Contrasting Population Dynamics of Haloxylon ammodendr on and Haloxylon persicum Along a Groundwater-Depth Gradient

Groundwater-table decline is an important environmental driver of desert vegetation degradation in arid regions, yet population structure and growth potential of co-dominant species along groundwater-depth gradients remain insufficiently compared. To examine differences in population dynamics between Haloxylon ammodendron and H. persicum, five sites with groundwater depths of 4, 7, 10, 13, and 16 m were selected along the southern margin of the Gurbantunggut Desert. Population stage and size-class structure, seedling survival, and reproductive characteristics were surveyed, and population dynamics were evaluated using life tables, population structural dynamic indices, demographic parameters, and Leslie matrix projections. Both species had abundant seedlings but relatively few non-current-year individuals, and seedling mortality approached 98%. The density of H. ammodendron individuals in stages III–IX was relatively similar across the 4–13 m sites but substantially lower at 16 m. In contrast, the density of H. persicum was lower across the 7–16 m sites than at 4 m. With increasing groundwater depth, the intrinsic rate of increase and finite rate of increase in H. ammodendron declined significantly, whereas its population doubling time increased significantly; none of the demographic parameters of H. persicum showed a significant relationship with groundwater depth. Under constant model parameters, Leslie matrix projections indicated that H. ammodendron populations generally declined at groundwater depths of 10–16 m, whereas H. persicum populations remained relatively stable or increased. The two species showed contrasting population patterns across the groundwater-depth gradient, but the spatial design does not establish groundwater decline as their sole cause, and differences in response strength between species require direct statistical testing. The projections describe conditional trajectories rather than observed temporal changes. The findings provide a basis for population maintenance of dominant desert shrubs and vegetation conservation under continued groundwater-table decline in the Gurbantunggut Desert.