water stress on photosynthesis, some of the most used models of CO2 assimilation in C3 and C4 functional types do not directly account for it. We discuss an extension of these models, which explicitly includes the metabolic and diffusive limitations due to water stress on photosynthesis. Functional relationships describing the photosynthetic processes and CO2 diffusion inside leaves are modified to account for leaf water status on the basis of experimental results available in the literature. Extensive comparison with data shows that the model is suitable to describe the reduction in CO2 assimilation rate with decreasing leaf water potentials in various species. A simultaneous analysis of photosynthesis, transpiration and soil moisture dynamics is then carried out to explore the actual impact of drought on different photosynthesis processes and on the overall plant activity. The model well reproduces measured CO2 assimilation rate as a function of soil moisture and could be useful to formulate hypotheses for detailed experiments as well as to simulate in detail transpiration and photosynthesis dynamics under water stress. [ABSTRACT FROM AUTHOR]

Droughts affect ecosystems at multiple time scales, but their sub‐seasonal legacy effects on vegetation activity remain unclear. Combining the satellite‐based enhanced vegetation index MODIS EVI with a novel location‐specific definition of the growing season, we quantify drought impacts on sub‐seasonal vegetation activity and the subsequent recovery in the Northern Hemisphere. Drought legacy effects are quantified as changes in post‐drought greenness and sensitivity to climate. We find that greenness losses under severe drought are partially compensated by a ∼+5% greening within 2–6 growing‐season months following the droughts, both in woody and herbaceous vegetation but at different timings. In addition, post‐drought sensitivity of herbaceous vegetation to hydrological conditions increases noticeably at high latitudes compared with the local normal conditions, regardless of the choice of drought time scales. In general, the legacy effects on sensitivity are larger in herbaceous vegetation than in woody vegetation. Plain Language Summary: Droughts are increasingly severe and widespread globally, but their impacts on the global carbon cycle remain unclear due to the complexity of vegetation response to climate under and after droughts. In particular, how much plant activity decreases during drought and how quickly it recovers in the following months or years is not well understood. Here, by employing large‐scale satellite‐derived vegetation information, we identify the existence of widespread drought legacy effects on vegetation activity in the Northern Hemisphere ecosystems. We find that vegetation becomes greener in 2–6 growing‐season months after severe droughts with a stronger response to climate. These findings are in general more prominent at mid‐ and high‐latitudes, in particular for the grass‐dominated ecosystems compared with the tree‐dominated ones. Our analyses extend the picture for understanding how vegetation responds to severe droughts and could provide useful information to assist the development of dynamic vegetation models toward assessing changes in ecosystem resilience under changing climates. Key Points: Prominent increases in the sensitivity of herbaceous vegetation to hydrological conditions after severe droughts in the high‐latitudesSignificant vegetation greening in the 2–6 growing‐season months following severe droughtsLarger drought legacy effects on the sensitivity of herbaceous vegetation to climate compared with woody vegetation [ABSTRACT FROM AUTHOR]

Boreal forests are often managed to maximize wood production, but other goals, among which climate change mitigation, are increasingly important. Hence, it is necessary to examine synergies and trade-offs between forest production and its potential for carbon sequestration and climate change mitigation in forest stands. To this aim, we develop a novel mass-balanced process-based compartmental model that allows following the carbon path from its photosynthetic fixation until its return to the atmosphere by autotrophic or heterotrophic respiration, or by being burnt as wood product. Following carbon in the system allows to account for how long forest ecosystems and wood products retain carbon away from the atmosphere (i.e., the carbon transit time). As example, we apply the model to four management scenarios, i.e., mixed-aged pine, even-aged pine, even-aged spruce, and even-aged mixed forest, and contrast metrics of performance relative to wood production, carbon sequestration, and climate change mitigation potential. While at the end of an 80 yr rotation the even-aged forests held up to 31% more carbon than the mixed-aged forest, the mixed-aged forest was superior during almost the entire rotation when factoring in the carbon retention time away from the atmosphere, i.e., in terms of climate change mitigation potential. Importantly, scenarios that maximize production or amount of carbon stored in the ecosystems are not necessarily the most beneficial for carbon retention away from the atmosphere. These results underline the importance of considering carbon transit time when evaluating forest management options for potential climate change mitigation. [Display omitted] • We evaluate wood production and climate change mitigation potential of boreal forests. • We combine an ecophysiological growth model with forest inventory tree allometries. • Higher carbon sequestration does not ensure higher climate change mitigation potential. • Potential climate change mitigation depends on carbon time away from the atmosphere. [ABSTRACT FROM AUTHOR]

