Ecology, Spatial Distribution, and Utilization of the Tugai Vegetation at the Middle Reaches of the Tarim, River Xinjiang, China (Tienda española)

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The Tugai vegetation, composed of Tugai forests, Tugai shrubs, and reeds, is the typical floodplain vegetation of the Central Asian deserts. The largest expansions of Tugai vegetation used to cover the floodplains of the Amu Darya, the Syr Darya, and the Tarim River. However, due to extensive land reclamation projects in the Soviet Union and China beginning in the 1950ies, most of the Tugai forests and reeds were cleared to make room for irrigation farmland. With increasing water demands for irrigation, the surface water runoff declined, the river discharges dwindled, and the groundwater levels dropped, so that large areas of Tugai forests, shrubs, and reeds became subjected to severe water shortages and drought. The largest and most natural Tugai forests stands are located at the middle reaches of the Tarim River, which was also the reason for establishing the Tarim Huyanglin Nature Reserve there in 1983. This is where the Tarim River has formed an inland delta with a rich mosaic of Tugai forest stands. During the time of this study in 2004, dikes were built all along the Tarim’s middle reaches except for the northern riverbank between Yengi Bazar and Iminqäk. Recently built weirs now regulate the influx of water into the river branches of this inland delta.
The key-stone species along the Tarim’s middle reaches are Populus euphratica for Tugai forests, Tamarix species for Tugai shrubs, and Phragmites australis for reeds. All these species are able to endure the arid climate as phreatophytic plants, i.e. plants which, instead of saving water, continuously tap the groundwater. The generative reproduction of these species takes place on moist riverbanks devoid of vegetation. The natural conditions of such germination sites are created during the annual floods which reshape the river course and moisten the riverbanks. Once rooted and established, Populus euphratica and Tamarix ramosissima can grow on sites with groundwater levels deeper than 10 m.
The nature reserve and the adjacent areas which are planned to become protected as well were selected as the study area for this thesis. The objective was to investigate the Tugai vegetation in its most natural state. It was hypothesized that groundwater depth and salt contents in the groundwater are the limiting site factors for individual plants of the Tugai vegetation once they have become established. It was also assumed that the typical desert river dynamics plays a crucial role in the reproduction and succession of the Tugai vegetation.
Therefore, groundwater depths and salt contents in the groundwater was measured on sample sites representing the whole range of the plant communities of the Tugai vegetation throughout the study area. On each site, soil profiles were drilled down to the groundwater layer, and the depth of closed capillary fringe was recorded as a proxy for the groundwater level. The salt content was measured in terms of electric conductivity in an extract of saturated soil taken from the closed capillary fringe. Accordingly, the ecological role of the river dynamics was studied along a model transect (Transect Ia). The plant communities were defined floristically. In addition, a land cover map was derived from satellite image (Landsat ETM+) classification. Vegetation changes since 1973 were traced through the Normalized Vegetation Index (NDVI) calculated from Landsat MSS, TM, and ETM+ images. Finally, the land use practices in the study area were recorded on the basis of interviews and were evaluated with regard to their impacts on the Tugai vegetation.
Populus euphratica and Tamarix ramosissima, i.e. the two main woody species of the Tugai vegetation, were found on comparable sites where the closed capillary fringe was as deep as 10.5 m. But, while the maximum electric conductivities in the closed capillary fringe under Tamarix ramosissima was 25.5 mS/cm, under Populus euphratica it only was 8.72 mS/cm. Thus, the salt concentration of the groundwater is the decisive factor, if Populus euphratica can remain on a site or not.
The study results show how the river dynamics and flood events play a central role in the reproduction of Populus euphratica and probably many other species of the Tugai vegetation as well. It is also proposed here that the periodical shifts in the river courses and subsequently declining groundwater levels actually constitute the driving forces of the vegetation succession. The succession starts with the species which have become established on a certain site. Yet as the river moves away, the groundwater levels decline, and the salt contents accumulate in the groundwater, increasingly more species disappear throughout the succession. During this succession no new plants species are able to enter the site. Only after a river has relocated and provided sufficient floodwater, are new plants able to germinate and possibly establish themselves firmly.
Tugai plant communities which are rich in species, usually with a total coverage of 50% or more, are restricted to sites with a depth to the closed capillary fringe of no more than 3.5 m. These sites all are closer than 1 km away from any surface water including periodically flooded areas. The land cover map revealed that dense Tugai vegetation with a total vegetation coverage of 50% or more, is confined to narrow zones no more than 1-2 km in width along river courses.
The changes in the NDVI of satellite images throughout the years reflect the effects of increased grazing activity in the eastern part of the nature reserve and land reclamation mainly in its core areas introduced sometime after 1992. But the image analysis also revealed an increase of vegetation coverage or vitality in the Tugai forests in the western part of the reserve between the settlements of Yengi Bazar and Iminqäk, probably due to increased moistening of soils along the river course.
The main land use practices are livestock prevailing throughout the study area and irrigated cotton on sites close to the Tarim River. Both land use types are accompanied by considerable impacts on the Tugai vegetation. The farmland has usually been reclaimed in the areas of Tugai forests and other Tugai vegetation. Over time, irrigation also leads to salinization and a degradation of soils. In addition, large areas of the nature reserve are subjected to grazing pressures that are not reconcilable with the conservation aims of protecting the Tugai vegetation. And yet, there are still some patches of non grazed areas to be found around Iminqäk, which fortunately could be used as a reference for studying undisturbed Tugai vegetation in its most natural state.
Finally, the stretch between Yengi Bazar and Iminqäk along the Tarim’s middle reaches could be identified as the most important area for the conservation of the Tugai vegetation. This is probably the only area where the river has retained its original hydrological dynamics, which in turn is a natural precondition for the germination and succession of the Tugai vegetation. Consequently, livestock herding should be strictly limited in this area in order to ensure the long-term reproduction of the various plant species. Sufficient water should also be allowed to enter the river branches in order to sustain the existing Tugai vegetation there. During strong floods, moreover, larger amounts of water should be flushed into the river branches to maintain at least some of the river dynamics and to conserve the characteristic sites for the generative reproduction of the Tugai vegetation. Last but not least, the farmland within the reserve should be converted into reed, which can also be used as regenerative fuel or as a raw material for paper production.