Groundwater Depth Drives Carbon Pools and Population Dynamics in Deep-Rooted Desert Plants of a Hyper-Arid Ecosystem

Groundwater plays a crucial role in supporting vegetation in hyper-arid regions, yet its regulatory effects on deep-rooted plants remain poorly understood. This study examined Alhagi sparsifolia Shap., a groundwater-dependent desert shrub at the southern Tarim Basin, over 4 years (2021-2024). We investigated how groundwater depth (2.5, 4.5, 11.0 m) influences population dynamics, biomass, and non-structural carbohydrates (NSCs). Our findings reveal that groundwater depth critically regulates carbon allocation and plant performance. NSC content exhibited nonlinear responses, with soluble sugars and starch reduced at 4.5 m depth, but elevated at 11.0 m. While total carbon content remained stable, its seasonal and interannual fluctuations were significant. Population density decreased, while individual biomass increased with groundwater depth, indicating a density-growth trade-off. At deeper depths, individual carbon pools expanded, but overall population biomass and carbon pools declined, suggesting a recruitment threshold. Random forest and PLS-PM analyses identified groundwater depth and soil moisture as key drivers of population demographic and metabolic traits. These results underscore the pivotal role of carbon metabolism and recruitment in shaping the ecological strategies of desert plants under water scarcity.