Seed predation can reduce the abundance and spread of unwanted vegetation in agricultural and other semi‐natural ecosystems. However, knowledge of how variations in seed predation rates affect plant species population dynamics is needed for decision making and knowledge‐based ecosystem management.We developed a stage‐classified stochastic matrix population model for Alopecurus myosuroides Huds. (blackgrass), an annual plant species thriving as a weed in temperate agroecosystems of Western and Northern Europe. The model was parameterised using empirical demographic data from long‐term experiments in Swedish winter wheat fields, including information on post‐dispersal seed losses by vertebrate and invertebrate seed predators.For agroecosystems with highly effective weed control measures (e.g. chemical and mechanical weed control), model simulations showed that seed losses via seed predation need to reach at least 78% at peak seed shedding to suppress population growth of A. myosuroides.The field experiment showed that vertebrates were most important for seed predation in July, at peak seed shedding. In August, after crop harvest, invertebrates were responsible for almost all seed predation. The model indicated that weed seed predation was much more important for weed regulation when it occurred before crop harvest in July. Vertebrates most strongly reduced population growth of A. myosuroides, although both vertebrates and invertebrates were needed to prevent it entirely.Synthesis and applications. We showed that weed seed predation by vertebrate and invertebrate seed predators is key for reducing the population growth of winter annual weeds like A. myosuroides in intensively managed agroecosystems. Therefore, protection of weed seed predators is essential for making management of unwanted vegetation less dependent on chemical and mechanical measures. [ABSTRACT FROM AUTHOR]

• On-farm ponds can sustainably provide water for irrigation, but impose trade-offs. • A minimalist model coupling soil water, crop and pond water storage is developed. • We identify the best pond size for different climates, soils and irrigation strategies. • Production maximization and low yield risk minimization require different pond sizes. • Future more extreme climates will make these goals even harder to reconcile. In many regions precipitation does not reliably meet crop water demands – a situation that climate change will likely exacerbate. Supplemental irrigation can help enhance and stabilize crop yields, but the need of water for irrigation has often led to groundwater over-exploitation. On-farm ponds can provide a more sustainable water source. Their use has often been promoted by local authorities, but, by converting a portion of cultivated area to water storage and reducing water availability downstream, on-farm ponds also imply constraints and trade-offs. For an effective exploitation of their potential benefits, they must be carefully designed and managed based on the local edaphic and climate conditions – a non trivial, task because of the cascading effects of rainfall unpredictability. Here we identify the most suitable on-farm pond size, according to two criteria: maximization of average yield (i.e., production maximization) and achievement of a minimum acceptable yield (i.e., risk minimization, accounting for the farmer's risk aversion). To this aim, we develop a minimalist model, requiring few, physically based parameters, coupling crop biomass, soil moisture, and water stored in the pond. While general, the model is here applied to a case-study in the Lower Mississippi River Basin (USA). Simulations show that yield maximization and risk minimization are goals hard to reconcile, regardless of climatic conditions, soil type and irrigation strategy, with smaller ponds allowing the maximum average yield at the cost of reducing its stability from year to year. Stress avoidance irrigation ensures higher yields than deficit irrigation, even if it implies a faster use of the stored water. Future, more extreme climates will result in lower maximum average yields and narrower ranges of pond sizes ensuring desirable minimum yields. [ABSTRACT FROM AUTHOR]

