Desert Riparian Ecosystem Research Articles

The precise implementation of the ecological water transfer project effectively promotes the enhancement of desert riparian ecosystem service value in the mainstream of Tarim River

Ecosystem service value (ESV), as a crucial indicator concerning human and regional ecological well-being, has always been a hot topic in academia. Selecting appropriate assessment factors for arid areas to comprehensively explore the spatial–temporal heterogeneity of ESV and proposing pathways for ESV enhancement are currently challenging in research. This study incorporates three driving indicators, namely, human disturbance intensity (HDI), vegetation growth status (NIRv), and hydrological wetness condition (TVDI), into ESV calculation, efficiently improving the pixel-scale spatial heterogeneity manifestation of ESV. The study was conducted in a representative desert region of the Tarim River Basin (TRB), where ecological water transfer has effectively facilitated vegetation restoration. Based on this analysis, the total ESV in the TRB grew by 12.73 % between 2000 and 2022. Among them, ESVHDI, ESVNIRv, and ESVTVDI increased by 4.10 %, 58.25 %, and 18.78 % respectively. ESV in the four source rivers and mainstream of TRB increased by 4.36 % (YR), 60.77 % (TR), 2.94 % (AR), 3.88 % (HR), and 2.11 % (KR), with the ESV enhancement areas highly coinciding (coincidence degree is as high as 60.64 % of the mainstream of TRB) with the Populus euphratica forest water transfer area. ESV in the ecological water transfer area increased by 51.27 %, especially in the mainstream of the TRB water transfer area, where ESV increased by 62.32 %. Based on the perspectives of water use benefit (ESV/ET) and efficiency (ET/NIRv), to improve the ecological water use efficiency and ESV even further, the water supply thresholds of forests, grasslands, and wetlands should be controlled within the ranges of [495.37 mm-553.45 mm], [474.25 mm-528 mm], and [1011.68 mm-1070.64 mm], respectively. This study not only provides new insights for ESV calculation in arid areas but also offers scientific guidance for precise management of arid ecosystem.

Impact of water supply on the restoration of the severely damaged riparian plants along the Tarim River in Xinjiang, Northwest China

Restoring degraded ecosystems in drylands is challenging because of low precipitation and high evapotranspiration. Water availability determines the existence and spatiotemporal distribution patterns of key endangered species struggling to survive in the severely damaged desert riparian ecosystem of the lower Tarim River (LTR) in NW China. Ecological water flow restoration measures with high costs and uncertainties have been implemented since 2000. The key task for scientists is to assess the success and drawbacks of this restoration project and propose specific recommendations for stakeholders to optimize the ecological benefits of the project in a cost-effective manner. In this study, Landsat time-series data from 1990 to 2020 were used to extract vegetation index values from river corridors along the LTR and explore the spatiotemporal patterns of desert riparian forest (DRF) vegetation. Based on land use and surface water data, we extracted the interannual spatial distribution range of vegetation and surface water, and selected several sample plots to demonstrate the influence of surface water and groundwater on vegetation change trends. The study results showed the following. 1) From 1990 to 1999, the vegetation slowly degraded; from 2000 to 2012, the extent of vegetation restoration was approximately 1 km from the river; and from 2013 to 2020, the extent of vegetation restoration at the Yingsu and Karday sections increased to 4.5–6.0 km, while that at the Arghan and Yiganbujima sections remained within 1 km from the river. 2) The vegetation growth rate increased substantially and was spatially heterogeneous when the groundwater level reached 4.0–5.0 m and was influenced by surface water divergence; the average vegetation expansion rates per square km for P1–P4 were 0.017 km2/a, 0.011 km2/a, 0.005 km2/a, and 0.002 km2/a, respectively. 3) Flooding by surface water was the main driver of vegetation expansion when groundwater was stable and elevations in surface vegetation area were often accompanied by the spreading of surface water. Surface water was significantly correlated with vegetation cover and normalized difference vegetation index (NDVI) (Water pixels–Vegetation cover [P1: R2 = 0.57, P < 0.001; P2: R2 = 0.45, P < 0.01]; Water pixels–NDVI [P1: R2 = 0.22, P < 0.05; P2: R2 = 0.40, P < 0.01]). The increase in surface vegetation has alleviated the desertification process in the LTR, and the survival of the poplar has been mitigated. The results of this paper can help decision makers to formulate reasonable ecological water conveyance strategies and realize sustainable management of DRF ecosystems in arid zones.

