利用大型环境生长箱研究了两种幼龄沙地优势灌木柠条（Caragana intermedia）和羊柴（Hedysarum mongolicum）对CO2浓度倍增和土壤干旱交互作用的响应。CO2浓度倍增并没有改善两种沙生灌木叶片的水分状况，而土壤干旱使叶片的相对含水量（RWC）显著降低。在土壤水分充足条件下，CO2浓度倍增促进两种沙生灌木植株生长，在干旱条件下则主要促进根的生长，提高根冠比。土壤干旱显著减少了植株生物量，但相对促进了根的生长，特别是显著提高了羊柴的根冠比。CO2倍增使稳定性碳同位素组分（δ13C）降低，但土壤干旱使之增加。两种沙生灌木叶片与根部的δ13C值呈极显著线性关系，羊柴的斜率大于柠条的，表明前者叶片与根部在光合产物分配上具有较高的生态可塑性，这和干旱条件下羊柴的根冠比增加相关联。羊柴的“源库”调节特性反映了对土壤水分胁迫具有较高的耐性。
Atmospheric CO2 concentrations are expected to double around the middle part of the 21st century. Plant growth might be favored by CO2 enrichment, but water limitation is a common stress for plant growth and productivity. At present, only a few studies have looked at the combined effects of CO2 enrichment and drought on plant ecophysiology. This experiment was conducted to investigate the responses of two dominant desert shrubs, Caragana intermedia and Hedysarum mongolicum, in western China to the interaction of doubled CO2 levels and soil drought in large environmental growth chambers (19 m2). In this paper, we employed different methods, including allometry and carbon isotope discrimination, to examine the effects of water availability on carbon allocation and stable carbon isotope composition (δ13C) of the two desert shrubs under two CO2 concentrations. The objectives included the following: 1) to investigate the effects of soil drought and CO2 enrichment on plant biomass and δ13C; 2) to investigate the effects of soil drought and CO2 enrichment on the allocation of dry matter and carbohydrates; and 3) to elucidate the adaptive strategies of C. intermedia and H. mongolicum to soil drought under doubled atmospheric CO2 concentrations. Compared to ambient CO2 concentrations, doubled CO2 concentrations did not improve the leaf water status, but soil drought significantly reduced the leaf relative water content (RWC). Doubled CO2 concentrations enhanced plant growth under well-watered conditions but increased root growth under drought conditions resulting in an increase in root to shoot ratios. Soil drought significantly reduced plant biomass and increased root to shoot ratios, especially for H. mongolicum. The δ13C values were reduced at doubled CO2 concentrations but increased under drought conditions. By plotting the leaf δ13C values against the root δ13C values, it was possible to assess carbon allocation and incorporation into roots in relation to present biomass. There was a significant and linear relationship between leaf δ13C and root δ13C values, and the slope of H. mongolicum was greater than that of C. intermedia indicated a higher plasticity in the ability to change carbon allocation patterns. This resulted in higher root to shoot ratios in H. mongolicum under drought conditions. The results indicated that both C. intermedia and H. mongolicum had a higher tolerance to severe water deficits under doubled CO2 conditions. Decreases in precipitation might accompany with future increases in atmospheric CO2 concentrations in the region dominated by these two species, suggesting that distribution ranges of C. intermedia and H. mongolicum might be constrained. Our results suggest that H. mongolicum has a higher tolerance to environmental stress than C. intermedia. Future work should emphasize how to enhance the drought tolerance of plants in semiarid region under conditions of CO2 enrichment.