No description available.

No description available.

No description available.

The coexistence of plant species in highly diverse communities is one of the enduring questions in ecology. The Hutchinsonian niche is defined as an n-dimensional hypervolume incorporating every aspect of the life history of a plant but in practice very few of those aspects have been examined in detail for a suite of coexisting tropical forest plants. In this thesis I examine the germination and seedling growth and mortality responses of up to 20 species of neotropical pioneers in relation to abiotic arid biotic factors in their environment. Variation in the germination response to the environmental indicators of the presence of a gap was strongly related to seed mass and the environmental constraints seed mass places on regeneration. Smaller seeded species required irradiance to stimulate germination and emergence occurs from the superficial depths of soil that receive irradiance. Both within the small-seeded group of species and in comparison with the larger seeded species, seed mass was positively related to the degree of discrimination in requirements to stimulate germination. Seed mass was related to the amount of resources and the amount of physical defences, both of which enhance seed longevity. Larger seeded species (seed mass > 1.5 mg) were able to persist in the seed bank and emerge from lower in the soil profile. Some large-seeded species possessed a positive germination response to increasing magnitude of diel temperature fluctuation, and measurements suggested that temperature fluctuations penetrated lower in the soil profile than irradiance. The seedling growth and mortality of three species in relation to gap size predicted the known differences in adult distribution for these species at my study site. Differences between species in response to gap size were enhanced but not created by herbivory. Trade-offs between rate of growth at high irradiance and growth and survival rates in low irradiance, and between growth at high irradiance and growth when water and light were simultaneously limiting contributed to the contrasts in response to gap size among six species. I found no evidence of maintenance of the rank of species' growth rates across irradiance treatments or for a trade-off between maximum rate of growth and susceptibility to herbivory. These results illustrate contrasting responses to environmental heterogeneity within the pioneer functional group at my study site. These differences represent a potential mechanism for coexistence of ecologically similar species at one site. I conclude that niche differentiation contributes to the maintenance of tropical forest diversity.

Examination of the imbibition characteristics of eighty chickpea genetic lines (53 Desi and 27 Kabuli types) and eleven cowpea cultivars revealed that in both species seeds with unpigmented testae imbibed more rapidly than the pigmented seeds. Rapid imbibition was associated with high solute leakage, indicative of possible imbibition damage. Further evidence of imbibition damage was revealed in cowpea where the rapidly imbibing unpigmented seeds had poor vital staining with tetrazolium chloride after 24h imbibition. In chickpea, all the genetic lines of white Kabuli seeds had similar rapid rates of imbibition. In contrast, the pigmented Desi genetic lines revealed a range of rates of water uptake although many imbibed more slowly than the Kabuli lines. The incidence of delayed imbibers, that is, seeds which failed to imbibe until after 8h in water, was the main reason for reduced rates of imbibition in Desi lines. The restriction to water uptake by the pigmented seeds was mainly influenced by the properties of the seed coat. In cowpea seeds, the permeability of the seed coat was the most important factor limiting water uptake whereas in chickpea the permeability of the seed coat as a whole and of the micropyle as well as the degree of adherence of the seed coat to the cotyledons were important in regulating the movement of water into the seed. Investigation of the rate of imbibition during the maturation of dwarf French bean, cowpea and chickpea seeds revealed that development of slow water uptake by coloured cultivars was clearly associated with the appearance of pigmentation. In chickpea, this also coincided with the development of the adherence of the seed coat to the cotyledons. In contrast, seeds from unpigmented cultivars imbibed rapidly at all stages of maturation. Examination of the seed coat anatomy of chickpea and cowpea indicated that the pigmented seeds tended to have shorter and narrower palisade cells and also shorter tracheid bars than the unpigmented seeds. Differences in water uptake by Kabuli and Desi seeds were not evident when seeds were stored at 100% relative humidity and 40<SUP>o</SUP>C for days. All seeds deteriorated at a similar rate. However, the importance of initial seed quality in influencing deterioration during storage was emphasised by the rapid loss of germination of the Kabuli seeds in which germination and vital staining revealed poor initial quality.

Niche-differentiation has been proposed to explain the coexistence of large numbers of tree species in tropical forests. Previous studies on established individuals have found little evidence for this. However, niche-differentiation may be apparent at the seed and seedling stages. This study investigated the effects of environmental variation in germination and seedling establishment for a range of Central Panamanian species. Firstly, topography has a large impact on soil water availability; slopes are wetter than nearby plateaux. The potential impact on seedling emergence and survival was studied by monitoring seedling emergence and survival over 15 months. This revealed that small-seeded species are unlikely to establish on plateaux, presumably because of the rider conditions. Secondly, the germination of four pioneer species in response to four environmental variables, related to canopy gap size, was investigated. Species exhibited consistent patterns of response suggesting adaptation for germination in particular gap sizes; Piper marginatum responded to nitrate, had a low base potential for germination and tolerated large temperature fluctuations, suggesting adaptation for germination in comparatively large gaps. Conversely, Pipe peltatum exhibited responses, which suggested adaptations for germination in small gaps. Finally, germination and seedling mortality for four pioneer species in two sizes of canopy gap was predicted using computer modelling. This indicated that large-seeded pioneers can germinate and establish in large gaps. Conversely, percentage germination and survival of small-seeded species was low in large gaps. However, small-seeded species produce greater quantities of seed than large-seeded species. Consequently they may "win by default" in large gaps, because of the absence of seeds of larger species. These results indicate that there are a number of axes of environmental variation along with nice-differentiation can occur. Furthermore, the response is variable between species. Thus niche-differentiation is important for fostering species coexistence, although density-dependent mortality and dispersal-limitation also contribute.

No description available.

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 123-132). Also available on the World Wide Web.

No description available.

No description available.