• We analyzed daily air temperature and soil moisture time series of 54 forest sites. • Moister soils supported cooler understory microclimates. • Temperature buffering was more sensitive to soil moisture in dry soils and on sunny slopes. • Forests can lose their temperature buffering function when soils become drier. Forest canopies can buffer the understory against temperature extremes, often creating cooler microclimates during warm summer days compared to temperatures outside the forest. The buffering of maximum temperatures in the understory results from a combination of canopy shading and air cooling through soil water evaporation and plant transpiration. Therefore, buffering capacity of forests depends on canopy cover and soil moisture content, which are increasingly affected by more frequent and severe canopy disturbances and soil droughts. The extent to which this buffering will be maintained in future conditions is unclear due to the lack of understanding about the relationship between soil moisture and air temperature buffering in interaction with canopy cover and topographic settings. We explored how soil moisture variability affects temperature offsets between outside and inside the forest on a daily basis, using temperature and soil moisture data from 54 sites in temperate broadleaf forests in Central Europe over four climatically different summer seasons. Daily maximum temperatures in forest understories were on average 2 °C cooler than outside temperatures. The buffering of understory temperatures was more effective when soil moisture was higher, and the offsets were more sensitive to soil moisture on sites with drier soils and on sun-exposed slopes with high topographic heat load. Based on these results, the soil–water limitation to forest temperature buffering will become more prevalent under future warmer conditions and will likely lead to changes in understory communities. Thus, our results highlight the urgent need to include soil moisture in models and predictions of forest microclimate, understory biodiversity and tree regeneration, to provide a more precise estimate of the effects of climate change. [Display omitted] [ABSTRACT FROM AUTHOR]

Purpose: Perennial crops have been suggested as a more sustainable alternative to the currently most common cropping systems. Compared with annual plants, perennial plants produce more biomass and have deeper roots, and are expected to lead to higher soil organic carbon (SOC). This hypothesis, however, has not been well tested for grain crops. Methods: Using perennial intermediate wheatgrass (IWG, Thinopyrum intermedium) and annual winter wheat (Triticum aestivum) as focal species, and native grassland as reference, we quantified the SOC accumulation via a process-based model, describing water and heat exchanges and carbon-nitrogen cycling in the canopy and soil to a depth of 2 m. The model includes C fixation via photosynthesis, plant biomass growth and litter production, physical protection of SOC, depolymerisation, C mineralisation, nitrification, denitrification, microbial growth, and necromass turnover in the soil. While of general applicability, we considered a sandy loam under warm-summer humid continental climate. Results: Following a conversion from native grassland, IWG reduced SOC losses by at least 38%, especially in the particulate organic carbon (POC) pool, within the top 2 m of soil, compared with annual wheat. Soil microbial biomass and soil respiration were higher in IWG than annual wheat. Shifting from annual wheat to high photosynthetic capacity IWG increased SOC by about 33 g C m−2 y−1 (averaged over a 4-year continuous IWG cropping), with a large fraction of SOC gain stemming from restoring POC. Conclusion: Compared with annual grains, perennial grains can increase soil carbon sequestration and maintain SOC at levels nearer to that of native grasslands. [ABSTRACT FROM AUTHOR]

Plant spring phenology has evolved to respond to seasonal variation of temperature. Bud break must occur early enough for the plant to take full advantage of days conducive to carbon fixation, but also late enough to limit the likelihood of damage from late frost events. The timing of spring leaf phenology is thus the result of a productivity-risk of damage trade off. But what level of risk is accepted by the plants? And can these accepted levels of risks be explained by plant functional types and climatic features (e.g., extent of temperature fluctuations and rate of spring temperature increase)? To answer these questions, data on bud break dates relative to tree species with a wide latitudinal range from the Pan European Phenology Database PEP725 are combined with meteorological data from the EObs dataset. Different measures of risk of late frost damage at the time of bud break are considered, based on available species-specific critical temperatures for lethal damage. Emerging patterns of these risk levels are explained, with reference to climatic and plant features. The obtained classification of species based on their acceptable risk of late frost damage is compared to those based on other aspects of plant strategies (early vs. late successional; fast vs. slow growing). [ABSTRACT FROM AUTHOR]

