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Thesis Abstract
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| Title: |
Biomass, growth and nutrient cycling in regenerating Eucalyptus regnans forest after fire. |
| Topic: |
Soils and Nutrition & E. regnans & Native forest |
| Year: |
1992 |
| Name: |
Hong Chen |
| Degree: |
Doctor of Philosophy |
| Uni: |
University of Melbourne |
| Abstract: |
The main theme of this thesis includes two aspects: (1) the biomass, growth, productivity and nutrient cycling in the regenerating Eucalyptus regnans F. Muell (Mountain Ash) forest at Britannia Creek after fire, and (2) to test the hypothesis that nutrient losses due to fire might lead to decline in stand productivity.
The studies in this thesis were based in the Britannia Creek Experimental Forest (Attiwill 1991a), which is a regenerating E. regnans forest on a logging coupe burned in the 1983 (Ash Wednesday) wild-fires. A total of 31 trees along the diameter range was harvested over four years from 1987 to estimate biomas and nutrient content. Monthly litter collections were made in the same period from 24 plots with different fertilizer treatments to assess changes in litterfall mass and nutrient returns. Fortnightly soil samples were taken to examine the impacts on fertilizer on nutrient availability, transformation and plant uptake.
E. regnans is a fire-climax species, and seedlings regenerate in profusion after fire. Thirty-nine months after regeneration started there were 16 700 stems ha–1, but the stocking density declined rapidly with an annual mortality rate of about 31% in numbers during study period. The annual growth of biomass, however, was 23 tonnes ha-1, an increase rate of 33% on average. By the end of 1989, at age 6.25 years, the forest accumulated 118 tonnes ha-1 in above-ground biomass of eucalypt trees, and 274 m3 ha-1 in volume. Nutrient contents in biomass increased with forest growth. At age 6.25, the above-ground biomass contained 294 kg ha-1 of N, 25 kg ha-1 of P, 311 kg ha-1 of K, 239 kg ha-1 of Ca, and 79 kg ha-1 of Mg.
Litterfall of 5.7 to 9.2 tonnes ha-1 year-1 is at the upper end of the range for eucalypt forests. The proportion of dead eucalypt leaves in litterfall is also large. Therefore, litterfall returned substantial amounts of nutrients back to the forest floor. The amounts returned in litterfall for N, P,K, Ca and Mg are 78, 4, 16, 61 and 13 kg ha-1 year-1 respectively in 1989. About 48% and 34% of the gross annual demands for N (126 kg ha-1 hear-1) and for P (9 kg ha-1 year-1) were met bay annual litter return and a little less than 20% by internal redistribution, although the demand for minerals from soil reserves was still large.
The net primary production (NPP) of 45 tonnes ha-1 year-1 (eucalypt trees alone) made this forest the most productive among any natural forests, including tropical forests, reported so far. There is certainly no evidence of decline in productivity following clearfelling and burning.
The addition of fertilizers temporarily increased the availability of applied nutrients. However, the increased concentration of available N returned to pre-addition level two months after fertilizer application, while the increased concentration of P was sustained for at least two years. Net N-immobilization occurred immediately after N fertilizer application, an it is apparently the most important process in the conservation of N fertilizer. A total of 24 kg ha-1 of the N, 11% of N added, was immobilized in the surface 5 cm of soil in the second month alone after fertilizer application. Only small proportion (5.5%) of added N was taken up by plants. Although greater concentrations of added nutrients were found in dead eucalypt leaves falling a litter, there was no difference in concentration in live leaves across the different fertilizer treatments.
The additions of up to 1000 kg ha-1 of N and 500 kg ha-1 of P, together with all other essential elements, have not increased either biomass of productivity in the E. regnans forest so far. The E. regnans forest at Britannia creek is therefore not limited by N or P or any other essential nutrients, and losses of nutrients due to fire have not resulted in a decline in productivity of the subsequent forest.
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