No description available.

This thesis looks into the use of historical macrophyte records to assess long term changes in macrophyte communities in lakes and potential reasons for these changes. In particular it uses historical records to assess changes in macrophyte communities in the Norfolk Broads and West Midland Meres, two sets of lowland, eutrophic lakes in England. It provides a critical examination of the use of historical records, highlighting some of the constraints common to such data such as variations in recording effort, and bias in species recording and site selection. Having acknowledged these issues we then go on to develop a robust way to interpret such data, using a “change index” based on species persistence over the last 200 years within individual lakes. Species with high change index values, which represented species which had persisted or increased within the lake districts, were those known to be characteristic of eutrophic lakes. Conversely species with low index scores, which had declined in both the broads and meres over the last 200 years, included species associated with less fertile conditions but also a selection of typically eutrophic species. Averaging of change index scores in present day survey data served to identify the historically least changed lakes and to rank lakes in order of degree of botanical change over the last century. We then analysed the ecological basis of the change index in order to better understand the processes behind the decline of some species and survival of others in the Norfolk Broads and West Midland Meres. Functional groups determined from morphological and regenerative traits displayed significant differences in change index values in both groups of lakes, but declining taxa occurred across a wide range of plant growth forms. Non-hierarchical clustering of species based on their ecological preferences, obtained from published literature, resulted in groups with distinct change index values, indicating that changes in the status of species could be partly explained by these preferences. Of these, trophic preference was consistently the most important, with species of less fertile habitats consistently experiencing the greatest declines. However, some characteristically eutrophic species have also declined significantly, particularly in the broads. In these cases increasing loss of shallow water, low energy habitats in the broads, or loss of fluctuating water levels and less alkaline backwaters in the meres, appear to have been contributory factors. In addition to the change index approach, we also used historical records at a site level to complement palaeolimnological analysis and investigate the change in macrophyte community composition and structure at Barton Broad, Norfolk. Sediment samples were extracted from the bottom of the broad and analysed for sub-fossil remains and pollen of macrophytes. The historical records and palaeolimnological analysis combined showed that early communities did not consist entirely of low growing, oligotrophic and mesotrophic species as previously thought, but in fact comprised a mixture of these and other more characteristically high nutrient species associated with taller, or free-floating growth habit. As eutrophication progressed throughout the last century, the community was increasingly dominated by these latter growth forms. Diversity was maintained, however, since encroaching reedswamp generated a mosaic of low energy habitats which supported a range of species unable to withstand the hydraulic forces associated with more open water habitat. When the reedswamp disappeared in the 1950s, many of the dependent aquatic macrophytes also declined resulting in widespread macrophyte loss. The thesis demonstrates not just the complexities of using historical records, but also ways in which these can be overcome to make useful observations about macrophyte community change and lake ecological integrity to inform conservation and lake management, both on a site and lake district level.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg 1992. / The aim of the study was to develop understanding of the relationships between vegetation types of the grassland biome of South Africa and the environment, with an emphasis on structural and functional characteristics. The grassland biome in South Africa has traditionally been divided into 'pure' grasslands, assumed to be climatically determined, and 'false' grasslands of recent anthropogenic origin. A review of literature from several disciplines including palaeobotany, archaeology, ecology and biogeography indicates that this is not a valid distinction. It is clear that the distribution of the grassland biome as a whole is poorly understood, but the general correlation between the distribution of biomes and climate elsewhere in the world suggests that this warrants more detailed investigation. A water balance approach was used to develop climatic incices that both predict the distribution of grasslands, and are easy to interpret biologically. The indices are the mean. number of days per annum when moisture is available for plant growth, tbe mean temperature on days when moisture is available for plant growth (wet season temperature),. and the mean temperature when moisture is not available for plant growth (dry season temperature). Based on these three.indices the grassland biome in South Africa call be distinguished from neighbouring biomes. The fynbos and succulent karoo biomes have rainfall in winter. The grassland, nama-karoo and savanna biomes have' rainfall in summer. The forest biome experiences rainfall throughout the year. Of the summer rainfall biomes, the quantity of water available in the grassland biome b greater than in the nama-woo, similar to savanna, but less than forest. Grasslands experience cooler dry season temperatures than savannas. The localised distribution of woody plants within the. grassland biome suggests that it is the effect of climate on the fire regime that may be of overriding importance h'l determining the distribution of the biome as a whole. Woody elements are restricted to sites that are either protected from fire, or experience fires of lower intensity than sites that support- grassland, The unifying feature of the grassland biome is its proneness to fire. The presence of a warm, moist season promotes plant production and leads to a high standing crop close to the ground. The prolonged dry season causes vegetation to dry out annually, rendering it flammable. More arid biomes have plants more widely spaced, making it difficult for fire to spread. In more mesic biomes where rainfall is less sea.sonal than in the grasslands or savannas, fuels do not dry out sufficiently to ignite, A number of additional climatic features may promote burning in the grassland biome, It has the highest lightning density of all South Africa's biomes. 