Fig. 5

Proportion of different OLDI levels by area. (a) Temporal variations in OLDI by grade level (2001–2023). (b)Proportion of different OLDI Levels Across Various Land Use Types in 2001. (c)Proportion of different OLDI levels across various land use types in 2010. (d)Proportion of different OLDI levels across various land use types in 2023.