Increased Humidity Improved Desert Riparian Ecosystems in the Tarim River Basin, Northwest China, from 1990 to 2020

Land use and land cover change (LULCC), along with the conversion of natural ecosystem cover into farmland, poses significant ecological challenges for desert riparian ecosystems. The Tarim River Basin (TRB), home to the world’s largest and most densely distributed and well-preserved desert riparian ecosystem, remains exceptionally susceptible to climate change. However, our understanding of the role of climatic factors (mean annual temperature (MAT); mean temperature during the warmest month (MWMT); relative humidity in September, October, and November (RH_SON); and the annual heat–moisture index (AHM)) in driving pattern changes in these ecosystems remains limited. To address this gap, we employed a transfer matrix approach coupled with geographically weighted regression models to conduct an extensive analysis of LULCC trends and their driving factors within the TRB from 1990 to 2020. The 30-year dataset on LULCC provided invaluable insights, revealing that the proliferation of farmland and shrubberies has precipitated the decline of arbor forests and grassland expanses. Furthermore, this expansion of farmland and shrubberies has resulted in heightened ecosystem fragmentation, particularly notable between 2005 and 2010. Our assessment indicates that artificial ecosystems are gradually transitioning back into natural states, encompassing 8.24% of the total area, chiefly attributed to the expanding shrubbery regions. Additionally, in-depth scrutiny of the impacts of climatic factors on ecosystem structure unveiled that moisture exerts the most pronounced influence on ecosystem patterns, followed by air moisture content during the growing season, while temperature exerts a relatively lesser impact. Overall, this study contributes to the realization of SDG 13 (Climate Action) and SDG 15 (Life on Land) by informing conservation efforts and sustainable land management practices in dryland desert riparian ecosystems.

Removing invasive giant reed reshapes desert riparian butterfly and bird communities

AbstractGiant reed (Arundo donax) is a prevalent invasive plant in desert riparian ecosystems that threatens wildlife habitat. From 2008 to 2018, under a United States–Mexico partnership, prescribed burns and herbicide applications were used to remove giant reed and promote native revegetation along the Rio Grande—Río Bravo floodplain in west Texas, USA, and Mexico. Our goal was to explore the effects of the removal efforts on butterfly and bird communities and their habitat along the United States portion of the Rio Grande—Río Bravo floodplain in Big Bend National Park, Texas. During spring and summer, 2016–2017, we surveyed butterflies, birds, and their habitat using ground‐collected and remotely sensed data. Using a variety of generalized linear and N‐mixture modeling routines and multivariate analyses, we found that the initial giant reed removal efforts removed key components of riparian habitat leading to reduced butterfly and bird communities. Within several years following management, giant reed levels remained low, while riparian habitat conditions and butterfly and bird communities largely rebounded, including many disturbance‐sensitive butterfly species and riparian‐associated bird species. Butterflies were most consistently associated with forb and grass cover, and birds with a remotely sensed index of greenness (the normalized difference vegetation index), several vegetation cover types, and habitat heterogeneity, habitat elements that were most common in locations that had the longest time to recover following management actions. Our results suggest that prescribed burns and herbicide applications, when used following protocols to minimize risk to wildlife, can limit the spread of giant reed in desert riparian systems and introduce habitat conditions that support diverse and abundant butterfly and bird communities.

