In corn fields, why is there a single tree in the middle?

You must be referring to an old farm. When plowing or harvesting with horses, it was nice to stop and rest the horses (and yourself) under the shade of a tree for lunch.

It is not always corn that is grown in the field, crops must be rotated and every now and again the field is left with no crops in it at all,(fallow) this is to allow the land to recover, and cows and sheep wil use it for grazing and sometimes a tree is left to provide shade for the animals.

The reason for the lone tree goes back thousands of years, before humans moved out of Africa. During the last ice age great sheets of ice came into what is now USA from the North pole and Canada This great ice flow carver out the great lakes, and picked up large stones of Granite and other hard heavy stones. As the ice melted the stones were left behind. When the land was settled and farmers of 1700s and 1800 cleared the land it was impossible to remove the largest rocks. These were left in the fields and planted around, This left a small spot of ground for a tree to grow.

Birds and crows and hawks need a place to set and guard the fields from all the insects and predators that seem to frequent the field. Trees also provide a little shade for farmer Brown also as he rests from hoeing all that corn.

That's news to me. I've never seen a single tree in a corn field and I've seen many. Do you live in Iowa. I've never been to Iowa. Is there a practice there to have a tree in the center, so as not to get lost.

It may be something to do with marginally cutting the amount of nitrate in the soil. The tree draws most of it up and thus protects the crop from being effected. Too many trees would mean that the crop would be lacking in water, but one actually helps the crop in the aforementioned problem.

I could be completely wrong, but that is my guess.

The Space Ship lands on the tree before the occupants of the ship get out to draw crop circles - silly.

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is it for crows to rest in after they eat your corn . that is my guess.