Fire as a Means of Controlling Velvet Mesquite, Burroweed, and Cholla on Southern Arizona Ranges

Abstract

A THOROUGH study needs to be made of the possibility of controlling undesirable shrubs or trees by fire on southern Arizona ranges. Other control methods, even if effective, are generally so costly as to prevent their general adoption on low-value range. Yet, it is on these low-value ranges that control is frequently most urgently needed. It has been known for many years that certain shrubs are readily killed by fire while others are very difficult to eradicate in this way (1, 6). Many observers of range conditions in the Southwest and elsewhere have suggested the use of fire as a tool to control undesirable trees and shrubs (1, 3, 6, 7). In 1907, Thornber (7) noted that when an area supporting burroweed (Aplopappus tenuisectus) was burned, all of the plants were killed, even when only partly charred. Such shrubs as catclaw (Acacia greggii), creosote bush (Larrea tridentata), Mormon tea (Ephedra trifurca), velvet mesquite (Prosopis velutina), and graythorn (Condalia lycioides), were also killed. Thornber, in 1910 (8), again reported on burning as a means of shrub control. He stated that burroweed, creosote bush, Mormon tea, and hackberry could be killed at very small expense by burning during the dry foresummer, i.e., May to June, inclusive. He noted further that charred stumps, occurring on certain areas, were an indication that fires had occurred commonly in the past and that velvet mesquite, formerly held in check by occasional fires, was at that time spreading. In the same year Griffiths (3) noted that areas formerly grass covered on the Santa Rita Experimental Range, by 1910 supported an abundant growth of young velvet mesquite and other shrubs. During the seven years that he had observed the area he noted a very definite increase in both shrubs and mesquite. He says in this connection: The probability is that neither protection nor heavy grazing has much to do with the increase of shrubs here, but that it is primarily the direct result of the prevention of fires.-The prediction is ventured that the time is coming when these foothill grassy areas, which now have only an occasional small shrub, will be as shrubby as the deserts and lower foothills below them, if not more so. He continues with the thought that although mesquite may have been spread to some extent on the Santa Rita Experimental Range by grazing animals, its increase was more probably due to grazing that removed the combustible ground cover, thus preventing fires. Griffiths concluded that before the area was grazed by domestic stock it probably produced more grass than in 1910 and that it was formerly burned at rather frequent intervals. In his opinion this burning had little effect on the grasses but almost entirely prevented establishment of undesirable shrubs. Bec8use of the slow growth rate of the shrubs he felt that they could be controlled by fires occurring only once in ten years. He believed also that were it not for recurrent fires the then grass-covered mesas would have been dominated by shrubs as were the more