Impacts of grazing and rainfall variability on the dynamics of a Sahelian rangeland revisited (Hein, 2006) – new insights from old data

Understanding the dynamics of semi-arid rangelands is a prerequisite for their proper management and long-term enclosure experiments are an important tool to investigate grazing impact. Hein (2006) presents findings from a 10 year enclosure experiment in the semi-arid Ferlo, Senegal. His main conclusion is that current high grazing pressure (0.15–0.20 TLU/h) negatively affects rain use efficiency and productivity especially in dry years because differences between treatments are significant for the whole period as well as for dry years only. A re-analysis under the framework of non-equilibrium theory of rangeland science leads to an alternative interpretation of the data: The vegetation on the more intensively grazed site possesses a remarkable resilience after the drought of 1983 & 1984. Standing crop recovers fast and for two years is even higher on the “high grazing” treatment than on the less intensively grazed treatment. Statistical analysis confirms this: a general linear model for standing crop against effective precipitation and grazing treatment finds a significant contribution of precipitation only (p < 0.0001). Thus, vegetation dynamics in the semi-arid Ferlo largely follows a non-equilibrium dynamic as it is rather driven by precipitation dynamics than by grazing. This also leads to different policy implications: droughts reduce livestock density and thus are important to allow the vegetation to rest for one or two years.