When rainfall does not meet crop water requirements, supplemental irrigation is needed to maintain productivity. On-farm ponds can prevent excessive groundwater exploitation – to the benefit of the whole community – but they reduce the cultivated area and require investments by each farmer. Thus, choosing the source of water for irrigation (groundwater vs on-farm pond) is a problem of collective action. An agent-based model is developed to simulate a smallholder farming system; the farmers' long-/short-view orientation determines the choice of the water source. We identify the most beneficial water source for economic gain and its stability, and how it can change across communities and under future climate scenarios. By using on-farm ponds, long-view-oriented farmers provide collective advantages but have individual advantages only under extreme climates; a tragedy of the commons is always possible. Changes in farmers' attitudes (and hence sources of water) based on previous experiences can worsen the economic outcome. [ABSTRACT FROM AUTHOR]

• Non-dimensional combinations of soil, plant, and climate variables reduce the complexity of plant water use strategies. • Plant water flux control relative to demand is the non-dimensional group that primarily influences plant water use performance. • Plant water use strategies that maximize long-term mean transpiration weighted by risks of water stress match soil moisture observations. • Combining non-dimensional groups and ecohydrological optimality offers constraints to quantify and understand plant water stress responses. Plant responses to water stress influence water and carbon cycles and can lead to feedbacks on climate yet characterizing these responses at ecosystem levels remains uncertain. Quantifying ecosystem-level water use strategies is complex due to challenges of upscaling plant traits and disentangling confounding environmental factors, ultimately limiting our ability to understand and anticipate global change in ecosystem dynamics and ecohydrological fluxes. We reduce the dimensionality of this problem and quantify plant water use strategies by combining plant traits with soil and climate variables into parameter groups that synthesize key eco-physiological tradeoffs. Using a parsimonious soil water balance framework, we explore variations in plant water uptake capacity, water stress responses, and water use performance via these non-dimensional parameter groups. The group characterizing the synchronization of plant water transport and atmospheric water demand emerges as the primary axis of variation in water use strategies and interacts with the group representing plant hydraulic risk tolerance, especially in arid conditions when plant water transport is limiting. Next, we show that specific plant water use strategies maximize plant water uptake (leading to carbon gain benefits) weighted by risks of water stress (leading to higher costs of water use). A model-data comparison demonstrates that these ecohydrologically optimal parameter groups capture observed soil moisture variability in 40 ecosystems and beyond aridity, rainfall frequency is an important environmental control for plant water use strategies. The emerging parsimonious link between ecohydrological performance and non-dimensional parameters provides a tractable representation of plant water use strategies, relevant to parameterize global models while accounting for ecological and evolutionary constraints on the water cycle. [ABSTRACT FROM AUTHOR]

In this stochastic rainfall scenario, we can exchange the long-term average in Equation 3 with the ensemble average so that the long-term mean net carbon gain rate defined with Equation 3 becomes (Lu et al., 2016) 10 HT ht Implementation In the single dry-down scenario (but not in the stochastic rainfall scenario), the simulations were run in discrete time with all the state variables updated daily. Stomatal regulation is a cornerstone process that links the exchange of carbon and water between vegetation and atmosphere, and how it is affected by plant water stress. [Extracted from the article]

