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1. Despite the fundamental significance of water to plants and the persisting question of how competing species coexist, this is the first review of hydrological niches. We define hydrological niche segregation (HNS) as: (i) partitioning of space on fine-scale soil-moisture gradients, (ii) partitioning of water as a resource and/or (iii) partitioning of recruitment opportunities among years caused by species specializing on particular patterns of temporal variance of water supply (the storage effect). 2. We propose that there are three types of constraint that lead to the trade-offs that underlie HNS. (i) An edaphic constraint creates a trade-off between the supply to roots of O2 on the one hand vs. water and nutrients on the other. (ii) A biophysical constraint governs gas exchange by leaves, leading to a trade-off between CO2 acquisition vs. water loss. (iii) A structural constraint arising from the physics of water-conducting tissues leads to a safety vs. efficiency trade-off. 3. Significant HNS was found in 43 of 48 field studies across vegetation types ranging from arid to wet, though its role in coexistence remains to be proven in most cases. Temporal partitioning promotes coexistence through the storage effect in arid plant communities, but has yet to be shown elsewhere. In only a few cases is it possible to unequivocally link HNS to a particular trade-off. 4. Synthesis. The field and experimental evidence make it clear that HNS is widespread, though it is less clear what its precise mechanisms or consequences are. HNS mechanisms should be revealed by further study of the constraints and trade-offs that govern how plants obtain and use water, and HNS can be mechanistically linked to its consequences with appropriate community models. In a changing climate, such an integrated programme would pay dividends for global change research.

Severe drought has the potential to cause selective mortality within a forest, thereby inducing shifts in forest species composition. The southern Sierra Nevada foothills and mountains of California have experienced extensive forest dieback due to drought stress and insect outbreak. We used high-fidelity imaging spectroscopy (HiFIS) and light detection and ranging (LiDAR) from the Carnegie Airborne Observatory (CAO) to estimate the effect of forest dieback on species composition in response to drought stress in Sequoia National Park. Our aims were (1) to quantify site-specific conditions that mediate tree mortality along an elevation gradient in the southern Sierra Nevada Mountains, (2) to assess where mortality events have a greater probability of occurring, and (3) to estimate which tree species have a greater likelihood of mortality along the elevation gradient. A series of statistical models were generated to classify species composition and identify tree mortality, and the influences of different environmental factors were spatially quantified and analyzed to assess where mortality events have a greater likelihood of occurring. A higher probability of mortality was observed in the lower portion of the elevation gradient, on southwest- and west-facing slopes, in areas with shallow soils, on shallower slopes, and at greater distances from water. All of these factors are related to site water balance throughout the landscape. Our results also suggest that mortality is species-specific along the elevation gradient, mainly affecting Pinus ponderosa and Pinus lambertiana at lower elevations. Selective mortality within the forest may drive long-term shifts in community composition along the elevation gradient.

The Three‐Georges Dam holds many records in the history of engineering. While the dam has produced benefits in terms of flood control, hydropower generation and increased navigation capacity of the Yangtze River, serious questions have been raised concerning its impact on both upstream and downstream ecosystems. It has been suggested that the dam operation intensifies the extremes of wet and dry conditions in the downstream Poyang Lake, and affects adversely important local wetlands. A floodgate has been proposed to maintain the lake water level by controlling the flow between the Poyang Lake and Yangtze River. Using extensive hydrological data and generalized linear statistical models, we demonstrated that the dam operation induces major changes in the downstream river discharge near the dam, including an average “water loss”. The analysis also revealed considerable effects on the Poyang Lake water level, particularly a reduced level over the dry period from late summer to autumn. However, the dam impact needs to be further assessed based on long‐term monitoring of the lake ecosystem, covering a wide range of parameters related to hydrological and hydraulic characteristics of the lake, water quality, geomorphological characteristics, aquatic biota and their habitat, wetland vegetation and associated fauna. Key Points The 3GD induces major changes in the downstream river discharge near the dam The 3GD causes considerable effects on the Poyang Lake water level The lake behavior is controlled by local factors modulated by the dam

