| The use of stable isotope techniques in plant ecological research has grown steadily during the past two decades. Carbon isotope composition (δ13C value) of plants are a useful index for assessing intrinsic water use efficiency and can even provide information on actual long term water-use efficiency (WUE), because δ13C value integrates photosynthetic activity throughout the period the leaf tissue was synthesized. Moreover, leaf δ13C values reflect the interplay among all aspects of plant carbon and water relations and are thereby useful as integrators of whole plant function. Water is the most important factor limiting plant growth and ecosystem productivity in semi-arid steppe in the Xilin River Basin. In this study, eight community types were selected along a soil moisture gradient, and δ13C values, leaf water content (LWC) and proline content etc. of major plant species were determined. Our objectives are to elucidate the physiologically adaptive strategies of plant species to different soil moisture habitats from plant species, plant functional group and community levels.1) The δ13C values and water-use efficiency of C3 plant species tended to increase as the occurrence frequency and relative biomass of the species in the entire basin increased; the mean foliage proline concentration of all species in a community was higher when the habitat was drier. In conclusion, two means might be employed by plants to adapt to arid habitats in Xilin River Basin. One is to enhance WUE through adjustment of stomatal conductance, the other is accumulating high level of proline to improve osmoregulative ability and keep a relative stable LWC.2) Plant species were classified into six plant functional groups (PFGs) based on their life forms, that is, trees, shrubs, sub-shrubs, perennial grasses, perennial forbs and annuals. In wetter habitats, forbs were more abundant and accounted for the majority of aboveground biomass, whereas grasses became more important in dryer habitats. Shrubs and sub-shrubs increased with decreasing soil water availability and their relative biomass rapidly increased in degraded steppe and sand dune; perennial grasses had significantly higher δ13C values than other PFGs; the δ13C values of perennial grasses and forbs increased with decreasing soil water availability, while those of shrubs/sub-shrubs showed an inverse trend.3) Based on their water ecological groups, plant species were grouped into six PFGs:xerophytes, mesoxerophytes, xeromesophytes, mesophytes, hygromesophytes and hygrophytes. In wetter habitats, hygromeso- and hygrophytes were more abundant and accounted for the majority of aboveground biomass, whereas xero- and mesoxerophytes became more important in dryer habitats; the mean 13C values of xerophytes was the highest, and those of meso-, hygromeso- and hygrophytes the lowest; From xerophytes to hygrophytes, their proline content markedly increased. Significantly positive relationships existed between proline and biomass or 13C values of different water ecological groups, respectively.4) There were large variations in the mean 13C values of different communities: typical steppe> degraded steppe > sand dune > restoring degraded steppe > meadow steppe > saline meadow > swamp meadow. The major determinants of the biomass-weighted 13C values in a given community were the presence or absence of C4 plants, the intrinsic variation among species, and the variation of the same species growing in different habitats with different soil moisture regimes and disturbance history.In addition, an experiment on the effects on N addition photosynthetic characteristics and WUE of Leymus chinensis and Stipa grandis were conducted. Results showed that distinctly opposite physiological strategies were employed to achieve high WUE and adapt to drought by two plant species, that is, higher net photosynthetic rate (A) and transpiration rate (E) for L. chinensis versus low A and E for S. grandis. At the same time, L. chinensis achieved higher WUE at the expense of decreasing nitrogen use efficiency (NUE),...
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