'tVarm, dry 'berg' winds desiccate fuels and 1 omote burning in the more mesic grasslands, The 'curing' of the grass sward due to dry season frost and temperature drop is important in establishing early dry season flammability. Savanna trees are fire tolerant, but they appear sensitive to the cold temperatures prevaient in the grassland biome in. the dry season, The relationship between the distribution of functional characters of grassland plants and environmental conditions was investigated. The distincrion between sweetveld, mixed veld and sourveld was recognised as one of the most Important functional features of South Africa's grasslands, The distribution of these vegetation types was examined in detail. Sweetveld occurs In warm, dry areas; sourveld in cool, moist areas. There Is overlap between these tyP.Js that Is dependant on soil nutrient status. Sweetveld that occurs in climatic conditions that would be expected to support mixed veld and sourveld, is on soils derived from basic parent material, including basalt, dolerite, gabbro and norite. Similarly, sourveld that occurs in areas that climatically would be expected to support sweetveld, is on soils derived from acid parent material such as sandstone and quartzite .. Soil nutrients that are most highly correlated to the occurrence of these three veld types are phosphoms availability and an index of nitrogen mineralization potential. 'l'here is an increase in bot; available phosphorus and the index of readily mineralizable nitrogen from sourveld to mixed veld to sweetveld. These features am inc01).10111tedinto a conceptual model that relates the distribution of these grassland types to carbon and nitrogen metabolism, with the role of phosphorus either similar to nitrogen, or else it may act indirectly by affecting the. rate of nitrogen mineralization, Nitrogen mineralization OCcursat lower water availability than carbon assimilation, and its temperature optimum is higher than that of carbon assimilation. Where nitrogen mineralization is favoured ielative to carbon assimilation, sweetveld is likely to (}C(.1\Xr. Where carbon assimilation is. favoured relative to; nitrogen mineralization, sourveld is likely to occur ....Soil texture affects the balance between these two processes in the degree to wm.r;h it protects soil organic matter, and thereforv the size of the nitrogen and ph_QSPllO_rOll.S pools. Changes in the rlj,stribution of South Africa's b~\omesfor a scenario of climate change are predicted using the biome model developed in this study. This illustrates the value of developing predictive models. / MT2017

A descrição dos padrões de acumulação de espécies com o aumento da área amostrada, através da curva de acumulação de espécies, tem diversas aplicações no estudo de comunidades vegetais. Porém o uso dessa relação como uma ferramenta para determinar a suficiência amostral em estudos fitossociológicos apresenta problemas metodológicos, como a arbitrariedade da ordem de entrada das unidades amostrais na construção da curva e a pressuposição de que um patamar será obtido no ponto em que o aumento da área amostrada não acrescentar novas espécies à amostra; e também problemas relacionados ao conceito de comunidade vegetal utilizado, que é considerada como uma entidade espacialmente discreta com composição de espécies fixa e definida. Em florestas tropicais a definição desses limites é dificultada pela alta riqueza de espécies e pela falta de estabilização da curva mesmo com grandes tamanhos de amostra. Utilizando dados de três formações florestais tropicais, foram obtidas curvas médias de acumulação de espécies e seus intervalos de confiança empíricos através de procedimentos de aleatorização, que enfatizaram o caráter assintótico da curva e evidenciaram a ausência de um ponto de inflexão para a determinação objetiva de um tamanho ótimo de amostra. Entretanto, o uso dos padrões de acumulação de espécies em estudos comparativos, seja em formações diferentes ou locais dentro de uma mesma formação, mostrou ser mais informativo. A análise das variações das estimativas de riqueza e do padrão de acumulação de espécies com tamanhos crescentes de amostra indicam que é possível identificar níveis de amostragem que permitem a distinção e a comparação entre locais. Por fim, considerando as limitações das medidas tradicionais de diversidade, como a dependência do esforço amostral, foram utilizados os índices de diversidade e distinção taxonômica para caracterizar e comparar a diversidade das diferentes formações estudadas. Além da vantagem de incorporar as diferenças taxonômicas entre as espécies à estimativa de diversidade, esses índices ainda apresentaram independência do esforço amostral e menor variabilidade, permitindo sua utilização na comparação entre áreas que tenham sido amostradas com diferentes intensidades. / The description of species accumulation patterns with the increase of sampled area using the species accumulation curve has many applications to plant community studies. The use of this relation as a tool to determine the sampling sufficiency in phytosociological studies, therefore, shows methodological problems such as the arbitrariness in the order of sampling units for the curve construction and the assumption that this curve tend to a flat line with the increase of sampled area. Another