(m) For eligible timberlands held by the taxpayer for at least a 22-year period beginning on or after January 1, 2005 but less than a 23-year period beginning on or after January 1, 2005, 13/15 of the gain recognized on the sale of the eligible timberlands;

(n) For eligible timberlands held by the taxpayer for at least a 23-year period beginning on or after January 1, 2005 but less than a 24-year period beginning on or after January 1, 2005, 14/15 of the gain recognized on the sale of the eligible timberlands; or

(o) For eligible timberlands held by the taxpayer for at least a 24-year period beginning on or after January 1, 2005, all of the gain recognized on the sale of the eligible timberlands.

(3) Taxpayers claiming this credit must attach a sworn statement from a forester licensed pursuant to Title 32, chapter 76 that the timberlands for which the credit is claimed have been managed sustainably. For the purposes of this subparagraph, "sustainably" means that the timberlands for which the credit is claimed have been managed to protect soil productivity and to maintain or improve stand productivity and timber quality; known occurrences of threatened or endangered species and rare or exemplary natural communities; significant wildlife habitat and essential wildlife habitat; and water quality, wetlands and riparian zones.

Upon request of the State Tax Assessor, the Director of the Bureau of Forestry within the Department of Conservation may provide assistance in determining whether timberlands for which the credit is claimed have been managed sustainably. When assistance is requested under this subparagraph, the director or the director's designee may enter and examine the timberlands for the purpose of determining whether the timberlands have been managed sustainably.