Fanjiang Zeng

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h−1) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.

Characterizing variations in soil particle size distribution in oasis farmlands-A case study of the Cele Oasis

Characterizing soil particle size distributions (PSD) and their variation is an important issue in environmental research. In this study, fractal theory was used to analyse the soil PSD and its variations in the Cele Oasis, which is located at the southern margin of the Tarim Basin. The characteristics of the soil PSD were then evaluated to identify the primary factors that influence soil PSD. The results showed that the fractal dimension (D) values ranged from 2.11 to 2.27, and that there were significant differences among groups. Furthermore, the D values showed a significant positive correlation with fine particles (<50 @mm) and soil organic matter contents. According to a comparative analysis of D values, the utilization years of farmlands had a significant influence on PSD, while the difference in the spatial distribution of farmlands did not. These results indicated that long-term and effective tillage management of the farmlands will be beneficial to keeping and improving the states of the soil PSD and other soil properties.

Leaf and whole tree adaptations to mild salinity in field grown Populus euphratica

Populus euphratica Oliv. is a highly salt tolerant tree species, and this study represents the first comprehensive investigation of salt tolerance mechanisms of mature trees of P. euphratica in the field. We measured NaCl concentration in xylem sap, NaCl accumulation in leaves, the effect of NaCl on leaf physiological parameters and osmotic adjustment and the allocation and distribution of NaCl between different plant organs on a whole plant level in trees exposed to mild saline groundwater (around 30 mM) in China. Populus euphratica showed three key mechanisms of salt tolerance. The primary mechanism had a strong control over Na(+) and Cl(-) uptake with effective exclusion mechanisms for Cl(-) with up to 99% of the external NaCl being excluded from the xylem. Secondly, the trees allocated large proportions of NaCl into the leaves, which served as a salt elimination mechanism as the leaves are ultimately shed at the end of the growing season. Thirdly, the trees tolerated high foliar Na(+) concentrations through a combination of osmotic adjustment using sucrose and probable sequestering of Na(+) in the apoplast. Our results indicate that the control of Na(+) and Cl(-) uptake and the regulation of Na(+) and Cl(-) delivery to the shoot are key to salt tolerance of P. euphratica in the field with tolerance of high Na(+) concentrations in leaves being a critical component.

Water relation characteristics ofAlhagi sparsifolia and consequences for a sustainable management

Water relation characteristics of the desert legumeAlhagi sparsifolia were investigated during the vegetation period from April to September 1999 in the foreland of Qira oasis at the southern fringe of the Taklamakan Desert, Xinjiang Uygur Autonomous Region of China. The seasonal variation of predawn water potentials and of diurnal water potential indicated thatAlhagi plants were well water supplied over the entire vegetation period. Decreasing values in the summer months were probably attributed to increasing temperatures and irradiation and therefore a higher evapotranspirative demand. Data from pressure-volume analysis confirmed thatAlhagi plants were not drought stressed and xylem sap flow measurements indicated thatAlhagi plants used large amounts of water during the summer months. Flood irrigation had no influence on water relations inAlhagi probably becauseAlhagi plants produced only few fine roots in the upper soil layers. The data indicate thatAlhagi sparsifolia is a drought-avoiding species that utilizes ground water by a deep roots system, which is the key characteristic to adjust the hyper-arid environment. Because growth and survival ofAlhagi depends on ground water supply, it is important that variations of ground water depth are kept to a minimum. The study will provide a theoretical basis for the restoration and management of natural vegetation around oasis in arid regions.

Characteristics of meteorological factors over different landscape types during dust storm events in Cele, Xinjiang, China

