Ximing Zhang

Clonal structure and genetic diversity of three desert phreatophytes

The objective of this paper was to assess clone sizes of three perennial desert plant species with AFLP markers and to relate them to clonal and genetic diversity and to hydroecology. The study was carried out at the southern rim of the Taklamakan Desert, where sexual regeneration is only possible shortly after rare flooding events, resulting in rarely established cohorts with subsequent extensive vertical growth and horizontal clonal spread. In this environment, repeated seedling establishment is excluded. We expected decreasing clonal and genetic diversity with increasing clone size and increasing distance to the groundwater table and a common response pattern among all study species. Maximum sizes of Populus euphratica and Alhagi sparsifolia clones were 121 ha and 6.1 ha, respectively, while Tamarix ramosissima clones reached a maximum size of only 38 m(2). In P. euphratica and A. sparsifolia, clonal diversity declined with increasing clone size and increasing distance to the groundwater table, while genetic diversity remained unaffected. Tamarix ramosissima differed from the other species because of a much smaller clonality. Clone size and clonal diversity were found to be good proxy variables for clone age. Despite the considerable age of the clones, genetic diversity is maintained in the populations.

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.

Stand structure and productivity of Populus euphratica along a gradient of groundwater distances at the Tarim River (NW China)

Aims Along central-Asian rivers in arid regions, lowering of the groundwater level constitutes a major threat to the riparian forests, whose tree layers are dominated by Euphrates poplar (Populus euphratica). Some of these fragile ecosystems are supplied with additional (‘ecological’) water for protection and conservation. We investigated interrelationships among groundwater distance, stand structure and above-ground wood production (at the tree and the stand level) in P. euphratica stands along a groundwater gradient (distances of 2.0–12.0 m) that also included a plot supplied with ‘ecological water’. Methods We measured the height, diameter at breast height and basal area of poplar trees and determined the annual above-ground wood production for the three most recent years on the basis of tree-ring analyses and allometric regressions at three sites at the upper, middle and lower reaches of the tarim River, Xinjiang, NW china. Important Findings the distribution of age classes differed significantly among the plots: in accordance with their life histories, stands growing at a larger distance to the groundwater harboured a larger fraction of old trees (>80 years) and, in most cases, a smaller fraction of young trees (≤20 years). the annual above-ground wood production per ha was highest on a plot with a short groundwater distance (2.0 m; maximum of the 3-year investigation period: 3.0 t ha−1 a−1) and lowest, at a plot with a large distance to the water table (6.6 m; minimum: 0.23 t ha−1 a−1). However, the plot located close to the groundwater (and to the river) at the middle reaches exhibited a relatively large interannual variation in above-ground wood production, which can be attributed to interannual variations in the river discharge. At the middle and the lower reaches, these interannual variations on plots with the most favourable water supply were even more obvious at the tree level. For the fraction of mature trees (60–99 years old), no significant differences in above-ground wood production were found between the plots with the most and the least favourable water supply. Overall, the productivity at the stand level was most closely correlated with the stand density (number of trees per ha, tree cover percentage). Productivity was negatively related to tree age, whereas groundwater distance alone was not a good predictor of above-ground wood production. In conclusion, vigorous growth of P. euphratica is possible at sites with groundwater distances of up to 12 m. Supply with ‘ecological water’ may be beneficial to trees growing at groundwater distances not larger than ~6 m. However, allocation of water should focus on stands with a short distance to the groundwater because only under these conditions, natural generative reproduction of the poplars is possible.

