2003

2010

2017

面积

占比

面积

占比

面积

占比

I级

1.65

7.14%

1.52

6.60%

2.19

9.29%

II级

1.47

7.37%

1.24

5.40%

3.01

12.73%

III级

5.43

23.56%

5.30

23.03%

6.48

28.11%

IV级

7.14

30.97%

7.01

30.44%

5.81

25.18%

V级

7.37

31.96%

7.95

34.55%

5.57

23.93%