Variations in life-form composition and foliar carbon isotope discrimination among eight plant communities under different soil moisture conditions in the Xilin River Basin, Inner Mongolia, China

Water is one of the key limiting factors for the survival and growth of plant species in arid and semi-arid steppe regions. Different plant functional groups (PFGs) based on life-forms differ in their strategies to cope with limited water availability. The foliar carbon isotope discrimination (Delta) value provides an integrated measurement of internal plant physiological and external environmental properties affecting photosynthetic gas exchange over the time interval when the carbon was fixed. In this study, we surveyed the composition and Delta values of various life-forms (shrubs, sub-shrubs, perennial grasses, perennial forbs and annuals) in eight different plant communities along a soil moisture gradient in the Xilin River Basin, Inner Mongolia, China. Our results showed that: (1) life-forms occurred variously in eight steppe communities with different soil moisture status; (2) in wetter habitats, forbs were more abundant and accounted for the majority of aboveground biomass, whereas grasses became more important in drier habitats. Shrubs and sub-shrubs increased with decreasing soil water availability and their relative biomass rapidly increased in degraded steppe and sand dune communities. (3) The numerical order of the mean Delta values of life-forms is as follows: perennial grasses (15.86 parts per thousand) < shrubs (16.10 parts per thousand) < perennial forbs (16.45 parts per thousand) = annuals (16.41 parts per thousand) < sub-shrubs (17.55 parts per thousand), reflecting their differences in water use efficiencies. The significant differences in the Delta values among these life-forms suggested that life-form-based PFGs not only represent a morphological classification of these plants, but could also represent a functional group integrating different physiological processes such as water use strategies, which may partially explain the differences in PFG composition and competitive ability of co-existing species along environmental gradients in the Xilin River Basin.