Hydraulic Sensitivity and Stomatal Regulation of Two Desert Riparian Species

AbstractCharacterizing plant response to water stresses is among the keys to understanding ecosystem functions. Despite increasing recognition of plant hydraulic and stomatal dynamics not captured by isohydricity, defined as the sensitivity of leaf water potential Ψl to soil water potential, it remains unclear which plant traits are critical to improved representation of plant hydraulic and stomatal dynamics beyond isohydricity, especially in dryland ecosystems. Here, we examine eco‐physiological responses of two typical desert riparian species, Populus euphratica and Tamarix ramosissima, to hydro‐meteorological variations. Based on measured hydraulic traits, variations of Ψl, leaf gas exchange, groundwater table, and soil moisture across vertical profiles, we investigated how subsurface hydraulic conditions control plant hydraulic sensitivity and stomatal regulation. We found both species exhibited anisohydric behaviors, which, however, were attributed to drying soil at different depths because of root distributions. The similar anisohydric behaviors also led to distinct stomatal regulation, due to the impact of atmospheric dryness decoupled from soil moisture and different sensitivities of stomatal conductance (gs) to Ψl. The latter was found to be dynamic throughout the growing season, which dominated the seasonal variation of gs and thus should not be neglected. The results suggest similar anisohydric behaviors could imply diverging hydraulic behaviors as the evaporate demand and flow regime change. Root morphological trait and stomatal sensitivity to Ψl are identified as keys to characterize responses to water stresses in desert riparian ecosystems. Our findings highlight that improved measurements and representations of these traits could contribute to better assessments of dryland ecosystem functions.

Resistance and Resilience of Desert Riparian Communities to Extreme Droughts

Changes in the resistance, recovery, and resilience of three desert riparian ecosystems to extreme droughts and the influence of ecological characteristics on them from 2000 to 2020 were analyzed within the Tarim River Basin, Xinjiang, China. The results showed that: (1) The resistance of the Populus euphratica forest community was the highest among the three plant communities. The recovery and resilience of the shrub community were the highest; (2) the resilience increased the most for the shrub community and decreased slightly for the Populus euphratica forest and herb communities as hydrological droughts waned; (3) the relative height and density significantly increased for Populus euphratica, decreased for herbs, and did not change for shrubs from 2000 to 2020; (4) the diversity indexes did not significantly change for the Populus euphratica forest and herb communities, and they increased for the shrub community from 2000 to 2020; (5) the resistance and resilience of the Populus euphratica forest, shrub, and herb communities were significantly affected by ecological characteristics, especially the Margalef richness index; and (6) the shrub community is more likely to rapidly recover (that is an increase of cover, height, and density) from future droughts in the Tarim River Basin, particularly with the implementation of new and more effective ecological restoration projects.

The coupled effect of soil and atmospheric constraints on the vulnerability and water use of two desert riparian ecosystems

Plant response to soil and atmospheric water stresses is the dominant control of Dryland ecosystem functions, affecting water resources, ecosystem stability and biodiversity. The link between water stresses and plant water status is regulated by plant hydraulics, of which the corresponding impact on plant water use and the susceptibility of Dryland ecosystems remains under-explored. We used a plant hydraulic model to describe hydraulic states and water flux of two species (Populus euphratica and Tamarix ramosissima) in desert riparian ecosystems. We optimized hydraulic parameters and tested the model using observed physiological states and ecosystem water flux. The optimized model was used to evaluate plant hydraulic sensitivity, tree mortality risk, and evapotranspiration under a wide range of water stress scenarios. The model captures the observed leaf water potential, sap-flow and ecosystem evapotranspiration. Our scenario analysis demonstrates that hydraulic sensitivities generally reduce as the water stresses intensify. The results highlight a strong coupled impact of the co-occurrence of soil and atmospheric water stresses on restricting ecosystem water flux and intensifying mortality risk. The assessment of multiple aspects of eco-physiological functions and the stress scenario analysis of desert riparian ecosystems will contribute to a better prediction of ecosystem functions and facilitate resource management under future climate.

