ROOT BIOMASS AND PRODUCTIVITY IN A GRAZING ECOSYSTEM - THE SERENGETI

Does grazing by large wild mammals, an intense form of aboveground her bivory, influence belowground productivity? The vast majority of liter ature data concentrate on short-term pot studies and indicate that cli pping consistently retards root growth. Field studies are few and cont radictory, but tend to indicate that grazing has little effect on gras sland belowground production. We sampled root-soil cores at 0-10 and 1 0-20 cm increments, at 11 locations across the Serengeti ecosystem, on 10 dates over an annual cycle, sampling monthly during the rainy and early dry seasons and every 2 mo during peak dry season. Fenced and un fenced plots were replicated (n = 2 or 3) at each location. Live roots , identified visually by brightness and texture, were sorted, washed, dried, and weighed. In addition, profiles were sampled at 10-cm increm ents to 50 cm in fenced and unfenced plots in short, mid-height, and t all grasslands, representing a gradient of grazing, during the month o f peak root biomass. Exclosures erected 22-25 yr previously were simil arly sampled in short and tall grasslands to a 30-cm depth. Root bioma ss reached a pronounced minimum in mid-wet season (February) and a dec ided maximum at the beginning of the dry season (June). Net productivi ty, based on maximum-minimum biomass, ranged from 100 to 600 g.m(-2).y r(-1) to a 20-cm depth, with minima ranging from 40 to 150 g/m(2) and maxima from 230 to 700 g/m(2), according to location. There was no evi dence that grazing reduced root productivity over the annual cycle. Ve rtical biomass profiles at peak standing crop were similar for short, mid-height, and tall grasslands, with root biomass dropping sharply wi th depth, except for short grasslands on soils that, atypically, lack a hardpan. In those grasslands, shallow root biomass was lower than in other grasslands, but biomass at depth was distinctly greater. For lo ngterm protected grasslands, root biomasses at peak were identical in short grasslands, whether fenced or unfenced, but fenced tall grasslan ds had a lower root biomass than grazed plots. We conclude that intens e herbivory does not inhibit root biomass or belowground productivity of Serengeti grasslands over either the short or the long term.