Mesquite trees, thorny shrubs and small trees native to the American Southwest and other arid regions, successfully invade rangelands and riparian zones. Their dominance is directly related to how they interact with underground water supplies, creating conflict with human and ecological water needs. While mesquite offers some ecological benefits, its aggressive water consumption and widespread proliferation make it a significant problem in areas relying on deep aquifers.
The Mesquite’s Deep Root System
The mesquite tree is classified as a phreatophyte, a plant species that relies on a perennial source of groundwater. This deep water access is achieved through a specialized and extensive root structure, giving the mesquite a distinct advantage over most native vegetation. The most remarkable feature is the taproot, which can penetrate dozens of feet into the soil to reach the permanent water table.
Documented taproots have been observed to reach depths exceeding 50 feet, with some exceptional examples recorded at up to 160 feet below the surface. This deep root grows quickly during the seedling stage, allowing the mesquite to survive extended periods of drought that would kill other plants. The plant effectively decouples itself from surface moisture, ensuring its survival even when surface soil is completely dry.
High Rates of Water Depletion
The mesquite’s access to a deep, reliable water source is problematic due to the high rates of water consumption. This process, known as evapotranspiration, involves the loss of water vapor from the leaves, moving water from the aquifer to the atmosphere. A single mature mesquite tree can transpire a significant volume of water daily, with rates ranging from 19 to over 46 gallons per day.
This high consumption rate means that mesquite effectively acts as a pump, continuously drawing water directly from the deep aquifer. The removal of this water lowers the water table over time, significantly reducing the permanent water supply. In Texas, one study estimated that removing 17 acres of established mesquite was equivalent to saving over 1.2 million liters of water annually, illustrating the collective impact of an infestation.
The mesquite also employs a secondary mechanism called hydraulic lift, which benefits the plant by moving water from the deep source. At night, the deep roots pull water from the saturated zone and release it into the drier, shallower soil layers. This redistribution benefits the mesquite’s own shallow lateral roots, but it also increases overall water loss through shallow evaporation and subsequent transpiration.
Ecological and Economic Impacts
The volume of water consumed by mesquite trees has negative consequences for the surrounding environment and human activities. Ecologically, the drawdown of the deep water table directly impacts native shallow-rooted vegetation, such as grasses and shrubs, which cannot reach the receding water source. The mesquite quickly outcompetes these native species, leading to a shift toward a mesquite monoculture.
This competition results in land degradation and the loss of biodiversity, as dense, thorny thickets of mesquite replace grassland habitat. The constant draw of water from deep layers to the surface for transpiration also contributes to soil salinization. As the water evaporates from the leaves, it leaves behind dissolved salts in the upper soil profile, making the ground less hospitable for native plants and crops.
Economically, the spread of mesquite reduces the productivity of rangelands by replacing forage grasses needed for livestock. Dense mesquite stands, armed with sharp thorns, can also form impenetrable barriers, complicating cattle mustering and blocking access to watering points. For agriculture, the lowering of the water table means irrigation wells must be drilled deeper, increasing costs and reducing water availability for human use.
Managing Mesquite Infestations
Controlling mesquite infestations is a challenge due to the plant’s aggressive root system and capacity to resprout. Effective management often requires a combination of methods, as simple cutting or top-killing techniques are usually ineffective. The plant maintains a root crown below the soil surface that can quickly generate new stems if the above-ground portion is removed.
Chemical control typically involves the application of specialized herbicides, often a mixture of triclopyr and clopyralid, applied as a foliar spray or directly to the stem. For successful root-kill, the herbicide must be applied during the plant’s optimal growth period when carbohydrates are actively moving downward to the roots.
Mechanical removal methods, such as root plowing or grubbing, physically remove the entire root crown but are often expensive and can disturb the soil significantly. Biological control, using natural enemies like seed-feeding beetles, is also being explored to limit the spread of seeds and control seedling establishment.