Population Ecology and Microhabitat Distribution of Dipteryx panamensis, a Neotropical Rain Forest Emergent Tree

Abstract

We report data on population structure, growth, mortality, and distribution among microhabitats of Dipteryx panamensis (Papilionoideae), an emergent tree of tropical wet forest at the La Selva Biological Station, Costa Rica. We studied all seedlings (1980 cohort) on a 1-ha plot from 7 months to 5 yr post-germination. Population structure was determined by complete inventory of non-seedlings on a 4-ha plot. Growth, microsite distribution, and 2-yr survivorship were measured for individuals obtained by systematic searches of > 150 ha of primary forest (-"extensive sample"). At 7 mo post-germination, 147 seedlings remained on the 1-ha plot; only two were in gaps (sensu Brokaw 1982). The 4-ha population averaged two individuals/ha > 300 mm in diameter, but included only one tree 100-300 mm in diameter. In the extensive sample, the 117 individuals > 300 mm in diameter showed a continuous distribution of diameters. Seedling mortality was 97 percent from 7 mo to 5 yr post-germination. Rates of mortality decreased with increasing diameter, 104 of 105 individuals .gtoreq. 100 mm in diameter survived 2 yr. Diameter growth of individuals .ltoreq. 10 mm in diameter averaged < 1 mm/yr, while individuals 100-700 mm in diameter averaged 5-8 mm/yr. Diameter growth was positively correlated with crown position (Dawkins and Field 1978) for individuals .ltoreq. 100 mm in diameter. Both diameter growth and height growth were higher in building-phase forest than under mature forest canopy. In the extensive sample, only one of 93 trees .ltoreq. 300 mm in diameter was in a gap. All but one juvenile .ltoreq. 100 mm in diameter (N = 83) had at least one crown of another plant above it, and most crowns were lit from the side, not from overhead. Five of the six Dipteryx 100-300 mm in diameter had vertically lit crowns. The continuous distribution of adult diameters and relatively rapid growth of large juveniles and small adults indicate that Dipteryx is regenerating in situ, and is therefore not a relict of past regional disturbance, as has been hypothesized for some emergents. Long-term measurements of juvenile growth and survival in relation to microhabitat are necessary to adequately describe the regeneration of tropical rain forest trees.