Tree cover homogenization in semi-open ecosystems worldwide and implications for ecosystem stability and conservation

Semi-open ecosystems, such as savannas and open woodlands, are biodiversity hotspots due largely to their heterogeneous tree cover (TC), which supports diverse habitats. However, increasing woody encroachment is altering TC heterogeneity, with unclear consequences for ecosystem stability. Using global satellite-based TC estimates (2000–2020), we reveal widespread TC homogenization (24.1%) in semi-open ecosystems, predominantly in temperate and boreal bioclimates undergoing substantial warming. Contrary to the assumption that vegetation heterogeneity promotes ecosystem stability, we find that TC homogenization has mixed correlations with ecosystem functional stability. Notably, positive relations dominate in water-abundant areas with increasing TC, while negative correlations prevail in water-constrained regions with stable or decreasing TC. Protected areas exhibit lower homogenization and greater stability than surrounding landscapes, underscoring their conservation value. Our findings highlight the need for nuanced land management strategies that balance biodiversity conservation, carbon sequestration, and ecosystem stability under global reforestation and restoration initiatives.