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.

Interaction between orchids and fungi belongs to little understood aspects of orchid biology. This thesis investigated an identity of mycorrhizal and other fungal symbionts of several European orchid species and also an influence of these fungal symbionts on ecology, distribution, and evolution of the studied orchid species. Diverse methodological approaches were used including in situ seed germination, culture-dependent and -independent techniques for fungal isolation and identification, molecular phylogenetics, stable isotope analyses, and transmission electron microscopy.

Common heliotrope (Heliotropium europaeum L.) is an herbaceous Mediterranean summer annual that grows on areas devoid of vegetation in the dry-land cropping region located in the north-west of the state of Victoria, Australia. This region is known as the Mallee, and common heliotrope is considered a weed here because it transpires soil water that could otherwise be used by ensuing crops, and is toxic to livestock. / In this study, laboratory experiments have shown that germination of seeds of common heliotrope is not limited by light, cold treatment, or a leacheable inhibitor. Temperature and water potential (and perhaps depth inhibition) are the principal environmental factors that limit germination. Although 100 % of seeds will germinate under optimal conditions, these conditions are rarely met in the field, and the percentage of seeds that germinate at sub-optimal conditions changes seasonally and between Australian populations. This is the principal mechanism of dormancy in the species, ensuring that seeds do not germinate when conditions are unfavourable for continued growth and that in the field, germination of a seed population is fractional, spreading risk temporally. Germination of seeds of common heliotrope does not conform to the assumptions of the hydrothermal time model frequently used to predict field emergence of weed species. / Field observations and simulated rainfall experiments on a consistent soil type (Calcarosol with a clay loam surface texture and clay subsoil) indicated that approximately 20 mm of rainfall is the minimum amount required for germination and emergence of common heliotrope. Field experiments using lysimeters indicated that this is also the amount required for minimal reproduction on the same soil type. Although common heliotrope can successfully reproduce upon the rainfall event which causes its germination, its growth is indeterminate, and further access to moisture will result in massively increased reproductive output. Plants will continue to grow and produce seed over summer until they are killed by drought or senesce in autumn. Laboratory and field studies showed that root growth in common heliotrope is relatively slow, and is opportunistic in areas where moisture becomes available. / Simulation of soil water and temperature fluxes showed that soil type has a large impact on the amount and duration of water potential that seeds and plants are exposed to. In the case of common heliotrope, this is critical in determining regional prevalence and distribution in the north-west of Victoria. It is recommended that studies of arid species reliant on isolated rainfall events for emergence should consider absolute water availability and not in terms of rainfall amount alone. / Analysis of long term summer rainfall data from the Victorian Mallee indicates that there is potential for reducing the cost of controlling common heliotrope by using residual pre-emergent sulfonylurea or triazine herbicides, instead of the traditional post-emergent herbicides or cultivation. More information is needed on the efficacy of such residual herbicides on common heliotrope and other summer weeds, and their impact on subsequent crops before the full potential of this management option is known. Reduced cost of control could also be achieved by economic analysis of the likely benefits of controlling each emerging cohort of common heliotrope, and a framework for decision making is suggested.

[Truncated abstract] Annual grasses have colonised a diverse range of environments in southern Australia. The “Silvergrasses” of the genus Vulpia are excellent examples being widely distributed, are prevalent weeds of agriculture and have had a long history to naturalise on the continent. Research was undertaken on Vulpia populations to identify if naturalising species have reproductive traits that provide propagules with the best chances of success. Furthermore, research aimed at investigating if these traits vary between species and their populations and how this variability related to the environment. A herbarium and field study was undertaken to establish what Vulpia species occur in SW Australia and to investigate environmental factors affecting their distribution. 169 herbaria specimens was examined and a botanical field survey of 189 sites was carried out in September 1998. Four species occur in the region: V. fasciculata, V. muralis, V. bromoides and two variants of V. myuros (V. myuros var. megalura and V. myuros var. myuros). V. bromoides and V. myuros were introduced early into the region while V. fasciculata and V. muralis more recently. It is plausible that Vulpia invaded the region via early seaport settlements and was spread by agricultural expansion. 96% of field sites contained V. myuros var. myuros, 79% V. myuros var. megalura, 50% V. bromoides, 6% V. fasciculata and 6% contained V. muralis. 90% of sites contained a mix of species and 9% of sites contained pure species stands. V. myuros var. myuros is the most widespread species and dominant form of V. myuros. It is found from high rainfall regions through to arid locations occurring on mostly light textured low fertility soils. V. muralis and V. fasciculata occur infrequently with the former widely dispersed and the later occurring predominantly on sands. V. bromoides occurs extensively in high rainfall regions but rarely extends to locations receiving less than 400-450mm annual rainfall and northward above 30°00’ latitude. It is predominantly on light to loamy textured soils that are fertile and acid. The most common species V. myuros and V. bromoides often coexist within sites but the dominance of one over the other is strongly correlated with growing season length and false break frequency. V. bromoides is positively correlated to growing season length and V. myuros is negatively related. The distribution of Vulpia species is strongly influenced by climate and soils. Variability in distribution is a reflection of the ecological differentiation between species to colonise different environments