Mauto (Lysiloma divaricatum (Jacq.) J. F. Macbr.; Fabaceae) is a thornless, arborescent legume that is abundant in tropical dry forests in northwestern Mexico. To test whether mauto allometry may be used as an indicator of cattle grazing pressure, we compared plant height, canopy cover, and basal trunk diameter between an area where cattle had been excluded for 12 years with an area under continuous heavy cattle grazing. Mauto plants that had mostly avoided grazing grew to 12 m in height, with an average basal trunk diameter of 11 cm. Under intense grazing, many plants appeared as a bonsai, that is, as small pruned trees with a relatively thick trunk. Such differences were expressed in the linearized (log-log) slopes of the height–diameter and cover–diameter allometric relationships, which varied significantly between the grazed and ungrazed areas. Basal trunk diameter increased faster per unit increase in plant height and canopy cover in the grazed area than in the ungrazed area. Therefore, these morphological or allometric relationships of mauto could be useful for quickly assessing cattle grazing pressure in tropical dry forests.