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SUMMARYWe Studied hemiparasitism in Okoubaka aubrevillei Pellegr. & Normand (Santalaceae), an uncommon West African rainforest tree of widespread distribution (Ivory Coast to Zaire) which attains heights of up to 40 m. It has a very large seed (mean fresh mass of 101 g), and slow accumulation of biomass characteristic of seedlings of shadetolerant rainforest trees. O. aubrevillei seedlings became hemiparasitic within 6 months when grown next to seedlings of the tree species Entandrophragma angolense (Wei.) DC, Pericopsis elata (Harms) Van Meeuwen, Pterygota macrocarpa K. Schum., and Tieghemella heckelii Pierre ex Chev. P. macrocarpa, and the nitrogenfixing legume, P. elata were most infected. Characteristically for hemiparasites, midday leaf water potentials in O. aubrevillei were at -2 MPa lower than those of the host seedlings. Stomatal conductance, however, was low, with a maximum of 111 mmol m^ s~^ After 1 yr, no significant effects of the hemiparasitism were observed on the grow^th of O. aubrevillei or on its nutrient status as measured by foliar element concentrations. By contrast, the two most infected host species showed increased mortality and/or reduced growth. Foliar element concentrations were not, however, affected in host plants of the surviving species. Although the resources of a large seed might enable seedlings of O. aubrevillei to grow^ independently from hosts for prolonged periods, their simultaneous strong effect on the mortality and growth of host seedlings might point to an alternative competitive function of hemiparasitism.

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