Influence of groundwater depth on species composition and community structure in the transition zone of Cele oasis

The paper analyzes the hypothesis that the distribution of dominant plant species and char- acteristics of plant communities are related to groundwater depth. The results showed that variations of groundwater depth impacted distributions and characteristics of dominant plant communities. However, besides groundwater depth, the community composition and species diversity were also influenced by physiognomy of the habitat. Based on the similarity coefficient, the differences between dominant plant communities were significant at different groundwater depths. Compared with other results relating to de- sert vegetation and groundwater depth, variations of community distribution were similar at the large spatial scale. However, in this extremely arid region, there were significant differences in community type and community succession when compared with other arid regions, especially in relationship to deep ground- water depth. With groundwater depth from deep to shallow, communities transformed with the sequence of Alhagi communities, Tamarix spp. communities, Populus communities, Phragmites communities, and Sophora communities. At groundwater depth of less than 6.0 m, the community type and composition changed, and the species diversity increased. Among these dominant species, Tamarix exhibited the big- gest efficiency in resource utilization according to niche breadth, which means it possessed the best adaptability to environmental conditions at the oasis margins.

Contributions to a Sustainable Management of the Indigenous Vegetation in the Foreland of Cele Oasis — A Project Report from the Taklamakan Desert

The ecological situation of oases at the southern border of the Taklamakan desert is shortly described, and the importance of a vegetation from indigenous species at the transition from the oases to the desert is emphasized. This vegetation serves as a shelter against sand drift and as a source of livestock feed as well as of fuel and construction material. Its destruction through overexploitation and other interventions during the last decades has considerably promoted sand drift and the deterioration of arable land. Therefore, a management of this protective vegetation is to be developped that leads to a sufficient regeneration and that ensures both its preservation and its use. A research project that is carried through jointly by Chinese and European scientists shall yield an ecological basis for this sustainable management.

Ecophysiological adaptation of Calligonum roborovskii to decreasing soil water content along an altitudinal gradient in the Kunlun Mountains, central Asia.

To understand the ecophysiological adaptation mechanisms of Calligonum roborovskii to altitude variation, this study analyzed chlorophyll a (Chl a), chlorophyll b (Chl b), Chl (a + b), carotenoid (Car), malondialdehyde (MDA), ascorbate (AsA), proline (Pro), membrane permeability (MP), reactive oxygen species (ROS), specific leaf area (SLA), leaf mass per area (LMA), leaf nitrogen content based on mass (Nmass), and the activities of peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX) in leaves of plants inhabiting different altitudes (A1: 2100 m, A2: 2350 m, A3: 2600 m) on the northern slope of the Kunlun Mountains. The results showed that Chl a, Chl b, Chl (a + b), SLA, Nmass, and the activity of CAT increased with increasing altitude. LMA, MP, MDA, Car, Pro, AsA, O2−, H2O2 and the activities of SOD, POD, and APX decreased with increasing altitude. The test results also showed that, changes in venvironmental factors along an altitudinal gradient are not obvious. Soil water content is the main ecological factor. With increasing altitude, soil water content increased significantly. More non-enzymatic and enzymatic antioxidants played an important role in eliminating intracellular ROS. They kept the cell membrane in a stable state and ensured the normal growth of C. roborovskii.

Clones or no clones: genetic structure of riparian Populus euphratica forests in Central Asia

Many riparian (Tugai) forests growing along rivers in arid and hyper-arid regions of Central Asia are dominated by the Euphrates poplar (Populus euphratica). Besides generative reproduction, which is only possible upon flooding events and at a distance to the groundwater of less than 2 m, this phreatophytic tree species also reproduces vegetatively by forming clones that can cover land surface areas of several hectares. Along a gradient of groundwater distances, we investigated whether the fraction of clones in P. euphratica stands (1) increases with increasing distance to the water table; (2) is higher if supplied with water via river cut-offs; and (3) approaches 100% at a short distance to the groundwater, but at high salt concentrations in the upper soil layers, which would prevent germination and establishment of seedlings. AFLP (Amplified Fragment Length Polymorphism) analyses were conducted on leaf samples taken from mature P. euphratica trees growing at the fringes of the Taklimakan Desert in stands with different distances (2–12 m) to the groundwater at two plots at the middle and the lower reaches of the Tarim River and in a stand close to Ebinur Lake, Xinjiang, China. Genetic diversity was large among plots, but considerably smaller within plots. We found the highest genetic diversity (caused by regeneration from seeds) at plots that have a short distance to the groundwater or are supplied with additional water. There was no significant relationship between groundwater distance and clonal fraction. All investigated trees at the saline Ebinur Lake site belonged to one single clone. Our results demonstrate that the genetic pattern of this widespread species is not easily predictable even over small distances as it is a result of a complex interplay of stand history and dispersal of propagules (pollen, seeds, and vegetative diaspores) by wind and water. In conservation and restoration schemes, P. euphratica stands with a high genetic diversity and stands that grow at short distances to the water table and are regularly subjected to flooding (which favors generative over clonal reproduction) should be prioritized.

