School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemDecline of river red gum (Eucalyptus camaldulensis Dehnh.) on grazing lands in Western Victoria( 1988)River Red Gum (Eucalyptus camaldulensis Dehnh.) is a common and extensively distributed tree in western Victoria. Widely-spaced stands of this species are a prominent feature of pastoral landscapes in that area. Many of these remnant stands are in a state of decline, with individuals being lost through deliberate clearing, senescence or the effect of rural dieback disease syndromes. This thesis reports on a study of the decline of one stand of E.camaldulensis trees growing on part of a grazing property near Buangor, in west-central Victoria. The rate of tree loss from agricultural land in the Buangor district was assessed from aerial photographs taken in 1947 and 1980. The average compounding rate of tree loss over that period was 1.1% p.a.. Rates of tree loss were greater from the remnant box-stringybark stands (1.9% p.a.) occurring in the district than from remnant River Red Gum stands (0.8% p.a.). The size and crown condition of all trees in the stand under investigation were assessed. Dieback-affected trees had relatively small and open crowns, that often had many dead branchlets and branches. The crowns of dieback-affected trees were largely of secondary or epicormic origin. Tree height and stem girth were not affected by contemporary crown health. The age of one tree was estimated by radiocarbon dating to be 440. ± 110 y B.P.. Detailed investigations of trees classified as either "healthy", "average" or "unhealthy" were undertaken. The amount of organic matter in soil declined as the quality of tree cover changed from "healthy" to "unhealthy" tree to open pasture. This change was probably linked with reduced additions of organic material through litterfall as tree crown cover deteriorated. The possible consequences of reduced litterfall and declining soil organic matter levels were that; (i) it may have disrupted biogeochemical nutrient cycling; and (ii) it may have increased the susceptibility of trees to root damage and other problems associated with animal traffic. Measurements of soil strength and bulk density indicated that soil under open pasture had been compacted, presumably by animal traffic. Soil under trees was harder during winter than soil under open pasture and was therefore less susceptible to puddling. Soil under trees was not as strong as soil under open pasture during spring and hence provided more favourable conditions for root growth. Investigations into the mineral nutrition of study trees found that "unhealthy" trees did not appear able to mobilize some macronutrient elements (N,P, K, Mg) to the same extent as "healthy" trees. The apparent nutrient deficiency may have been due to disruptions to the biogeochemical nutrient cycle as dieback progressed. The nutrient deficiency may exacerbate this disruption, since it was associated and possibly linked with delayed new foliage production in "unhealthy" trees and reduced area growth in that foliage. Measurements of leaf water potential, diffusive conductance and transpiration showed that atmospheric, rather than soil water deficits had the greatest influence on water relations in the study trees. The measurements showed that "unhealthy" and "average" trees did not greatly restrict transpiration during periods of high evaporative demand. "Healthy" and "average" trees transpired more water and assimilated more carbon than did "unhealthy" trees. Differences in gas exchange were due only to large differences in leaf area. Soil under "healthy" and "average" trees remained drier than soil under "unhealthy" trees and open pasture throughout the study period. However, there was little difference in soil water depletion during the summer, even though the annual pasture had died off. Measurements provide evidence of deep infiltration of water beyond plant root zones, particularly under open pasture and "unhealthy" trees. The healthier trees appeared to have a beneficial impact on the local water balance. Continued rural dieback and/or tree loss is likely to exacerbate disruptions to the local water balance that commenced with clearing for agriculture.
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ItemA study on damping-off in a regeneration mixed eucalypt forest( 1986)Although damping-off is known to cause considerable loss before and after emergence of seedlings in forest nurseries, little is known of its role in seedling mortality after direct seeding in artificial regeneration of mixed eucalypt forests. Findings reported in this thesis are the result of a survey for the presence of damping-off fungi in soils and dying seedlings, and tests of their pathogenicity to a range of eucalypts found in the Wombat State Forest of Victoria. This study was carried out in conjunction with a research programme established by the Victorian Department of Conservation, Forests and Lands to determine which factors govern the successful establishment of E. obliqua and E. radiata in three seed-bed types under five levels of overwood. Propagule counts for Pythiaceous fungi in burnt and unburnt soils from field experiment, plots prior to sowing indicated that the inoculum level was low and differed with levels of overwood retained following site preparation in summer (Chapter 2). Baiting the soils with E. sieberi cotyledons, apples or directly sowing E. sieberi seed in samples of the field soil showed that both burnt and unburnt soils were variably infested with Pythium spp., Fusarium spp., and Cylindrocarpon destructans. A small portion of the seed to be sawn in the field plots without any fungicidal pre-treatment was found to harbour a range of fungi, sane of which had the potential to inhibit germination of the eucalypts sown on the plots (Chapter 3). The fungi having this ability were however not isolated from damped off seedlings in the experimental plots. Field studies showed that the proportion of germinated seedlings killed as a result of damping-off was lower than that attributable to frost heave, droughting following freezing and insect damage (Chapter 4). Damping-off was found to be widespread over the entire study site and the rate of damping-off to differ with seed-bed type. Overwood density did not affect the disease level. Although Pythium spp. were responsible for most deaths, Cylindrocarpon destructans and Fusarium spp. also caused some mortality. Pathogenicity tests (Chapter 5) confirmed that the fungi associated with seedling mortality in the field can cause significant pre- and post-emergence damping-off in a range of eucalypts found at the study site. The fungi differed in their pathogenicity to the eucalypts and sane induced feeder root necrosis after the two leaf stage of seedling development. No differences in susceptibility to post-emergence damping-off were found between the tree species sown on the plots and others of economic importance in the forest. The overall findings and their implications for direct sowing as a management option in future field regenerations are discussed in Chapter 6.