SEED, SEEDLING AND SUBCANOPY SURVIVAL IN MONODOMINANT AND MIXED FORESTS OF THE ITURI-FOREST, AFRICA

The central Ituri Forest of Zaire is characterized by monodominant stands of Gilbertiodendron dewevrei (mbau forest) within a matrix of mixed forest where Julbernardia seretii is subdominant. Both species produce supra-annual, synchronous (mast) seed crops. The relationship of mast seed production to monodominance for the two tree species was evaluated by investigating: (1) the relationship between seed density and seed survival, (2) the relative impact of mammal versus insect seed predators in high and low seed density areas, and (3) post-germination survival in the understorey. For the latter, tagged seedlings were monitored over 10 years and both species were censused in different understorey size classes. Percentage seed survival was significantly greater for Julbernardia (but not for Gilbertiodendron) in areas of high conspecific seed density. Generalist fruit- and seed-eating mammals were the principal predators on seeds of both species when removed to al eas without seed-fall. In mast areas high seed densities appeared to satiate mammalian seed predators but not the specialized beetle seed predators that were the major cause of seed destruction. Gilbertiodendron seedlings survived longer in the forest understorey than did seedlings of Julbernardia. Furthermore, Gilbertiodendron made up a greater proportion of the larger subcanopy size classes in monodominant forest than did Julbernar, dia in mixed forests. These results suggest that, irrespective of mortality to seeds, the ability to persist in the understorey may confer on Gilbertiodendron a greater probability of reaching and dominating the canopy.