California Code, Revenue and Taxation Code - RTC § 38204

(a) On or before December 31, 1976, and periodically thereafter as determined by the board, the board after consultation with the Timber Advisory Committee and after public hearings held pursuant to the Administrative Procedure Act, shall designate areas containing timber having similar growing, harvesting, and marketing conditions to be used as timber value areas for the preparation and application of immediate harvest values. The board may designate areas for timber standing on lands owned by local agencies and on timberland as defined in Section 51104 of the Government Code and designate separate areas for timber standing on national forest lands owned by the United States government. On or before March 1, 1977, for timber harvested between April 1 and December 31, 1977, and on or before December 31, 1977, and on June 30 and December 31 of each year thereafter for timber harvested during the succeeding two calendar quarters, the board, after consultation with the Timber Advisory Committee, shall estimate the immediate harvest values of each species or subclassification of timber within those areas as of the initial date of the period.These values shall be determined under rules adopted pursuant to the Administrative Procedure Act from the best evidence available, including (1) gross proceeds from sales on the stump of similar timber of like quality and character at similar locations, or (2) gross proceeds from sales of logs, or of finished products, adjusted to reflect only the portion of those proceeds attributable to value on the stump immediately prior to harvest, or a combination of (1) and (2), and shall be determined in a manner that makes reasonable allowance for differences in age, size, quality, cost of removal, accessibility to point of conversion, market conditions and other relevant factors.

(b) The board, either on its own motion after consultation with the Timber Advisory Committee or in response to application from a timber owner, may modify the immediate harvest values to reflect material changes in timber values that result from fire, blowdown, ice storm, flood, disease, insect damage or other cause, for any area or part thereof in which damaged timber is located. The board shall specify any additional accounting or other requirements to be complied with in reporting and paying the tax on that timber.