School of Agriculture, Food and Ecosystem Sciences - Theses

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    Photosynthetic responses to light, nitrogen, phosphorus and pruning of Eucalyptus in south-eastern Australia
    Turnbull, Tarryn Louise ( 2005)
    Eucalypts frequently grow faster after additions of fertiliser, but more slowly in the shade or following `green pruning'. The coupling of rates of growth to environmental factors is at least partly due to acclimation of photosynthetic processes. Photosynthesis rarely proceeds at maximum rates in natural environments as photosynthetic processes and the supply of basic requirements of photosynthesis (CO2, H20, light, phosphorus and nitrogen) vary at both short (minutes to hours) and longer (days to months) time scales. Currently we lack mechanistic explanations for how these variables, alone and in combination underpin changed growth rates in Eucalyptus. This study examined growth and photosynthetic characteristics in glasshouse-grown seedlings and field-grown trees of Eucalyptus species that are commonly planted for forestry and revegetation purposes in central Victoria. Acclimation to light (among seedlings and within canopies), nutrient availability (phosphorus and nitrogen) and increased sink-strength for photosynthates were the primary foci of the study. In each instance I examined distribution of leaf nutrients within a canopy and allocation of N to Rubisco and chlorophyll to assess the degree to which nutrients limit photosynthesis in Eucalyptus. A novel technique was introduced to quantify the allocation of inorganic phosphorus within cells (cytoplasm versus vacuole), followed by an assessment of inorganic phosphorus allocation in response to a long-term reduction in phosphorus supply. In all circumstances, rates of growth were responsive to environmental conditions. Growth responses were underpinned by altered patterns of biomass partitioning and changed leaf morphology more than by rates of photosynthesis per se. There was little difference in adaptive strategies implemented by seedlings and trees: both were oriented towards the accumulation of nutrients rather than increasing rates of photosynthesis. Photosynthesis was reduced by shading (among different plants and within the canopy of a tree) and reduced phosphorus supply whereas N had little effect on photosynthesis. Analysis of pools of inorganic P revealed that adequate supplies were maintained for photosynthetic processes regardless of P supply, therefore reduced photosynthesis follows, rather than leads, a more general leaf-level response to reduced P. Similarly, changed partitioning of nitrogen between Rubisco and chlorophyll was unnecessary as leaf nitrogen concentrations were consistently maintained at well above published minimum levels. Hence, photosynthesis was not up-regulated following increased nitrogen or phosphorus supply; instead excess nutrients were accumulated and used to support increased biomass. One exception was after defoliation, when up-regulation of photosynthesis was observed, presumably to ensure the demand for photosynthates could be met by a reduced leaf area. Sensitivity analyses consistently revealed variation in photosynthetic rates owed more to altered biochemical activity (e.g. Jmax and Vcmax) rather than stomatal conductance regardless of growth condition (glasshouse versus field). Hence, whilst Eucalyptus has considerable photosynthetic potential, faster rates of carbon fixation are only exhibited in the short-term. In part, this is due to the multiplicity of factors involved in `optimisation' of photosynthesis and their individual and collective responses to environmental conditions. In the long term however, increased canopy photosynthetic capacity follows only an increased photosynthetic area.
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    An evaluation of fuel-reduction burning in the dry sclerophyll Wombat State Forest
    Wibowo, Ari ( 1994)
    Each year, between one and three per cent of the total forested area in Victoria is fuel-reduced as a part of the Department of Conservation and Natural Resources' program to implement its overall fire protection policy. The dry sclerophyll Wombat State Forest has been fuel-reduced since the 1960's, - a practise considered important because of its susceptibility to fire and its highly flammable plant community. This thesis presents the results of an evaluation of fuel-reduction burning in the Wombat State Forest, commencing with a review of forest conditions, principles of fuel-reduction burning, fire behaviour and fire effects. This is followed by observations and measurements in the field to document current practices of fuel-reduction burning, to select an appropriate fire-behaviour prediction model, to identify time-intervals between successive burns and to evaluate the effects of fuel-reduction burning on forest trees. Historical data of wildfires were used to evaluate the effectiveness of fuel-reduction burning in limiting the number, area and losses from wildfires. "Multi-criteria analysis" was then applied to select priority areas for fuel-reduction burning. The fuel-reduction burning program which is conducted 'every year during autumn and spring requires detailed planning and preparation, because it can only be carried out under certain prescriptions for weather, fuel and fire behaviour. Depending on the extent and conditions of each area, either ground or aerial ignition is applied. Large areas ignited from the air often result in variations of fire behaviour and intensity that leave many un-burnt patches within the broad areas of burnt forest. In order to predict fire-behaviour, comparisons were made between the Control Burning Meter, the McArthur Fire Danger Meter and the Fire Behaviour Tables for Western Australia (the Red Book). From a study of small trial plots, it was shown that the fire behaviour model incorporated in the Control Burning Meter for messmate-gum or silvertop forest type provided quite realistic predictions of low-intensity prescribed fires compared with predictions based on the McArthur Forest Fire Danger Meter and the Fire Behaviour Tables for Western Australia. Since its inception, fuel-reduction burning in the Wombat State Forest has decreased the quantity of litter and twigs, but it has had no significant effect on the quantity of humus and coarse fuels. This reduction of flammable fuels has reduced the potential for major fires and the study showed that a fuel-reduction burning cycle of less than five years is required to restrict the build up of fine fuels to acceptable levels. With regard to the impact of fuel-reduction burning, it was shown that it causes scorches on most trees, and that there is a significant relationship between scorch-height and tree diameter for the two dominant species, messmate (Eucalyptus obliqua L'Herit) and peppermint (E. radiata Sieb. ex. DC.). However, because of their different bark types, average scorch height was higher for messmate. Due to the considerable variation in the severity of fire seasons, hence in the frequency and severity of fire, no significant differences were detected in the number of wildfires and the extent of areas burnt, before and after the application of fuel-reduction burning in the Wombat State Forest. However, analysis of the historical fire data led to the conclusion that fuel-reduction burning has achieved its objective of limiting the severity of wildfires. Most of the wildfires occurred on areas that either had never been fuel-reduced or that had been fuel-reduced for more than five years. The costs of fuel-reduction burning have been relatively modest in comparison with the potential losses. Therefore, when properly planned and implemented, fuel-reduction burning is a valuable management tool for protecting forests and wider community values. This study has also provided a general idea on the applicability of "multi-criteria analysis" for identifying priority areas for fuel-reduction burning in the Wombat State Forest, with priority areas being selected on the basis of level of fire hazard (fuel, weather and topographical conditions) and values of particular sites (significant values, timber quality, distance from the nearest township and extent of the area). The result was a ranking of areas according to their priorities for burning.
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