As a warmer climate enables an increase in atmospheric humidity, extreme precipitation events have become more frequent in the Northeastern United States. Understanding the impact of evolving precipitation patterns is critical to understanding water cycling in temperate forests and moisture coupling between the atmosphere and land surface. Although the role of soil moisture in evapotranspiration has been extensively studied, few have analyzed the role of soil texture in determining ecosystem‐atmosphere feedbacks. In this study, we utilized long term data associated with ecosystem water fluxes to deduce the strength of land‐atmosphere coupling at Harvard Forest, Petersham, MA, USA. We found a 1.5% increase in heavy precipitation contribution per decade where high‐intensity events compose upwards of 42% of total yearly precipitation in 2023. Intensifying precipitation trends were found in conjunction with a long‐term soil drying at the Harvard Forest despite no significant increase in evapotranspiration over 32 years. This suggests that soil water holding capacity is a key mediating variable controlling the supply of water to ecosystems and the atmosphere. We found that these land surface changes directly impacted the lifted condensation level (LCL) height over Harvard Forest which was found to be increasing at a rate of 6.62 m per year while atmospheric boundary layer (ABL) heights have fallen at a modest rate of 1.76 m per year. This has amplified dry feedbacks between the land surface and the atmosphere such that 80% of observed summers ending in a water deficit also had an anomalously low soil water content in the spring. Plain Language Summary Warm air holds more water. As air temperatures increase due to climate change, this warmer air evaporates more water from the soil and releases it in more intense rain events. Since the soils at Harvard Forest have a low water holding capacity, most of this rainfall quickly seeps deep into the ground and becomes unavailable for trees to take up through shallow roots or for the atmosphere to evaporate. Thus, despite more intense and total rainfall, the surface soils are becoming drier. These drier soils do not contribute as much water back to the atmosphere as is needed to maintain the conditions necessary for rain as often as wetter soils. As a result, drier soils do not create as much rain as wetter soils, and become even drier, creating an amplifying feedback loop. These results suggest that the amount of water available to the atmosphere within a soil is an important factor in creating the conditions necessary for strong interactions between the land and atmosphere in this region. Key Points Land‐atmosphere coupling at Harvard Forest is principally controlled by the low amount of precipitation retained as soil moisture rather than evapotranspiration More extreme rainfall resulted in a greater percentage of water becoming unavailable to evapotranspiration due to low soil water holding capacity Drier soils are less capable of maintaining the necessary conditions for rain, which strengthens a amplifying dry soil moisture feedback loop

Atmospheric dryness (i.e., high vapor pressure deficit, VPD), together with soil moisture stress, limits plant photosynthesis and threatens ecosystem functioning. Regions where rainfall and soil moisture are relatively sufficient, such as the rainfed part of the U.S. Corn Belt, are especially prone to high VPD stress. With globally projected rising VPD under climate change, it is crucial to understand, simulate, and manage its negative impacts on agricultural ecosystems. However, most existing models simulating crop response to VPD are highly empirical and insufficient in capturing plant response to high VPD, and improved modeling approaches are urgently required. In this study, by leveraging recent advances in plant hydraulic theory, we demonstrate that the VPD constraints in the widely used coupled photosynthesis‐stomatal conductance models alone are inadequate to fully capture VPD stress effects. Incorporating plant xylem hydraulic transport significantly improves the simulation of transpiration under high VPD, even when soil moisture is sufficient. Our results indicate that the limited water transport capability from the plant root to the leaf stoma could be a major mechanism of plant response to high VPD stress. We then introduce a Demand‐side Hydraulic Limitation Factor (DHLF) that simplifies the xylem and the leaf segments of the plant hydraulic model to only one parameter yet captures the effect of plant hydraulic transport on transpiration response to high VPD with similar accuracy. We expect the improved understanding and modeling of crop response to high VPD to help contribute to better management and adaptation of agricultural systems in a changing climate. Key Points Coupled photosynthesis‐stomatal conductance models alone underestimate vapor pressure deficit (VPD) stress effects on crop stomatal conductance and transpiration Limited plant hydraulic transport capability could play a role in plant response to high VPD A simplified representation of plant hydraulic model for capturing VPD stress on plants is proposed

Mountains are vital to ecosystems and human society given their influence on global carbon and water cycles. Yet the extent to which topography regulates montane forest carbon uptake and storage remains poorly understood. To address this knowledge gap, we compared forest aboveground carbon loading to topographic metrics describing energy balance and water availability across three headwater catchments of the Boulder Creek Watershed, Colorado, USA. The catchments range from 1800 to 3500 m above mean sea level with 46–102 cm/yr mean annual precipitation and −1.2° to 12.3°C mean annual temperature. In all three catchments, we found mean forest carbon loading consistently increased from ridges (27 ± 19 Mg C ha) to valley bottoms (60 ± 28 Mg C ha). Low topographic positions held up to 185 ± 76 Mg C ha, more than twice the peak value of upper positions. Toe slopes fostered disproportionately high net carbon uptake relative to other topographic positions. Carbon storage was on average 20–40 Mg C ha greater on north to northeast aspects than on south to southwest aspects, a pattern most pronounced in the highest elevation, coldest and wettest catchment. Both the peak and mean aboveground carbon storage of the three catchments, crossing an 11°C range in temperature and doubling of local precipitation, defied the expectation of an optimal elevation‐gradient climatic zone for net primary production. These results have important implications for models of forest sensitivity to climate change, as well as to predicted estimates of continental carbon reservoirs.

