Get reliable rangeland science

Live honey mesquite (Prosopis glandulosa) trees create �islands of fertility� beneath their canopies. Our objective was to determine the impact of herbicide induced mesquite mortality on soil nutrients and herbaceous communities beneath the canopies and in interspaces between trees. The study was a randomized complete block design, blocked by site type (two blocks: four upland sites, two lowland sites). Each site had one replicate of an untreated control, one aerial broadcast herbicide treatment in 2013 and one in 2014. In each replicate, three 1x2 meter grazing exclosures were placed under the canopies and three in the interspaces. During peak cool- and warm-season herbaceous standing crop, herbage was clipped by functional group to ground level in a 0.125 m2 frame in the exclosure, dried in a forced-air oven at 60�C for 48�h, and weighed. In addition, soil cores are taken at 0-15.25 cm and 15.26-30.5 cm and composited under the canopies of trees and in the interspaces. Light measurements were taken with a photometer under the canopies of one tree in each rep. In 2015, production of cool season perennial grasses was at least 40% higher and total perennial grass production was at least 35% higher in treated areas versus control areas; however, warm season grass production was not significantly different between treatments and controls. During peak cool season standing crop in 2016, production of annual grasses and annual forbs were twice as high in the 2013 treatment than in the control. Inorganic nitrogen levels were significantly higher during this sampling time. This, along with rainfall timing and amounts, likely contributed to the flush of annuals, which in turn suppressed the growth of perennials in the 2013 treatment. While mesquite mortality results in increased herbaceous production, the proportional difference in plant functional group productivity may vary.