Every now and then I find myself without a gray card in the middle of the woods, which by definition tend to be rather green in the summer, so I usually go looking for birch bark. I've learned to be wary of the auto white balance on my Nikon D300, and the reason for this should be fairly obvious from the pictures above. The shot on the left is what auto white balance was giving me at Meech Lake this afternoon, straight out of the box, no saturation. The one on the right is what Lightroom's white balance pick tool proposed in post-processing, which was +1 temperature and a whopping +69 tint.
Like my late father used to say, "I don't care how fancy your camera is, you're still smarter than it is".