constraint is the plant community concept adopted, where the plant community is saw like a spatially discrete entity with fixed species composition. In tropical forests, the identification of communities boundaries is particularly difficult and, due to their high species richness, the species accumulation curves do not become flat, even with large sample sizes. Data from a sample of 5,74ha in three tropical forests showed a mean species accumulation curves, with empiric confidence intervals obtained by randomization procedures, which emphasized the asymptotical character of the curve. The curve also stressed the absence of a inflection point and indicated that it is not possible to objectively define a optimum sample size. Therefore the use of species accumulation patterns is more informative in comparative studies, either within or between plant formations, than in the description of a given individual study area. The analysis of the variations in richness estimates and species accumulation patterns with increasing sample sizes indicated that is possible to identify optimal sampling sizes in the comparison of different forest areas that allows to distinguish them. Finally, considering the limitations of traditional diversity measures, such as sampling effort dependency, diversity and distinctness taxonomic indexes were used to characterize and compare the diversity of the three different plant formations. Besides the advantage of incorporating the taxonomic differences among species, these indexes showed independence of sampling sizes and had estimates of low variability, which allows their utilization in comparisons of areas sampled with different intensities.

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 98-108). Also available on the World Wide Web.

<p>I have, in this thesis, studied the interactions between gray-sided voles (Clethrionomys rufocanus) and tundra vegetation, on islands in, and mainland sites close to the lake Iešjávri, in northern Norway. As isolated islands are virtually free of predation, I have been able to compare plant-herbivore interactions in the presence and absence of predators. I transplanted vegetation from an island with predators and voles, to predator-free islands with and with out voles. The results reveal the existence of a terrestrial trophic cascade as voles had a severe impact on the transplanted vegetation on the predator-free islands, but only minor effects on the mainland where predators are present. Moreover, this study shows that plant defence was only a successful strategy when predators were present. Voles reduced the abundance of all available plants during winter on the predator-free islands. The results imply that cascading effects of predation are most important for well-defended plants with grazing-sensitive morphology as these plants escape herbivore impacts in the presence of predators but are vulnerable in their absence</p><p>I studied the recovery of intensively grazed vegetation by building exclosures on islands that have been heavily grazed by voles for almost a decade.This study shows that the collective biomass of vascular plants recovered completely on three years, when voles were excluded. Although most species that are dominating the ungrazed vegetation recovered rapidly in the exclosures, the vegetation did not simply return to its ungrazed state. Herbaceous plants increased and there were pronounced differences in response among evergreen dwarf-shrub species. The semi-prostrate and tannin rich crowberry (Empetrum nigrum), showed the strongest recovery of all species, while the erect lingonberry (Vaccinium vitis-idaea) only showed weak signs of recovery. Thus, growth form determined the potential of plants to recover, whereas the trade-off between defensive investments and capacity to recover was weak or absent.</p><p>I studied the interaction between gray-sided voles and their main winter food plant, bilberry (Vaccinium myrtillus) on islands in and mainland sites close to the lake Iešjávri. I compared the abundance, population structure and palatability of bilberry ramets between vole-free islands, islands with voles but no predators and mainland sites with both voles and predators. Voles decreased the abundance of bilberry on the mainland, but the effect was much stronger on predator-free islands. Bilberry was fairly tolerant to grazing as it partially compensated for the lost tissue by producing more new ramets. Moreover, a cafeteria experiment showed that voles preferred the ramets from predator-free islands, which is inconsistent with conjectures emphasizing inducible plant defenses. The vole-bilberry interaction lacks features of delayed density dependence that could explain the vole cycles. I conducted a clipping and fertilization experiment to further investigate the effects of herbivory on palatability of bilberry shoots. Fertilization decreased the concentration of condensed tannins in shoots of bilberry and voles preferred fertilized and clipped shoots. I found no indication of induced defense that could reduce the palatability of bilberry twigs in response to herbivory.</p><p>The relationships between gray-sided vole densities, levels of invertebrate herbivory and chemical quality of leaves of Northern willow (Salix glauca) were studied on islands and mainland sites with contrasting vole densities. I found a positive correlation between level of invertebrate herbivory and vole density. The number of leaves per shoot, leaf size and leaf nitrogen content were also positively correlated with vole densities, while leaf C/N ratios were negatively correlated with vole densities. The positive correlation between vole densities and level of invertebrate herbivory is probably due to a facilitative effect of voles on invertebrate herbivores, mediated through changes in plant chemistry.</p>

