Water uptake from different soil depths for halophytic shrubs grown in Northern area of Ningxia plain (China) in contrasted water regimes

In order to understand the contributions of groundwater and deep soil water to the growth of halophytes in salinity-affected area, water use strategies of four shrubes, i.e. 20-year-old Tamarix ramosissima Ledeb., three-year-old T. ramosissima., Lycium barbarum L., and Atriplex canescens (Pursh) Nutt. were studied under contrasted water regimes in Northwest China. The result showed that there was a vertical gradient in soil δ¹⁸O and δD profiles resulted from evaporation and irrigation. The 20-year-old T. ramosissima mainly used water from middle (40–140 cm) and deep (140–200 cm) under both water regimes indicating its phreatophytic nature. Soil water in upper profile (0–40 cm) was the dominant water source for the three-year-old T. ramosissima before irrigation. After irrigation, the three-year-old T. ramosissima and L. barbarum switched their water sources to middle soil profile. Our experiment revealed phreatophytic tendency for the three-year-old A. canescens, which was not responsive to irrigation enlighten by photosynthetic parameters and stem water potentials.     

沙漠腹地天然绿洲不同林龄胡杨水分利用来源

了解干旱荒漠绿洲区主要植被的水分利用来源,能为该区域植被保护和水资源的合理分配提供科学依据.本研究以达里雅布依天然绿洲胡杨幼龄木(胸径DBH≤10 cm)、成熟木(10 cm＜DBH≤40 cm)和过熟木(DBH＞40 cm)为对象,测定不同林龄胡杨木质部水和潜在水源(地表水、0～3m土层土壤水、地下水)的氧同位素,运...     

Hydraulic Lift in Cork Oak Trees in a Savannah-Type Mediterranean Ecosystem and its Contribution to the Local Water Balance

The aim of this study was to identify the sources and depth of water uptake by 15-years old Quercus suber L. trees in southern Portugal under a Mediterranean climate, measuring δ¹⁸O and δD in the soil–plant-atmosphere continuum. Evidence for hydraulic lift was substantiated by the daily fluctuations observed in Ψ_s at 0.4 and 1 m depth and supported by similar δ¹⁸O values found in tree xylem sap, soil water in the rhizosphere and groundwater. From 0.25 m down to a depth of 1 m, δD trends differed according to vegetation type, showing a more depleted value in soil water collected under the evergreen trees (−47‰) than under dead grasses (−35‰). The hypothesis of a fractionation process occurring in the soil due to diffusion of water vapour in the dry soil is proposed to explain the more depleted soil δD signature observed under trees. Hydraulically lifted water was estimated to account for 17–81% of the water used during the following day by tree transpiration at the peak of the drought season, i.e., 0.1–14 L tree⁻¹ day⁻¹. Significant relationships found between xylem sap isotopic composition and leaf water potential in early September emphasized the positive impact of the redistribution of groundwater in the rhizosphere on tree water status.     

Water utilization of Pinus sylvestris var. mongolica in a sparse wood grassland in the semiarid sandy region of Northeast China

Key message

Mongolian pine in a sparse wood grassland stand maintains a relatively stable state under the current water conditions by varying its water use sources at a constant water use efficiency.

Abstract

In a semi-arid sandy ecosystem, water is one of the most important factors influencing survival and growth of Mongolian pine in a sparse wood grassland (MPSWG); however, the mechanism of water utilization of MPSWG has been yet poorly understood. Stable isotopes of hydrogen and oxygen of water in xylem, soil, precipitation and groundwater were analyzed to determine water sources used by MPSWG from April to October in two consecutive years (1.2 and 0.89 times of long-term annual precipitation in 2010 and 2011, respectively). Needles δ¹³C were measured simultaneously to assess water use efficiency. Results showed that MPSWG used soil water (sampling depth up to 60 cm) between April and July, and in October. However, water sources used by MPSWG exhibited significant differences in August and September between 2 years. In 2010, MPSWG only utilized soil water in August and September due to higher soil water content, but they used both soil water and groundwater in 2011 because of lower soil water availability, suggesting that MPSWG could utilize groundwater to replenish soil water deficiency. No significant differences in mean δ¹³C among months and between 2 years indicated that MPSWG did not suffer severely water stresses. In conclusion, MPSWG only used soil water during higher precipitation year, whereas they primarily utilized soil water and relied a little on groundwater during lower precipitation year. Water obtained by MPSWG (including water stored in trunk) basically satisfied its requirement for growth. Thus, MPSWG stand was relatively stable presently in the semi-arid sandy region.     

