Plant Reproductive Ecology of a Secondary Deciduous Tropical Forest in Venezuela

Abstract

The breeding systems, reproductive efficacies, and densities of selected trees, shrubs, vines, and hemiparasites of a secondary decduous mid-elevation tropical forest in Venezuela are investigated. A total of 77.17 percent of the species studied are adapted for obligate outbreeding, 54.45 percent through the possession of genetic self-incompatibility, and 22.72 percent by way of dioecy or functional dioecy. Reproductive densities (density of flowering individuals) vary from 2.0 to 165.9 individuals per species per hectare with a mean of 10.7 and median of 5.0. Hemiparasites and vines form denser populations than do trees and shrubs. The densities of tree and shrub species are similar. All species, whether obligately outbred or genetically self-compatible, set abundant seed under the ecological conditions of the community. Distance between the individuals of a population does not limit seed set under obligate outbreeding. The reproductive efficacies of dioecious species are higher than those of genetically incompatible hermaphrodites. Non-autogamous compatible hermaphrodites set a lower percentage of seed than do autogamous hermaphrodites. The proportion of outbreeding species in the secondary tropical forest is similar to that reported for a speciose tropical deciduous forest in Costa Rica; however, the species of the secondary forest exhibit higher reproductive efficacies. Factors favoring the maintenance of high proportions of obligately outbred species in species-rich tropical ecosystems are discussed. RECENT INFORMATION and interest on coadaptations between plants and animals for pollination have produced a wealth of speculation and many perhapspremature conclusions concerning the breeding systems of tropical plants. Fedorov (1966) proposed a novel explanation for high species diversity in tropical ecosystems based on accelerated speciation resulting from genetic drift in small isolates of predominantly self-fertilized plants. Baker (1959), stressing the hindering effect low population density and structural complexity might have on inter-individual pollinator movement, and hence crosspollination, argued for self-compatibility in tropical plants. Percival (1974) recently inferred autogamy in 33 species in tropical scrub in Jamaica, this inference was, however, in the absence of appropriate genetic tests. It is not surprising that controlled experimental work on the breeding systems of tropical plants on an ecosystems basis is essentially lacking, for such studies have rarely been attempted in lowdiversity temperate communities. In the one landmark tropical study that exists to date, Bawa (1974) demonstrated high percentages of self-incompatibility and dioecism among the trees of a lowland deciduous forest in Costa Rica. This paper reports breeding system studies and other aspects of plant reproductive ecology of a secondary deciduous forest in Venezuela. We initiated the study with several objectives in mind: firstly, to obtain breeding system data on a community basis; secondly, to test whether intra-specific population density affects inter-individual pollinator movement and hence seed set under obligate outbreeding, and thirdly, to determine the kinds of pollinator strategies which occur in the secondary tropical forest. CLIMATE AND ECOLOGICAL HISTORY OF STUDY SITE All experimental work was conducted in the Biological Reserve (Arboretum de la Escuela de Biologia Universidad Central de Venezuela), Colinas de Bello Monte, Edo. Miranda, Venezuela (elevation ca. 1100 m, 100 30' N, 660 53' W), which is part of the low mountainous area fringing the southwestern side of the Valley of Caracas. The reserve contains two hectares of continuous, secondary deciduous forest (Tropical Dry Premontane Forests, Holdridge et al. 1971), considered successional (Ewel and Madriz 1968), consisting of two poorly defined strata: 1) a tree strata of 5-8 m height with no single dominant, and 2) a lower strata of shrubs, herbaceous perennials, and intermixed agavaceous and cactaceous succulents, these last probably representing elements of lPresent address: Instituto Universitario de Tecnologi'a, Coro, Edo. Falcon, Venezuela. 2Present address: Departamento de Biologia, Facultad de Ciencias, Universidad de Chile, Santiago, Chile. BIOTROPICA 10(3): 221-23