Influence of sediment, soil, and micro-relief conditions on vitality of Populus euphratica stands in the lower Tarim Riparian Ecosystem

Excessive withdrawal of water for farmland irrigation, and water losses from water reservoirs and channels, have caused water deficit problems along the Tarim River in Northwestern China. Especially, the lower reaches have been severely threatened since the 1950s, and are experiencing a drastic increase in the mineralisation of the river water. Additionally, large scale deposition of aeolian sand and dust is another major threat for the Tarim Riparian Ecosystem (TaRiEc). This development reached its “climax” with the desiccation of the Taitema Lake in 1972, and the “climax” was well expressed by severe degradation of the TaRiEc, which is in turn predominately characterized by the dying of Populus euphratica stands. All these developments led to the initiation of the “Ecological Water Diversion Project” (EWDP). The main objective of this project is to ensure a constant water supply to the lower reaches of the Tarim River via the water transfer both from the Bosten Lake catchment and from Tarim's upper and middle reaches. In this study, the processes and trends of the vegetation change during this period of intermittent water transfer were examined on the basis of remote sensing data. The results show that with the water transfer only some of the P. euphratica trees reached a more vital state.Common hypotheses about factors influencing the tree vitality under stressful conditions deal with the water level in the river, the groundwater level and the distance of the trees from the river. However, soil and sediment, as well as micro-relief conditions also play an important role and take influence on the vitality of the trees. The main objective of the present study was to analyse the features of soil and sediment concerning water passage and water storage processes in near surface and subsurface positions of the lower Tarim Riparian Ecosystem. For that purpose, 2010 and 2011 field and laboratory analyses of soil and sediment profiles were performed. Laboratory results comprise grain size measurements, pore size distribution, total organic content (TOC) and data from XRD-measurements on the soil's mineral composition for all studied sites within the lower Tarim study area and for selected ecosystem types as well. These parameters allow conclusions about water storage and water passage conditions, which play a key role for water availability and water supply of P. euphratica trees. Besides single soil and sediment parameters, such as salt content and mineral composition, layer types (vertical sequence of soil horizons and sediment layers) have been identified as leading factors determining near surface and subsurface water passage and water storage features of the Tarim Riparian Ecosystem.