Root dynamics of Melaleuca halmaturorum in response to fluctuating saline groundwater

Melaleuca halmaturorum is a salt and waterlogging tolerant tree and thus often occurs in saline areas fringing permanent wetlands and in ephemeral swamps. The dominance of this tree in natural groundwater discharge areas may result in M. halmaturorum transpiration making a major contribution to groundwater discharge. To quantify this the seasonal changes in tree water sources in response to fluctuating soil salinity and waterlogging were examined. This study was conducted in a natural system where seasonally fluctuating saline groundwater (64 dS m^–1; 0.3–1.2 m deep) allowed the patterns of M. halmaturorum root water uptake to be followed over a 15 month period. Tree water sources were examined using the naturally occurring stable isotopes of water, while new root growth was examined using a field root observation window and from soil cores. The presence of isotopic fractionation of ²H under conditions of soil salinity and waterlogging was tested in a glasshouse experiment. Measurements of soil and leaf water potential were also made to examine the possible water sources and limits to water uptake. No isotopic fractionation was found by tree roots under conditions of salinity and waterlogging. M. halmaturorum trees were active in taking up groundwater at most times and combined this with a shallower soil water source replenished by rainfall in winter. Water uptake was concentrated in the deeper parts of the soil profile when the groundwater was at its deepest and salt had accumulated in the surface soils, at the end of summer. When groundwater rose, at the end of winter, roots responded by extracting water from near the soil surface (0–0.1 m), at the new watertable. This pattern of water uptake in response to groundwater fluctuations and salt accumulation in the surface soil was also reflected in new root tip appearance at the root observation window. Fluctuations in leaf water potential fallowed fluctuations in surface soil (0.1 m depth) water potential at all times. In winter leaf water potential reflected the absolute values of the surface soil water potential but in summer it was between surface soil and groundwater water potentials. We conclude that M. halmaturorum used groundwater in summer and a combination of rainfall and groundwater from the surface soils in winter. The ability to take up water from saline substrates through the maintenance of low leaf water potential, combined with this ability to rapidly alter root water uptake in response to changes in soil water availability contributed to the survival of M. halmaturorum in this saline swamp.     

Drought impact on water use efficiency and intra‐annual density fluctuations in Erica arborea on Elba (Italy)

Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environments. The eco‐physiological plasticity of this species was evaluated by studying plants growing at two sites with different soil moistures on the island of Elba (Italy), through dendrochronological, wood‐anatomical analyses and stable isotopes measurements. Intra‐annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site‐specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipitation patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the ¹³C‐derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increasing C assimilation. The variation in vessel size and the different absolute values of δ¹⁸O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor seasonal environmental differences through tree‐ring width, but also added valuable information on E. arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics.     

基于树木年轮技术的塔里木河下游河岸胡杨林生态需水量研究

生态需水量计算是干旱脆弱生态区恢复重建的一个关键问题。分析了塔里木河下游荒漠河岸林关键种-胡杨树木年轮近90年来的变化特征及对气候水文过程的响应,并基于树木年轮技术提出了维系荒漠河岸林不同恢复状态的生态需水量。研究结果表明,塔里木河下游胡杨树木年轮主要承载的是区域水文历史变化信息,可以作为定量评估生态输水工程的生态恢复效应和定量计算植被生态需水量的新指标;胡杨标准年轮宽度指数与生长年生态输水量呈显著正相关(P0.05),近15年生态输水对胡杨年轮宽度指数增加的平均贡献率为42.96%;若要维持塔里木河下游英苏段垂直河道300 m范围内的胡杨达到近90年来的平均生长水平,则需在生长年下泄生态需水量0.84×10~8m~3,若要达到断流前(1933—1974年)的平均径向生长,则每个生长年内生态需水量应达到0.91×10~8m~3。     

