School of Agriculture, Food and Ecosystem Sciences - Theses

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Type: PhD thesis × End date: 2022 × Author: Dignan, Paul. ×

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    A functional approach to stream buffer design in mountain ash forest
    Dignan, Paul. (University of Melbourne, 2001)
    Buffer strips, zones of vegetation retained along stream banks in, and adjacent to, logged areas, are used to maintain water quality and to protect a range of values pertaining to stream and riparian resources. They are commonly applied as strips of undisturbed forest of constant width from the stream, but their effectiveness varies with topographic, edaphic, vegetation and climatic factors. The use of variable buffer widths based on functional criteria has been proposed as a more rational basis for the design of stream buffers. The purpose of this thesis is to test the application of this approach to stream buffer design in the mountain ash forests of south-eastern Australia. It examines some of the main problems encountered in the design of stream buffers, namely temporal and spatial variation in the extent of soil saturation and overland flow, the deterioration of established buffers and the frequency and nature of unmanaged sediment flows into buffers. It also looks at the ability of buffers to maintain the light environment in riparian forest and develops a light penetration edge effects model for use in buffer design. Periodic field survey of a network of streams around a logged area found considerable variation in the spatial and temporal extent of surface soil saturation and overland flow, extending laterally up to 190 m from the main channel. Large areas of surface hydrological activity were particularly evident at the confluences of streams and towards the lower ends of the stream catchments. The deterioration of buffer vegetation was also highlighted as a factor to be considered when looking at effective buffer width. Eighteen percent of the buffers surveyed were found to be severely degraded and 69% of these were as a result of penetrating fire during broadcast slash burning. Where penetrating fire was not evident, edge aspect had the major influence on buffer overstorey crown health, with 33% of trees in north facing edges classed as severely degraded compared to 10%, 11% and 14% for south, east and west facing edges respectively. Catastrophic windthrow was evident where edges faced the direction of damaging winds. Maintenance of the light environment is an important ecological function of buffers in riparian forests. Measurements of the understorey light environment using hemispherical photographs were used to characterise the natural light regime in riparian and upslope forest. A matched set of post logging photographs were used to develop a model of the response of the light regime to edge creation. The natural understorey light environment around the streams was influenced by proximity to the streamline to about 50 m upslope, light penetration increasing on average at a relative rate of about 9% for every 10 m from the streamline to the upslope eucalypt forest. Light penetration was also strongly influenced by height above ground, with steeper gradients associated with lower order streams. Logging of the upslope forest resulted in major changes in light penetration close to the cut edge, reducing with distance into the forest. Distance from the cut edge was the main factor influencing light penetration edge effects, with most of the change occurring within 10-30 m of the edge. Manipulation of the edge orientation using the hemispherical images enabled models of the change in light penetration following edge creation to be developed for edges facing north, east and west, with distance from the edge as the only significant site factor. The importance of buffers for the control of unmanaged sediment flows from logged areas was examined by surveying 7 km of buffer edge to determine the frequency and nature of these flows. Seven instances of sediment flow into buffer were noted and all were due to the failure of in-coupe erosion control measures. This led to the conclusion that sediment control was a valuable function of buffers but it was not a suitable design criterion for multi-purpose buffers. The site specific empirical models of light penetration were combined with conceptual models developed in other forest types for Large Woody Debris (LWD) recruitment and stream bank stability to compare fixed width buffers with multi-purpose functional models in terms of resource use and the protection of stream and riparian zone values. Fixed width buffers of 20 m were inconsistent in the level of protection afforded to the light environment both in the vicinity of the stream and in the riparian zone. They generally provided about 60- 80% maintenance of LWD potential. Variable width buffers provided a consistent level of protection for both these functions, although the cost in terms of resource requirement was moderately high. The fixed width buffers occupied 13-22.5% of the available area compared to 31-37% for the variable width buffers. Measurement of the buffer from the outer edge of the riparian zone proved to be effective in increasing the level of maintenance of light in this zone for quite a small increase in the land area required.