• The Jet Stream controls vegetation greenness over large parts of Europe. • The Jet Stream modulates growing season length over large parts of Europe. • The jet's role is specific to climate zone, land class growing season sub-period. • A locally-defined growing season is essential to capture the jet–vegetation link. Jet streams are a key component of the climate system, whose dynamics couple closely to regional climate variability. Yet, the link between jet stream variability and vegetation activity has received little attention. Here, we leverage our understanding of the mid-latitude jet stream dynamics over the Euro-Atlantic sector to probe climate–vegetation interactions across Europe. We link indices related to the meridional location of the jet and the large-scale zonal wind speed with remotely-sensed vegetation greenness anomalies during locally-defined growing seasons. Correlations between greenness anomalies and jet latitude anomalies point to a control of the jet stream's variability on vegetation activity over large parts of Europe. This potential control is mediated by the jet latitude anomalies' correlations with temperature, soil moisture and downward surface solar radiation. The sign and strength of these correlations depend on location and time of the year. Furthermore, jet stream variability modulates conditions at the onset and end of the growing season. The link between jet latitude anomalies and vegetation greenness is not only specific to the climate zone, but also to the landclass and subperiod within the growing season. It is thus important to use a locally-defined growing season for interpreting the atmospheric controls on regional vegetation phenology. Results consistent with the correlation analysis emerge when focussing on local high or low greenness months only or on zonal wind speed anomalies, confirming the relevance of jet variability for vegetation activity. [ABSTRACT FROM AUTHOR]

When modelling forest growth, capturing the effects of climate change is needed for reliable long-term predictions and management choices. This remains a challenge because commonly used mensurational forest growth and yield models, relying on inventory data, cannot account for climate change effects. We developed hybrid physiological/mensurational basal area growth and yield models, which combine physiological response to climatic conditions and empirical relations. We included climate and site effects by replacing time with light sums of photosynthetically active radiation and modifying the latter with monthly soil water, vapour pressure deficit, temperature, and frost days. When parameterised with permanent sample plot data for Scots pine and Norway spruce across Sweden, the hybrid models could reproduce observations well, although with no increase in precision compared with time-based mensurational models. When considering different climate scenarios, a significant impact on productivity from climate change emerged. For example, a 2 °C warming enhanced Scots pine production by up to 14% in regions where temperatures were originally cooler and soil water deficit was low (i.e. northwest Sweden), but depressed it, up to 9%, elsewhere. Hence, climate-sensitive models that take local variations into account are necessary for accurate predictions and sustainable forest management. [ABSTRACT FROM AUTHOR]

• The ecological niche space for nitrogen (N) use traits was defined via n -dimensional hypervolumes. • Trait space was influenced more by the cultivar identity than crop mixing. • High N uptake was associated with a reduced overlap in niche-space in the mixtures. • Initial seed N pools contributed to N accumulation, shoot and tiller production. More efficient resource use, especially nitrogen (N) in agricultural fields could considerably reduce the losses and spillover effects on the environment. Cereal-legume mixtures can lead to more efficient uptake of growth-limiting resources, and increase and stabilize yields, due to the variation in functional traits that facilitate partitioning of niche space. Here we identify crop mixtures with functional traits that facilitate optimal N resource use in two selected cereal-legume mixtures by using the multi-dimensional trait space concept. Combinations of pea-barley and faba bean-wheat crops were grown in the field as pure cultures and mixtures in Central Sweden, during two years with contrasting weather. The ecological niche space was defined via the n -dimensional hypervolumes represented by N pool, tiller/branch number, shoot biomass, and grain yield functional traits. Regressions and correlations allowed quantifying the relations between functional traits and plant N pools. Differences in trait space were not a result of crop mixing per se , as similar hypervolumes were found in the pure culture and mixture-grown crops. Instead, the trait space differences depended on the cultivar identities admixed. Furthermore, cereals increased their efficiency for N uptake and therefore benefitted more than the legumes in the mixtures, in terms of accumulated N and grain yields. Tiller and shoot biomass production in cereals was positively correlated to N pool accumulation during the season. Resource acquisition through increased N uptake in the mixture was associated with a reduced overlap in niche-space in the mixtures, and initial seed N pools significantly contributed to within-season N accumulation, shoot and tiller production. [ABSTRACT FROM AUTHOR]