Landscape characteristics influence meteorological factors, thus affect the occurrence and nature of dust storm events. The present study investigates the spatiotemporal characteristics of six meteorological factors (wind velocity, wind direction, air temperature, relative humidity (RH), photo synthetically active radiation (PAR), and solar radiation) over different landscape types (shifting-sand frontier, semi-fixed sandy land, fixed sandy land, and the inner region of an oasis) before and after dust storms during four typical dust storm events in an oasis-desert ecotone in Cele, Xinjiang, China. The results show that the average wind velocity decreased significantly from the shifting-sand frontier to the inner oasis, which was mainly attributable to the vegetation coverage. Before the dust storm events, there were obvious differences in air temperature and RH either in the horizontal or vertical direction over the different landscape types. However, these factors were very similar during and following the dust storm events. PAR and solar radiation were significantly reduced during the dust storm events and the subsequent sand-blowing and floating-dust conditions. This effect was much stronger than during similar weather conditions without dust storm events such as sand-blowing and overcast and/or rainy days. Additionally, the variation in the meteorological factors among the different landscapes was also affected by the prevailing wind direction during the dust storm events. However, the landscape type slightly changed the prevailing wind direction, with the greatest dispersion distribution of wind direction in the inner oasis. The findings of this study are helpful for understanding the function of landscape types in the occurrence of dust storms, as well as for providing a theoretical basis for prevention of dust storms.

Root characteristics of Alhagi sparsifolia seedlings in response to water supplement in an arid region, northwestern China

The effect of variation in water supply on woody seedling growth in arid environments remain poorly known. The subshrub Alhagi sparsifolia Shap. (Leguminosae), distributed in the southern fringe of the Taklimakan Desert, Xinjiang, northwestern China, has evolved deep roots and is exclusively dependent on groundwater, and performs a crucial role for the local ecological safety. In the Cele oasis, we studied the responses of A. sparsifolia seedling roots to water supplement at 10 and 14 weeks under three irrigation treatments (none water supply of 0 m3/m2 (NW), middle water supply of 0.1 m3/m2 (MW), and high water supply of 0.2 m3/m2 (HW)). The results showed that the variations of soil water content (SWC) significantly influenced the root growth of A. sparsifolia seedlings. The leaf area, basal diameter and crown diameter were significantly higher in the HW treatment than in the other treatments. The biomass, root surface area (RSA), root depth and relative growth rate (RGR) of A. sparsifolia roots were all significantly higher in the NW treatment than in the HW and MW treatments at 10 weeks. However, these root parameters were significantly lower in the NW treatment than in the other treatments at 14 weeks. When SWC continued to decline as the experiment went on (until less than 8% gravimetric SWC), the seedlings still showed drought tolerance through morphological and physiological responses, but root growth suffered serious water stress compared to better water supply treatments. According to our study, keeping a minimum gravimetric SWC of 8% might be important for the growth and establishment of A. sparsifolia during the early growth stage. These results will not only enrich our knowledge of the responses of woody seedlings to various water availabilities, but also provide a new insight to successfully establish and manage A. sparsifolia in arid environments, further supporting the sustainable development of oases.

Responses of root growth of Alhagi sparsifolia Shap. (Fabaceae) to different simulated groundwater depths in the southern fringe of the Taklimakan Desert, China

Alhagi sparsifolia Shap. (Fabaceae) is a spiny, perennial herb. The species grows in the salinized, arid regions in North China. This study investigated the response characteristics of the root growth and the distribution of one-year-old A. sparsifolia seedlings to different groundwater depths in controlled plots. The ecological adaptability of the root systems of A. sparsifolia seedlings was examined using the artificial digging method. Results showed that: (1) A. sparsifolia seedlings adapted to an increase in groundwater depth mainly through increasing the penetration depth and growth rate of vertical roots. The vertical roots grew rapidly when soil moisture content reached 3%–9%, but slowly when soil moisture content was 13%–20%. The vertical roots stopped growing when soil moisture content reached 30% (the critical soil moisture point). (2) The morphological plasticity of roots is an important strategy used by A. sparsifolia seedlings to obtain water and adapt to dry soil conditions. When the groundwater table was shallow, horizontal roots quickly expanded and tillering increased in order to compete for light resources, whereas when the groundwater table was deeper, vertical roots developed quickly to exploit space in the deeper soil layers. (3) The decrease in groundwater depth was probably responsible for the root distribution in the shallow soil layers. Root biomass and surface area both decreased with soil depth. One strategy of A. sparsifolia seedlings in dealing with the increase in groundwater depth is to increase root biomass in the deep soil layers. The relationship between the root growth/distribution of A. sparsifolia and the depth of groundwater table can be used as guidance for harvesting A. sparsifolia biomass and managing water resources for forage grasses. It is also of ecological significance as it reveals how desert plants adapt to arid environments.

