(i) A forest management and harvest plan, as defined in section 573, subsection 3-A, has been prepared for the eligible timberlands and has been in effect for the entire time period used to compute the amount of the subtraction modification under this paragraph; and

(ii) The taxpayer has received a written statement from a licensed forester certifying that, as of the time of the sale, the eligible timberlands have been managed in accordance with the plan under subdivision (i) during that period.

(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;