While many agricultural practices have resulted in loss of soil organic carbon (SOC), paddyrice production often presents favourable conditions for carbon sequestration. At the sametime, it is a major source of methane emissions and a large water user. The introduction ofwater saving irrigation (WSI) practices, such as alternate wetting and drying (AWD) andmid-season drainage (MSD), have been suggested as strategies for decreasing the large waterdemands of paddy rice production. With the removal of anaerobic conditions during thegrowing season, they may also lower CH4 production and reduce the global warmingpotential of rice production. However, the long-term impact of WSI on SOC - an indicator ofsoil health and fertility - has been little explored. Through a meta-analysis we assessedthe effects of WSI on SOC storage and GHG emissions. AWD and MSD reducedemissions of CH4 by 52.3%, and increased those of CO2 and N2O by 44.8 and 30%respectively. WSI reduced CO2-equivalent emissions by 52% but increased thesoil-to-atmosphere flux of carbon by 24% when compared to flood irrigation. Moreover, WSIdecreased SOC concentrations by 8.2% when compared to flood irrigation. WhileWSI affects SOC and greenhouse gas emissions, it has been shown to potentiallyhave little effect on rice yield. However, evidence of yield stability is limited toshort-term (1-2 years) experiments, whereas soil properties change over decades.Therefore, care should be taken when assessing the long-term sustainability of theseirrigation practices because WSI can decrease soil fertility. The development offuture irrigation strategies should consider long-term effects and trade-offs betweenwater savings, fertility, other ecosystem services and climate change mitigation. [ABSTRACT FROM AUTHOR]

Despite the appeal of the iso/anisohydric framework for classifying plant drought responses, recent studies have shown that such classifications can be strongly affected by a plant's environment. Here, we present measured in situ drought responses to demonstrate that apparent isohydricity can be conflated with environmental conditions that vary over space and time. In particular, we (a) use data from an oak species (Quercus douglasii) during the 2012–2015 extreme drought in California to demonstrate how temporal and spatial variability in the environment can influence plant water potential dynamics, masking the role of traits; (b) explain how these environmental variations might arise from climatic, topographic, and edaphic variability; (c) illustrate, through a "common garden" thought experiment, how existing trait‐based or response‐based isohydricity metrics can be confounded by these environmental variations, leading to Type‐1 (false positive) and Type‐2 (false negative) errors; and (d) advocate for the use of model‐based approaches for formulating alternate classification schemes. Building on recent insights from greenhouse and vineyard studies, we offer additional evidence across multiple field sites to demonstrate the importance of spatial and temporal drivers of plants' apparent isohydricity. This evidence challenges the use of isohydricity indices, per se, to characterize plant water relations at the global scale. Metrics of plant isohydricity have been used to describe plant water regulation with limited success. Here, using in situ measurements of Quercus douglasii under drought, we highlight the importance of environmental variations over space and time in influencing the apparent isohydricity of plants. [ABSTRACT FROM AUTHOR]

Many recent studies on drought‐induced vegetation mortality have explored how plant functional traits, and classifications of such traits along axes of, for example, isohydry–anisohydry, might contribute to predicting drought survival and recovery. As these studies proliferate, the consistency and predictive value of such classifications need to be carefully examined. Here, we outline the basis for a systematic classification of plant drought responses that accounts for both environmental conditions and functional traits. We use non‐dimensional analysis to integrate plant traits and metrics of environmental variation into groups that can be associated with alternative drought stress pathways (hydraulic failure and carbon limitation), and demonstrate that these groupings predict physiological drought outcomes using both synthetic and measured data. In doing so, we aim to untangle some confounding effects of environment and trait variations that undermine current classification schemes, advocate for more careful treatment of the environmental context within which plants experience and respond to drought, and outline a pathway towards a general classification of drought vulnerability. [ABSTRACT FROM AUTHOR]

