Oxalic acid is the standard means of trying to remove such stains, regardless of what wood it is. However, bear in mind that it doesn't work equally well on all woods. It may get rid of most of the stain, or not, and it may also affect the wood in such a way as to cause a light spot when you go to stain. It's worth a try, but don't expect a perfect result.

Oxalic acid is not a "spot treatment" I'd do the whole board to prevent a light "spot" Here are some directions and information: http://www.woodcentral.com/bparticles/woodbleach.shtml   I believe _some_ deck brighteners are oxalic acid (others can be Sodium percarbonate
(like Oxyclean) that generates hydrogen peroxide when mixed with water. Check the label or MSDS.

I tried using Barkeepers Friend (which has oxalic acid listed as an ingredient) on one of the stains. tI did remove the stain, but there is now a bright spot where the stain was. I just wanted to see how well the oxalic acid works, it seems to do a good job. I think and hope that by applying the wood brightener to the entire surface of the deck, the spot should blend in, as the rest of the wood lightens too. After reading so much about deck stains, I don't want to have to use a solid stain if possible, I'd prefer a semi transparent so the stains/bright spots need to go. Is the sodium percarbonate better than the oxalic acid? If possible, I'd prefer to use whatever is less harmful to the surrounding grass, plants and bushes.

Just to update, I used the brightener on the entire deck, I had to go over the floor boards twice which lightened the stains a great deal, although didn't completely remove them. They were light enough that I felt they wouldn't be noticeable once stained. I chose Armstrong and Clark driftwood gray semi transparent stain, just finished the project yesterday and it looks great. The tannin stains are not visible under the new stain color.

Thanks for the update. You must have done a good job because stuff like this is hard to make unnoticeable.