<p>Topography in mountain areas redistributes the incoming solar radiation and determines water availability, creating microclimates and forming distinct habitat conditions within short distances. How this local topography modulates the impacts of climate change on the growth of vegetation on mountains at large scales remains unclear. Here we quantify the difference in vegetation density between polar-facing and equatorial-facing slopes (defined as aspect asymmetry) across the Northern Hemisphere and find a weakening trend of aspect asymmetry from 2003 to 2024. For regions that show higher vegetation density on the polar-facing slopes, the magnitude, area and seasonal duration of aspect asymmetry all decreased, implying a reduction in water control on vegetation growth. Further analyses show that the observed changes in vegetation aspect asymmetry are attributable to hydrothermal conditions, and are dominated by solar radiation and temperature in particular. Our findings highlight changes in an underexplored but important part of mountain ecosystems, with implications for ecological stability under climate change.</p>