Linze Inland River Basin Research Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
Acta Ecologica Sinica,2008,28(4):1616～1624.Abstract： The inland Heihe River basin of arid region in northwestern China is composed by three great landscapes: the mountain, the oasis and the desert. The upriver mountain is a sensitive region to the environmental change, which suffers little human interference; in the midriver and downriver regions, the oases and deserts are ecological fragile regions, and their environmental transition is the focus that people pay attention to. Plant is an indicator of environmental change, and compared with the other organs, photosynthetic organ is more sensitive, as its characteristics can imply the influence of the environmental factor change or the adaptation to the environment.
Along the Heihe River basin, from the water source collection area to the water source vanishing area, leaves or assimilation shoots of plants were collected to determine the stable carbon isotope ratio. The results showed that stable carbon isotope ratio (δ13C) of the plants on the mountain regions were between -23‰ --29‰, with the mean value of -26.3‰; the δ13C of the plants in oases were between -26‰ --30‰, with the mean value of -27.2‰; while the δ13C of the plants in deserts were among -23‰ --28‰ and -12‰ --15‰, with the mean values of -26.0‰ and -138‰, respectively. The δ13C was higher in the harsher environments. For the same species in different habitats, it showed that the δ13C was higher when the habitat was harsher. For Populus euphratica, tree species of desert riparian forest, the δ13C of its willow leaves were lower than that of its poplar leaves. Either in the mountain region or in the desert, along the increasing altitude, stable carbon isotope discriminations (Δ) of some plants reduced, while some were not notable. The △of Picea crassifolia significantly linearly decreased with the increasing altitude.
It can be concluded that the C4 plants occupy certain proportion in the desert vegetations. Water use efficiency (WUE) of plants in harsh habitat was higher than in suitable habitat. For P. euphratica, WUE of its linear leaves was the lowest, and that of the orbicular leaves was the highest among the different leaf morphologies; WUE rose as the willow leaves in seedling period grew to poplar leaves. P. crassifolia may be used as an important indicator plant to the environmental change of the mountain area of the upriver region of Heihe River. Among the trees in the Qilian Mountain area, the WUE of the evergreen P. crassifolia is the highest, the high WUE may be the reason that P. crassifolia is the dominant species in the Qilian Mountain area. Exorbitant δ13C of the same species indicates the decline of the plant and the severe stress of the habitat. Plant adapting arid environment is along the direction helping to improve the water use efficiency.