蒸发条件下潜水埋深对土壤-柽柳水盐分布的影响

盐水矿化度下模拟设置4个潜水埋深(0.9、1.2、1.5、1.8 m),分析不同土层的土壤相对含水量(RWC)、含盐量(S_C)和土壤溶液绝对浓度(C_S)等水盐参数,及柽柳叶片和新生枝条的含水量及Na~+含量,探讨盐水矿化度下土壤-柽柳水盐参数对潜水埋深的响应规律。结果表明:各土层RWC与潜水埋深呈负相关,0.9 m潜水埋深下各土层的RWC均最高,且各土层RWC随土层深度的增加呈先降低后增加的趋势,其它潜水埋深下各土层RWC均逐渐增加,1.2 m是地下水所能上升且保持柽柳柱体土壤表层湿润的最高高度。各土层S_C和C_S与潜水埋深呈抛物线型,均表现为先增加后降低,潜水埋深1.2 m时,各土层S_C均最高。随土层深度的增加,各潜水埋深下S_C先降低后增加,而C_S呈现减少趋势;潜水埋深越高,土层间C_S变化幅度越激烈。潜水埋深对柽柳叶片和新生枝条的含水量无显著影响(P0.05),而随潜水埋深的增加,柽柳叶片Na~+含量逐渐增加,新生枝条Na~+含量则先增加后降低。从整个柽柳土柱看,随潜水埋深的增加,整个土壤剖面的RWC均值逐渐降低,而S_C和C_S均值先增加后降低,潜水埋深1.2 m是盐分变化的分界点,建议栽植柽柳的潜水埋深大于1.2 m。     

Tracing water uptake by jarrah (Eucalyptus marginata) trees using natural abundances of deuterium

 Seasonal change in the δ²H content of water from twig sap, soil, rainfall and groundwater were measured to determine the water sources accessed by jarrah (Eucalyptus marginata) trees at three sites in Western Australia with differing soils and depths to water table. During winter and spring the main contributor to the water uptake of the trees was stored water in the surface layers of the soil replenished by predominantly winter rainfall. With the onset of summer drought jarrah became more reliant on water from deeper down the profile. There was no clear evidence that jarrah could tap water from groundwater more than 14 m deep in deep sands. Defining the source of water for trees in deep lateritic soils using stable isotopes is hampered by the uniform deuterium profiles down most of the unsaturated zone and into the groundwater. There was a limited response in the δ²H values of sapwater in twigs to changes in the δ²H of the upper layers of the deep sand following input of rainfall in autumn. The damped response was related to the small variation in the δ²H composition of rainfall in most events during the year and the mixing in the tree of water extracted from different locations in the soil profile. Received: 21 August 1995 / Accepted: 3 December 1995     

Modelling carbon isotope fractionation in tree rings based on effective evapotranspiration and soil water status

Environmental influences on carbon isotope fractionation in tree rings require further elucidation in order to use this parameter as a biological marker of climatic variations. δ¹³C values in tree-ring cellulose of beech (Fagus sylvatica L.) were analysed for the period from 1950 to 1990. A bioclimatic model of water balance was used to give the actual evapotranspiration as well as the soil water content on a daily basis. δ¹³C shows a significant decrease from –24·5‰ to –25‰ over this period. Internal CO₂ concentration changes from 200 to 220 ppm in relation with the rise of atmospheric CO₂. Beside a slight non-significant inter-individual variation, a large year-to-year variation exists. The relative extractable soil water of July, combined with the value of δ¹³C for the previous year, predicts as much as 70% of this variance. Air temperature or precipitation accounted for less variation. δ¹³C is strongly correlated with basal area increment, but appeared a more reliable indicator of water status at the stand level.     

