Tex. Tax Code § 23.78

The taxable value of qualified timber land appraised as provided by this subchapter may not be less than the appraised value of that land for the taxing unit in the 1978 tax year, except that the taxable value used for any tax year may not exceed the market value of the land as determined by other generally accepted appraisal methods. If the appraised value of timber land determined as provided by this subchapter is less than a taxing unit's appraised value of that land in 1978, the assessor for the unit shall substitute the 1978 appraised value for that land on the unit's appraisal roll.