By partitioning mass and energy fluxes, soil moisture exerts a fundamental control on basin hydrological response. Using the design characteristics of the Biosphere 2 hillslope experiment, this study investigates aspects of soil moisture spatial and temporal variability in a zero‐order catchment of a semiarid climate. The hydrological response of the domain exhibits a particular structure, which depends on whether topography‐induced subsurface stormflow is triggered. The occurrence of the latter is conditioned by topography, soil depth, and pre‐storm spatial distribution of moisture. As a result, a non‐unique behavior of soil moisture spatial heterogeneity emerges, manifested through a hysteretic dependence of variability metrics on mean water content. Further, it is argued that vegetation dynamics impose a “homogenizing” effect on pre‐storm moisture states, decreasing the likelihood that a rainfall event will result in topographic redistribution of soil water. Consequently, post‐rainfall soil moisture dynamics associated with the effect of topography that could lead to the enhancement of spatial heterogeneity are suppressed; a potential “attractor” of catchment states emerges. The study thus proposes several hypotheses that will be testable within the framework of long‐term hillslope experiments.

Successful use of stable isotopes (δ2H and δ18O) in ecohydrological studies relies on the accurate extraction of unfractionated water from different types of soil samples. Cryogenic vacuum distillation (CVD) is a common laboratory‐based technique used for soil water extraction; however, the reliability of this technique in reflecting soil water δ2H and δ18O is still of concern. This study examines the reliability of a newly developed automatic cryogenic vacuum distillation (ACVD) system. We further assessed the impacts of extraction parameters (i.e. extraction time, temperature and vacuum) and soil properties on the recovery of soil water δ2H and δ18O for the ACVD and traditional cryogenic vacuum distillation (TCVD) systems. Finally, we investigated the potential influence of CVD (ACVD and TCVD) technique on the prediction of plant water uptake through a sensitivity analysis. Both ACVD and TCVD similarly extracted water from the rewetted soils, but none of the CVD systems successfully recovered the isotopic signatures of doped water from soil materials. Mean δ2H offsets of extracted soil water were −2.6 ± 1.3‰ and −2.4 ± 1.7‰ for ACVD and TCVD, respectively, while mean δ18O offsets were −0.16 ± 0.14‰ and −0.39 ± 0.37‰. The isotopic offsets of CVD systems were positively correlated with soil clay content, and negatively correlated with soil water content. Using corrected soil data (with CVD offsets) could improve the prediction of plant water uptake based on its high correlation with the environmental factors. This study identifies the isotopic offsets of CVD systems (i.e. ACVD and TCVD) and provides possible solutions for better predicting plant water sources. Even though, the wide use of CVD techniques probably induce noticeable uncertainties in the prediction of plants water uptake depths. The dataset of soil water extraction in this study will have implications for the technological development of CVD techniques. 摘要 若要将氢 (δ2H) 和氧 (δ18O) 稳定同位素成功应用到生态水文学研究中，准确提取不同类型土壤中液态水而不发生同位素分馏效应是至关重要的技术环节之一。低温真空提取 (CVD) 是一种实验室内广泛使用的土壤水抽提技术，然而，CVD在提取土壤水δ2H和δ18O时的可靠性却饱受争议。. 本研究检验了一种新近发明并商业化的全自动低温真空提取 (ACVD) 技术的可靠性。我们首次系统评价了提取系统参数 (如，抽提时长、抽提温度和真空度) 和土壤质地对ACVD提取土壤水的影响，并进一步将该技术与传统低温真空提取 (TCVD) 技术进行了实验室间比较。在此基础上，我们以水分溯源研究案例为切入口探讨了两种低温真空提取 (包括ACVD和TCVD) 技术对植物水分获取深度预测的潜在干扰。. 本研究表明，ACVD和TCVD技术在提取及恢复浸润土壤水时的可靠性表现不相上下。但是，二者均未能成功复原添加到干土壤中液态水δ2H和δ18O的初始值。ACVD和TCVD提取土壤水δ2H的平均偏差分别是 −2.6 ± 1.3‰和 −2.4 ± 1.7‰，二者提取土壤水δ18O的平均偏差分别是−0.16 ± 0.14‰和 −0.39 ± 0.37‰。进一步地，上述土壤水δ2H和δ18O提取偏差与待提取土壤黏土含量和土壤含水量分别成正、反相关。敏感性分析证明，合理校正ACVD或TCVD提取的土壤水δ2H和δ18O数据可显著提升植物水分溯源结果的科学性和准确性。. 本研究证实了CVD技术在提取土壤水过程中极可能引入难以忽视的同位素偏差。与此同时，我们尝试提供了一种弱化该同位素偏差对相关研究影响的校正方法。尽管如此，我们仍担忧当前各实验室内广泛使用的CVD方法会提升稳定同位素生态水文学研究结果的不确定性。鉴于本研究仍存诸多不足，殷切期望更多科研工作和专业人员不断努力以完善土壤水提取技术或设备。.

