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Structure and function of small, headwater streams flowing through wet Eucalypt Forest in Southern Tasmania and the impact of clearfell forestry

posted on 2023-05-26, 02:28 authored by Burrows, RM
Clearfell, burn and sow (CBS) forestry is a major disturbance to headwater streams flowing through wet eucalypt forests in southern Tasmania involving clearfelling trees around them and the burning of remaining slash within a year of harvest. The aim of this research was to assess the short-term (<19 years) effects of CBS forestry on several key structural (woody debris and dissolved organic matter source and composition) and functional (nutrient uptake and organic matter processing) characteristics of headwater streams in southern Tasmania. I evaluated these using a combination of replicated space-for-time surveys and an MBACI (multiple before-after control-impact) experiment in headwater stream reaches flowing through old-growth and CBS-affected forest. My findings show that CBS forestry increased available light, elevated water temperatures (between 0.25 and 0.94°C), and significantly increased the quantity of woody debris situated in the stream channel. I also used fluorescence characterisation of dissolved organic matter (DOM) to show that forest harvesting did not affect the relative contributions of autochthonous and allochthonous stream DOM despite the major reach-scale disturbance that clearfell forestry represents. However, there was conflicting evidence for changes in DOM composition after harvesting. It is likely that catchment-scale processes are more important than reach-scale processes (i.e. forest harvesting) in determining stream DOM biogeochemistry, because only a small proportion of the total channel length (<100 m) is affected by clearfell forestry. The large physical structural changes to headwater streams caused by CBS forestry led to changes in stream function. Nutrient addition experiments showed greater phosphorus uptake in CBS-affected relative to old-growth (OG) stream reaches, which was likely due to increased biotic activity (algae and bacterial biofilms) related to greater in-stream light availability and quantity of in-stream woody debris. However, sorption to sediment and charred woody debris may also have contributed to the greater phosphorus uptake after harvesting. The impact of CBS forestry on organic matter decomposition differed among years and benthic habitats, with evidence for an increase in bacterial carbon production (BCP) in fine sediment habitat but a decrease in BCP and cellulose decomposition in coarse gravel habitat. Contrary to most previous research, increasing contribution of terrestrial DOM was the strongest variable driving in situ benthic BCP. Some of the structural changes from CBS may be beneficial in reducing impacts at the catchment-scale. For instance, the observed increase in the amount of woody debris and light availability after harvesting may prevent elevated phosphorus export to downstream ecosystems by increasing phosphorus uptake and retention in headwaters. While these effects characterised the short-term responses to CBS in these headwater streams, the longer-term (>19 years) and catchment-scale impacts require further research. Many variables (e.g. the quantity of woody debris) will take decades to recover to pre-disturbance levels and the cumulative impacts of harvesting multiple coupes throughout the landscape needs to be determined to ensure that CBS operations are managed in space and time to minimise impacts on downstream ecosystems.


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