Did you know farmers in Iowa cut down trees around their fields to minimize shade? The graph you provided is completely irrelevant to this discussion. Corn yield growth is resultant from political decisions to fund biofuels.
The yield increases have mainly come from two sources: increased use of chemicals for pest control and fertilizer, and genetic engineering. Both of those are problematic, with consequences only now slowly dawning on most people, and heavy resistance by the industry to any naysaying in public places.
One day, we may recognize the problem and pass laws to limit or prohibit the use of chemicals. (I think the horse is out of the barn on genetic modification.) That will mean lower yields. Further, if California's droughts continue, the Midwest may need to start growing a greater variety of crops beyond corn and soybeans. There is no other place in the U.S. with soil as potentially productive and useful. Wasting it on wind farms when literally any other place will have less potential impact on agriculture is ridiculous.
