Ficus Stupenda Germination and Seedling Establishment in a Bornean Rain Forest Canopy

Factors limiting seed germination and seedling establishment of the hemiepiphyte Ficus stupenda were investigated with two field experiments conducted in a Bornean rain forest canopy. In Experiment 1, seeds of F. stupenda were planted in potential establishment sites averaging 31 m above the ground in each of 45 dipterocarp trees. Twenty seeds each were planted in a total of 336 sites. Germination, survivorship, and growth were monitored over 1 yr, and examined in relation to microsite and host tree characteristics. The presence of substrate with good moisture retention (soil, rotting wood, or moss) was the most important factor for germination. Such substrates were most frequently associated with knothole sites in the canopy, which had the highest level of establishment success. Seedling survival to 12 mo was low (1.3% of planted seeds), especially considering that the best available sites in each tree were selected. Only 0.04% of seedlings showed vigorous growth after 12 mo. A seed—harvesting ant in the genus Pheidole significantly reduced germination success, and herbivory and desiccation killed many seedlings at later stages. In Experiment 2, seedling growth in natural canopy sites was compared with growth of seedlings in artificial planter boxes raised into the canopy in order to assess the relative quality of canopy sites. Planters were superior to natural sites for seedling survivorship and supported much more rapid seedling growth with a positive response to light level. Higher growth rates in planters compared with natural sites showed that water stress appeared to be the critical factor limiting seedling growth in the canopy, even in this very wet climate. Sites in the canopy with the optimal combination of conditions for fig seedling establishment appear to be very scarce. Ficus population densities may be limited by both biotic and abiotic factors reducing early recruitment success.