Demographic Bottlenecks and Size-Dependent Competition Shape Populus euphratica Persistence

Although Populus euphratica forests are key components of arid riparian ecosystems, the demographic and competitive processes associated with their population persistence remain insufficiently understood. We investigated population structure, demographic dynamics and intraspecific competition in a natural P. euphratica population within the Tarim River Basin, northwestern China. Field data from 2690 individuals were analyzed using diameter-class structure, population dynamic indices and the Hegyi competition index. The population exhibited an inverse J-shaped diameter distribution: Class I contained 1601 individuals (59.5% of the total population), and Classes I–III together accounted for 83.9%, indicating active recruitment but limited transition into intermediate diameter classes. Competition intensity was negatively correlated with DBH, tree height and crown area (r = −0.42, −0.41 and −0.31, respectively; p < 0.001) but positively correlated with local neighbourhood density (r = 0.39; p < 0.001). The multiple regression model explained 27.1% of the variation in competition intensity, and the DBH power-law model provided the lowest residual standard error (RSE = 4.887). These findings suggest that demographic bottlenecks and size-dependent competition are closely associated with P. euphratica persistence and provide field-based evidence for competitive filtering in arid riparian tree populations.