Impact of groundwater depth and soil salinity on riparian plant diversity and distribution in an arid area of China

Riparian plant diversity in arid regions is sensitive to changes in groundwater. Although it is well known that groundwater has a significant influence on plant diversity, there have been few studies on how groundwater and soil salinity impact plant community in desert riparian ecosystems. Therefore, we surveyed 77 quadrats (100 m × 100 m) to examine the relationship between groundwater depth, groundwater salinity, soil salinity and plant community in the upper reaches of the Tarim River. Data were analyzed with two-way indicator species analysis (TWINSPAN), detrended canonical correspondence analysis (DCCA) and principal component analysis (PCA). The results indicated that Populus euphratica, Tamarix ramosissima, and Phragmites australis were the dominant plants among trees, shrubs and herbs, respectively. Five plant community types were classified. There were significant differences in species diversity, soil moisture, soil salinity, groundwater depth and groundwater salinity across the community types. The composition and distribution of plant community are significantly influenced by groundwater depth, groundwater salinity, soil moisture, distances from the river to the quadrats, soil pH, electrical conductivity, total salt, CO32−, Cl−, SO42−, Ca2+, Mg2+, Na+ and K+. Shallow groundwater depth, low groundwater salinity, and high soil moisture and soil salinity were associated with higher plant diversity.

Response of riparian ecosystem to dike construction on the Middle Reaches of the Tarim River, Northwest China

AbstractStudies have demonstrated that water management engineering may bring ecological degradation to the riparian ecosystems. However, most such works focus on dams, and the impact of dikes has been less studied. We investigated how dikes interfere with the desert riparian ecosystem on the middle reaches of the Tarim River, Xinjiang, China. We collected data of vegetation species and coverage, groundwater depth, total dissolved solids (TDS), and soil salinity, along eight transects crossing the river channel, from 2001 to 2010. We conducted spatio‐temporal analyses of the data from the transects and calculated correlations between the ecological indices and the groundwater measurements and between the ecological indices and the soil properties. We found that since the construction of dikes in 2001, both the depth of and TDS in groundwater had been increasing so did soil salinity; their spatial distributions changed from irregular to increasing along with the distance from the dikes, and the width of solute‐dilution belt became narrower. The overall species richness had been declining, and spatially, it had decreased as the distance to the dikes increases; meanwhile, the species evenness had been increasing. The correlation between the species diversity and the groundwater measurements was not significant before 2003 but became highly significant for 2006–2010, after the dikes were fully established. A shrinkage of arboreal cover is accompanied by expansions of shrubby and herbaceous covers. Particularly for the Populus euphratica community, the main arboreal community in the region, the ratio between seedlings and young trees had been declining; the structure of the community became simpler as the diversity decreased; and the lateral width of the community changed from 1,500 m to 800–1,000 m. This community degradation appears to be a consequence of the dike constructions, which prevented overbank floods, resulting in a shortage of water for groundwater recharging, an increase of soil salinity leaching, and a loss of proper habitats for seeds germination.

Responses of two desert riparian species to fluctuating groundwater depths in hyperarid areas of Northwest China

AbstractIn the hyperarid region of Northwest China, frequent variations in hydrological environments present challenges to the persistence of riparian plants. The main objective of this study was to determine whether two desert riparian species (Populus euphratica and Tamarix ramosissima) differed in their water uptake patterns and ecophysiological responses to fluctuating groundwater depths (GWDs). This study was conducted in typical desert riparian ecosystems in the downstream Heihe River basin, Northwestern China, where the GWD continuously increases during growing season. Stable oxygen composition (δ18O) in xylem water, soil water, and groundwater, as well as leaf water potential and gas exchange were monitored. Results showed that P. euphratica used a higher ratio of soil water, whereas T. ramosissima relied more on groundwater and deep soil water. As the GWD increased during the growing season, both species modified their water use patterns, but they did so differently, P. euphratica extracted an increasing proportion of deep soil water and groundwater, whereas T. ramosissima took an increasing ratio of groundwater at critical growth stages. P. euphratica exhibited decreases in its daily maximum photosynthetic rate (Amax) and stomatal conductance (gmax) as the GWD increased, whereas those of T. ramosissima changed little. In summary, both species shift to use greater ratio of more reliable water sources with the increasing GWD, but the switching of water sources could not sufficiently compensate for the impact of drought stress on gas exchange for P. euphratica.

