Southern Arizona ranchers have been battling mesquite trees for almost 100 years. The principal reason is that mesquite trees and shrubs suck up the water and thereby degrade the range making it less suitable for raising livestock. It also makes the grasslands less accommodating to wildlife.
Mesquite is a very hardy plant that produces an abundance of seed pods. The seeds and pods are collected and stored by rodents. Many animals, including livestock and deer, eat the seed pods, but the seeds themselves pass through undigested and are deposited with some fertilizer.
The problem as described to me by a southern Arizona rancher:
Southern Arizona is characterized by intermittent drought. This results in marked death loss of the perennial forage grasses, however, drought seldom causes death of whole mesquite plants. Mesquite have a long tap root enabling it to reach underground moisture and can tolerate drought. Grasses do not have that advantage. As the tree grows it demands more moisture. The result is each year, depending on the rainfall, less and less forage is produced as the tree shades the ground and quickly takes up the moisture. As the mesquite grow, noxious plants also become established with their deeper roots which make them drought resistant. These include burroweed and snakeweed. These also crowd out grass. As the ground is bare, when a heavy rain falls there is not grass and grass roots to hold the soil so erosion becomes an issue.
According to University of Arizona Technical Bulletin 74, published in 1938:
Certain range lands of the grassland type in southern Arizona are undergoing an invasion by the mesquite tree and noxious shrubs to the extent that the native stand of palatable forage is being materially reduced. The development of this problem has taken place at a pace gradual enough that its seriousness was not fully realized by stockmen until the cumulative effects of some forty years’ transition in the vegetation type of the affected areas became increasingly apparent.
The report goes on to discuss various methods of mesquite eradication including use of petroleum products, arsenic, acid sprays, and other chemical means. These methods proved ultimately ineffective. Petroleum did not kill the roots so the mesquite soon sprouted new growth. The chemicals remained on the stumps and were thus dangerous to livestock and wildlife. Burning individual trees was also ineffective. Only sodium arsenite solution would kill the roots, but it was difficult to prepare and handle.
The US Department of Agriculture weighed in with Circular 908 published in 1952:
One of the most serious and perplexing problems in southeastern Arizona is mesquite invasion of grasslands. Mesquite occurs there in varying degrees of abundance on 9 million acres of range land. The problem is likewise serious elsewhere in the Southwest. Mesquite is now firmly established on considerably more than 70 million acres of range in Texas, New Mexico, and Arizona. An estimated half of the area now occupied by mesquite has been invaded since the advent of domestic livestock. The increase of mesquite is viewed with ever-increasing alarm by range operators.
The principal reasons for concern are : (1) Mesquite, even under moderate grazing use, is still persistently increasing both by invading open grassland and by thickening of old stands. (2) Cutting mesquite, especially in bottom-land areas, usually results in an impenetrable thicket of sprout regrowth and new seedlings. In many of these “jungles,” grazing has had to be abandoned. (3) Livestock handling costs are increased, especially in dense upland mesquite thickets where it is difficult to gather livestock for market or to find screwworm-infested animals for treatment. (4) Increases in mesquite are usually accompanied by decreases in quantity and quality of perennial grass forage and corresponding reductions in livestock production. (5) Still more serious from a long-time viewpoint is the accelerated erosion generally found on uplands as well as bottom lands wherever mesquite has encroached.
The USDA further opines on the cause of the mesquite invasion:
The probability is that neither protection nor heavy grazing has much to do with the increase of shrubs here, but it is primarily the direct result of the prevention of fires.
I spoke with several ranchers who are battling mesquite in southern Arizona. For many years they have been using mechanical means to cut the trees and shrubs and digging out the roots. They try to remove at least 80% of the mesquite. Studies at the Santa Rita Experimental Range show that removing about 80% and leaving some mature trees makes the range more amenable to wildlife than thick mesquite stands or open range.
Before removal work can be done, the ranchers have to make surveys for endangered species such as the Pima Pineapple Cactus, which was listed in 1993 (and there is still no recovery plan). They also have to survey for cultural resources.
The mechanical method means bulldozers to knock down the trees and dig up the roots. One rancher told me it costs $300-$400 per acre and can get only one acre per hour. This is an expensive and tedious operation.
More recently, ranchers have been experimenting with chemical warfare again. Dow Chemical has developed an herbicide that is specific for mesquite. It is deployed by helicopter spraying. This costs about $106 per acre and can cover 80 acres per hour. This is similar to crop dusting operations used on farms.
The ranchers say another main reason for mesquite removal is to reduce soil erosion and restore native grasses. Since chemical removal has begun, ranchers have noticed return of many native grass species.
After reading a version of this article published in the Arizona Daily Independent, another southern Arizona rancher send me these comments:
Mesquite proliferation can be a problem, but they multiply and then decrease quite apart from grazing. It’s fairly cyclic and occurs even where no grazing has occurred (there’s a very deep and large area surrounded by cliffs in Mexico–never grazed–where scientists measured mesquite increase. It increased at the same rate as in the grazed pastures elsewhere.
Mesquites do consume a lot of water and at greater than 20% cover they are problematic, however, at lower than 20% overall, they are a major feed item for both cattle and wildlife–the beans are highly nutritious and so are the leaves. Also, they make a rich, nitrogen-enhanced (they are nitrogen fixers) soil beneath the tree which grows and maintains good native perennials, just so long as the tree cover is not beyond 20-30% and sunlight reaches the area below the tree for some good part of the day. Shade is also good! The upshot is: these trees are natives and are highly adapted to this climate; they have significant value for both wildlife and cattle as forage and as shade for natives that prefer less direct sun like plains bristlegrass and viney mesquite (that’s a grass in spite of the name) and others.
Here we are not at war with mesquites; however there are areas where their density exceeds desirable. Right now the cost of control (just private land–forget trying to do anything on federally managed land) and the issues with flood plains or critical habitat for assorted species are overwhelming and make constructive control financially impractical.
The screw worm issue mentioned in that 1952 publication is past tense; screw worms were effectively controlled decades ago by the propagation and dissemination of sterile male screw worms by the federal govt. and ranchers. It used to be a huge problem in the summer–now not, We can only surmise that such control would be met by howls of opposition from those who would prefer not to eradicate pest insects.
Controlling mesquite makes the range more productive, saves water, and benefits wildlife.
For information on traditional use of mesquite trees see: