(1) The determination of forest costs in ARM 42.20.725 represent the average costs for reforestation, fire assessment, slash disposal, timber stand improvement, timber harvest, forest practices, administration and the severance tax over the base period specified in ARM 42.20.725. Forest costs, with the exception of the fire assessment fee and the severance tax, are calculated from the actual expenditures for those activities conducted by the Department of Natural Resources and Conservation, Forestry Division (DNRC) . The average forest cost in each forest valuation zone is derived from DNRC land management areas. The fire assessment fee will be the average fee the DNRC charges landowners over the base period. The severance tax is the average severance tax that is paid by landowners who harvest timber over the base period in each land management area. Forest costs shall be deducted from the per acre gross timber income.