The Jornada Range Reserve, on which this study was made, was established in 1912 and has been conducted by the U.S. Forest Service as a range experiment station since 1915. It is a typical semi-desert range adjacent to the Rio Grande Valley in Dona Ana County, New Mexico, about 50 miles north of the Mexico boundary. There are about 145,000 acres (58,000 hectares) in the mesa portion of which approximately one-fourth is covered by the Prosopis sand dune type, consisting of low, evenly distributed dunes occupied principally by mesquite (Prosopis glandulosa Torr.), with " blow outs " (wind formed hollows) in between the dunes. The soils of the mesa are mostly Quaternary alluvium with older sands and gravels (Darton, '22), and the Prosopis type is always found in the loose, wind-blown phase of the more sandy soils. The community is rather extensive in southern New Mexico, although it is not mentioned by Watson ('12) in his description of the northern part of the state.