Seed Fate in a Tropical Mistletoe: The Importance of Host Twig Size

Abstract

1. In a bird-dispersed neotropical mistletoe two aspects of seed deposition are hypothesized to influence seedling establishment on host trees: the size of the host stem on which seeds are deposited and whether seeds are deposited singly or in small groups. 2. In Monteverde, Costa Rica, seeds of Phoradendron robustissimum (Viscaceae) were planted on 10 host trees (Sapium oligoneuron; Euphorbiaceae) and seedling establishment was followed for 3 years. 3. Seed clumping (groups of five vs single seeds) had no effect but stem size had a strong effect on seed persistence and seedling establishment. 4. Seedling establishment was most frequent on twigs in the 10-14mm diameter class; however, at least one seedling established on all but the largest (>80 mm diameter) of seven size classes of stems used in the experiment. 5. Causes of seed loss and seedling mortality showed directional patterns across the range of stem sizes; small twigs died frequently, leading to death of seedlings on them, whereas germinated seeds often died in situ on large branches, apparently unable to penetrate the thick bark of the host tree. 6. In the smallest two size classes of host stems (<10 mm diameter), twigs with mistletoe seeds died more frequently than those without mistletoe seeds, suggesting that mistletoe seeds can induce death of small host twigs. 7. Many seeds disappeared shortly after being planted and an exclusion experiment suggested that missing seeds may have been eaten by arboreal seed predators. 8. Thus, the size of twigs on which mistletoe seeds were deposited strongly influenced seed fate