RESPONSES OF DROSERA-CAPENSIS AND D-BINATA VAR MULTIFIDA (DROSERACEAE) TO MANIPULATIONS OF INSECT AVAILABILITY AND SOIL NUTRIENT LEVELS

Insectivory, growth, and nutrition of Drosera binata var. multifida an d D. capensis were studied in long-established plantings in a greenhou se. Exclosures were used to manipulate insect availability to leaves, and substrate phosphorus and nitrogen fertiliser was added to increase these mineral nutrients. Neither species benefitted significantly fro m insect capture on a nutritional or energetic basis in nutrient-poor or -rich soils. However, both species benefitted from soil nutrient ad ditions, and accumulated N and P in vegetative organs. Phosphorus leve ls in shoots were especially high in P fertilised plots for both speci es. In addition, P concentrations increased significantly in the roots of D. binata. Nitrogen levels in all tissues were slightly higher in N fertilised plots, and statistical differences were observed in roots of D. capensis and in shoots of D. binata. Drosera capensis showed in creased growth with N and P additions, especially in root biomass. How ever, leaf number and rosette diameter increased where N and P were ad ded together. This study adds to the growing evidence that insectivory is not as an important nutrient source to insectivorous plants, compa red to inorganic soil-acquired nutrients, especially when the soil nut rient concentrations are high.