Wasserhaushalt und Wassernutzungseffizienz von vier perennierenden Pflanzenarten im Vorland einer zentralasiatischen Flussoase

Abstract

At the southern fringe of the Taklamakan desert, the annual precipitation is only 33 mm, and the potential evaporation 2600 mm. The oases are supplied with water through rivers, which run from the Kunlun Mountains and carry high water during snow melt in summer. A belt of natural vegetation marks the transition zone between oasis and desert, and serves as a shelter against sand drift and as a source of fodder, firewood and construction material. Taking the oasis Qira as an example, the four key species of the foreland vegetation: Alhagi sparsifolia Shap., Tamarix ramosissima Lebed., Populus euphratica Oliv. and the C4 species Calligonum caput-medusae Schrenk. were investigated on their water status and water use efficiency. The studies were conducted at fenced research sites that were dominated by one of these plant species each. The effects of experimental irrigation and of harvest of above-ground biomass (to mimic the use of biomass by the local farmers) on the water status of the species was explored. The results should contribute to create the ecological basis for the restoration and sustainable use of the foreland vegetation. The plants of the foreland vegetation exhibited a high water use (sap flow sum of the vegetation period of 1999: C. caput-medusae, 183 kg H2O*m-2; A. sparsifolia, 391 kg H2O*m-2; T. ramosissima, up to 441 kg H2O*m-2; for P. euphratica a value of 392 kg H2O*m-2 was calculated using the PENMAN-MONTEITH equation). This high demand of water can only be met by ground water use, as the soil water content is negligible except for flooding periods, and as most of the foreland area is not reached by natural summer floods. The water potentials did not indicate severe drought stress, and neither transpiration nor sap flow were impaired during midday. An experimental irrigation did not have any positive effect on the water status of the investigated species. On the non-irrigated plots, water use was lower than on their irrigated counterparts. Compared to the irrigated plots, productivity was higher on the non-irrigated plots of A. sparsifolia and C. caput-medusae, but lower on those of T. ramosissima and P. euphratica. These discrepancies were not due to irrigation but to general differences between the research plots that affected water use and productivity: in C. caput-medusae and T. ramosissima, these differences were due to a longer distance to ground water on the non-irrigated plots; in P. euphratica and maybe also in A. sparsifolia, both being species with a pronounced clonal growth, genetic differences between plant clusters (genets) on the respective plots may have been effective. The harvest of above-ground biomass had a negative effect on productivity of A. sparsifolia in the following year, but a positive effect on productivity of the remaining shoots of the other species. Water use efficiency of production (WUEp) was calculated from productivity and annual water use measured by sap flow. Compared to plants in other arid regions, the investigated species exhibited a high productivity. The C4 species C. caput-medusae showed the highest WUEp (1,16 g DM*kg H2O-1), whereas WUEp of A. sparsifolia was lowest (0,60 g DM*kg H2O-1). T. ramosissima attained 0,82 g DM*kg H2O-1. These WUEp values are rather low in comparison to other species. Irrespective of irrigation, the carbon isotope ratio (δ13C) showed a decline through the vegetation period that is due to metabolic processes. On the basis of these results, neither a restricted water availability throughout the year nor a positive effect of irrigation on the water status of the plants could be assumed. Apparently, a natural regeneration of the foreland vegetation takes place only near riverbeds. Therefore, for the restoration of most of the foreland area, plantings and irrigation are necessary. Regeneration measures can only be successful if the plants are able to gain access to ground water. Being the species with the highest water use efficiency, C. caput-medusae is recommended for sand fixation. However, the needs of the local population have to be considered: A. sparsifolia is a valuable fodder plant, and T. ramosissima is important as a highly productive source of wood for fuel and construction material; therefore, these species should be promoted as well. P. euphratica as a protected species should be fostered as well; once the stands have recovered, they represent a valuable resource of wood and a shelterbelt around the oases that can contribute to the prevention of sand drift. A management plan for grazing and use of the foreland vegetation is a prerequisite for a sustainable development. An intact foreland vegetation in combination with planted shelterbelts offers the most effective shelter against sand drift.