The influence of harvest systems on sediment delivery on the Moutere gravels.
Degree GrantorUniversity of Canterbury
Degree LevelBachelors with Honours
Degree NameBachelor of Forestry Science
Fine sediment suspended in waterways is one of the most significant pollutants associated with New Zealand plantation forests, adversely impacting the quality of downstream aquatic ecosystems, and tarnishing public perception of the forestry industry. Nelson Management Limited identified in their plan to improve sedimentation performance that a quantification of the ground disturbance for common harvest systems would help identify the sediment delivery risks for each system on the Moutere gravels. With a combination of ground survey and aerial photography techniques, soil disturbance patterns and sediment breakthroughs were observed over 16 harvested settings that had been exposed to at least one significant rainfall event. Of these sites, 11 sites were cable yarded, and five were ground-based.
For cable yarding sites, breakthroughs were observed every 190 m of ephemeral stream or every 4.55 ha of harvest area. For ground-based sites, on average a breakthrough was found for every 107 m of ephemeral stream or for every 1.82 ha over harvest area. All breakthroughs observed were into ephemeral streams. The majority of sediment breakthroughs were associated with earthworks or harvesting related soil disturbance, rather than landslides. For cable yarding systems, there were large areas of scalping observed, but this had little bearing on sediment delivery as 16 of the 26 breakthroughs were due to machine tracking on the slopes. For ground-based sites, 15 of the 25 breakthroughs were primarily caused by skid and spur roads. Roading density was the only significant predictor of sediment breakthroughs at the significance level α ≤ 0.05. On the Moutere gravels; slope, stream length per ha, crew and extraction method were not found to be significant predictors of sediment breakthroughs per ha. However, due to the limited selection of harvest settings, there was limited replication of the crew and extraction method factors, making any statistical differences difficult to detect.
The image classification method developed to estimate the bare soil percentage for a site was found to be insufficiently reliable to allow conclusions to be made from the data that was collected for each setting.
Managers should focus on reducing roading density through careful road placement and focusing on breaking the connectivity between sediment generated from earthworks and streams. Further study that focuses on quantifying the rate that sediment is delivered to ephemeral streams gets transported to perennial streams would show how significant the breakthroughs to ephemeral streams are to total sediment yields from harvested catchments.