Mesquite trees, belonging to the genus Prosopis, are a complex feature of arid and semi-arid landscapes across North and South America. These drought-tolerant species are well-adapted to environments with limited water availability. The question of whether mesquite is an invasive species or merely an aggressive one is complicated by its native status within much of its range, particularly the southwestern United States. Although native to these ecosystems, the rapid increase in mesquite density over the last 150 years has transformed vast areas of grassland into dense woodlands. This population surge presents significant management challenges for landowners and conservationists alike.
The Critical Distinction Between Aggressive and Invasive
The term “invasive species” has a precise ecological definition. An invasive species is non-native to the ecosystem under consideration and causes or is likely to cause economic or environmental harm, or harm to human health. By this standard, mesquite species like honey mesquite (Prosopis glandulosa) or velvet mesquite (Prosopis velutina) are not truly invasive within the US Southwest, as they are native to this region.
Mesquite is more appropriately classified as an aggressive native species in these areas. Aggressive native plants rapidly increase their population density and distribution, often in response to human disturbance or changes in the natural environment. The extensive spread of mesquite in historic grasslands is largely attributed to factors like the suppression of natural fire regimes and the widespread use of domestic livestock. In parts of the world where mesquite species have been introduced, such as in South Africa or Australia, they are genuinely invasive and pose a severe threat to local biodiversity.
Biological Factors That Drive Mesquite Proliferation
The ability of mesquite to dominate a landscape stems from highly adaptive biological features. As a phreatophyte, the tree develops an extremely deep taproot system, which can extend over 45 feet downward to access groundwater sources unavailable to shallow-rooted grasses. This deep-water access allows mesquite to maintain metabolic activity through extended dry periods.
Mesquite produces numerous seed pods, and the seeds within these pods possess an exceptionally hard outer coat, known as an exocarp. Successful germination requires a process called scarification, which weakens this hard coating. The most efficient dispersal and scarification mechanism involves the consumption of the pods by livestock and wildlife, with cattle being particularly effective vectors.
The seeds remain viable for long periods, sometimes decades, creating a persistent seed bank in the soil that allows for reinfestation years after control efforts. Mesquite is difficult to kill because it can rapidly resprout from a protected bud zone located just below the soil surface. If the above-ground portion is cut or burned without destroying this subterranean woody structure, the plant will quickly regrow with multiple stems.
Environmental and Economic Impacts of Mesquite Dominance
The unchecked proliferation of mesquite causes significant negative consequences, even as a native species. A mature mesquite tree can consume a substantial amount of water, with some adult honey mesquite trees using up to 20 gallons of water per day during peak growing season. This high water usage has a pronounced effect on local hydrology, contributing to the depletion of shallow water tables in arid environments.
The expansion of dense mesquite thickets is a major economic concern for rangeland management. Mesquite outcompetes desirable forage grasses for light, water, and nutrients, leading to a substantial reduction in the land’s capacity to support livestock. The expansion of mesquite has diminished the value of grazing lands and negatively impacted habitat for various wildlife species. The alteration of habitat structure from open grassland to dense shrubland also shifts local biodiversity.
Methods Used for Mesquite Population Control
Land managers employ a combination of strategies to mitigate the spread and density of mesquite populations. Chemical control is a common approach, involving the targeted application of herbicides like triclopyr and clopyralid. These chemicals are applied as a foliar spray to the leaves or as a basal bark treatment, aiming to achieve a high percentage of root-kill rather than just top-kill.
Mechanical removal methods include grubbing and root plowing, which use heavy machinery to physically extract the plant and its root crown from the soil. For this method to be effective, the cut must be made below the bud zone to prevent the plant from resprouting. These methods are often expensive and can cause significant soil disturbance.
Biological control, though controversial in native ranges, utilizes natural enemies to limit population growth. In areas where mesquite is truly invasive, seed-feeding insects like bruchid beetles are sometimes introduced to reduce the number of viable seeds produced. Prescribed fire is another tool, but its effectiveness is limited. Fire can kill young seedlings, but mature mesquite will often resprout unless the fire is intense enough to destroy the root crown.