Changes in soil nutrients, vegetation structure and herbaceous biomass in response to grazing in a semi-arid savanna of Ethiopia Z.K. Tessema a, c, * , W.F. de Boer a , R.M.T. Baars b , H.H.T. Prins a a Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3a, Wageningen 6708PB, The Netherlands b Van Hall Larenstein University of Applied Sciences, Wageningen University, Droevendaalsesteeg 2, Wageningen 6708PB, The Netherlands c School of Animal and Range Sciences, Haramaya University, PO Box 138, Dire Dawa, Ethiopia article info Article history: Received 3 December 2009 Received in revised form 2 February 2011 Accepted 6 February 2011 Available online xxx Keywords: Grazing pressure Perennial grasses Plant abundance Soil composition Species richness Woody species abstract The effect of grazing was studied on vegetation structure, herbaceous biomass, basal and bare ground covers, together with soil nutrient concentrations in two locations in an Ethiopian semi-arid savanna. The lightly grazed sites had significantly higher herbaceous diversity, total abundance, basal cover and aboveground biomass, and a lower percentage of bare ground compared with the heavy grazed sites. Grazing pressure had no effect on the density and number of woody species as well as on the proportion of encroaching woody species. The light grazing sites had higher organic carbon, phosphorus and exchangeable bases, and therefore a higher pH and higher electrical conductance, indicating an improved soil nutrient status compared with heavy grazing sites, mainly attributed to the higher basal cover and standing biomass at light grazed sites, and the export of nutrients through grazing and dung collection from the heavily grazed sites. There were significantly higher soil nutrients, species diversity, above- ground biomass and basal cover in the light grazing sites compared with heavy grazing sites. We concluded that changes in herbaceous vegetation, standing biomass and soil compositions are caused by interactions between grazing, soil and vegetation, and these interactions determine the transitions of semi-arid savannas. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Savannas, characterized by the coexistence of trees and herba- ceous vegetations (van Langevelde et al., 2003), occupy a fifth of the earth’s land surface and support a large portion of the world’s human population and most of its livestock and wild herbivores (Sankaran et al., 2005). The semi-arid savannas have been described as stable ecosystems around one or more steady states (Rietkerk et al., 1996), but they are highly dynamic, and transitions from one state to the next often occur under influence of distur- bances, such a through grazing or bush fires (van de Koppel and Prins, 1998). Semi-arid savanna grazing systems can therefore be described by state-and-transition models (Rietkerk et al., 1996). Two extreme states are often described, the first one being a state with ample herbaceous cover, perennial grasses and scattered trees (Scholes and Archer, 1997; Simioni et al., 2003), the second one as a state with a poor cover of annual grasses, absence of perennial grasses, a high proportion of bare soil and/or often bush encroached (Richter et al., 2001; Roques et al., 2001). Moreover, there are feedbacks within these stable states that perpetuate or maintain stability (Rietkerk et al., 1996). The vegetation structure is influ- enced by the soil composition (Augustine, 2003), under influence of the grazing and fire regimes (Moussa et al., 2009). However, the processes leading the vegetation from one state to the next are still poorly understood, especially the role of the soil nutrients in rela- tion to the vegetation, and how grazing influences this interaction. Heavy grazing leads to excessive defoliation of herbaceous vegetation, reducing standing biomass, basal cover and plant species diversity, often triggered by a decline in net primary productivity (NPP), as the intensity of grazing increases (Bilotta et al., 2007). The decline in NPP under heavy grazing is attributed to a reduction of plant material available for photosynthesis. Moreover, heavy grazing alters the herbaceous vegetation compo- sition through an increase in the abundance of annual species with a decline in perennials (Hoshino et al., 2009). Smith (1979) reported that heavy grazing reduces the growth rate and reproductive potential of perennial grasses, and influences the competitive relationships among the different species, so that the heavily grazed perennial grass species loose competitive power over the lightly grazed ones, and subsequently, unpalatable and grazing tolerant annual species become dominant in heavily grazed patches. At heavy grazing pressures, grazing intolerant species disappear because they are highly nutritious and eaten before seed * Corresponding author. Fax: þ31 317 484845. E-mail address: tessemaz@yahoo.com (Z.K. Tessema). Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv 0140-1963/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2011.02.004 Journal of Arid Environments xxx (2011) 1e9 Please cite this article in press as: Tessema, Z.K., et al., Changes in soil nutrients, vegetation structure and herbaceous biomass in response to grazing in a semi-arid savanna of Ethiopia, Journal of Arid Environments (2011), doi:10.1016/j.jaridenv.2011.02.004