0869-B1
Xiangning Jiang[1], Shasheng Wang, Xuemei Chen, Andrea Polle, Thomas Teichmann, Meng Chen, Yill-Sung Park and Bruce Pendrel
This paper outlines preliminary results from a survey of natural Euphrat poplar (Populus euphratica) forests in Luntai County, Xinjiang, P.R. China. The distribution and health status of P. euphratica is reported. An extremely stress-tolerant tree species, P. euphratica, is found in flood plains and arid areas with less than 100 mm annual precipitation. In Xinjiang, natural P. euphratica forests along the Tarim River play a key role in the prevention of land degradation and desertification. To understand the importance of this species, integrated studies covering a broad range of aspects from the cellular and molecular levels to whole-tree ecophysiology and landscape ecology are needed. With this information, strategies to protect natural P. euphratica forest resources and to prevent further desertification of semi-arid lands may be developed. In addition to the key role of this species as a natural shelterbelt against erosion and desertification, its superb stress tolerance makes it an interesting genetic resource for detecting novel traits required for stress tolerance in trees.
Worldwide, 45 million km2 of land are classified as arid or semi-arid and therefore vulnerable to desertification. China is one of the countries in the world most vulnerable to desertification. Geographically, China’s arid, semi-arid and dry sub-humid areas lie well inland. The long distance from oceans, the intersecting mountain ranges and, in particular, the uplift of the Qinghai-Tibet Plateau, block the movement of moisture-laden air from the oceans. Consequently, these areas have become the driest in China with a fragile eco-environment characterized by the lowest precipitation and the highest evaporation for that latitude in the world. About 80% of the land in this region has become to some extent desert. The desert-affected land now totals 1.7 million km2 or 17.6% of China’s territory and directly affects a population of about 400 million people. The situation could be would appear even more beak if assessed according to the standard and definition of desertification of the United Nations Convention to Combat Desertification. Against these criteria, the affected area would total to 2.6 million km2, accounting for 27.3% of the territory.
Despite a growing awareness of environmental issues, the consequences of forest loss cannot simply be reversed by improving silvicultural practices and land-use management or by eliminating the sources of forest destruction. This is because the fundamental growth conditions for plants in these areas have been changed and the favorable microclimates that had been created earlier by perennial woody plants, no longer exist. Prevailing conditions are characterized by extreme temperatures and drought. If afforestation in these areas is to be economically feasible, stress-resistant trees need to be selected. Tree species must be found that are specialists in surviving extreme temperatures, drought and salinity, and furthermore that can be grown commercially.
In China, poplars are widely used in large-scale plantations and agroforestry. The importance of Populus euphratica Oliv. as a stress-tolerant tree has long been recognized because this species is able to exist under extreme environmental conditions. Natural populations show varying degrees of tolerance to saline soils, periodic waterlogging, cold, and arid conditions. P. euphratica has been acclaimed for its superior survival and biomass production in the arid areas of China, Pakistan, Iraq, and Iran (Sharma et al. 1999). It is a valuable genetic resource for the biotechnological exploitation of tolerance characteristics in woody plants. To protect and use P. euphratica genetic resources, the ecological functioning now found in natural P. euphratica forests must be retained and environmental conditions in areas where large-scale forest dieback occurs must be improved. In 1994, the International Poplar Commission (IPC 1994) published the following recommendations concerning P. euphratica:
Enlarge fundamental knowledge of P. euphratica, particularly in the fields of its ecology, physiology, and genetics;
Conserve natural stands in areas where they are diminishing seriously;
Establish at least one Populetum euphraticum for ex situ conservation;
The goals of the present review are to increase our understanding of the occurrence and ecology of P. euphratica in China and to elucidate traits related to stress tolerance.
P. euphratica is a large deciduous tree known for its high tolerance to salt and atmospheric drought. Adult plants can grow normally on soils containing up to 2% salt and can survive up to 6% salt at annual rainfall and evapotranspiration rates of about 30 mm and 3,500 mm, respectively. It is generally propagated by seed; however, conventional rooting of cuttings can be used.
P. euphratica is naturally distributed over a wide longitudinal stretch from China to Spain and from Kenya in the south to Kazakhstan in the north. In its range along sections of the Euphrates River in Western Asia, it is the most common tree, forming dense stands along riverbanks that are likely clones formed from runners of a parent tree. The leaves are polymorphic, having leaves with strikingly different shapes on the same tree or even the same branch. The reasons for this foliar polymorphism are not well known. Genetic comparison of P. euphratica with other Populus and Salix species revealed that it was intermediate between the two families (Tuscan, pers. Comm.). Lanceolate-like leaves, which resemble those of Salix, are typically found on juvenile plants up to the age of 6 to 8 years, however, re-sprouts on adult trees may also display lanceolate leaves. It has further been speculated that stress may result in the formation of new leaves with changed morphology.