The forecasted increased frequency and intensity of extreme climatic events may strongly affect ecosystem structure and function in the future. It is unclear how ecosystems will function in the long run over a large spatial scale under a new extreme water cycle. This open question calls for a conceptual framework as a fundamental basis for theoretical and experimental exploration of ecosystem function on a large scale driven by an extreme climate envelope. To assess the problem on a large scale, we investigated hyper‐arid ecosystems (HAEs) as natural tangible models that already function under an extreme climatic envelope. Our new assertion is that if extreme climate change drives arid lands to function under alternate extreme conditions, then arid land ecosystems will function like an HAE as an alternative state, rather than progress to desertification. To support our assertion, we developed a conceptual framework of HAEs that includes a geo‐hydrological “abiotic engine” that drives HAE function by soil moisture diversity and plant functional groups. Based on this conceptual framework, we suggest incorporating two new hypotheses in climate change studies to advance our understanding of responses of large‐scale, water‐limited ecosystems: (1) Hydro‐climatic extremes in water‐limited ecosystems will reduce the degree of resource conservation by slope ecosystems due to reduction in plant cover and soil. The decreased ecosystem function on the slope will be compensated for by increasing the effect of the abiotic engine on the ephemeral stream, thus enhancing meta‐ecosystem functioning in the ephemeral stream. (2) In water‐limited ecosystems, climate change toward hydro‐climatic extremes will rescale the dominant hydro‐ecological processes of pulse–reserve, source–sink, and connectivity along the semiarid, arid, and HA gradients in two ways: (i) shrinking of both spatial and temporal dimensions; and (ii) shrinking in the temporal dimension and expanding in the spatial dimensions. The first rescaling trajectory is related to biodiversity–ecosystem function and the second to the abiotic engine processes.

Increasing vapor pressure deficit (VPD) increases atmospheric demand for water. While increased evapotranspiration (ET) in response to increased atmospheric demand seems intuitive, plants are capable of reducing ET in response to increased VPD by closing their stomata. We examine which effect dominates the response to increasing VPD: atmospheric demand and increases in ET or plant response (stomata closure) and decreases in ET. We use Penman‐Monteith, combined with semiempirical optimal stomatal regulation theory and underlying water use efficiency, to develop a theoretical framework for assessing ET response to VPD. The theory suggests that depending on the environment and plant characteristics, ET response to increasing VPD can vary from strongly decreasing to increasing, highlighting the diversity of plant water regulation strategies. The ET response varies due to (1) climate, with tropical and temperate climates more likely to exhibit a positive ET response to increasing VPD than boreal and arctic climates; (2) photosynthesis strategy, with C3 plants more likely to exhibit a positive ET response than C4 plants; and (3) plant type, with crops more likely to exhibit a positive ET response, and shrubs and gymniosperm trees more likely to exhibit a negative ET response. These results, derived from previous literature connecting plant parameters to plant and climate characteristics, highlight the utility of our simplified framework for understanding complex land‐atmosphere systems in terms of idealized scenarios in which ET responds to VPD only. This response is otherwise challenging to assess in an environment where many processes coevolve together. Plain Language Summary Plants can sense increasing dryness in the air and close up the pores on their leaves, preventing water loss. However, drier air also naturally demands more water from the land surface. Here we develop a simplified theory for when land surface water loss increases (atmospheric demand dominates) or decreases (plant response dominates) in response to increased dryness in the air. This theory provides intuition for how ecosystems regulate water in response to changes in atmospheric dryness. According to the theory, ecosystems are capable of broad range of behavior in response to increased atmospheric dryness, from strongly reducing water loss to allowing large increases in water loss. Ecosystem behavior depends both on environmental conditions and plant type. Key Points We derive a simplified analytical model for ecosystem‐scale evapotranspiration response to changes in vapor pressure deficit Ecosystems exhibit a range of behavior, from reductions to increases in evapotransipration, in response to increasing vapor pressure deficit The choice of stomatal conductance model fundamentally alters the relationship between evapotranspiration and vapor pressure deficit