The influence of rainfall and grazing on the demography of some African savanna grasses: a matrix modelling approach.
Two savanna sites in Gazankulu, South Africa, were fenced in 1984 and subjected to heavy or light grazing for 4 growing seasons. Size-structured matrix models were constructed for Aristida bipartita, Bothriochloa insculpta, Digitaria eriantha, Heteropogon contortus, Setaria incrassata and Themeda triandra. The asymptotic population growth rate varied between years in relation to rainfall trends, but there were few marked differences between grazing treatments. Aristida had the highest asymptotic growth rate (λ1 = 1.624) over a single 1-year transition period, while the heavily grazed population of Heteropogon had the lowest (λ1 = 0.552). Most species declined in the final dry year. The stable stage distribution was generally a reversed 'J'. The geometric mean of the 4 1-year transition periods indicated that most populations were relatively stable (λ1 about 1), although the populations of Aristida (mean λ1 = 1.301 and 1.135) and Digitaria (mean λ1 = 1.154 and 1.176) increased while that of Setaria (mean λ1 = 0.911 and 0.937) and Heteropogon (heavily grazed, λ1 = 0.914) decreased. The survival of the smallest size-class was important for declining populations or populations whose largest size-classes were collapsing, while the fraction either remaining in a size-class or increasing was important for stable or growing populations. Reproductive output varied widely, but had a minor effect on population growth. The population growth rates of most species were positively correlated, indicating that an extrinsic force, presumably rainfall, had the greatest effect on population growth.