Intrinsic effects of species on leaf litter and root decomposition: a comparison of temperate grasses from North and South America

A simple model of water uptake by vegetation is used to aid the discrimination of plant water sources determined with isotope data. In the model, water extracted from different soil depths depends on the leaf–soil potential difference, a root distribution function and a lumped hydraulic conductance parameter. Measurements of plant transpiration rate, and soil and leaf water potentials are used to estimate the value of the conductance parameter. Isotopic ratios in soil water and xylem are then used to constrain the root distribution. The model is applied to field measurements of transpiration, leaf water potential and ¹⁸O composition of xylem water on Corymbia clarksoniana, Lophostemon suaveolens, Eucalpytus platyphylla and Melaleuca viridiflora, and soil water potential and ¹⁸O composition of soil water to 8.5 m depth, in an open woodland community, Pioneer Valley, North Queensland. Estimates of the water uptake from various depths below the surface are determined for each species. At the time of sampling, the proportion of groundwater extracted by the trees ranged from 100% for C. clarksoniana to <15% for L. suaveolens and E. platyphylla. The advantages of the model over the traditional approach to determining sources of water used by plants using isotope methods are that it: (1) permits more quantitative assessments of the proportion of water sourced from different depths, (2) can deal with gradational soil water isotope profiles (rather than requiring distinct values for end-members), and (3) incorporates additional data on plant water potentials and is based on simple plant physiological processes.     

Differential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community

We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado. The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C₄ shrub) and Bouteloua gracilis (a C₄ grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δ¹⁸O value of plant stem water (for comparison with potential water sources) and the δ¹³C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized by the shrubs. Based on stem water δ¹⁸O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees were exclusively using groundwater, and the consistent δ¹³C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δ¹⁸O, shrubs may have also had access to groundwater. However, their overall δ¹⁸O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently, our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition by shrubs. Received: 23 December 1997 / Accepted: 16 September 1998     

宁夏平原北部地下水埋深浅地区不同灌木的水分来源

宁夏平原北部引黄灌区地下水埋深浅是该地区土壤盐碱化的主要原因, 种植耐盐植物可以吸收利用地下水, 在降低地下水位的同时可以减少对地面灌溉的依赖。为了分析银川平原北部4种灌木对不同水源的利用特征, 于2010年生长季测定了灌溉前后20年生多枝柽柳(Tamarix ramosissima)、3年生多枝柽柳、3年生宁夏枸杞(Lycium barbarum)和3年生四翅滨藜(Atriplex canescens)木质部水及不同潜在水源稳定氧、氢同位素组成(δ¹⁸O和δD), 应用IsoSource同位素线性混合模型估算了不同灌木对不同水源的利用率。同时测定了0-200 cm土壤剖面的全盐含量、含水量和pH值以及灌溉前后光合气体交换参数。结果表明: 不同深度土壤水δ¹⁸O和δD值存在较大差异, 并呈规律性变化。土壤水δ¹⁸O和δD值随深度加深呈逐渐降低的趋势。灌溉后80 cm以上土壤水δ¹⁸O和δD值低于灌溉前。无论灌溉前还是灌溉后, 20年生多枝柽柳与3年生灌木相比具有更低的δ¹⁸O和δD值。灌溉前, 3年生多枝柽柳、宁夏枸杞和四翅滨藜主要利用表层土壤水(70.1%、52.3%和48.9%); 20年生多枝柽柳对地下水的利用率最高(21.5%)。灌溉后, 3年生多枝柽柳和宁夏枸杞对80-140 cm土壤水利用率较高(59.5%和58.8%)。20年生多枝柽柳对地下水的利用率最高(18.3%)。灌溉前, 20年生多枝柽柳净光合速率、气孔导度和蒸腾速率显著高于其他3种灌木, 灌溉后3年生四翅滨藜净光合速率最高。灌溉对3年生多枝柽柳和宁夏枸杞的净光合速率和气孔导度有显著影响。无论灌溉前还是灌溉后, 3年生四翅滨藜瞬间水分利用效率均高于其他3种灌木。研究表明, 不同灌木在不同水分条件下水分利用策略不同, 这主要与植物种类及树龄有关。灌溉前幼龄多枝柽柳凭借其对干旱较强的忍耐能力利用浅层不饱和土壤水, 灌溉后其又转而利用中层土壤水, 表现出潜水湿生植物的特征, 主要吸收利用深层土壤水分, 对灌溉反应不明显。     