Effects of cutting and burning on regeneration of Alhagi sparsifolia Shap. on the southern fringe of the Taklamakan Desert, North-west China

Indigenous vegetation such as Alhagi sparsifolia Shap. (Fabaceae) has been severely damaged in recent years because of the growing population and increasing land use on the southern margin of the Taklamakan Desert. Alhagi sparsifolia plays an important role in supporting the fragile ecosystem in the oasis foreland as it has multiple ecological and economic functions. Cele Oasis, located on the southern rim of the Taklamakan Desert, was used to investigate the impact of human disturbance on regeneration of A. sparsifolia in the oasis–desert ecotone. Observations of A. sparsifolia in response to cutting and burning were conducted in 2010 and 2011. The results showed that burning in spring significantly decreased height and biomass in comparison to cutting in the fall. Moreover, biomass was decreased by spring burning more than by spring cutting. Burning in spring is no advantage for the growth and survival of A. sparsifolia. Cutting in fall appears to be a useful treatment for increasing the production from and survival of A. sparsifolia that could facilitate the sustainable development of the Cele Oasis.

Water but not photosynthates integration exists between mother and daughter ramets of a root-derived clonal shrub

Clonal integration in rhizomatous and stoloniferous clonal plants has been studied for many years, but the mechanisms of clonal integration of root-derived plants remain largely unknown. Alhagi sparsifolia is a typical root-derived clonal plant in the extremely dry land of the Taklamakan desert, and it usually cannot reproduce sexually in natural and non-irrigated environments; clonal reproduction is the primary way for this plant to maintain and extend its populations. After determining that A. sparsifolia clonally integrates its ramets, we tested whether that capability could enhance the species’ survival in dry conditions, using both a spacer-severed group and a control spacer-connected group, by applying both supplementary water and 13C labeling to mother ramets and old daughter ramets. Our results showed that the midday water potential of both mother and daughter ramets increased significantly after spacer severance. The water potential of the mothers significantly increased after water supplementation, under both the spacer-severed and spacer-connected treatments. The water potential of daughter ramets in the spacer-connected treatment group also increased significantly, but that of the spacer-severed treatment group did not. Leaf relative water content and increases in height, crown width, number of branches, and basal diameter of the daughter ramets in the spacer-connected group were significantly higher than those in the spacer-severed group. δ13C values of mother (and of old daughter) ramets significantly increased after 13C labeling, but δ13C values of neither mother, old daughter nor young daughter ramets changed. We concluded that clonal integration of water takes place between mother and daughter ramets of A. sparsifolia and that the mother can transfer water to daughter ramets through the spacers, but that no clonal integration of photosynthates occurs among the ramets of A. sparsifolia. These results have important implications for vegetation restoration and the utilization of water resources in this area.

Ecological characteristics of Alhagi sparsifolia Shap. seedling roots under different irrigation treatments

An trench profile method was used to study seasonal variation of root ecological characteristics of Alhagi sparsifolia Shap. seedlings under different irrigation treatments. The results indicated the following: (1) Root morphology: under excellent soil moisture conditions, A. sparsifolia seedlings developed many horizontal roots and root sprouts to compete for light; but under poor soil moisture, the vertical root system expanded its resource space into deeper soil. Plasticity of root morphology is an important strategy to capture water and adapt to the hyperarid environment. (2) Root/shoot ratio: root/shoot ratio increased with declining soil moisture, and this trend was more obvious later in the growing season. Increase of root/shoot ratio is a strategy for adapting to drought. (3) Growth of root system: The seedlings prefer to develop roots in shallower surface layers with less water availability. The growth depth and vertical growth rate of roots increased with decreased soil moisture. (4) Accumulation of root biomass: biomass and surface area of the root system decreased with increased soil depth, with the roots distributed in an ‘inverted pyramid’ in vertical section view. The formation of root biomass conformed to a logistic ‘slow-quick-slow’ growth curve, and total biomass decreased with intensity of soil drought. The findings will provide data useful for effective restoration of A. sparsifolia and better utilization of water resources in hyperarid regions.