Shrub Invasion of Southern Arizona Desert Grassland

Abstract

E NCROACHMENT of mesquite, burroweed, and other noxious shrubs has seriously reduced the carrying capacity of thousands of acres of desert grassland ranges. Two principal theories have been advanced for this invasion-grazing and control of fires. In the early part of the century Griffiths (1910), and Thornber (1910) observed the large mesquites along the water courses, and the presence of young mesquites on the ridges separating them. Griffiths predicted that both mesquite and other shrubs would increase in size and number on these ridges. Both expressed the opinion that fire, formerly unchecked and with abundant fuel, had maintained the grassland in its original shrub-free state. They believed that livestock grazing had controlled fires indirectly through removal of fuel by trampling and grazing. Later, Wooton (1916) observed that Griffiths' prediction was coming true. He noted that occasional fires were the only check on the increasing shrubs. Leopold (1925) advanced this same theory for the short-grass areas of northern Arizona and called attention to the encroachment of oak and juniper from the higher elevations. This invasion followed the cessation of fires which was shown by burn scars to have been some 40 years earlier. He doubted, however, that the lower levels of the desert grassland ever supported sufficient vegetation for widespread or frequent fires. Humphrey (1949) called attention to the earlier work of Thornber, Griffiths and Wooton. In a study of two burned areas near Tucson, Arizona, he observed that burroweed and snakeweed were effectively controlled by fire. He also recorded a moderate degree of control of such shrubs as jumping cholla; mesquite and palo verde. The grazing theory also has many adherents (Allred 1948, Upson et al 1937, Young et al 1948). In general, they believe that breakdown of the sod and compaction of the soil surface through grazing pressure and drought has led to the invasion of shrubs, causing a grazing disclimax. In refutation of the fire theory of shrub control, Allred (1948) has pointed out that some of the most frequently recurring fires are in the Texas brush country where mesquite is densest. It should be kept in mind that Allred's observations pertain primarily to Texas while the workers referred to in the previouis paragraphs obtained their results in Arizona. Further support is lent to the grazing theory by the fire-resistant nature of mature mesquites. Although the top is readily killed by hot grass fires, the plant is a stump sprouter, and usually sprouts following severe injury to the top. Many other desert plants are similar to mesquite in this respect. Neither of these theories has been adequately proved. Doubtless both fire and grazing have played an important part in the ecology of the desert grassland. Further information concerning the rate of shrub invasion, its relation to management practices and forage production, and the successional relationships of the invaders is necessary for control of the