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4–6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9–20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the δ¹⁸O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ¹⁸O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ¹⁸O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar δ¹³C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2–3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.     

Tree age,site and climate controls on tree ring cellulose δ18O: A case study on oak trees from south-western France

A main concern of dendroclimatic reconstruction is to distinguish in the tree ring proxy the influence of the climate variables of interest from other controlling factors. In order to investigate age, site and climate controls on tree ring width and cellulose δ¹⁸O, measurements have been performed in nearby groups of young (145 years old) and older (310–405 years old) oak trees in south-western France, covering the period 1860–2010.Within a given site, inter-tree deviations are small, pointing to a common climatic signal. Despite a similar inter-annual variability, the average level of cellulose δ¹⁸O in the young tree group is ∼0.8‰ higher than in the old trees. Such offsets might be caused by different soil properties and differences in the fraction of the source water used by trees from different depths. The δ¹⁸O of water in the top soil layer is directly related to the current growing season precipitation, while deeper water can have a lower and more constant δ¹⁸O. Local cave drip waters at 10 m depth indeed show a constant isotopic composition, which corresponds to pluri-annual mean precipitation.A 2‰ increasing trend is observed in cellulose δ¹⁸O of young trees in the first 30 years of growth, during a period when no trend is visible in older trees. This increase can be quantitatively explained by humidity gradients under the forest canopy, and a changing microclimate around the crown as trees grow higher.While relationships between tree ring width and climate appear complex, the isotopic composition of cellulose is strongly correlated with summer maximum temperature, relative humidity and evapotranspiration (r ≈ 0.70). Weaker correlations (r ≈ 0.40) are identified with precipitation δ¹⁸O from a 15-year long local record and from the REMOiso model output. These results imply that leaf water enrichment has a stronger control on the inter-annual variability of cellulose δ¹⁸O than the δ¹⁸O of precipitation.This study demonstrates the suitability of oak tree ring cellulose δ¹⁸O for reconstructing past summer climate variability in south-western France, provided that the sampling and pooling strategy accounts for the fact that trees from different sites and of different age can introduce non-climatic signals.     

内蒙古西鄂尔多斯荒漠区降水入渗氘同位素特征

探讨我国干旱半干旱地区大气降水在土壤剖面中的时空分布特征将为西鄂尔多斯荒漠退化生态系统恢复和维持提供科学依据.本研究利用氘同位素技术研究了内蒙古西鄂尔多斯荒漠的大气降水、土壤水、地下水中的氘同位素值(δD),运用二元线性混合模型计算降水对各层土壤水的贡献率,并结合土壤含水量分析了不同降水条件下土壤剖面各层土壤水δD的时空分布特征.结果表明: 雨后9 d内,小雨(0～10 mm)影响0～10 cm土壤含水量和土壤水δD值,对表层土壤(0～10 cm)的贡献率在30.3%～87.9%;中雨(10～20 mm)影响0～40 cm土壤含水量和土壤水δD值,对0～40 cm土壤水的贡献率为28.2%～80.8%;大雨(20～30 mm)和特大暴雨(＞30 mm)影响0～100 cm土壤含水量和土壤水δD值.降水对100～150 cm深层土壤水δD值影响不显著.西鄂尔多斯荒漠土壤水δD介于大气降水δD与地下水δD之间,表明西鄂尔多斯荒漠土壤水主要来源于大气降水与地下水.在同一降水强度下,表层土壤水(0～10 cm)受降水的直接影响显著,随着土壤深度的增加,土壤水δD变化幅度降低,100～150 cm深层土壤水δD基本趋于稳定.降水强度越大,对土壤水δD影响的时间越长,影响的土壤深度也越深.     