Water relations of four perennial plant species at the southern periphery of the Taklimakan desert

The investigations started from the assumption that perennial plants in the foreland of river oases in the Taklimakan desert are able to take up water from soils of low water content and low water potential during long drought periods. In order to check this assumption, P-V curves were established and selected water relation parameters were derived. Alhagi sparsifolia, Calligonum caput-medusae, Tamarix ramosissima, and Populus euphratica, growing in the foreland of Qira oasis were investigated. Contrary to the expectation, the osmotic potentials both at plant water saturation and at incipient plasmolysis are high in comparison with other desert plants and reveal that the investigated plants do not need to take up water against a low soil water potential. The osmotic potential at incipient plasmolysis is moderately decreasing during the growing season in three of the species, but this seems to be an endogenous process that is not caused by drought stress. Small differences between osmotic potentials at incipient plasmolysis and the minimum plant water potentials that are reached during daily courses point to a transpiration control that accepts very low turgor potentials. A plasmolysis risk is possibly diminished by an increase of cell wall elasticity during the growing season. The results support the conclusion that all investigated species have permanent access to groundwater and live as phreatophytes in the foreland of Qira oasis.

Comparative study on the water characteristics of Populus euphratica with different depths of underground water in the foreland of Takelamakan Desert

The effect of depth to ground water on water relations of obligate phreatophyte Populus euphratica (Salicaceae) was investigated in the foreland of the hyper-arid Takelamakan Desert in Xinjiang, NW China. P. euphratica grow on sand dunes of different height (0--15.4m) above ground water that was the only water source for the species. The depth to ground water from the surface of the lowest sand dune was 6.8m under Populus. The water relation data illustrated the negative impact of sand dune height on water status of the species. Predawn and midday water potentials decreased significantly with increasing sand dune height. Water relation data suggest that decreases in hydraulic conductance with increasing distance to ground water have caused decreases in stomatal conductance due to a diminished water supply in plants on higher sand dunes. Results of this study indicate that whole-tree hydraulic conductance for Populus is heavily influenced by below ground hydraulic conductance. Whether the variations in hydraulic conductance are a function of increased path length or if they are the result of morphological changes in the conducting tissues remains further investigation. A detailed knowledge of hydraulic parameters in the root system is required to assess the effects of hydraulic limitations on plant water relations.

Water relation characteristics of Calligonum and consequences for a sustainable management of vegetation in the southern fringe of Takelamakan Desert

On field sites, water physiological characteristics of the desert legume Calligonum were investigated during the vegetation period in 1999(2000) in the southern fringe of the Takelamakan Desert, Xinjiang, NW China. The seasonal variation of predawn water potentials and of diurnal water potential indicated that Calligonum plants were well water supplied and it has stronger ability to maintain the water balance within the body over the entire vegetation period. Water relationship parameters from pressure-volume analysis confirmed that Calligonum plants were not drought stressed and xylem sap flow measurements further indicated that Calligonum plants were well water supplied during the summer months. The data indicate that artificial flooding only had limited effects on the water relations of plant. Long-term adjustment to the hyper-arid environment and its growth (survival) of Calligonum maybe is dependent on permanent contact with ground water. Therefore, it is important that variations of ground water depth are kept to a minimum. The study will provide theoretical basis for the restoration and management of natural vegetation in arid and semi-arid regions.