THE NUTRIENT CAPITAL OF INDIGENOUS SORGHUM SPECIES AND OTHER UNDERSTOREY COMPONENTS OF SAVANNAS IN NORTH-WESTERN AUSTRALIA

Indigenous Sorghum species dominate the understorey of the savannas of tropical north-western Australia. The nutrient capital of these grasses was investigated and the consequences for nutrient cycling, the effects of fire, and herbivory were discussed. The macronutrient concentrations in two annual species, S. intrans and S. stipoideum indicated that their growth was nutrient limited. The breakdown of litter may provide up to one quarter of the N yield of S. intrans, hence losses of N due to dry season fires, which consume this litter, may reduce grass growth. However, this effect was not apparent after one fire. From the first sampling, approximately 60 days after germination, until maturity, the herbage of S. intrans and S. stipoideum contained insufficient N and P to maintain the liveweight of cattle. Translocation to the inflorescence during seed set left the herbage grossly deficient for livestock maintenance during the dry season. The relative lack of large native herbivores is consistent with this poor herbage quality. Nutrient concentrations in the seeds of nine Sorghum species (including two perennials) were measured. The concentrations were relatively high, which accounts for the importance of the seeds of the indigenous Sorghum species for granivorous rodents, birds, and ants. Macronutrient concentrations were similar in both annual and perennial species.