ROOT HEMIPARASITISM IN A WEST-AFRICAN RAIN-FOREST TREE OKOUBAKA-AUBREVILLEI (SANTALACEAE)

We studied hemiparasitism in Okoubaka aubrevillei Pellegr. & Normand ( Santalaceae), an uncommon West African rainforest tree of widespread d istribution (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 accumu lation of biomass characteristic of seedlings of shade-tolerant rainfo rest trees. O. aubrevillei seedlings became hemiparasitic within 6 mon ths when grown next to seedlings of the tree species Entandrophragma a ngolense (Wel.) DC., Pericopsis elata (Harms) Van Meeuwen, Pterygota m acrocarpa K. Schum., and Tieghemella heckelii Pierre ex Chev. P. macro carpa, and the nitrogen-fixing legume, P. elata were most infected. Ch aracteristically for hemiparasites, midday leaf water potentials in O. aubrevillei were at -2 MPa lower than those of the host seedlings. St omatal conductance, however, was low, with a maximum of 111 mmol m(2) s(-1). After 1 yr, no significant effects of the hemiparasitism were o bserved on the growth of O. aubrevillei or on its nutrient status as m easured by foliar element concentrations. By contrast, the two most in fected host species showed increased mortality and/or reduced growth. Foliar element concentrations were not, however, affected in host plan ts of the surviving species. Although the resources of a large seed mi ght enable seedlings of O. aubrevillei to grow independently from host s for prolonged periods, their simultaneous strong effect on the morta lity and growth of host seedlings might point to an alternative compet itive function of hemiparasitism.