Rows of Osage Orange (a. k. a., hedge apple, horse apple, bois d'arc, bodark) trees planted in western Oklahoma during the 1930s were called:

Shelter belts.

Explanation:

Trees are one of the important natural resources of the earth. Trees perform different important functions like purifying air, production of oxygen, reduction of the amount of carbon dioxide and consumptive use.

The shelter belts may be defined as the vegetative barriers producted by planting the trees in the specific rows. They are mainly designed to reduce the wind effect, its speed. The shelter belts also provide sheltered areas of the windward.

Thus, the answer is shelter belts.