OOS 63-3 - Assessing aboveground vegetation dynamics alongside seed bank sampling in response to shrub management

Woody species such as honey mesquite (Prosopis glandulosa Torr.) have progressively encroached into historically grass-dominated ecosystems across much of the southwestern United States, aided by factors including extensive livestock grazing and drought. The transition from grassland to shrubland reduces the abundance of native grass and forb species, increases soil erosion and redistribution of ecosystem resources, and ultimately decreases the ecological and economic services provided by these lands. Though P. glandulosa may be effectively reduced by control measures, the recovery of desirable herbaceous plant species could be dependent on encroachment severity prior to management, and after a threshold is crossed, difficult to achieve even with active intervention. To examine plant community dynamics along a P. glandulosa encroachment gradient following herbicide treatment, we established twenty sets of paired herbicide-treated and untreated 5-ha plots across a gradient of relatively low to high P. glandulosa encroachment on the Jornada Basin Long-term Ecological Research site and Experimental Range in southern New Mexico, USA. Annual vegetation and soil seed bank surveys were conducted during peak biomass production along three 50-m transects per plot prior to (2020), immediately following (2021), and one year following (2022) aerial herbicide application in late summer 2021 to control P. glandulosa.

Aboveground plant community composition differed between years, with annual forb, annual grass, and perennial forb cover increasing in 2021 compared to the year prior to treatment (2020), regardless of herbicide treatment or initial encroachment severity. Perennial grass and shrub cover, including P. glandulosa, remained similar between these years. One year following application (2022), aboveground community composition was significantly different in response to herbicide, dependent upon functional group. Groups known to be sensitive to broad-leaf herbicide application, including P. glandulosa and perennial forbs, were reduced on treated plots, whereas annual forb and grass cover increased on these plots, and secondary perennial grass cover increased regardless of herbicide treatment. Most functional group responses were negatively related to the encroachment gradient (perennial grasses, annuals), however perennial forb cover was unrelated. By comparison, the soil seed bank density of perennial forb and grass species, two groups of primary concern post-management, were generally highest in moderately-encroached plots. Collectively, these results suggest that initial P. glandulosa encroachment severity does impact plant community dynamics following management, with varied responses across functional groups. Priority functional groups, and their relationship to encroachment severity both above and belowground, should therefore be considered prior to herbicide application to best meet management goals.