Selective impacts on the vigour and mortality of Aristida junciformis (subsp. junciformis)

Abstract

The aim of grazing management is to maximise livestock production by maintaining high sward quality. Many southern African grasslands have become degraded allowing grass species unfavourable for livestock production, such as Aristida junciformis subsp. junciformis, to become dominant thereby reducing the available sward quality. Aristida junciformis persists once established and is remarkably understudied. Three studies were conducted to investigate the dynamics of this grass and to find focused management techniques to control and manage A. junciformis. The studies compared the impact of a high density graze (HDG), targeted herbicide application and a control on the survivorship and productivity of A. junciformis tufts, on the species and cover composition and on the post-treatment seedbank. Tufts exposed to herbicide had a lower probability of survival (p = 0.887) than those subjected to a HDG (p = 1.000) or control (p = 1.000). After treatment implementation, grazed tufts were significantly (p = 0.0018) shorter than control tufts. The tufts displayed a linear growth rate under the control (F1,8 = 456.84; P < 0.001), increasing steadily over time, and a quadratic growth rate under the HDG (F2,7 = 125.35; P < 0.001), initially growing rapidly then declining towards the end of the growing season. There was no significant difference in the height (p = 0.9481) and the aboveground net primary productivity (ANPP) (p = 0.7053) between the tufts in the control and HDG paddocks. The plant species composition (p = 0.4169) and cover composition (p = 0.4169) did not differ among treatments, however there were significant shifts in species composition (p = 0.0002) and cover composition (p = 0.0005) over time (p = 0.0002). The directional shift in species and cover composition were similar in all paddocks. Most of the grazing resistant perennial grasses, or ‘mtshiki’ species (Eragrostis curvula, E. plana, Sporobolus pyramidalis and S. africanus) and A. junciformis increased and Themeda triandra decreased over time. Total vegetation cover increased across all paddocks for all grass and forb species such that the bare soil cover was reduced from 53% to 34%. No A. junciformis seedlings emerged from the seedbank study. Most of the seedlings emerging from the seedbank (92%) and field studies (40%) were forbs. In the field study A. junciformis (30%) was second most dominant, followed by T. triandra (13%). The plant species composition of emerged seedlings did not differ among treatments (p = 0.8134). Aristida junciformis is and remains a persistent, indigenous weed that is difficult to eradicate. More research is required to prevent its establishment in areas not yet dominated but prone to its invasion and to eradicate it in veld where it is already dominant.