P. euphratica is found in the Xinjiang Autonomous Region, located in the western part of People’s Republic of China. The Xinjiang Autonomous Region encompasses the second largest desert in the world after the Sahara Desert in Africa, far from the sea and surrounded by mountains in the hinterland of Eurasia (Lu and Lu 2000). Xinjiang covers an area of 1.7 million km2 with 49.5% mountains, 28% plains, and 22.5% deserts. The Tarim Basin, located in the temperate desert region of southern Xinjiang, has annual average temperatures of 9.0 to 11.4°C with annual rainfall of 20-80 mm with 2,300-2,900 mm in annual evapotranspiration. In northern Xinjiang, the Zhuenherl Basin situated in the cold temperate desert region has an average temperature of 3.6-5.7°C with annual rainfall of 100-200 mm and annual evaporation of 1,000-2,000 mm. The ecosystems in Xinjiang have been experiencing severe environmental disturbances from sandstorms, desertification, salinization, and degradation of natural forests as a consequence of having large areas of desert, high radiation, limited water resources, and pressure from an increasing population.
In Luntai County at the edge of the Taklamakan Desert along the Tarim River (Xinjiang Province, P.R.China), half a million hectares (8,000,000 Mu) of original P. euphratica forests exist, which protect the southern part of Xinjiang Province from further desertification. Since the beginning of 1990s, Luntai County has become a “hot spot” of industrial development in northwestern China due to exploitation of its oil reserves. Land-use and land-cover changes are taking place; the riverbed has been changed, farmland is expanding, and forests are disappearing. Remote sensing has shown major land-cover changes in this ecosystem, such as degradation of natural poplar forests around the Tarim River due to water overuse for agriculture and a consequent decline of underground water levels. From the 1950s to early 1980s, the extent of forest lost amounted to 3,000 km2. Soil salinization was also increased by thousands of square kilometers from 1964 to 1994 as a result of secondary salinization caused by a rise in the local water table after the overuse of water for irrigation; salinization has, however, stabilized from 1994 to 2000. The agricultural land surface during the whole period has been stable, since the largest scale land reclamation was done in 1950s.
The Taklamakan Desert, which lies in a vast intermountain basin in the south of Xinjiang, at 337,600 km2, is the second largest sand desert with moving dunes in the world, occupying 82.2% of its area. It is well known for its harsh natural conditions: high temperatures, high evaporation, and minimal precipitation. The Tarim River runs along the northern border of the desert and is the longest inland river in China, extending for 2,200 km in Xinjiang Uygur Autonomous Region. It is fed by streams coming from the melting snow of Tianshan Mountains and disappears in the desert. The whole drainage area of the Tarim covers 198,000 km2 and is densely forested with P. euphratica on its banks, creating a huge oasis in the desert. Its tributaries supply water to the farmland, cotton, and watermelon fields and to poplar forests. Since the 1960s, human activities, such as unrestrained cultivation and forest harvesting on the upper river reaches have caused large-scale drying along the lower reaches. As a result, the river is now 300 km shorter than it was three decades earlier and the eco-environment in the valley has been rapidly deteriorating. In the past few years, the nearby Kuruk Desert has moved 60 km toward the valley and, in some parts, is separated by just 2 km from the Taklamakan Desert. Environmental experts warn that the Tarim River would have disappeared completely in a short time had the desertification tendency continued.
Tissue culture and micropropagation systems are required to maintain the P. euphratica genetic resource and for rapid production of suitable clonal material. Research into organogenesis and plantlet regeneration of in vitro-grown P. euphratica has been carried out in various laboratories in China, Israel, Germany, Spain, and elsewhere. Tissue culture uses shoots from mature trees, leaves from juvenile plants, or roots as initial explants. With manipulation of plant growth regulators in tissue culture media, high plant regeneration rates were achieved, depending on the explant types, ranging from 83 to 100%. Somatic cell cloning techniques for P. euphratica were also established in vitro at the Beijing Forestry University, where problems of vitrifiation and deterioration of the sub-cultured shoots were overcome.
As P. euphratica displays remarkable stress tolerance, several laboratories in Europe and in China have begun research to understand its mechanisms of stress tolerance. Although P. euphratica occurs in arid conditions, it is currently not known whether drought tolerance is due principally to morpohological adaptations, such as the formation of deep rooting systems with access to ground water, or a supply of water through clones connected by root suckers, or whether biochemical mechanisms also contribute to this feature. In addition, its salt tolerance is well documented (Chen et al. 2001). At the biochemical level, salt tolerance is thought to depend on three mechanisms: exclusion of excess sodium, ability to cope with elevated solute concentrations, and ability to protect tissues from increased oxidative stress through enhanced antioxidant capacity (Zhu et al. 2001). At the molecular level, global stress responses can be studied using molecular analysis, such as EST-sequencing and expression profiling on macro or micro arrays. Such a project is current underway (http://www.gwdg.de/~establis).
