I'll check but I think it's primarily Poplar with a few Red Maples mixed in. The larger root came from an area that received a bit more light than the rest of last years plantings did. The heat in July put them down early but they still put on plenty of growth. Wow Brad, judging by your karma, your getting more popular every minute!
I say walnut because I have noticed a correlation between large roots and walnut trees. I have talked to some other growers, and they have also noticed a connection. If you get the chance, plant a patch next to a big mature walnut and see how they do. Now, walnuts leaf out late and drop early, so the additional light might have some bearring on this too.
I always thought it had to do with the acidity of the soil making for a prime growing location for ginseng. I also talked to a guy from southern Ohio in the strip mine area and he said he noticed not only the walnuts, but coal banks too produced big ginseng.