Ask Mr. Smarty Plants

Sorry, but, after an extensive search, Mr. Smarty Plants couldn't find a precise answer to your question.  We can tell you that, according to the USDA Plants Database, the pH requirements for its growth are 4.0 minimum and 6.5 maximum and we can point you to the US Forest Service account with a wealth of information about bald cypress.

We do wonder why you asked the question.  Are you considering using Taxodium distichum (bald cypress) needles as mulch for your garden and hoping to lower the pH of your soil to help particular plants? If that's the case then there isn't an easy answer to the exact results.  The overall effect of bald cypress needles on basic soils should be acidifying, but there are many factors that will affect how much the pH could be changed—the amount of needles and how they're used, original soil pH, soil mineral fractions, water quality and its pH, amount of rainfall, soil biota, etc.

There is some experimental evidence about the effect of cypress mulch on soil acidity.  Cypress mulch is the shredded whole tree (including the needles).  A study in Florida by Stephen H. Brown of the Lee County Cooperative Extension Service ("Response of Hibiscus to Organic Mulches", Proc. Fla. State Hort. Soc. 109:30-33. 1996) compared the effects of five organic mulches—needles of Pinus elliottii (slash pine), municipal solid waste (MSW) composed of refuse from professional landscapers and homeowner lawn organic waste, and shredded mulch from three trees—Taxodium distichum (bald cypress) and two trees that are native to Australia, Melaleuca quinquenervia (Melaleuca) and Eucalyptus sp. (no particular species named) on the growth of Chinese hibiscus (Hibiscus rosa-sinenesis).  Interestingly, they found that all applications decreased the pH of the soil (made it more acidic—going from a range of 7.37-7.63 to a range of 6.47-6.8), but the amounts of the decrease were not significantly different for the 5 treatments or the control with no mulch added.  The cypress mulch lowered the pH of the soil from 7.37 to 6.47 in one year.