Abstract:

Environmental change of oases in the arid regions of China is a subject of recent concern. Although many of the changes have been recorded qualitatively through the use of comparative photography and historical reports, little quantitative information has been available on regional scale. Geographical information systems (GIS) and remote sensing (RS) technologies provide the basis for developing landscape composition and pattern indicators as sensitive measures of large-scale environmental change and thus may provide an effective and economical method for evaluating the condition of oases in relation to human and natural disturbances. This paper discusses the dynamic changes of the Qira oasis over a 40 year period by using 3 types of imagery data: aerial photographs (1956), TM (1990) and SPOT (1998). Remote sensing data were interpreted and classified using ARC/INFO and ERDAS software and landscape indices of satellite imagery were calculated using FRAGSTATS 3.0 software. The change in the spatial extent of landscape forms and vegetation types around Qira oasis was analyzed by comparing SPOT satellite image from 1998 with aerial photographs from 1956. The results showed that the area covered by vegetation of the Qira oasis increased by nearly 20% from 1956 to 1998, but the distribution of vegetation distribution changed greatly and the river bed shifted about 400 m. Dune and desert encroachment was successfully combated near the oasis border but increased in extent at the outward border of the surrounding vegetation. The area covered with Populus trees was smaller in 1956 than today due to some new forests established in 1977 north of the oasis. Subfossil wood and leaf remnants of Populus euphratica found in the surrounding areas must have originated from forests destroyed before 1956. 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 factor forming the oasis landscape pattern but also provides the only new regeneration sites for plant species.Quantitative analysis of the Qira oasis, the oasis-desert ecotone, desert and dunes from 1990 to 1998 showed that the area of the oasis increased by 6 km2 but patch decreased 256 blocks; the ecotone decreased in size by 2.5 km2 but patch increased 1 055 blocks; the area of desert decreased by 3.4 km2 but patch increased 608 blocks; and the area of dune did not change but patch increased 671 blocks. At the class level, Patch Density (PD), Edge Density (ED), Interspersion and Juxtaposition Index (IJI), Clumpiness Index (CLUMPY) and Aggregation Index (AI) were all calculated. At the landscape level, PD, ED, IJI, AI, Contagion Index (CONTAG), Diversity Index and Evenness Index were calculated. The indices of PD and ED increased, IJI, CONTAG and AI decreased. Diversity Index and Evenness Index changed slightly. The Qira oasis has changed considerably over the last 40 years due to natural flooding and vegetation degradation by human overexploitation. The trend of a decrease in the width of the ecotone (indigenous vegetation belt), which resulted from the advancing desert and expansion of arable land, is particularly alarming, because a decreased protective function against shifting sand can be expected in the future.