Competitive Interactions Among Populus euphratica Seedlings Intensify Under Drought and Salt Stresses

Plant interactions and their responses to stress environments are important ecological processes for ecosystem stability and biodiversity formation, but how plant intraspecific relationships respond to environmental stresses remains to be studied in depth. In this study, annual <i>Populus euphratica</i> seedlings were planted in singles or doubles, and two stress treatments were set up: two drought levels (0.7 and 0.4 L) and two salinity levels (200 and 400 mmol L^-1). <i>P. euphratica</i> seedlings' total and part biomass, root/shoot ratio, net photosynthetic rate, stomatal conductance, nonstructural carbohydrate concentration, and proline content were measured. Relative interaction indices were calculated to clarify their intraspecific relationships. The results of the study showed that compared to the single-planted <i>P. euphratica</i>, the double-planted <i>P. euphratica</i> was more significantly inhibited by drought and salt stress, the total biomass decreased, photosynthesis declined, proline content increased, and non-structural carbohydrates changed, which reflected a competitive intraspecific relationship. Secondly, as drought and salt stress intensified, the relative interaction index indicated that the intraspecific relationship of <i>P. euphratica</i> seedlings gradually shifted from neutrality to competition, which indicated that the intraspecific competitive relationship of <i>P. euphratica</i> seedlings was exacerbated by environmental stresses. These findings highlight the need to account for stress-mediated competition in <i>P. euphratica</i> seedlings during ecological restoration in arid environments.