This study consisted of two research projects in the Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh) alliance, the most extensive of the big sagebrush complex in the Intermountain West. In the first project, we intensively sampled 107 relatively undisturbed, late seral Wyoming big sagebrush sites across the High Desert, Humboldt, and western Snake River Ecological Provinces to investigate vegetation heterogeneity and the relationship of environmental factors with vegetation characteristics. Vegetation characteristics were highly variable across the region. Perennial grass and total herbaceous cover varied more than six and sevenfold, respectively between minimum and maximum values. Sagebrush cover averaged 12%, but ranged between 3 and 25%. With the exception of perennial grass cover (p<0.0001, r²=0.52), limited variability in other vegetation characteristics was explained by environmental variables. In the second project, we investigated the functional role of Wyoming big sagebrush by using undisturbed and sagebrush removed (with burning) treatments and comparing vegetation and microsite characteristics under (subcanopy) to between sagebrush canopy (interspace) zones. Wyoming big sagebrush influenced associated vegetation and microsites. On sites receiving high incidental radiation, perennial grass and total herbaceous cover and density were greater in the subcanopy than interspace zones (p<0.05). On north aspects, these differences were not as pronounced suggesting sagebrush's influence on associated vegetation is site dependent. Temperature extremes were mediated and soil water content was greater in the subcanopy than interspace zones during the growing season. Results indicated that the subcanopy zone can be a more favorable environment to herbaceous vegetation than the interspace zone. Wyoming big sagebrush is important to community resource capture and use. Plots with sagebrush had greater soil water content at the start of the growing season and produced more total biomass compared to where sagebrush had been removed in both post-fire years (p<0.05). However, higher Thurber's needlegrass photosynthetic rates and greater herbaceous cover and production where sagebrush had been removed suggested that more resources were available to herbaceous vegetation in the absence of sagebrush. / Graduation date: 2006

I have, in this thesis, studied the interactions between gray-sided voles (Clethrionomys rufocanus) and tundra vegetation, on islands in, and mainland sites close to the lake Iešjávri, in northern Norway. As isolated islands are virtually free of predation, I have been able to compare plant-herbivore interactions in the presence and absence of predators. I transplanted vegetation from an island with predators and voles, to predator-free islands with and with out voles. The results reveal the existence of a terrestrial trophic cascade as voles had a severe impact on the transplanted vegetation on the predator-free islands, but only minor effects on the mainland where predators are present. Moreover, this study shows that plant defence was only a successful strategy when predators were present. Voles reduced the abundance of all available plants during winter on the predator-free islands. The results imply that cascading effects of predation are most important for well-defended plants with grazing-sensitive morphology as these plants escape herbivore impacts in the presence of predators but are vulnerable in their absence I studied the recovery of intensively grazed vegetation by building exclosures on islands that have been heavily grazed by voles for almost a decade.This study shows that the collective biomass of vascular plants recovered completely on three years, when voles were excluded. Although most species that are dominating the ungrazed vegetation recovered rapidly in the exclosures, the vegetation did not simply return to its ungrazed state. Herbaceous plants increased and there were pronounced differences in response among evergreen dwarf-shrub species. The semi-prostrate and tannin rich crowberry (Empetrum nigrum), showed the strongest recovery of all species, while the erect lingonberry (Vaccinium vitis-idaea) only showed weak signs of recovery. Thus, growth form determined the potential of plants to recover, whereas the trade-off between defensive investments and capacity to recover was weak or absent. I studied the interaction between gray-sided voles and their main winter food plant, bilberry (Vaccinium myrtillus) on islands in and mainland sites close to the lake Iešjávri. I compared the abundance, population structure and palatability of bilberry ramets between vole-free islands, islands with voles but no predators and mainland sites with both voles and predators. Voles decreased the abundance of bilberry on the mainland, but the effect was much stronger on predator-free islands. Bilberry was fairly tolerant to grazing as it partially compensated for the lost tissue by producing more new ramets. Moreover, a cafeteria experiment showed that voles preferred the ramets from predator-free islands, which is inconsistent with conjectures emphasizing inducible plant defenses. The vole-bilberry interaction lacks features of delayed density dependence that could explain the vole cycles. I conducted a clipping and fertilization experiment to further investigate the effects of herbivory on palatability of bilberry shoots. Fertilization decreased the concentration of condensed tannins in shoots of bilberry and voles preferred fertilized and clipped shoots. I found no indication of induced defense that could reduce the palatability of bilberry twigs in response to herbivory. The relationships between gray-sided vole densities, levels of invertebrate herbivory and chemical quality of leaves of Northern willow (Salix glauca) were studied on islands and mainland sites with contrasting vole densities. I found a positive correlation between level of invertebrate herbivory and vole density. The number of leaves per shoot, leaf size and leaf nitrogen content were also positively correlated with vole densities, while leaf C/N ratios were negatively correlated with vole densities. The positive correlation between vole densities and level of invertebrate herbivory is probably due to a facilitative effect of voles on invertebrate herbivores, mediated through changes in plant chemistry.

