Methods Monocultural and mixed R. soongarica and S. passerina communities were selected in Lingwu, Zhangye and Jiuquan, representing a drought stress gradient varying from desert steppe habitat to extreme desert habitat. Measurements were made on the vertical structures of root traits including specific root length (SRL) and specific root surface area (SRA). Root samples were collected by trench method by layers.

Important findings Both SRL and SRA had greater values in mixed R. soongarica-S. passerina communities than in monocultural communities of either species within given habitat, suggesting a mutually beneficial relationship between the two species and the enhanced ability of roots to absorb soil nutrients and water when in mixture. Reaumuria soongarica had deeper roots and a greater root extinction coefficient in vertical distribution than S. passerina in either monocultural or mixed communities, indicating the belowground niche segregation between the two species. We also found that greater the environmental stress, the higher values of SRL and SRA in both R. soongarica and S. passerina. The root extinction coefficient was about the same between the monocultural and mixed communities for R. soongarica, but changed a little for S. passerina. With increasing drought stress, the belowground niche segregation became more apparent, conforming to the stress gradient hypothesis. The observed patterns of aggregation aboveground and segregation belowground between R. soongarica and S. passerina in the mixed communities might be an adaptive strategy to drought environment of these two desert plant species.