Probably the tree most commonly planted in treeless regions has been some species of cottonwood. Lombardy poplar and Balm of Gilead have been great favorites. Cottonwood grows rapidly and is hardy against frost, but requires a never-failing supply of water within five to twenty feet of the surface. Because of its demands for moisture it will not grow on uplands, but thrives along water courses or where there is plentiful supply of moisture below the surface. Its fuel value is not high, though the quantity of its wood production compensates for its poor quality, nor does it make good fence posts. Where quick growth is the main consideration, however, it is a good tree to plant. The varieties known as Norway and Carolina poplar are the best. Green ash and hackberry are also hardy against both cold and moisture, but of slow growth. Their wood is durable in contact with the soil, making them suitable for fence posts. Where it succeeds black locust combines many of the desirable qualities to the highest degree. It is a rapid grower, makes excellent fence posts and has high fuel value. It is not as hardy against frost as cottonwood and ash, and while it has been planted successfully in sheltered locations on high plateaus, its success where frosts occur during the summer months is problematical. A closely related species, honey locust, is more frost-hardy but less desirable in other respects, though an excellent tree nevertheless. Other fairly hardy and drought-resistant trees are osage orange and Russian mulberry. Their value for fuel and fence posts is high, but they will not succeed in the most severe situations. Box elder is hardy and has been widely planted, but it is of low fuel value and short lived. In favorable localities at low altitudes, where moisture is abundant either through natural precipitation or from irrigation, the number of species which are adapted to woodlot planting is largely increased. Black walnut, black cherry and hardy catalpa are probably the most valuable of these. The latter, however, is sensitive to early and late frosts. WINDBREAKS The planting of windbreaks and shelter belts around dwellings and fields is of prime importance to the settler in an open country. Nothing adds more to the comfort of the dweller than a belt of timber about the home to protect it from the wind. Orchards need windbreaks to save them from injury in a wind-swept country, and gardens are more successful when surrounded by trees. One of the most important functions of the windbreak, however, is the saving of soil moisture within the protected area, for it is a well established fact that evaporation takes place more rapidly when there is a movement of the atmosphere than when it is calm. It is safe to say that a windbreak is effective in preventing evaporation for a distance equal to ten to fifteen times its height.