Plant Community Recovery following Chemical Control of Mesquite

Molly Reichenborn

Aided by a host of factors including extensive livestock grazing and drought, woody species such as Honey mesquite (Prosopis glandulosa Torr.) have progressively encroached into historically grass-dominated ecosystems in the southwestern United States. This transition from grassland to shrubland causes concomitant reduction in native grass and forb species, increased soil erosion and redistribution of ecosystem resources, and ultimately decreases the ecological and economic services provided by these lands. Though mesquite may be effectively removed or reduced by control measures, the recovery of desirable herbaceous plant species could be dependent on degradation severity prior to management, and after a threshold is crossed, difficult to achieve even with active intervention. To examine this, we established twenty sets of paired treated and untreated 5-ha plots across a gradient of relatively low to high mesquite encroachment on the Jornada Experimental Range. Pre-treatment vegetation data were collected on three, 50-meter transects within each plot in the fall of 2020 prior to upcoming herbicide application in 2021. We captured a range of initial community conditions across the encroachment gradient including 109 observed species. Overall shrub cover, largely comprised of mesquite, ranged between 3 and 28%, perennial grass cover between 2 and 43%, perennial forb cover between 0 and 10%, and bare soil between 17 and 78%. As expected, species richness, evenness, and perennial plant cover declined with increasing encroachment and bare soil cover. Following herbicide application, these data will be utilized to identify thresholds in degradation severity at which herbicide application gives the most the return on restoration investment, and which pre-treatment variables are most important in predicting long-term plant community recovery.