Journal of Range Management, Volume 53, Number 2 (March 2000)

Abstract

POSTMASTER: Return entire journal with address change -R E T U R N P O S TA G EG U A R A N- T h e Journal of Range Management is a publication of the Society for RangeManagement. It serves as a forum for the presentation and discussion of facts, ideas, and philosophies pertaining to the study, management, and use of rangelands and their several resources. Accordingly, all material published herein is signed and reflects the individual views of the authors and is not necessarily an official position of the Society. Manuscripts from anyone-nonmembers as well as members-are welcome and will be given every consideration by the editors. Editorial comments by an individual are also welcome and, subject to acceptance by the editor, will be published as a "Viewpoint." In AbstractWe compared the herbage standing crop on 31 farms along a rainfall gradient in Namibia (southwestern Africa) in 1997 with the results attained for the same gradient by Walter (1939). We found that the slope for the regression of herbage yield on mean annual rainfall in 1997 was 5.93, i.e. 5.93 kg herbage was produced per hectare for every 1 mm increase in rainfall along the gradient. This regression slope is considerably lower than that in Walter's (1939) study (slope = 10.34). Thus, current grassland productivity per unit of rainfall in Namibia is about half that of 50 years ago. There is no evidence of a change in annual rainfall over this period, nor is there any evidence that either short-term (current) or longer-term (11 years) stocking densities affect current herbage yield. We conclude that, while desertification has taken place, grazing over the last decade has not been the cause of this reduced productivity.Key Words: Africa, rangelands, historical records, grazing, rainfall There is widespread concern over desertification (i.e. a longterm decline in productivity) of the semi-arid grasslands of the world (Schlesinger et al. 1990, 1996, Hall and Scurlock 1991, Strohbach 1992, Parton et al. 1995. However, it is often difficult to unequivocally determine whether desertification has occurred, because different types of information (e.g. on soil quality, carbon isotopes, vegetation quality and quantity) may give conflicting results (Hoffman et al. 1995, Parton et al. 1995, Parsons et al. 1997. Also, logistical problems such as variability in annual rainfall cause great natural fluctuations in herbage growth, making it difficult to differentiate pattern in declining productivity from noise caused by random or event-driven fluctuations (Hoffman and Cowling 1990, Sullivan 1996, Hoffman 1997, Ward et al. 1998, Ward et al. 1999. Long-term cycles in rainfall can further exacerbate this problem. For example, Hoffman and Cowling (1990) and Hoffman (1997) have shown that Acocks ' (1953) claim that desertification was rampant in the semi-arid Karoo region of South Africa was probably caused by the long dry period in the 1950's. When one compares photographs of the vegetation taken in 1925 by I.B. Pole Evans with photographs taken at the same ...