(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.

In the case of timberlands owned by an entity that is treated as a pass-through entity for income tax purposes, the land must be treated as eligible timberland if ownership and use of the land by the pass-through entity satisfies the requirements of this paragraph. If the owner of the eligible timberlands is an S corporation, the taxpayer must subtract the owner's pro rata share of the gain. If the owner of the timberlands is a partnership or limited liability company taxed as a partnership, the taxpayer must subtract the taxpayer's distributive share of the gain, subject to the percentage limitations provided in this paragraph.

This modification may not reduce Maine taxable income to less than zero. To the extent this modification results in Maine taxable income that is less than zero for the taxable year, the excess negative modification amount may be carried forward and applied as a subtraction modification for up to 10 taxable years. The entire amount of the excess negative