Managing biodiversity for ecosystem stability amid environmental change is crucial, but local-scale knowledge often fails to align with regional management needs. Variation in species composition among communities (β-diversity) likely stabilizes ecosystem functions at larger spatial scales by promoting spatial asynchrony—where different communities respond asynchronously to environmental fluctuations. Both β-diversity and spatial asynchrony are scale-dependent, yet the role of spatial extent in shaping these relationships remains unclear. Using simulated landscapes from a highly replicated grassland biodiversity experiment, we demonstrate that when environmental heterogeneity is minimized, β-diversity promotes spatial asynchrony, enhancing stability at larger spatial scales. However, these relationships are scale-dependent, weakening with increasing spatial extent. Additionally, local-scale diversity remains influential even after accounting for its covariance with β-diversity. Our results, therefore, emphasize the importance of preserving β-diversity, particularly in small, abiotically homogeneous landscapes, and advocate for multi-scale conservation strategies that enhance ecosystem stability amid global environmental change.