Multidecadal landscape evolution and natural recovery of mangroves beyond protected areas

A significant portion of global mangrove conservation relies on de facto conservation areas that sustain biodiversity despite lacking formal protection. Assessing their ecological integrity is essential for recognizing them as Other Effective area-based Conservation Measures under global biodiversity targets. Here we show that six decades of historical aerial photographs, satellite imagery, institutional maps, and 183 field inventory plots reveal mangrove forest expansion driven by natural regeneration that more than compensates for local losses. This results in a net area increase and the persistence of seven species communities. We identify a “Xylocarpus Paradox” in which remnant Xylocarpus granatum trees disproportionately enhance structural complexity and support locally rare species despite low abundance. Assessment against internationally recognized criteria for Other Effective area-based Conservation Measures demonstrates long-term ecological persistence, effective hybrid governance, and biodiversity conservation, indicating that de facto conservation landscapes can contribute to global biodiversity targets. Although focused on a Thai ecosystem, such areas are globally vital for long-term conservation.