Study Region: The Northern Tianshan Urban Agglomeration, an oasis-dominated corridor along the northern foothills of the Tianshan Mountains in arid north-west China, combines alpine catchments, irrigated plains, and rapidly expanding cities. Sharp gradients in climate, topography, and land use make it an ideal natural laboratory for examining how ecosystem services (ESS) respond to coupled natural-human pressures. Study Focus: We quantified the spatio-temporal dynamics (2003-2023) of five key ESS-water yield, soil retention, food supply, habitat quality, and carbon storage-using multi-source remote sensing and the InVEST model. An integrated ESS supply index (ESSD) captured trade-offs and synergies, while boosted regression trees (BRT) revealed nonlinear marginal effects and critical thresholds in 15 natural and anthropogenic drivers. New Hydrological Insights for the Region: Mean ESSD remained > 0.95, clustering in mountain-urban-oasis ecotones. Between 2003 and 2023, water yield and soil retention declined by 45.2 % and 54.5 %, and habitat quality by 6.1 %, whereas food supply rose by 120.5 % and carbon storage stayed stable (+0.64 %). Habitat quality and carbon storage were strongly synergistic (r = 0.88-0.90), while food supply showed a persistent trade-off with soil retention (r = -0.08 to -0.09). NDVI dominated system behaviour (75.31 %), with marginal effects plateauing above 0.88. Thresholds emerged for PET (similar to 400 mm), precipitation (250-500 mm), slopes > 20 degrees, elevations 2000-2500 m, GDP > 3467 yuan km(-)(2) and human-activity intensity (HAI) > 16; the last two triggered ESS decline. These thresholds guided delineation of three management zones and targeted spatial-optimisation measures, providing actionable guidance for sustainable planning of arid-region urban agglomerations.