Work with the model plant Arabidopsis has shown that mechanisms to prevent the accumulation of toxic sodium concentrations are probably most important to ensure plant vitality. In order to understand the role played by proteins in stress tolerance, tests were carried out with cell cultures of P. euphratica using a liquid culture medium with increased concentrations of osmotic agents (NaCl and PEG-6000) to simulate the cell response to salt and drought stress. Compared with controls, the treatments with 0.3% NaCl and PEG promoted cell proliferation and resulted in increase protein concentrations, but when the NaCl level was higher than 0.3%, the cell proliferation and protein content started to decrease. Cell proliferation was suppressed and the protein content in the cell was reduced by 30% at a 1.8% NaCl concentration. Protein analysis (SDS-PAGE) showed that two protein bands (28KD and 66KD) were highly expressed in the cell suffering higher salt stress, whereas only the 28 KD protein was highly expressed in drought-stressed cells. Physiological and molecular genetic studies of P. euphratica could lead to a greater understanding of stress-tolerance in woody plants that may be used in combatting desertification.
A field survey conducted in October 2002 shows that P. euphratica forests in Xinjiang province are endangered by changes in land-use practices. Continuous forests have been degraded to scattered smaller stands, which have increased the risk of further desertification. Techniques to combat the expansion of deserts have been developed in China, including dune fixation; sand stabilization with biological and mechanical measures along railways, highways and in mining facilities; use of surplus flood water during the rainy season to flatten sand dunes; and air-seeding of bushes and grasses to revegetate shifting sand areas. Still, the region and its inhabitants are not well protected from catastrophic events such as sandstorms. Natural forests do offer some protection and currently P. euphratica forms a natural barrier in front of Taklamakan Desert. To prevent further desertification, the international scientific community needs to pay more attention to the natural resources of P. euphratica. Multidisciplinary and multinational cooperation should be fostered with the goal of developing strategies to prevent further deterioration of P. euphratica forests and to disseminate these results.
Therefore, we urge the adoption and promotion of environmentally friendly land-use practices to protect and expand P. euphratica forests. In addition to its paramount role in one of the Earth’s most interesting and fragile ecosystems, P. euphratica is also recognized as an important genetic resource for detecting genes involved in mediating stress tolerance in woody plants. We call on the international scientific community, industries, and governments to pay more attention to this unique forest resource in desert regions, to support its study, conservation, exploration and utilization, not only for countries in this region and for our generation, but for countries around the world and future generations.
This paper is based partly on results from Drs. Ruisheng Gu, Qunlu Liu, and Huancheng Ma. The authors acknowledge that some information has been obtained from the Internet sources. The research was supported by grants from the NSF of P.R. China to S.S. Wang, X.N. Jiang, from the National Education Committee for Talented Young Scholar to X.N. Jiang, from the National Science, and Technology through the 9th Five-Year Project to Stress-Tolerant Plant Species Selection to X.N. Jiang, and from the EU under contract number QKL5-CT-2000-0349, the German Science Foundation and the BML to A. Polle.
Chen, S., J. Li, S, Wang, A. Hüttermann, and A. Alteman. 2001. Salt, nutrient uptake and ABA of Populus euphratica and a hybrid in response to increasing NaCl. Trees 15: 186-194
IPC, 1994. News from the International Poplar Commission. Forest Genetic Resources No 23.
Lu, Y., and J. Lu, 2000. Oasis Forestry in Xinjiang, IDRC: Library: Documents: Agroforestry Systems in China
Sharma, A., B.N. Dwivedi, B. Singh, and K. Kumar, 1999. Introduction of Populus euphratica in Indian semi-arid trans-Gangetic plains. Annals of Forestry 7(1):1-8
Zhu, J.K., 2001. Plant salt tolerance. Trends Plant Sci 6: 66-71
http://web.odu.edu/webroot/instr/sci/plant.nsf/pages/poplar Euphrates and White Poplar
http://www.geo.ucl.ac.be/Recherche/Teledetection/Projects/China_Tarim.html (Remote sensing of land-cover change: Investigations and design of a dynamic monitoring system in northwest China: Case study in the Tarim Basin, Xinjiang
Email: [email protected]
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[1] College of Life Sciences
and Biotechnology, Beijing Forestry University, Beijing 100083, P.R. China.
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