Spatial variation in leaf nutrient traits of dominant desert riparian plant species in an arid inland river basin of China.

Understanding how patterns of leaf nutrient traits respond to groundwater depth is crucial for modeling the nutrient cycling of desert riparian ecosystems and forecasting the responses of ecosystems to global changes. In this study, we measured leaf nutrients along a transect across a groundwater depth gradient in the downstream Heihe River to explore the response of leaf nutrient traits to groundwater depth and soil properties. We found that leaf nutrient traits of dominant species showed different responses to groundwater depth gradient. Leaf C, leaf N, leaf P, and leaf K decreased significantly with groundwater depth, whereas patterns of leaf C/N and leaf N/P followed quadratic relationships with groundwater depth. Meanwhile, leaf C/P did not vary significantly along the groundwater depth gradient. Variations in leaf nutrient traits were associated with soil properties (e.g., soil bulk density, soil pH). Groundwater depth and soil pH jointly regulated the variation of leaf nutrient traits; however, groundwater depth explained the variation of leaf nutrient traits better than did soil pH. At the local scale in the typical desert riparian ecosystem, the dominant species was characterized by low leaf C, leaf N, and leaf P, but high leaf N/P and leaf C/P, indicating that desert riparian plants might be more limited by P than N in the growing season. Our observations will help to reveal specific adaptation patterns in relation to the groundwater depth gradient for dominant desert riparian species, provide insights into adaptive trends of leaf nutrient traits, and add information relevant to understanding the adaptive strategies of desert riparian forest vegetation to moisture gradients.

Groundwater Depth and Soil Properties Are Associated with Variation in Vegetation of a Desert Riparian Ecosystem in an Arid Area of China

Groundwater is a major driving force for plant community distribution in arid areas worldwide. Although it is well known that groundwater has a significant impact on soil and vegetation, there is little information on how groundwater depth affects soil and vegetation in an arid inland basin desert riparian ecosystem. Therefore, quantitative analysis of the relationships among groundwater depth, soil properties and plant community distribution is necessary. A desert riparian ecosystem in the lower reaches of the Heihe River in an arid area of Northwest China was used to determine quantitative relationships among groundwater depth, soil and vegetation. Groundwater depth significantly increased with increased distance from the river. Soil and vegetation characteristics showed a significant trend with increasing groundwater depth. With increasing groundwater depth, soil water content, soil total nitrogen, soil total carbon, soil available phosphorus and soil available potassium decreased, while the soil bulk density and soil carbon:nitrogen (C:N) ratio increased. Soil pH and soil electrical conductivity followed quadratic function relationships with groundwater depth. Species richness, aboveground biomass, community coverage, community height, foliage projective cover and leaf area index all significantly decreased with increased groundwater depth. Groundwater depth and soil were associated with vegetation variance, explaining 85.8% of the vegetation variance. Groundwater depth was more important in explaining vegetation variance than soil properties (soil bulk density) and soil pH. Our observations indicate that changes in groundwater depth would have a significant influence on desert riparian forest vegetation, and that maintaining appropriate groundwater depth is necessary to preserve the riparian ecosystem.

