How is a cost basis calculated for sale of timber only from real estate gifted by parent in 2006?

There are specific rules that apply to timber.

See https://www.irs.gov/publications/p544#en_US_2021_publink100072532

Generally, your basis is going to depend upon whether you have a gain or a loss.

See https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/property-basis-sale-of-home-etc/property-basis-sale-of-home-etc

Presumably, the sale of timber will result in a gain and that gain should be a capital gain (as the sale of a capital asset). As a result, your basis would be a carryover basis (i.e., you take your parent's basis - the donor's basis). 

here's a link to an article. it's put out by the University of Missouri

https://extension.missouri.edu/publications/g5055 

Cost basis for timber is usually zero. 

For a discussion on Timber Basis, see https://www.extension.purdue.edu/extmedia/FNR/FNR_FAQ_2.pdf

Additional References:http://www.timbertax.org/developments/Federal%20Income%20Tax%20on%20Timber-%20A%20Quick%20Guide%20for%20Woodland%20Owners.pdf

https://www.fs.fed.us/spf/coop/library/taxpubfaqs.pdf

https://ttlc.intuit.com/questions/2894323-where-do-i-enter-a-1099-s (1099-S for timber)

https://ttlc.intuit.com/community/entering-importing/help/where-do-i-enter-income-from-a-timber-sale/00/26620(TT FAQ timber)