Pub. Resources Code, § 4583

A timber harvesting plan shall conform to all standards and rules which are in effect at the time the plan becomes effective. Except for stocking standards in effect at the time of commencement of timber operations under a timber harvesting plan, which shall remain in effect for any timberland harvested under such plan, all timber operations shall conform to any changes or modifications of standards and rules made thereafter unless prior to the adoption of such changes or modifications, substantial liabilities for timber operations have been incurred in good faith and in reliance upon the standards in effect at the time the plan became effective and the adherence to such new rules or modifications would cause unreasonable additional expense to the owner or operator.