Effects of Drought Stress on Annual Herbaceous Plants under Different Mixed Growth Conditions in Desert Oasis Transition Zone of the Hexi Corridor

Annual herbaceous plants are frequently layered under the artificial sand-fixing forest within the desert oasis transition zone of the Hexi Corridor. The effect of drought stress on annual herbaceous plants is of great significance to the restoration of artificial vegetation as well as the stability of the ecosystem in the desert oasis transition zone. Setaria viridis , Chloris virgata , Halogeton arachnoideus , and Bassia dasyphylla are the typical annual herbaceous plants that occur naturally in the Caragana korshinskii forest and were used as the research subject in this study. Concentration gradient tests were conducted under different mixed growth conditions: 0 (blank control group), 5, 10, and 15 C. korshinskii seeds, and different drought stress conditions: 0%, 2%, 5%, 10%, and 15%, in order to explore the interactive effects of drought stress on annual herbaceous plants. The results demonstrated that the germination percentage and germination rate of annual herbaceous plants was significantly affected by the number of C. korshinskii seeds ( p < 0.05), whereby the germination effect was optimal when no C. korshinskii seeds were present. Furthermore, we found that the germination percentage and germination rate of the annual Gramineae was higher than that of the Chenopods. In the growth stage, the biomass and root-shoot ratio of the chenopods were significantly affected by the number of C. korshinskii seeds and drought stress ( p < 0.05). We found that the biomass of annual herbaceous plants was the highest at 2% drought stress, and the root-shoot ratio displayed a positive correlation with an increase in drought stress. Notably, the survival rate of annual herbaceous plants was higher when grown in combination with five C. korshinskii seeds, thus indicating a positive interaction; in contrast, the survival rate decreased significantly when they were grown in combination with more than five C. korshinskii seeds, indicating a negative interaction. We observed a decreasing trend in root activity and chlorophyll content when annual herbaceous plants were grown in combination with an increasing number of C. korshinskii seeds and drought stress. The reduced root activity and decline in photosynthetic ability resulted in the inhibition of seedling growth. Furthermore, we found that the root activity and chlorophyll content of the Gramineae was ~1.3–2.0 times higher than that of the Chenopods, which may be the reason behind the lower survival rate of the chenopods.