Topographic separation of two sympatric palms in the central Amazon - does dispersal play a role?

Despite broadly overlapping geographic distributions in the central Amazon basin, two congeneric palm species (Attalea attaleoides and Attalea microcarpa) have topographically separated distributions on a local scale in Reserva Ducke near Manaus. Our aim here was to determine if this local scale separation can be linked to (1) seedling stage specialization to different habitat conditions of the two species, and/or (2) environmentally-controlled seed dispersal. We assessed the role of these potential drivers by mapping the local distribution of the two species over a 25-km(2) grid and testing for correlation to seed removal and seed germination patterns using seed sowing experiments. 360 seeds of each species were sown in 30 uniformly distributed plots (12 seeds of each species in each plot), and seed removal and germination were subsequently monitored. Adult populations of the two species showed opposite distribution patterns linked to topography. However, there was little evidence for specialization to different habitat conditions at the seedling stage: after 11 months, 26.1% of seeds of A. microcarpa had germinated along the entire topographic gradient, albeit with a tendency toward higher germination in more inclined areas. For A. attaleoides, only 2.2% seeds had germinated, and again along the entire topographic gradient. In contrast, there was evidence for environmentally-controlled seed dispersal: for both species, seed removal was higher in flat areas. Presence of adults did not affect germination or seed removal. Our results suggest that topographically differentiated distributions of A. attaleoides and A. microcarpa may be reinforced by steep slope avoidance by their seed dispersers. A direct environmental control mechanism remains to be identified to explain the consistent topographic associations, but our results show that this mechanism does not work at the seed germination stage. (C) 2012 Elsevier Masson SAS. All rights reserved.