A Species-Specific and spatially-Explicit Model for Estimating Vegetation Water Requirements in Desert Riparian Forest Zones

Balancing human demands for water with environmental requirements to maintain functioning ecosystems requires the quantification of ecological water requirements. In arid regions, high spatial variability of vegetation cover and different water consumption of plant species make it different to estimate reasonable ecological water requirements. We developed a simple and practical approach that estimates the vegetation water requirements (VWRs) of desert riparian ecosystems. This model is species-specific and spatially-explicit; it considers the water consumption characteristics required by different species and highlights the impacts that high vegetation cover spatial variability has in arid regions on evapotranspiration. The model was parameterized based on the observation of the water consumption of two typical desert riparian species, Populus euphratica and Tamarix spp., in the lower basin of the Tarim River in northwestern China. Comparisons between the modeling results and measured data for two mature Populus and Tamarix stands indicate that the model is reasonable predictive. A case study in the lower basin of the Tarim River demonstrated the model’s practicality and transferability. This model could run based on near real-time or forest weather data and spatial vegetation patterns, and provides a continuous estimation of the temporal and spatial variations of the VWR. Particularly, this model forecasts VWRs under different vegetation spatial distribution and coverage scenarios, and evaluates the impacts and consequences of different management actions. This model can serve as a useful tool for management agencies interested in improving their decisions to allocate river water between human activities and natural ecosystems in arid regions.

Species Introductions and Their Cascading Impacts on Biotic Interactions in desert riparian ecosystems.

Desert riparian ecosystems of North America are hotspots of biodiversity that support many sensitive species, and are in a region experiencing some of the highest rates of climatic alteration in North America. Fremont cottonwood, Populus fremontii, is a foundation tree species of this critical habitat, but it is threatened by global warming and regional drying, and by a non-native tree/shrub, Tamarix spp., all of which can disrupt the mutualism between P. fremontii and its beneficial mycorrhizal fungal communities. Specialist herbivorous leaf beetles (Diorhabda spp.) introduced for biocontrol of Tamarix are altering the relationship between this shrub and its environment. Repeated episodic feeding on Tamarix foliage by Diorhabda results in varying rates of dieback and mortality, depending on genetic variation in allocation of resources, growing conditions, and phenological synchrony between herbivore and host plant. In this article, we review the complex interaction between climatic change and species introductions and their combined impacts on P. fremontii and their associated communities. We anticipate that (1) certain genotypes of P. fremontii will respond more favorably to the presence of Tamarix and to climatic change due to varying selection pressures to cope with competition and stress; (2) the ongoing evolution of Diorhabda's life cycle timing will continue to facilitate its expansion in North America, and will over time enhance herbivore impact to Tamarix; (3) defoliation by Diorhabda will reduce the negative impact of Tamarix on P. fremontii associations with mycorrhizal fungi; and (4) spatial variability in climate and climatic change will modify the capacity for Tamarix to survive episodic defoliation by Diorhabda, thereby altering the relationship between Tamarix and P. fremontii, and its associated mycorrhizal fungal communities. Given the complex biotic/abiotic interactions outlined in this review, conservation biologists and riparian ecosystem managers should strive to identify and conserve the phenotypic traits that underpin tolerance and resistance to stressors such as climate change and species invasion. Such efforts will greatly enhance conservation restoration efficacy for protecting P. fremontii forests and their associated communities.

Heliotropic leaf movement of Sophora alopecuroides L.: An efficient strategy to optimise photochemical performance

We studied the survival adaptation strategy of Sophora alopecuroides L. to habitat conditions in an arid desert riparian ecosystem. We examined the responses of heliotropic leaf movement to light conditions and their effects on plant photochemical performance. S. alopecuroides leaves did not show any observable nyctinastic movement but they presented sensitive diaheliotropic and paraheliotropic leaf movement in the forenoon and at midday. Solar radiation was a major factor inducing leaf movement, in addition, air temperature and vapour pressure deficit could also influence the heliotropic leaf movement in the afternoon. Both diaheliotropic leaf movement in the forenoon and paraheliotropic leaf movement at midday could help maintain higher photochemical efficiency and capability of light utilisation than fixed leaves. Paraheliotropic leaf movement at midday helped plants maintain a potentially higher photosynthetic capability and relieve a risk of photoinhibition. Our findings indicated the effective adaptation strategy of S. alopecuroides to high light, high temperature, and dry conditions in arid regions. This strategy can optimise the leaf energy balance and photochemical performance and ensure photosystem II function.

