Observation: Long-term increases in mesquite canopy cover in a North Texassavanna

It is necessary to quantify rates of woody plant encroachment on southweste rn USA rangelands to determine the economic feasibility of treatments desig ned to manage these plants. This study observed changes in honey mesquite ( Prosopis glandulosa Torr.) canopy cover over a 20-year period (1976-1995) i n 2 treatments: an untreated area that initially had a moderately dense mes quite stand (14.6% cover), and an area cleared of mesquite with root-plowin g in 1974. Canopy cover of mesquite was estimated from scanned color-infrar ed aerial photograph images by manually delineating mesquite canopies with a computer using ArcView software. During the 20 years, mesquite cover in t he untreated area increased (P less than or equal to 0.05) from 14.6 to 58. 7%, averaging 2.2 percentage units per year. Cover in the root-plow treatme nt also significantly increased during the same period from 0 to 21.9% (1.1 percentage units per year), but the rate of increase was significantly low er than in the untreated area because mesquite growth was from new seedling s instead of established plants and/or new seedlings as occurred in the unt reated area. Rate of increase was significantly lower from 1976 to 1990 (1. 6 and 0.2 percentage units per year) than from 1990 to 1995 (4.1 and 3.7 pe rcentage units per year) in the untreated and root-plow treatment, respecti vely. These differences were attributed to precipitation which was near nor mal from 1976 to 1990 but 25% above normal from 1991 to 1995.