Numerous studies of morphological and physiological traits (reviewed in 3, 57, 95, 132, 135) as well as the more recent studies of enzyme loci (reviewed in 21, 52, 63) have generally shown the existence of significant amounts of genetic variation both within and between plant populations. These reviews (21, 52, 63) have also demonstrated considerable variation among species in intrapopulation allozyme variation. For instance, Hamrick (63) found that populations of woody plants contained higher levels of genetic variation than did populations of herbaceous species; Gottlieb (52) and Brown (21) demonstrated that predominantly outbreeding species maintain higher levels of intrapopulation genetic variation than predominantly inbreeding species. Nevo (111) found that habitat generalists and animal species with cosmopolitan or tropical distributions were typically more variable than species with specialized habitat preferences or temperate distributions. While these results are interesting and informative, they consider the influence of only a few evolutionarily important characteristics on the main-

https://www.jstor.org/stable/info/2096789

Relationships Between Life History Characteristics and Electrophoretically Detectable Genetic Variation in Plants

Diversity has been given a central role in attempts to develop a general theory of ecology (Johnson and Raven, 1970). Much attention has been focused on large-scale trends, such as the increase in species diversity with decreasing latitude (Pianka, 1966; Mac Arthur, 1965; Fischer, 1960; Tramer, 1974), but there is surprisingly little documentation of the equally striking changes in diversity within the tropics. Moreover, additional information on tropical plant species diversity patterns is critically needed because of their potential importance in resolving some of the fundamental and theoretically significant differences of opinion as to the nature and manner of regulation of species diversity.

Patterns of neotropical plant species diversity.

Upper Amazonian data for tree species richness in 1-hectare plots are reported. All plants ≥10 cm diameter were censused and identified in six plots in Amazonian Peru and one on the Venezuela-Brazil border. The two plots from the everwet forests near Iquitos, Peru, are the most species-rich in the world, with ≈300 species ≥10 cm diameter in single hectares; all of the Peruvian plots are among the most species-rich ever reported. Contrary to accepted opinion, upper Amazonian forest, and perhaps Central African ones, have as many or more tree species as comparable Asian forests. Very high tree species richness seems to be a general property of mature lowland evergreen forests on fertile to moderately infertile soils on all three continents.

https://www.pnas.org/content/pnas/85/1/156.full.pdf

Tree species richness of upper Amazonian forests

Gene flow via seed and pollen is a primary determinant of genetic and species diversity in plant communities at different spatial scales. This paper reviews studies of gene flow and population genetic structure in tropical rain forest trees and places them in ecological and biogeographic context. Although much pollination is among nearest neighbors, an increasing number of genetic studies report pollination ranging from 0.5-14 km for canopy tree species, resulting in extensive breeding areas in disturbed and undisturbed rain forest. Direct genetic measures of seed dispersal are still rare; however, studies of fine scale spatial genetic structure (SGS) indicate that the bulk of effective seed dispersal occurs at local scales, and we found no difference in SGS (Sp statistic) between temperate (N=24 species) and tropical forest trees (N=15). Our analysis did find significantly higher genetic differen- tiation in tropical trees (FST=0.177; N=42) than in temperate forest trees (FST=0.116; N=82). This may be due to the fact that tropical trees experience low but significant rates of self-fertilization and bi-parental inbreed- ing, whereas half of the temperate tree species in our survey are wind pollinated and are more strictly allogamous. Genetic drift may also be more pronounced in tropical trees due to the low population densities of most species.

/pdf/spatial-scales-of-pollen-and-seed-mediated-gene-flow-in-37tuu2yw5j.pdf

Spatial Scales of Pollen and Seed-Mediated Gene Flow in Tropical Rain Forest Trees

The first systematic observation of a general flowering, a phenomenon unique to lowland mixed-dipterocarp forests in Southeast Asia, is presented. During general flowering, which occurs at irregular intervals of 3‐10 yr, nearly all dipterocarp species together with species of other families come heavily into flower. We monitored reproductive phenology of 576 individual plants representing 305 species in 56 families in Sarawak, Malaysia. Observations continued for 53 mo from August 1992 and covered one episode of a general flowering cycle. Among 527 effective reproductive events during 43 mo, 57% were concentrated in the general flowering period (GFP) of 10 mo in 1996. We classified 257 species into flowering types based on timing and frequency of flowering. The most abundant type was ‘‘general flowering’’ (35%), which flowered only during GFP. The others were ‘‘supra-annual’’ (19%), ‘‘annual’’ (13%), and ‘‘sub-annual’’ (5%) types. General flowering type and temporal aggregation in reproductive events were commonly found among species in various categories of taxonomic groups, life forms, pollination systems, and fruit types. Possible causes for general flowering, such as promotion of pollination brought about by interspecific synchronization and paucity of climatic cues suitable for flowering trigger, are proposed, in addition to the predator satiation hypothesis of Janzen (1974).

Plant reproductive phenology over four years including an episode of general flowering in a lowland dipterocarp forest, Sarawak, Malaysia

IN the tropical rain forest, many tree species grow close together, and they are apparently equivalent in their edaphic requirements and spatially interchangeable. Most species occur at low density although they may be geographically widespread. No other plant community contains as many species of a single life form as do certain mixed dipterocarp forests of the aseasonal wet lowlands of Malesia1–3. Such a population structure and level of diversity makes it hard to see how speciation has occurred. Spatial isolation of individuals and the abundant, normally hermaphrodite flowers simultaneously available for fertilisation in the canopy of the dipterocarp emergents suggest the possibility of self-fertilisation, but many of the understorey species are dioecious. As part of a comparative study of the reproductive biology of rain forest tree species, intra-specific genetic variation of isozymes has been demonstrated for the first time in an emergent and an understorey species of rain forest trees. Polymorphism is common in both species and there is evidence for short-range spatial heterogeneity in gene frequency and also in leaf morphology.

Genetic variation in wild populations of rain-forest trees

Starch gel electrophoresis has been carried out on leaf extracts of Shorea leprosula, S. ovalis, and Xerospermnum intermedium, rain forest trees growing in South-west Malaysia. A high level of intra-population genetic variation is indicated by electromorphic frequencies obtained from S. leprosula and X. intermedium extracts, but not so for S. ova/is extracts which yielded remarkably uniform electromorph patterns. Evidence is presented for genetic differences between age classes. IN THE TROPICAL RAIN FOREST there is a great abundance of tree species which grow close together, appear to be equivalent in their edaphic requirements, and are apparently spatially interchangeable. These features are most dramatically displayed in certain mixed dipterocarp forests of the aseasonal, wet lowlands of Malesia and thereby pose problems as to the origin of such an immense species diversity. Although spatial isolation and the abundance of normally hermaphrodite flowers in the canopy of the very tall, emergent species suggest that self-fertilization may occur, many understory species are dioecious.

Yik-Yuen Gan

Papers

Genetic variation in wild populations of rain-forest trees

Isozyme Variation in Some Rain Forest Trees