Abstract:Haloxylon ammodendron is a dominant shrub widely distributed in the desert in the Xinjiang Uygur Autonomous Region, northwest China. In the present study, we investigated the variation in the stable carbon isotope composition (δ13C) in an assimilating branch of H. ammodendron and the relationship of δ13C from H. ammodendron with environmental factors (altitude, duration of sunshine, evaporation, mean annual precipitation, and mean annual temperature) using a dataset collected from 101 communities and 23 ecosystem sites. Our results showed that: (1) δ13C in the assimilating branch of H. ammodendron averaged 14.15‰ with a range from -13.14 to -15.38‰ at a 95% confidence interval, which indicated that H. ammodendron was characterized by a C4 photosynthetic pathway. (2) Along an environmental gradient, δ13C values of H. ammodendron was negatively and significantly correlated with mean annual precipitation and mean annual temperature, but positively correlated with altitude, duration of sunshine, and evaporation. We inferred that the different response patterns of δ13C values to environmental factors were possibly caused by stomatal limitation in the assimilating branch of H. ammodendron, which was considered an adaptation to the dry desert conditions. (3) δ13C values of H. ammodendron varied significantly among different habitats. The highest δ13C values appeared when H. ammodendron was found with Nitraria tangutorum and Reaumuria Linn. in grey and grey-brown desert soils, and distributed in plain, mountainous and hill habitats, whereas the lowest δ13C values were observed when H. ammodendron was found with Ceratoides arborescens and Calligonum mongolicum located in aeolian sandy and chisley soils, and distributed in Junggar Basin. Together, our results demonstrated that water use efficiency of H. ammodendron varied significantly along environmental gradients and habitats, indicating a divergent adaptation strategy for water use efficiency.