Answer: In general, the problem is not one of pH, but rather one of possibly tying up the nitrogen in the soil. As the bark decomposes, it requires a steady supply of nitrogen and can temporarily "rob" the surrounding soil of it to the detriment of nearby plants. This is important in a vegetable garden because those plants grow so quickly that they can't afford a lapse in nutrient availability. Some studies show that this problem doesn't arise unless the mulch is mixed into the top few inches of soil, but it's good to be on the lookout for signs of nitrogen deficiency, just the same.
In practice, this can be compensated for by adding a bit of extra nitrogen to the soil. Usually this would be applied before the mulch is put down, but since it is already done you might consider a side dressing. In this case, let your plants' health be the guide.
As always, if in doubt, perform a soil test. Your County Extension staff can help you interpret the results and make suggestions for any needed amendments. In the long run, the bark is a great source of organic matter for the soil!
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