Hossdaniels advice echoes my personal experience.
In both 2011-12 and 2012-13, a good friend of mine spent about $2000 attempting to establish 4 cool weather food plots in the midst of a stand of large, thinned pines. The plots total acreage ... probably 2 acres. He has abandoned the effort.
This is in SE Alabama. The mix he tried in 2 strips was a combination of Crimson clover and purple top turnips. In 2 strips he tried "Big Buck" blend ... a premix of oats, wheat, grain rye, Crimson clover.
The plots had been pH corrected and fertilized each fall according to soil testing. Planting was done by an experienced planting service using a large "Do All" style drill planter. To be "kind", the plots were fair. The "Big Buck" blend was exceptionally weak ! Of the crops, the turnips fared best, with the clover doing fjust OK.
I'd say based upon this that it is all about the availbility of adequate sunlight.