The arid and semiarid Southwest has frequent droughty periods that have a marked effect on the vegetation. Precipitation is extremely variable, both temporally and spatially (Martin and Cable 1974, Paulsen and Ares 1962). Perennial grasses provide the most reliable forage on grass-shrub ranges (Martin 1975). Cover for most species varied greatly from year to year in southern Arizona (Martin and Cable 1974). During the 1916–18 drought, black grama’s basal cover on the Jornada Experimental Range (JER) in southern New Mexico declined to 42% of predrought cover (Nelson 1934). Black grama was the major plant species on lighter-textured soils. It practically recovered its stand during two years of favorable precipitation (1919–20), then dropped very suddenly in the following two years of drought to the extremely low point of 11% of the original cover. Black grama cover remained at a low level for the remainder of the 1921–25 drought, then increased to its original size when summer rainfall was above average from 1926 to 1929. The accumulated deficits from 1915–1988 headquarters from the 1921–25 drought were 185 mm.2 From 1926 to 1929, July-September rainfall was 115 mm above average. Nelson (1934) found that black grama cover is mainly influenced by plant vigor at the start of the current growing season, as determined by rainfall from the previous summer. Changes in black grama basal area were closely correlated with precipitation during a 15-month period from July 1 of one year through September 30 of the next year.