(2) To the extent they are included in the taxpayer's federal adjusted gross income, the following amounts must be subtracted from federal adjusted gross income:

(a) For eligible timberlands held by the taxpayer for at least a 10-year period beginning on or after January 1, 2005 but less than an 11-year period beginning on or after January 1, 2005, 1/15 of the gain recognized on the sale of the eligible timberlands;

(b) For eligible timberlands held by the taxpayer for at least an 11-year period beginning on or after January 1, 2005 but less than a 12-year period beginning on or after January 1, 2005, 2/15 of the gain recognized on the sale of the eligible timberlands;

(c) For eligible timberlands held by the taxpayer for at least a 12-year period beginning on or after January 1, 2005 but less than a 13-year period beginning on or after January 1, 2005, 1/5 of the gain recognized on the sale of the eligible timberlands;

(d) For eligible timberlands held by the taxpayer for at least a 13-year period beginning on or after January 1, 2005 but less than a 14-year period beginning on or after January 1, 2005, 4/15 of the gain recognized on the sale of the eligible timberlands;

(e) For eligible timberlands held by the taxpayer for at least a 14-year period beginning on or after January 1, 2005 but less than a 15-year period beginning on or after January 1, 2005, 1/3 of the gain recognized on the sale of the eligible timberlands;

(f) For eligible timberlands held by the taxpayer for at least a 15-year period beginning on or after January 1, 2005 but less than a 16-year period beginning on or after January 1, 2005, 2/5 of the gain recognized on the sale of the eligible timberlands;