POLLEN DISPERSAL IN LOW-DENSITY POPULATIONS OF 3 NEOTROPICAL TREE SPECIES

Studies of mating patterns of tropical trees, typically involving comm on species, have revealed that most species are outcrossed and that, i n some cases, a significant fraction of outcross pollen moves long dis tances. We evaluated mating systems and effective pollen dispersal for three hermaphroditic, insect-pollinated Neotropical tree species, Cal ophyllum longifolium, Spondias mombin, and Turpinia occidentalis, all of which occurred at low adult densities at the study site. Mating pat terns were estimated for each maternal tree within 84-ha populations o f C. longifolium and S. mombin in 1992 and 1993 and within a 50-ha pop ulation of T. occidentalis in 1993. Each population was 100% outcrosse d. Multilocus paternity exclusion analyses indicated that in C. longif olium, a minimum of 62% of effective pollen moved at least 210 m. For S. mombin, estimates of apparent pollen flow greater than 300 m were 5 .2% and 2.5% in 1992 and 1993, respectively. For all species, pollen d ispersal was strongly affected by the spatial distribution of reproduc tive trees. Where flowering adults were evenly spaced, a large fractio n of effective pollen moved at least a few hundred meters and well bey ond the nearest reproductive neighbors. Conversely, where flowering tr ees were clumped, the majority of matings were among near neighbors. T he minimum area required to encompass a natural breeding unit was esti mated for each population.