Impact of nitrate enrichment on wetland and dryland seed germination and early seedling development

AbstractQuestionsAtmospheric and agricultural inputs of nitrogen have increased significantly. Because riparian zones act as buffers for nutrient transfer and treated municipal effluent and untreated agricultural run‐off are released into streams, and because desert riparian ecosystems contain a wide range of functional groups with respect to water relations, we asked if wetland and dryland species respond differently to increasing levels of soil nitrate. Specifically, we asked if seed germination rate, total percentage germination and seedling development varied by species and functional group in response to increasing nitrate levels.LocationSouthwestern US.MethodsWe subjected seeds of common wetland and dryland species to distilled water (control), 0.025 M (low), 0.05 M (intermediate) and 0.075 M (high) KNO3 treatments for 30 days. We determined seed germination rates, total percentage germination and seedling development for each species. Generalized linear mixed models tested for differences between treatments.ResultsSeeds of wetland species germinated rapidly and in high percentages, and developed into seedlings regardless of nitrate treatments. Dryland species had unique responses to nitrate treatments, with some species exhibiting high germination rates in control treatments and others showing strong germination responses at low to intermediate nitrate levels. A larger number of dryland germinants developed into seedlings under control to low nitrate treatments.ConclusionsRiparian plant community dynamics arising from germination responses could change under increased nitrogen inputs, as some species are seemingly unaffected and others significantly impacted. Based on our limited species pool and treatment conditions, the functional group ‘wetland species’ produces a consistent germination and early seedling response regardless of nitrate concentration, while ‘dryland species’ responses are variable and species‐specific. Species known to be nitrophiles can be inhibited by high nitrogen concentration as germinants, highlighting the importance of investigating plant community response to changing conditions at a variety of life stages.

Osmolyte accumulation in leaves of Tamarix ramosissima growing under various soil conditions in the Colorado River basin

Tamarix ramosissima is a dominant species in desert riparian ecosystems in the western USA. It is a phreatophytic halophyte, with salt glands on the leaves. While osmoregulation is essential for turgor maintenance under high salinity, the dose–response relationship to salinity of various osmolytes in plants with salt glands is still unknown. We profiled crude leaf extracts of T. ramosissima to identify the metabolic compounds that contribute to its salt tolerance. We compared leaf cation, soluble sugar, amino acid, and betaine content among T. ramosissima samples from five points along the Colorado River. The leaf sodium content of T. ramosissima trees increased with increasing soil salinity. Under high salinity conditions, soluble sugar and betaine content did not increase, but amino acids did. The increase in proline accumulation was highly and positively correlated with leaf sodium content. Thus, proline appears to be the essential osmolyte that T. ramosissima accumulates in response to severe salt stress in desert riparian areas of the USA.

Long‐term decrease in satellite vegetation indices in response to environmental variables in an iconic desert riparian ecosystem: the Upper San Pedro, Arizona, United States

AbstractThe Upper San Pedro River is one of the few remaining undammed rivers that maintain a vibrant riparian ecosystem in the southwest United States. However, its riparian forest is threatened by diminishing groundwater and surface water inputs, due to either changes in watershed characteristics such as changes in riparian and upland vegetation, or human activities such as regional groundwater pumping. We used satellite vegetation indices to quantify the green leaf density of the groundwater‐dependent riparian forest from 1984 to 2012. The river was divided into a southern, upstream (mainly perennial flow) reach and a northern, downstream (mainly intermittent and ephemeral flow) reach. Pre‐monsoon (June) Landsat normalized difference vegetation index (NDVI) values showed a 20% drop for the northern reach (P &lt; 0·001) and no net change for the southern reach (P &gt; 0·05). NDVI and enhanced vegetation index values were positively correlated (P &lt; 0·05) with river flows, which decreased over the study period in the northern reach, and negatively correlated (P &lt; 0·05) with air temperatures in both reaches, which have increased by 1·4 °C from 1932 to 2012. NDVI in the uplands around the river did not increase from 1984 to 2012, suggesting that increased evapotranspiration in the uplands was not a factor in reducing river flows. Climate change, regional groundwater pumping, changes in the intensity of monsoon rain events and lack of overbank flooding are feasible explanations for deterioration of the riparian forest in the northern reach. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Ecological response and hydrological mechanism of desert riparian forest in inland river, northwest of China