No description available.

The aim of this study was to describe the plant communities within the eastern portion of the Mkuze Wetland, including only those systems that have a substantial input of water from the Mbazwane Stream, and to determine environmental factors that control distribution. The Mbazwane Wetland has a catchment of reworked sandy marine sediments, and its gradient is very shallow, such that water moving downstream through this system is free of suspended sediment and is clear. In addition, it has low conductivity suggesting a low solute load. A total of nine communities were identified in the study, based on the TWINSPAN cluster analysis. These are: Ficus trichopodalScleria angusta Swamp Forest Community; Rubus rigidus Disturbed Swamp Forest; Phragmites australis/Ficus trichopoda Precursor Swamp Forest; Typha capenis/Pycreus mundii/Leersia hexandra Marsh Community; Phragmites australis/Cladium mariscus/Ficus verruculosa Swamp Community; Cyperus papyrus Swamp Community; Echinochloa pyramidalis Floodplain Community; Ischaemum fasciculatum/Centella asiatica Floodplain Community; Paspalum distichum Floodplain Community. These communities can be divided into three broad groups, swamp forest, emergent herbaceous swamp/marsh and grassland floodplain communities. One of the main environmental variables influencing distribution was duration and depth of flooding, which separated the permanently flooded swamp and emergent swamp/marsh communities from the seasonally/ periodically inundated floodplain communities. Environmental variables that account for further division of the communities within these two groups, appear to be disturbances from fire and substratum differences related to sediment deposition from the Mkuze River, which enters the lower Mbazwane Wetland from the west. The permanently flooded swamp forest communities, are largely restricted to the northern parts of the study area, while the emergent swamp/ marsh communities, are restricted to the southern part of the study area. The swamp forest community is fringed to the west by an extremely high and steep dune, while the swamp/marsh communities are fringed to the west by much lower and more gently sloping dunes. It is suggested here that wildfires in conjunction with topography influence the distribution of these two groups of communities. An analysis of diurnal variation in temperature in the winter months (June - August) for winds (greater than l.0m/s) reveals that winds blowing from the west to north-west are associated with extremely high temperatures that persist during the day and well into the night. These are berg wind conditions that have been strongly linked to the desiccation of vegetation and promoting its susceptibility to burning in wildfires, and it is during these conditions that fires are most likely to occur in the study area. Swamp forest is situated in areas that are protected from direct exposure to these winds by the high, steep dune immediately to the west. These are thus likely to be naturally protected from fire. In contrast the herbaceous swamp/marsh communities are not protected from wind or fire by a similar topographic feature. Mature swamp forests were restricted to these 'berg wind shadow' areas, where there is complete protection from fire. Precursor and disturbed swamp forests occur where they are less protected and thus are infrequently exposed to fire. The distribution of the permanently flooded swamp/marsh communities in the areas exposed to fire appears to be related to the input of nutrients. The Cyperus papyrus Swamp Community was rooted in clay rich peat in the area around the Mkuze Delta that receives an input of clay from the Mkuze River during very high floods. In contrast, the Phragmites australis/Cladium mariscuslLeersia hexandra Swamp Community was rooted in peat with low ash content, as there is little or no input of clay from the Mkuze River, even during high floods. The seasonally/periodically- flooded communities included the Echinochloa pyramidalis Floodplain Community, the Ischaemum fasciculatum/Centella asiatica Floodplain Community and the Paspalum distichum Floodplain Community. The Echinochloa pyramidalis Floodplain Community was restricted to seasonally flooded areas receiving an input of clastic sediment from the Mkuze River during high floods, while the other floodplain communities occurred in areas receiving water from the relatively sediment free Mbazwane Stream. The distribution of these two communities appears to be influenced by the duration of inundation, with the lschaemum fasciculatum/Centella asiatica Floodplain Community being at higher elevation and therefore less frequently flooded than the Paspalum distichum Floodplain Community. / Thesis (M.Sc.)-University of Natal, Durban, 2000.