SEED DISPERSAL, SEED PREDATION, AND SEEDLING RECRUITMENT OF A NEOTROPICAL MONTANE TREE

Abstract

Postdispersal fate of seeds from Ocotea endresiana (Lauraceae), a bird-dispersed Neotropical montane tree, was studied in Costa Rica to determine the influence of seed dispersers, seed predators, and microhabitat characteristics on seedling recruitment. Particular emphasis was placed on finding naturally dispersed seeds in order to study the link between dispersal and postdispersal fate of seeds. Four species of birds (Emerald Toucanet, Aulacorhynchus prasinus; Resplendent Quetzal, Pharomachrus mocinno; Three-wattled Bellbird, Procnias tricarunculata; and Mountain Robin, Turdus plebejus) dispersed the seeds by regurgitation, and one species (Black Guan, Chamaepetes unicolor), by defecation. Most seeds (80%) were dispersed within 25 m of parent trees and under high (>92%) canopy cover. Bellbirds deposited 52% of the seeds they dispersed under habitual song perches in standing dead trees on the edges of treefall gaps >25 m from parent trees. In contrast, the other four species dispersed only 6% of the seeds they dispersed >25 m from parent trees, and <3% of the seeds were dispersed to gaps. Thus, male bellbirds provided predictable, nonrandom dispersal to a different microhabitat than the other four species. Seed predation, germination, and one-year seedling survival were assessed for naturally dispersed seeds and for seeds placed at randomly located sites. Seed predation during the first 12 mo after dispersal was extremely high (99.7%). At least 50% of seed removal was attributable to small rodents, such as Peromyscus mexicanus, with no evidence of scatterhoarding or secondary dispersal. Seeds dispersed >20 m from parent trees were removed more slowly than seeds directly under the crown, but over the course of the fruiting season seed predation approached 100%, throughout the study site. Seed predation did not differ between gaps and forest understory. Germination of seeds protected from mammals was high (70–90%), regardless of microhabitat or seed size. Seeds regurgitated by birds and seeds removed from fruits by hand germinated equally well (98%), but seeds in intact fruits did not germinate. Seeds defecated by guans and regurgitated by other species in the field had similar germination success. Thus, dispersal by birds is important for removal of pulp but does not otherwise affect germination. Seedlings were taller in gaps (<90% canopy cover) than in closed-canopy forest (>90% cover). Fungal pathogens killed fewer seedlings in gaps than in understory, while other causes of seedling mortality were similar among microhabitats. Consequently, bellbirds, the most likely species to disperse seeds to gaps, had a disproportionate influence on seedling recruitment. Nevertheless, the absolute number of seedlings was higher in closed canopy forest, because the great majority of seeds (85%) landed there. The overall pattern of recruitment, averaged among trees, had a large peak corresponding with bellbird song perches in gaps, as well as a smaller peak near the parent tree corresponding with the seeds dispersed by other species. The relative height of these peaks is opposite that of the original pattern of dispersal. Frugivorous birds that use habitual perches may have disproportionate effects on plant recruitment. To test this idea further, it is necessary to study each stage of the recruitment process, including seed dispersal, postdispersal seed survival, and seedling survival.