Abstract: The sustainable management of unwanted vegetation in agricultural fields through integrated weed control strategies requires detailed knowledge about the maternal formation of primary seed dormancy, to support the prediction of seedling emergence dynamics. This knowledge is decisive for the timing of crop sowing and nonchemical weed control measures. Studies in controlled environments have already demonstrated that thermal conditions and, to some extent, water availability during seed set and maturation has an impact on the level of dormancy. However, it is still unclear if this applies also under field conditions, where environmental stressors and their timing are more variable. We address this question for Alopecurus myosuroides in south‐western Sweden. We quantified the effects of cumulated temperature and precipitation as well as soil water potential during the reproductive growth phase of A myosuroides on primary seed dormancy under field conditions. Empirical models differing in focal time intervals and, in case of soil water potential, focal soil depths were compared regarding their predictive power. The highest predictive power for the level of primary dormancy of A. myosuroides seeds was found for a two‐factorial linear model containing air temperature sum between 0 and 7 days before peak seed shedding as well as the number of days with soil water potential below field capacity between 7 and 35 days before peak seed shedding. For soil water potential, it was found that only the top 10 cm soil layer is of relevance, which is in line with the shallow root architecture of A. myosuroides. We conclude that for this species the level of dormancy depends on the magnitude and timing of temperature and water availability during the reproductive growth phase. Water availability appears to be more important during maternal environmental perception and temperature during zygotic environmental perception. [ABSTRACT FROM AUTHOR]

Background and Aims Water and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration (E) can be decreased by reducing leaf stomatal conductance (gs) at the expense of lowering CO2 uptake (A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when gs declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration (N) varies on time scales much longer than gs, an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO2 concentration (ca). Methods Optimization theory is employed to quantify interactions between resource-use efficiencies under elevated ca and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, λ = (∂A/∂gs)/(∂E/∂gs) and η = ∂A/∂N, respectively. The relationship between the two efficiencies and related variation in intercellular CO2 concentration (ci) were examined using A/ci curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO2 Enrichment experiment (North Carolina, USA). Key Results Optimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where η is complementary to the square-root of λ. The data support the model predictions that elevated ca increased η and λ, and at given ca and needle age-class, the two quantities varied among needles in an approximately complementary manner. Conclusions The derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of η, and assist in predicting how increasing ca influences ecosystem water use. [ABSTRACT FROM AUTHOR]

• APSIM was calibrated and validated for pure cultures of wheat and faba bean. • APSIM was evaluated for wheat-faba bean pure cultures and intercrops in Europe. • APSIM reproduced pure culture aboveground dry matters and, for wheat, yields. • APSIM overestimates faba bean performance relative to wheat in intercrops. • APSIM must incorporate an alternative approach to simulate height growth. Cereal-legume intercropping can increase yields, reduce fertilizer input and improve soil quality compared with pure culture. Designing intercropping systems requires the integration of plant species trait selection with choice of crop configuration and management. Crop growth models can facilitate the understanding and prediction of the interactions between plant traits, crop configuration and management. However, currently no existing crop growth model has been calibrated and tested for cereal-legume intercrops throughout Europea. We calibrated the Agricultural Production Systems sIMulator (APSIM) for pure cultures of wheat and faba bean using data from Dutch field trials, and determined the phenological parameters to simulate pure cultures and intercrops from seven field experiments across Europe. APSIM successfully reproduced aboveground dry matters and, for wheat only, grain yields in pure cultures. In intercrops, APSIM systematically overestimated the aboveground dry matter and grain yield of faba bean and underestimated those of wheat. APSIM was reasonably capable of simulating plant heights in pure cultures, but respectively overestimated and underestimated the height of faba bean and wheat in intercrops. In order to simulate wheat-faba bean intercrops better, APSIM should be improved regarding the calculation of biomass partitioning to grains in faba bean and of height growth in both species. [ABSTRACT FROM AUTHOR]

