School of Botany - Theses
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ItemSilver wattle (Acacia dealbata): its role in the ecology of the mountain ash forest and the effect of alternative silvicultural systems on its regeneration( 1999)The key objectives of this thesis were to evaluate the affects of alternative silvicultural systems on regeneration of silver wattle and to determine whether nitrogen-fixation by silver wattle replaced losses of nitrogen by timber harvesting and burning and whether regeneration of silver wattle affected the growth of mountain ash. Germination of silver wattle is greatest after heating seeds to 80°C for 5 minutes. Heating to 100°C for more than one minute kills the seeds and lower temperatures fail to rupture the coats of many seeds. However, 30 - 35% of seeds germinate with no pretreatment. Sunlight and severe mechanical abrasion stimulate germination of a significant proportion of seeds. At Tanjil Bren the average number of seeds of silver wattle in the top 10 cm of soil is 350 per m2 . Few seeds germinate from below 10 cm in the soil. The present method of timber harvesting by c1earfelling followed by broadcast burning favours silver wattle. The optimum seed-bed for establishment of silver wattle is undisturbed soil burnt by high intensity fire. Establishment is poorest on unburnt seedbeds with undisturbed litter or heavy accumulation of slash. Although germination is less and initial growth is slower after mechanical disturbance than after slash-burning, the differences between the treatments decreases with time. Stocking, height and cover of silver wattle are significantly reduced by retained overwood, smaller coupe sizes and logging slash. Reducing coupe size to 0.5 hectares or increasing the proportion of retained overwood to 30% reduces height growth of silver wattle by 50- 75% over the first three years. Rates of nitrogen-fixation by silver wattle at Tanjil Bren are sufficient to replace all losses of nitrogen due to timber harvesting and burning 10 years after disturbance. Silver wattle increases the concentration of nitrogen in the surface soil. Using the l5N natural abundance technique it was shown that 30% of nitrogen, equivalent to 800 kg nitrogen per hectare, in soil at 0-10 cm is derived from nitrogen-fixation under 6000 dominant and codominant stems per hectare after five years. Although dominant silver wattles reduce growth of mountain ash in the short term, they occur over a relatively small area of a coupe and decrease with time. Silver wattles may ultimately increase the growth of dominant mountain ash by effectively thinning out suppressed trees and increasing the amount of nitrogen in the ecosystem. Total biomass and nutrient content of the regeneration are greater with silver wattles present. Growth of dominant silver wattles initially exceeds that of dominant mountain ash, but later decreases and silver wattles are eventually suppressed by mountain ash.
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ItemThe ecology of Agrostis spp. (bent grass) invasion into temperate pastures( 1996)The grass, commonly called 'bent grass', has been rated by meat producers and consultants working in industry as one of the worst weeds of perennial grasses in temperate pastures in Victoria. Bent grass in Victorian pastures has traditionally been identified as Agrostis capillaris. However it has become clear that Agrostis castellana rather than A. capillaris is predominantly found in Victorian pastures. Management techniques to reduce the proportion of A. castellana in pastures must be modified based on the assumption that we are dealing with A. capillaris. The vegetative morphology of A. castellana populations sampled from diverse locations within Victoria was similar. A. castellana spreads vigorously by rhizomes which will reduce the success of presently-accepted management techniques (e.g. mob stocking) but will enhance the success of techniques that focus on reducing the rhizomatous capability of A. castellana. Flowering times differed between A. castellana populations such that genotypes of populations from drier climates completed flowering earlier or within a shorter time than genotypes from other populations. Techniques which focus on preventing floral development (e.g. spray-topping) will require refinement for different locations in Victoria. The cost of renovation combined with limited success to reduce the proportion of Agrostis spp. in the long term (> 10 years) has reduced the adoption of pasture renovation in some areas. Agrostis spp. regenerated from seedlings after pasture renovation, at variance with general conceptions. Thus pasture renovation techniques must reduce the regenerative ability of Agrostis spp. from seed. Agrostis castellana regenerates from viable rhizome fragments remaining after cultivation, in addition to seedling regeneration. Rhizomes of A. castellana displayed apical dominance in that lateral buds on rhizomes remain dormant. Presently-accepted pasture renovation does not take into account the fact that apical dominance in rhizomes ensures continuative regeneration. The technique of pasture renovation must ensure that the regenerative ability of bent grass is severely reduced, and this was achieved when rhizomes were broken into small 1-node fragments, buried beyond 75 mm and a rapidly establishing pasture sown. The dominance of bent grass in pastures indicates that bent grass competes successfully with other pasture species under current agricultural practices in Victoria. The mechanism of competition under A. castellana-dominant pastures supported the resource competition theory in that a greater proportion of the nitrogen absorbed under A. castellana pasture remained in the roots than in shoots, reducing the availability of nitrogen to be re-cycled in faeces and urine by the animal. Thus a greater proportion of nitrogen was returned by plant death and decomposition in A. castellana-dominant pasture than in L. perenne-dominant pasture. Decomposing shoots, roots and rhizomes are slow to release mineral nitrogen which is confirmed by reduced gross mineralization and reduced concentrations of nitrate in soil solution under A. castellana-dominant compared with L. perenne-dominant pasture. N uptake by A. castellana was as nitrate, in preference to ammonium, further depleting the concentration of nitrate in soil solution. When L. perenne grows in association with A. castellana, plant nitrate requirements are unlikely to be met and L. perenne pasture may then degrade to A. castellana-dominance. The likelihood of A. castellana dominance in L. perenne pastures may be reduced by improving litter quality and rates of mineralization.
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ItemAbiotic edge effects in wet sclerophyll forest in the Central Highlands of Victoria, Australia( 1997)Edge effects are the physical and biotic gradients which occur in an ecotone intersecting two dissimilar ecosystems. This research aimed to characterize abiotic edge effects in wet sclerophyll forest adjacent to recently logged areas in the Central Highlands of Victoria, Australia. Changes in photosynthetically active photon flux density (PPFD), air temperature, and ambient vapor pressure (ea) were monitored in the first 100 m of two east-facing edge environments during late summer and early winter of 1997. Microclimatic variables were measured on a relatively fine scale in the edge environment by using a rope and pulley system which moved the sensors along the selected edge transects. Consistently declining gradients across the edge ecotone were observed for PPFD and air temperature. These radiation and temperature gradients were influenced by edge orientation, time of day, forest canopy structure, and cloud cover. A negative exponential regression was fitted to radiation data, and from this model, an index for measuring the distance of edge radiation penetration was developed. This distance, designated as d10, is the distance at which PPFD reaches 10% of interior forest values. Average d10 values on overcast and clear summer days were 68 m and 118 m, respectively. Temperature was typically still declining at the last point of measurement, which indicates that the width of the temperature edge zone is greater than 100 m. Current forest practice in Victoria provides rainforest buffer zones of 20 to 100 m of sclerophyll forest. This study shows that if rainforests are to be protected from edge-induced micro climatic change, these buffer widths may need to be increased.