Many topple when they can no longer reach the watertable. Many from 1960 to 1984 have been landcleared (Programa Nacional de Desmontes). This deforestation was financed by the World Bank. Desertification is based most on the civic demand for more freshwater than can be obtained. The foliage of mesquites in the Mexican state of Sonora reacts to lowering the watertable by rapidily producing smaller leaves to reduce the heat load.
Natural selection is by heat load/water stress. Also for some 40 years in Sonora, civil and agricultural engineers have known that if they pump too much they will cause seawater intrusion. Incidentally, mesquites are famous for their ability to manage on and to reclaimed sodic soils.