Habitat Specificity of Two Palm Species: Experimental Transplantation in Amazonian Successional Forests

Large-seeded plant species tend to be restricted to late-seral and mature forests, yet the reason for this habitat specificity is often not understood. The typical absence of these species from younger forests may be due to either their ineffective seed dispersal to early-seral habitats or "postdispersal" factors such as life history traits and physical and biotic variables, or both. Two large-seeded palm species, Iriartea deltoidea and Astrocaryum murumuru var. javarense, were used to experimentally evaluate the role of postdispersal limitation by testing whether the species exhibit differential seedling survivorship in forest stands of different ages. Under natural conditions, both palm species are largely absent from early- and mid-seral floodplain forest but are common in late-seral and mature forest. Iriartea seedlings and juveniles are found occasionally in early- and mid-seral forest, whereas Astrocaryum are almost never found there. To assess the degree to which postdispersal factors could account for these patterns, seeds of the two palm species were transplanted into four zones in the successional floodplain forest of Manu National Park, Peru. Three experiments quantifying seedling establishment and survivorship were carried out: a 3-yr experiment using Iriartea, in which flooding during the Ist yr was lower than average (Iriartea Dry Experiment); a l-yr experiment using Iriartea, in which flooding during the Ist yr was higher than average (Iriartea Wet Experiment); and a 2-yr experiment using Astrocaryum, in which flooding during the Ist yr was also higher than average (Astrocaryum Wet Experiment). For each experiment, four or five plots, each containing 75 seeds, were placed in each of four zones from early to late succession. The entire design was replicated in two successional transects that were separated by 6 km. Seedling survivorship of Astrocaryum was highest in the late-seral zones of both transects, which corresponded closely to its restricted natural distribution. Hence, postdispersal limitations, perhaps in conjunction with restricted seed dispersal, apparently contributed heavily to the general absence of Astrocaryum from the early- and mid-seral floodplain forests. Flooding did not appear to influence Astrocaryum seedling survivorship. Postdispersal factors also seemed to influence strongly Iriartea establishment, as evidenced by the significant variation in seedling survivorship across seral zones and between transects. Moreover, in the Iriartea Dry Experiment, seedling survivorship was negatively correlated with level of flooding. During the Iriartea Wet Experiment, survivorship was substantially lower in all seral zones, probably due to the heavy flooding. Thus, flooding, as a postdispersal factor, appeared to pray a major role in shaping the natural distribution of Iriartea in the successional floodplain forests. Because seedling survivorship in one mid-seral zone was substantially higher than would be predicted from the natural Iriartea seedling density, results from transplant experiments also suggest that limited seed dispersal may contribute to this palm's early-seral distribution. For Iriartea, seed dispersal limitation may play a role complementary to that of flooding. In Amazonian primary-successional floodplain forest, the habitat specificity of at least one, and perhaps two, large-seeded species appears to result from both dispersal limitation and postdispersal limitation factors.