  • Item
    No Preview Available
    A functional approach to stream buffer design in mountain ash forest
    Dignan, Paul. (University of Melbourne, 2001)
    Buffer strips, zones of vegetation retained along stream banks in, and adjacent to, logged areas, are used to maintain water quality and to protect a range of values pertaining to stream and riparian resources. They are commonly applied as strips of undisturbed forest of constant width from the stream, but their effectiveness varies with topographic, edaphic, vegetation and climatic factors. The use of variable buffer widths based on functional criteria has been proposed as a more rational basis for the design of stream buffers. The purpose of this thesis is to test the application of this approach to stream buffer design in the mountain ash forests of south-eastern Australia. It examines some of the main problems encountered in the design of stream buffers, namely temporal and spatial variation in the extent of soil saturation and overland flow, the deterioration of established buffers and the frequency and nature of unmanaged sediment flows into buffers. It also looks at the ability of buffers to maintain the light environment in riparian forest and develops a light penetration edge effects model for use in buffer design. Periodic field survey of a network of streams around a logged area found considerable variation in the spatial and temporal extent of surface soil saturation and overland flow, extending laterally up to 190 m from the main channel. Large areas of surface hydrological activity were particularly evident at the confluences of streams and towards the lower ends of the stream catchments. The deterioration of buffer vegetation was also highlighted as a factor to be considered when looking at effective buffer width. Eighteen percent of the buffers surveyed were found to be severely degraded and 69% of these were as a result of penetrating fire during broadcast slash burning. Where penetrating fire was not evident, edge aspect had the major influence on buffer overstorey crown health, with 33% of trees in north facing edges classed as severely degraded compared to 10%, 11% and 14% for south, east and west facing edges respectively. Catastrophic windthrow was evident where edges faced the direction of damaging winds. Maintenance of the light environment is an important ecological function of buffers in riparian forests. Measurements of the understorey light environment using hemispherical photographs were used to characterise the natural light regime in riparian and upslope forest. A matched set of post logging photographs were used to develop a model of the response of the light regime to edge creation. The natural understorey light environment around the streams was influenced by proximity to the streamline to about 50 m upslope, light penetration increasing on average at a relative rate of about 9% for every 10 m from the streamline to the upslope eucalypt forest. Light penetration was also strongly influenced by height above ground, with steeper gradients associated with lower order streams. Logging of the upslope forest resulted in major changes in light penetration close to the cut edge, reducing with distance into the forest. Distance from the cut edge was the main factor influencing light penetration edge effects, with most of the change occurring within 10-30 m of the edge. Manipulation of the edge orientation using the hemispherical images enabled models of the change in light penetration following edge creation to be developed for edges facing north, east and west, with distance from the edge as the only significant site factor. The importance of buffers for the control of unmanaged sediment flows from logged areas was examined by surveying 7 km of buffer edge to determine the frequency and nature of these flows. Seven instances of sediment flow into buffer were noted and all were due to the failure of in-coupe erosion control measures. This led to the conclusion that sediment control was a valuable function of buffers but it was not a suitable design criterion for multi-purpose buffers. The site specific empirical models of light penetration were combined with conceptual models developed in other forest types for Large Woody Debris (LWD) recruitment and stream bank stability to compare fixed width buffers with multi-purpose functional models in terms of resource use and the protection of stream and riparian zone values. Fixed width buffers of 20 m were inconsistent in the level of protection afforded to the light environment both in the vicinity of the stream and in the riparian zone. They generally provided about 60- 80% maintenance of LWD potential. Variable width buffers provided a consistent level of protection for both these functions, although the cost in terms of resource requirement was moderately high. The fixed width buffers occupied 13-22.5% of the available area compared to 31-37% for the variable width buffers. Measurement of the buffer from the outer edge of the riparian zone proved to be effective in increasing the level of maintenance of light in this zone for quite a small increase in the land area required.