Seed and Seedling Ecology of Neotropical Melastomataceae

Abstract

Models of gap—phase regeneration in tropical wet forests are based primarily on studies of canopy trees. These paradigms are derived from studies of a small number of species whose life history traits place them easily into "pioneer" or "climax" species Woody plants of the forest understory, subcanopy trees and shrubs, are known to affect significantly regeneration dynamics in treefall gaps, yet we lack fundamental information on life history characteristics of understory plants that could permit their incorporation into gap regeneration models. We investigated intra— and interspecific variability in early life history characteristics of understory Melastomataceae species of different growth forms and adult distributions from the lowland tropical rain forest at La Selva Biological Station, Costa Rica. We examined germination patterns in vitro (22 species), effects on germination of passage through birds (5 species), seedling establishment in the field (3 species), and growth in different light environments (8 species). Established seedlings were significantly more abundant in gabs than in nearby understory sites. This pattern was generally applicable to all melastomes studied, regardless of the habitat where adults were normally most abundant. Seedlings were more common in the root pits and on the root mounds of uprooted trees, even though there was less light available to the seedlings in these microsites than elsewhere in the gaps. In a series of germination trials in vitro, most seeds germinated readily within 1—2 wk. Comparisons of germination rates between seeds collected from bird droppings and seeds extracted from fruits by hand gave varied results. With one exception, seeds of shade—tolerant species germinated more slowly following passage through birds, while seeds from high—light—demanding species germinated more rapidly after passing through birds. Seeds of three common species were sown directly into different microsites within gaps and understory control sites. In contrast to the germination trials, emergence percentages in the field ranged from 4% in understory sites to 21% in gap centers. Survival after 1 yr was low (0.1%—0.4%). After 1 yr, these seedlings were small (<2 cm tall) and produced few leaves. In similar light environments in the shade—house, melastome seedling survival was between 80% and100%, and growth increased with increasing light levels from 2% (corresponding to understory sites) to 20% (small gap centers) of full sun. With the exception of two species common to abandoned pastures and river banks, the species used in the shade—house growth experiments did not show significant increases in growth with increases in light form 20% to 40%. Suites of early life history traits described for tropical tree species are not consistent with the variation in pattern seen within this one family of understory species.