Grasslands, covering over 40% of terrestrial land surfaces, play a critical role in regional water cycling through their greening processes. However, the decoupling mechanisms between grassland greening and water availability (WA) changes across the Northern Hemisphere, along with their future trajectories, remain poorly understood. Here, we integrated multi-source satellite observations with CMIP6 model ensembles to systematically assess the spatiotemporal evolution and trend divergence of leaf area index (LAI) and WA across Northern Hemisphere grasslands from 2000 to 2100. Our results showed that grassland LAI exhibited sustained growth during 2000–2020, with 55.28% of regions showing significant increasing trends. However, 73.67% of grassland regions experienced declining WA during the historical period, revealing widespread decoupling between grassland greening and water deficit. Future scenario projections indicated a reversal to increasing WA trends, with 57.51% of regions showing significant increases under SSP5–8.5. Furthermore, 61.87% of grasslands exhibited greening-driven drying (GDD) characteristics during the historical period, while greening-driven wetting (GDW) regions were projected to expand to 72.44% in the future. Analysis along aridity gradients revealed that humid zones contributed most prominently to LAI and WA changes. Mechanistic decomposition demonstrated that grassland WA changes shifted from precipitation-dominated control (53.60%) in the historical period toward a regime jointly governed by precipitation dominance and coupled precipitation–evapotranspiration drivers in the future. Concurrently, the dominant factor controlling grassland greening transitioned from vapor-pressure deficit (VPD) to temperature (TEM) control. Additionally, driving factors exhibited pronounced differentiation patterns along aridity gradients during the historical phase: arid zones were dominated by soil moisture (SM) and semi-arid zones displayed dual control by SM and VPD, while humid zones were governed by coupled TEM-VPD regulation. This study reveals the divergent trends between grassland greening and WA and unravels their driving mechanisms, offering important scientific evidence for formulating regionally differentiated ecological water resource management strategies.

Shrub encroachment represents a widespread shift in vegetation structure, yet its influence on the relationship between vegetation greening and ecosystem productivity across global aridity gradients remains poorly understood. Focusing on global grasslands as the study domain, we systematically examined shrub-encroached areas across diverse climatic zones spanning aridity gradients from arid to humid regions. Here, we integrated multi-source remote sensing datasets, including MODIS land cover, leaf area index (LAI), and gross primary productivity (GPP), with a global aridity index to systematically detect shrub encroachment. By combining trend analysis, Pettitt change-point detection, and a moving-window pairwise comparison approach, we characterized the spatiotemporal dynamics of encroachment and quantified its differential effects on vegetation dynamics. Our results showed that the global extent of shrub encroachment expanded continuously from 2002 to 2022, with an abrupt change detected between 2010 and 2014. Relative to comparable non-encroached coverage pixels (NSEC), comparable shrub-encroached coverage pixels (CSEC) exhibited generally increasing trends in LAI, fractional vegetation cover (FVC), and GPP, with enhancement magnitudes showing strong aridity gradient dependence, peaking in semi-arid and dry sub-humid regions, indicating that shrub encroachment is substantially regulated by water limitation. Furthermore, joint trend analysis revealed that the concurrent increase in both LAI and GPP (LAI+GPP+) was the dominant pattern, observed in approximately 70% of encroached areas, although decoupling persisted at high latitudes and in certain humid regions (LAI+GPP−). These findings demonstrate that shrub encroachment is fundamentally regulated by moisture gradients and exhibits pronounced spatial heterogeneity, providing new evidence for understanding carbon cycling dynamics and informing grassland ecosystem management under increasing aridity.