I have also wondered about the phrase "hickory wind." I can't imagine its a seasonal weather phenomenon like the mistral. I wondered whether the flowers of the tree might scent the air in a distinctive way, but I've been told this is not the case. However, after much thought, I've concluded that the phrase alludes to the scent of hickory smoke in the air. Hickory has a distinctive and pleasing scent when it burns and that is why it is prized for barbecues. So, when I hear that song, I imagine he is nostalgic for the scent of fires being revived in woodstoves in the morning, scenting the air as it blows through a valley or mountainside.