Seasonal plant water uptake patterns in the saline southeast Everglades ecotone

The purpose of this study was to determine the seasonal water use patterns of dominant macrophytes coexisting in the coastal Everglades ecotone. We measured the stable isotope signatures in plant xylem water of Rhizophora mangle, Cladium jamaicense, and Sesuvium portulacastrum during the dry (DS) and wet (WS) seasons in the estuarine ecotone along Taylor River in Everglades National Park, FL, USA. Shallow soilwater and deeper groundwater salinity was also measured to extrapolate the salinity encountered by plants at their rooting zone. Average soil water oxygen isotope ratios (δ ¹⁸O) was enriched (4.8 ± 0.2‰) in the DS relative to the WS (0.0 ± 0.1‰), but groundwater δ ¹⁸O remained constant between seasons (DS: 2.2 ± 0.4‰; WS: 2.1 ± 0.1‰). There was an inversion in interstitial salinity patterns across the soil profile between seasons. In the DS, shallow water was euhaline [i.e., 43 practical salinity units (PSU)] while groundwater was less saline (18 PSU). In the WS, however, shallow water was fresh (i.e., 0 PSU) but groundwater remained brackish (14 PSU). All plants utilized 100% (shallow) freshwater during the WS, but in the DS R. mangle switched to a soil–groundwater mix (δ 55% groundwater) while C. jamaicense and S. portulacastrum continued to use euhaline shallow water. In the DS, based on δ ¹⁸O data, the roots of R. mangle roots were exposed to salinities of 25.4 ± 1.4 PSU, less saline than either C. jamaicense (39.1 ± 2.2 PSU) or S. portulacastrum (38.6 ± 2.5 PSU). Although the salinity tolerance of C. jamaicense is not known, it is unlikely that long-term exposure to high salinity is conducive to the persistence of this freshwater marsh sedge. This study increases our ecological understanding of how water uptake patterns of individual plants can contribute to ecosystem levels changes, not only in the southeast saline Everglades, but also in estuaries in general in response to global sea level rise and human-induced changes in freshwater flows.     

Seasonal water use characteristics of tall wheatgrass [Agropyron elongatum (Host) Beauv.] in a saline environment

It is essential to characterize the water use of plants that have potential for the stabilization of rising saline ground-water which could lead to increases in soil salinity. In this study, several techniques were used to determine the seasonal water use characteristics of the perennial grass Agropyron elongatum (tall wheatgrass) growing in a moderately saline, dryland environment with a fluctuating shallow groundwater table varying in electrical conductivity between 0 and 10 dS m^?1. Soil conditions were examined in terms of water potential measurements, plant water sources were identified using a stable isotope of water (deuterium, ²H) and evapotranspiration was estimated using hydrological and ventilated chamber methods. Seasonal changes in soil water potential were caused by salt accumulation and soil moisture leading to changes in plant water availability, particularly in the surface soil region over summer and autumn. Evapotranspiration in A. elongatum was high over summer during the peak period of growth (4 mm d^?1), with evidence of water use from groundwater and from specific regions of the soil profile. Evapotranspiration was low during the period that A. elongatum was partially senescent in autumn (< 0.5 mm d^?1) and the lowest leaf water potential of -3 MPa that was measured occurred during this period of moderate water stress. Intermediate levels of water use (1.5 mm d^?1) were measured during winter when the entire soil profile was available for water uptake. Based on physiological characteristics, including aspects of summer water use, root morphology and salt tolerance in A. elongatum, we conclude that this species is suitable for stabilizing the level of moderately saline groundwater in parts of southern Australia, which could reduce the potential for soil salinization.     

