Elements of a theory of vegetation dynamics in arid rangelands

ABSTRACT Under grazing pressure the vegetation of rangelands is often altered. Range managers traditionally have thought of these changes as retrogression or disturbance, setting climax vegetation back to a seral stage. Unfortunately, removing the grazing pressure often does not reverse the changes in the way the succession model would predict. Here four elements are sketched out of a theory which can deal with these exceptions. First, which plant growth-forms win competition often depends not only on the growth-forms but also on which is present as established adults, or on which is most abundant. Second, vegetation changes can sometimes induce soil changes. The extent to which reversing a vegetation change depends on soil restoration varies greatly, and the speed with which soil structure can be restored varies from rapid to negligible. Third, the impact of grazing on different life-forms varies greatly in different situations; in particular, the nature of and scope for selectivity is very different in ...