School of Botany - Theses

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Author: May, Barrie × End date: 1999 × Start date: 1980 × Subject: Victoria ×

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    Silver wattle (Acacia dealbata): its role in the ecology of the mountain ash forest and the effect of alternative silvicultural systems on its regeneration
    May, Barrie ( 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.