• Comprehensive review about the water and carbon cycles in perennial Kernza plants. • Kernza has a relatively high water-use efficiency through the whole growing season. • Kernza tends to have higher evapotranspiration rates when compared to annual counterparts. • Kernza has shown to act as carbon sink on a long-term basis. Perennial crops have been proposed as a more sustainable alternative to annual crops, because they have extended growing seasons, continuous ground cover, reduced nutrient leakage, and sequester more carbon in the soils than annual crops. One example is intermediate wheatgrass (Thinopyrum intermedium), a perennial crop that has been used as a cool-season forage throughout the USA and Canada and also across its native range in Eurasia. Since the 1980′s, intermediate wheatgrass has been under domestication to improve seed fertility and grain yield. Commercial products are being sold under the trade name Kernza, owned by The Land Institute, located in Salina, Kansas, USA. This review provides a comprehensive framework about the physical and biological aspects involving the water and carbon cycles in Kernza plants. The main aspects we highlight here are based on previous findings regarding Kernza: i) the ability of maintaining a relatively high water-use efficiency throughout the whole growing season, which is beneficial to mitigate water stress, representing an important physiological mean to acclimate under severe, unfavorable weather conditions, and ii) its higher evapotranspiration (ET) and net carbon uptake rates, particularly when compared to annual counterparts. Only a thorough multifaceted assessment of the repercussion for carbon and water fluxes of a shift from annual crops to Kernza will allow assessing the perspectives of such novel perennial crop to support food security and a number of ecosystem services, particularly under future climates. [ABSTRACT FROM AUTHOR]

Dry spells are sequences of days without rainfall and can cause adverse consequences for the environment, economy and societies. Up until today, climate impact research has mainly focused on changes in the frequency of hydrometeorological extremes. Yet, the changes in the timing can pose a real threat, for example in agriculture. In this research, we analysed changes in the within-year timing of extreme dry spells using a consistent global land surface rainfall dataset, covering a period of 60 years, with main focus on the United States, Europe, China and Australia. Our method is based on a novel combination of weather generation techniques and statistical analyses, generally applicable to any rain gauge data. We contrast two thirty-year periods: 1956-1985 and 1986-2015. Our study reveals statistically significant shifts in the timing of extreme dry spells at about 40% of all rain gauge locations. Recognizable patterns exist in some regions, but the changes are generally characterized by a high spatial variability. Our study is a starting point for a better understanding of future challenges in risk management and disaster risk reduction due to changes in the timing of extreme events. [ABSTRACT FROM AUTHOR]

Using a stomatal optimization model, an abrupt change is foundnd in the relationship between carbon and water fluxes along the evolutionary gradient from C3 to C4 photosynthetic types.C4 photosynthesis evolved independently numerous times, probably in response to declining atmospheric CO2 concentrations, but also to high temperatures and aridity, which enhance water losses through transpiration. Here, the environmental factors controlling stomatal behaviour of leaf-level carbon and water exchange were examined across the evolutionary continuum from C3 to C4 photosynthesis at current (400 μmol mol–1) and low (280 μmol mol–1) atmospheric CO2 conditions. To this aim, a stomatal optimization model was further developed to describe the evolutionary continuum from C3 to C4 species within a unified framework. Data on C3, three categories of C3–C4 intermediates, and C4Flaveria species were used to parameterize the stomatal model, including parameters for the marginal water use efficiency and the efficiency of the CO2-concentrating mechanism (or C4 pump); these two parameters are interpreted as traits reflecting the stomatal and photosynthetic adjustments during the C3 to C4 transformation. Neither the marginal water use efficiency nor the C4 pump strength changed significantly from C3 to early C3–C4 intermediate stages, but both traits significantly increased between early C3–C4 intermediates and the C4-like intermediates with an operational C4 cycle. At low CO2, net photosynthetic rates showed continuous increases from a C3 state, across the intermediates and towards C4 photosynthesis, but only C4-like intermediates and C4 species (with an operational C4 cycle) had higher water use efficiencies than C3Flaveria. The results demonstrate that both the marginal water use efficiency and the C4 pump strength increase in C4Flaveria to improve their photosynthesis and water use efficiency compared with C3 species. These findings emphasize that the advantage of the early intermediate stages is predominantly carbon based, not water related. [ABSTRACT FROM PUBLISHER]