ABSTRACTThe interactive relationships of ecological and hydrological processes on individual plant growth and development as well as community succession have been a main focus of research in plant ecohydrology. In this paper, we firstly explore the spatial difference of the desert riparian ecosystems as distance to the river in the arid inland Tarim River Basin and then investigate the response of vegetation communities to emergent ecological water conveyances. Results showed that the decreasing groundwater table is the main driving force for vegetation degradation. Herbs exhibit degradation when the groundwater table is 4–6 m below the surface, whereas trees do not exhibit degradation until the groundwater table exceeds 6 m from the soil surface, in the lower reaches of the Tarim River. In addition, the structure of the plant community is controlled by the depth of the groundwater table. The mixed trees/shrubs/herbs structure is distributed in areas where the groundwater table is between 2 and 4 m in depth, the trees/shrubs structure is distributed in areas where the groundwater depth is 4–8 m and the simple structure of degraded Populus euphratica/Tamarix chinensis dominates the areas where the groundwater depth is &gt;8 m. After ecological water conveyance, the Phraginites communis leaf weight, length and width decrease with the increasing distance from the river channel. The length of current‐year twigs and average number of leaves of P. euphratica are significantly greater in the upstream section of the lower reach than in the downstream section of the lower reach. We concluded that the ecologically suitable groundwater depth for P. euphratica and T. chinensis is 2–4 m and the critical groundwater depth for drought stress is about 9 m. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Dryland Riparian Ecosystems in the American Southwest: Sensitivity and Resilience to Climatic Extremes

Drylands make up over 40% of the earth’s land surface (Noy-Meir 1973; Reynolds and others 2007). In these arid and semiarid landscapes, surface and subsurface water flows create mesic riparian environments (Kingsford 2006; Stromberg and Tellman 2009). Dryland riparian areas sustain water-limited plants and animals that cannot withstand upland conditions year-round, thereby supporting regional diversity (Sabo and others 2005; Lite and others 2005). Dryland rivers are also characterized by occasional large, high-intensity floods that rework the bed geomorphology and increase riparian heterogeneity. This combined variation in water availability and flood disturbance creates spatiotemporally complex and unique landscapes (Soykan and Sabo 2009; Soykan and others 2012). The hydrology of desert riparian ecosystems has been extensively altered by human activities including stream diversion and groundwater extraction, and is expected to undergo further change in response to changing climates (Patten 1998). Globally, increasing temperatures associated with climate change are expected to intensify the water cycle, leading to greater rates of evapotranspiration, more water in the atmosphere, and more-frequent intense storms (Huntington 2006; IPCC 2007). More intense storms are likely to increase flood intensity over the next 50 years (Dominguez and others 2012). In the American Southwest in particular, warmer sea surface temperatures could spur stronger and more-frequent El Nino-associated winter storms (Garfin and Lenart 2007). Perhaps of greater importance are projected increases in aridity. Average precipitation is expected to increase at a global scale, but some regions, including the American Southwest, are expected to experience substantial decreases in average annual rainfall as well as increases in frequency and intensity of droughts (Seager and others 2007). As aridity increases, streams and riparian zones undergo declines in surface flow and depth to groundwater; there are declines, as well, in the extent of river segments with perennial (vs. intermittent or ephemeral) stream flow. Given that rivers and their riparian corridors function as keystone elements of arid and semiarid landscapes, climate change will have far reaching effects on regional biota (Stromberg and others 2010). Nevertheless, the extent of biotic change for riparian ecosystems in coming decades will be tempered by the fact that for desert rivers, climatic and hydrologic variation is the norm (Milly and Received 30 May 2012; accepted 13 September 2012