Defoliation effects on basal cover and productivity in perennial grasslands of Ethiopia

This study aimed to investigate how perennial grass species in Omo National Park (ONP), Ethiopia tolerated defoliation under varying amounts of rainfall. Perennial grasses that have evolved with grazing appear to be generally tolerant to defoliation, although how rainfall influences this tolerance is unclear. Research was conducted in three perennial grasslands where there is a rainfall gradient from north to south (800 – 500 mm yr⁻¹). Grasslands were characterized as either wet, intermediate or dry sites according to their relative position along the rainfall gradient. The wet, intermediate, and dry sites were dominated by two, five, and two grass species, respectively, which comprised 98% of total plant basal cover at each site. Six exclosures containing a total of 12 defoliated and 12 non-defoliated plots (2 × 2 m) were constructed at each site. Hand-clipped defoliation treatments were imposed bimonthly for 18 months (i.e., four rainy seasons, three dry seasons). Repeated measurements of basal cover and biomass production were analyzed for overall response and by species. Basal cover increased (P < 0.05) or remained unchanged for all but one perennial grass species. Biomass production indicated trend for some species but was sensitive to annual rainfall. Overall results indicated that dominant perennial grasses of ONP were tolerant to defoliation, and this tolerance was expressed under all three rainfall levels. In addition, a decrease (P < 0.05) in basal cover was found for grasses in non-defoliated plots for five of nine cases, indicating a negative response to protection from grazing and fire. This revised version was published online in June 2006 with corrections to the Cover Date.