RAINFALL AND GRAZING EFFECTS ON NET PRIMARY PRODUCTIVITY IN A TROPICAL SAVANNA, INDIA

The effects of rainfall amount, rainfall distribution, and grazing intensity on net primary production and herbivory were examined for a dry tropical savanna. Above-ground net primary production in permanently protected savanna ranged from 329 to 741 g.m-2.yr-1 and was positively related to annual rainfall. It was not related to early rainy season rainfall, but was related to rainfall of the later part of the rainy season. Grazing stimulated 4-45% greater net primary production, the stimulation being higher at lightly and moderately grazed sites. Consumption, as estimated by comparing net primary production inside temporarily fenced and outside the fenced plots, was 476-734 g-m-2.yr-1. Consumption in the present savanna was positively related to net primary production, and to annual rainfall. Green biomass, being a function of soil moisture, was greater in the high rainfall year. Belowground net primary production in permanently protected savanna (BnpP) ranged from 404 to 838 g.m-2.yr-1 and was significantly related to annual rainfall. BnpP was not related to early rainy season rainfall but was related to late rainy season rainfall. Total net primary production ranged from 836 to 1579 g.m-2.yr-1 and was also related to annual rainfall, and was more sensitive to late rainy season rainfall than early rainy season rainfall. Grazing reduced belowground net primary production by 25-65% in grazed savanna (BnpG). Maximum reduction occurred in heavily grazed savanna. BnpG was inversely related with rainfall. Total net primary production (herbaceous + woody vegetation) of the savanna ranged from 11.9 to 19.1 Mg.ha-1.yr-1. The total net primary production of the savanna was similar to that of the native dry deciduous forest; however, the production of herbaceous species was 61% lower in the forest. The total net primary production of savanna was higher than that of the dry tropical forest when the belowground net primary production of the 0-50 cm layer was considered.