School of Ecosystem and Forest Sciences - Theses

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    An evaluation of timber drying problems in terms of permeability and fine structure
    Kininmonth, John Alexander (1931-) ( 1970)
    The relationships of difference in rate of drying to permeability and wood structure were determined for two angiosperms and one gymnosperm. These investigations took two particular drying problems as a basis for study and attempted to explain why: - heartwood of Nothofagus fusca (red beech) takes many times longer to dry than sapwood. - green sapwood of Pinus radiata (radiata pine) dries readily but, if dried and pressure-treated with water-borne preservatives, its subsequent drying is greatly retarded. Test material was used from 14 trees of N.fusca from New Zealand, four trees of Eucalyptus regnans (mountain ash) and seven trees of P.radiata from Victoria, Australia and the experimental work was carried out under three headings: (a) Unidirectional drying. Small specimens, sealed on all except one pair of grain faces, were dried in a laboratory kiln at temperatures up to 60C. Comparisons were made between radial and tangential drying in sapwood and heartwood or in green and resaturated specimens; effects of treatments such as steaming were also assessed. Moisture gradients were determined to show the contribution of free water movement to overall drying. (b) Permeability studies. A method was developed to measure the transverse permeability of green wood to the flow of micro-filtered water; established methods were used for longitudinal permeability. Data for P.radiata met the requirements allowing application of Darcy's Law for flow of fluids through inert porous media and N.fusca approximated them. Pathways of flow were determined with chemical stains. (c) Wood structure. The transmission electron microscope was used to compare the appearance of pit membranes and the cell walls in sapwood and heartwood of N.fusca. In P.radiata, emphasis was on determining the percentage of bordered pits that were aspirated in sapwood - green, after drying and resaturation and after various treatments - and relating this to differences in drying and permeability. The main conclusions drawn from this study are: (a) The green sapwood of N.fusca and E.regnans is permeable to micro-filtered water in the radial and tangential directions. After drying and resaturation, the permeability of N.fusca is unchanged but that of E.regnans is drastically reduced, particularly in the tangential direction. The heartwood of both species is impermeable when tested at a pressure differential of 40 cm.Hg. (b) Differences in the permeability of N.fusca can be explained by differences in the appearance of pit membranes in sapwood and heartwood: in heartwood, the membrane surfaces are usually completely occluded when viewed as replicas in a transmission electron microscope; in sapwood, the surfaces are always less occluded often exhibiting a clean primary well texture. It is inferred from studying the effects of various extraction treatments that the pit membrane surfaces in sapwood are less occluded than indicated by the appearance of replicas. (c) Plasmodesmata may provide pathways for mass movement of liquids in the radial direction in the wood, but, in other pits, without obvious pores, permeability probably results from movement through the general structure of the pit membrane. (d) Heartwood of N.fusca takes several times longer to dry than sapwood because of its reduced permeability coupled with lower rates of moisture diffusion. (e) Contrary to previous reports, at least 80 percent of the bordered pits in green sapwood of P.radiata are open, irrespective of distance from the outside of the tree. After drying and resaturation most pits are aspirated and the wood is much less permeable than in the green state. (f) The condition of the bordered pits has an effect on the rate of drying in the tangential direction - causing a marked reduction in resaturated material - but has no appreciable effect on radial drying which is little different in green or resaturated wood.
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    Regeneration of river red gum Eucalyptus camuldulensis Dehn
    Dexter, Barrie Donald ( 1970)
    The aims of this study were to investigate the main factors influencing the regeneration of river red gum Eucalyptus camaldulensis Dehn., in Barmah forest and to use the results to develop procedures for establishing regeneration primarily for wood production. Factors influencing germination and survival of seedlings were examined. These included seed supply, seasonal conditions, seed beds, availability of soil moisture, the influence of over-topping trees, flooding and grazing. Natural seed supply is variable because the intensity of flowering varies widely and unpredictably from year to year and about 45 per cent of flowers fail to mature. Seasonal conditions are a major factor affecting germination of seed and survival of seedlings especially in the absence of flooding. On unflooded areas germination is confined to the wetter, cooler months and survival is highest if there are good summer rains. Germination and survival following flood recession are often high. When flood recession occurs late in summer, however, hot conditions may kill most seeds or young germinates. The combination of winter-spring flooding and above average summer rains favours germination and survival. In very dry years few seedlings develop and these are restricted to the most receptive sites. Ash bed and cultivated seed beds are the most receptive sites for seedling establishment and grassed and hard bare earth sites are the least receptive. Seedling establishment is also severely restricted where weeds or over-topping trees compete with seedlings for moisture. Prolonged flooding kills large numbers of young seedlings especially if they are completely immersed for some months. Flooding is uncontrollable so it is advantageous to promote rapid seedling growth and so minimize deaths or severe flood injury. During drought periods red gum seedlings may be destroyed by rabbits, kangaroos, wild horses and cattle. When feed is abundant, however, the adverse effect of all these animals is slight. Extensive grazing of cattle on regeneration areas keeps weeds that are competing with seedlings for moisture in check, and seedling mortality due to soil drought is much less than on ungrazed areas. Several techniques for regenerating river red gum were developed from the fundamental studies and were tested on an operational scale. Of the procedures based on direct seeding clear felling followed by aerial seeding is the cheapest and most flexible, and is recommended as the technique to be used generally. The major costs involved are in seed bed preparation, poisoning non-merchantable trees, collecting seed and in aerial seeding. None of these operations is expensive. Sowing rates and time of sowing are determined on the bases of expected flooding and seed bed quality. Seed beds are made receptive by removing grass and other vegetation and preparing the ground surface by slash burning and cultivation. Procedures based on natural seed supply involve preparation of seed beds and inducement of seed fall during summer and autumn, followed by utilization of merchantable and poisoning of non-merchantable trees. Because careful timing of each phase is required to suit such factors as seed maturation, seed fall and germination, the induction of seed fall can be costly and difficult to organize. Various factors that may influence the choice of the regenerative procedure are discussed. Finally, it is concluded that the provisional stocking standards are compatible with other forest values.