I'm not a rub expert, but I know they like pines, maples, willowsand poplar trees. At least in my area they do. It seems to me that as individuals, the bucks have their own preference. Some prefer pines and some prefer maples. Just like some prefer to live in the open fields (white racks) and some prefera dark jungle of pines (dark racks). This can happen on the same piece of ground. Some say it is the aroma, I'm not sure. I think that they like a tree that can hold their scent the best. I think that sign post rubs happen to be the best tree in the area with the most buck activity, what ever that tree happens to be. More than likely, it'll be one of the ones that I mentioned (in my area).