Effects of dispersal pattern and mammalian herbivores on seedling recruitment for Virola michelii (Myristicaceae) in French Guiana

The effects of dispersal pattern (seeds in small clumps vs. seeds scattered in pairs) and distance to the nearest Carapa procera (Meliaceae; a tree that produces seeds preferred by terrestrial vertebrates) on survival of seeds and seedlings were examined for the animal-dispersed tree species Virola michelii (Myristicaceae) in a mature forest at Paracou, French Guiana, in 1992 and 1993. We assessed the putative role of ground-dwelling mammalian herbivores, rodents, and ungulates that filter the seed shadow, acting either as dispersers or predators and thus modifying the original pattern of seed dispersal made by frugivores. We measured the effects of simulated seed burial by rodents using marked seeds and quantified the effect of protecting seeds and seedlings from ground-dwelling vertebrates on seedling germination and survival with fence exclosures in 1992. Dispersal pattern had short-term but no long-term effects on the proportion of V. michelii seeds that survived one year later as seedlings. In the short term, within six weeks, clumped seeds survived better than scattered seeds in both years. Marked seeds that were removed from their site of dispersal were eaten; rodents only rarely buried seeds of V. michelii, and seed burial reduced seed and seedling survivorship. The combined effect of the factors year and Carapa proximity significantly affected seed survival within six weeks. Although six-week seed survival was greater in 1993 than in 1992, seedling establishment was lower in 1993 than in 1992 following a lower rainfall regime during the key period of seed germination (February). One-year seed and seedling survivorship was similar between treatments and years. Seed survival and seedling establishment in V. michelii was dependent on vertebrates in the short term and on climate in the long term. Overall, seed and seedling survivorship depended on a combination of these factors.