Yes, timber sales are treated as long term capital gains. So, any capital losses, you have, short or long term, can offset some of the gain from the timber sale. As the other reply said, whether you should sell, depends on your overall tax situation. For many people, the long term capital gain tax rate is 0%. The long term capital gain tax rate is 0%, until your total taxable income* reaches the 22% tax rate ($81,050 for Married Filing Jointly). Even then, the remaining capital gains are taxed at 15% (until the next threshold is reached).
For most people, the cost basis, in the timber sold, is zero. So, the entire amount of the timber sale is (usually) your capital gain. However, depending on the accounting used by the timber partnership, the timber may have a cost basis and your capital gain may be something less than the full sale amount of the timber.
References:
https://www.extension.purdue.edu/extmedia/FNR/FNR_FAQ_2.pdf
http://www.timbertax.org/developments/Federal%20Income%20Tax%20on%20Timber-%20A%20Quick%20Guide%20fo...
https://www.fs.fed.us/spf/coop/library/taxpubfaqs.pdf
*Taxable income is after adjustments and deductions, including your standard deduction.
