Vegetation changes in a river oasis on the southern rim of the Taklamakan Desert in China between 1956 and 2000.

Theindigenousvegetation surroundingtheriveroasesonthesouthern rimofthe Taklamakan Deserthasdrasticallydiminished dueto overexploitation asasourceoffodder, timber and fuel for the human population. The change in the spatial extent of landscape formsandvegetationtypesaroundtheQiraoasiswasanalyzedbycomparingSPOTsatellite images from 1998 with aerial photographs from 1956. The analysis was supplemented by field surveys in 1999 and 2000. The study is part of a joint Chinese-European project with theaim ofassessingthecurrentstateoftheforeland vegetation,ofgatheringinformationon the regeneration potential and of suggestingprocedures for a sustainablemanagement. With 33 mm of annual precipitation, plants can only grow if they have access to groundwater, lakes or rivers. Most of the available water comes into the desert via rivers in the form of seasonal flooding events resulting from snow melt in the Kun Lun Moun- tains. This water is captured in canal systems and used for irrigation of arable fields. Among the eight herbaceous and woody vegetation types and the type of open sand without any plant life that were mapped in 2000 in the oasis foreland, only the latter, the oasis border between cultivated land and open Populus euphratica forests and Tamarix ramosissima-Phragmites australis riverbed vegetation could be clearly identified on the photographs from 1956. Thecomparison oftheimagesrevealed that theoasis increased in areabetween 1956and 2000. Shifting sand was successfully combated near to the oasis borders but increased in extentattheoutward borderoftheforelandvegetation.In contrastto expectations,thearea covered with Populus trees was smaller in 1956 than today due to some new forests in the north oftheoasisthathavegrownup since1977.Subfossilwoodand leafremnantsof Popu- lus euphratica that were found in many places in the foreland must have originated from forests destroyed before 1956. In the last 50 years, the main Qira River has shifted its bed significantly northward and developed a new furcation with a large new bed in 1986. The natural river dynamics are not only an important factorin forming the oasis' landscapebut also in providing theonly possible regeneration sites for all occurringplant species. The conclusion of the study is that the oasis landscape has changed considerably in the last50yearsdueto naturalfloodingsandto vegetationdegradation by humanoverexploita- tion. The trend towards decreasing width of the indigenous vegetation belt resulting from the advancing desert and the expansion of arable land is particularly alarming because a decreasein itsprotective function against shifting sand can be expected in the future.