Changes in leaf photosynthetic characteristics and water use efficiency along with tree height of 4 tree species

As main photosynthetic organs, leaves are very sensitive to exterior environments. Water deficiency obviously affects the biological and physiological characteristics of leaves. Xylem pathways increase when trees grow tall, which results in the increase in water gravity as well as pathway resistance. Accordingly, the physiological characteristics of leaves change along with tree height. In this research, the photosynthetic characteristics and carbon isotope ratio (δ¹³C) in the leaves of 4 tree species, Platanus hispanica, Robinia pseudoacacia, Fraxinus chinensis and Ginkgo biloba, were measured. The results showed that the leaf photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Cond) and internal CO₂ concentration (Ci) reduced along with tree height, while the leaf δ¹³C increased along with tree height. The One Way ANOVA and LSD tests showed that the leaf photosynthetic characteristics and δ¹³C varied significantly at different tree heights (P < 0.05). The decrease in leaf photosynthetic capability and the increase in δ¹³C along with tree height indicate that the leaves at the tree tops suffer from water stress. These results support the hydraulic limitation hypothesis.     

Identifying groundwater‐fed climate refugia in remote arid regions with citizen science and isotope hydrology

1. Groundwater and surface water‐fed systems act as biodiversity hotspots and ecological refuges and evolutionary refugia in arid regions. Groundwater‐dominated systems are sustained by underground aquifers that are recharged by rain that has fallen in the distant past, while surface water‐dominated systems are fed by recent local rain or floods. Some waterbodies are fed by a mixture of these sources. Perennial, groundwater‐dominated systems will act as refuges and refugia under future rainfall declines associated with global warming. We sought to identify climate refugia, based on groundwater dominance, by using isotope hydrology to characterise water samples collected by citizen scientists across arid central Australia.
2. There is a linear relationship between hydrogen isotopes (²H/¹H, δ²H) and oxygen isotopes (¹⁸O/¹⁶O, δ¹⁸O) in rainfall. This relationship is known as the meteoric water line (MWL). By comparing our samples with the Australian MWL, and developing a local evaporation line, we were able to test the hypotheses that groundwater‐dominated systems will follow the Australian MWL while temporary systems follow the local evaporation line, and, accordingly, distinguish between groundwater and surface water‐dominated systems. The isotopic composition of samples collected over a 36‐month period was determined using isotope ratio infrared spectrometry. The electrical conductivity of each sample was recorded to determine where freshwater is available for biota within this arid region.
3. Over 240 water samples were collected from 62 waterbodies and seven bores (groundwater wells) spanning an area of more than 250,000 km². Approximately 75% of the samples were collected by citizen scientists and 25% by research scientists.
4. Twenty groundwater‐dominated waterbodies, characterised by a small range of δ²H and δ¹⁸O values (c. ?55 to ?20‰ and c. ?9 to ?3‰, respectively) clustered around the long‐term mean composition of rainfall (δ²H = ?37.5‰, δ¹⁸O = ?6.4‰), were identified as future evolutionary refugia. These sites are likely to contain water through the most severe of droughts and will be critically important for the persistence of water‐dependent species.
5. Based on their isotopic composition, we identified 45 waterbodies (rockholes/waterholes) as temporary or ephemeral (δ²H c. ?40 to ?100‰ and δ¹⁸O c. ?4 to +25‰), that is, with no evidence of groundwater inflow. These, together with waterbodies supported by a mix of groundwater and surface water, can act as stepping stones and form part of the aquatic mosaic that is critical to supporting species in arid regions. Over two‐thirds of the waterholes sampled were very fresh (electrical conductivity <0.8 mS/cm), indicating that they provide the freshwater needed to support much of the regional aquatic and terrestrial fauna.
6. All evolutionary refugia are located within protected areas (i.e. national parks or Indigenous Protected Areas), but some are subject to the impacts of feral animal species and invasive plants. Our findings indicate where control programmes and restoration actions can be prioritised to support biodiversity conservation and climate change adaptation. Our approach, combining citizen science and isotope hydrology, can be used to identify future refugia in other remote and arid regions where water scarcity is likely to increase under global climate change.