Benefits of riparian planting : A case study of lowland streams in the Lake Ellesmere catchment

Abstract

Freshwater is a globally important resource, yet the continued availability of high quality water is at risk. In New Zealand, around half of lowland water bodies do not meet water quality standards. One of the major threats to water quality in New Zealand is the widespread conversion and intensification of land use. Since the European settlement of New Zealand, more than 13 million hectares (around 50 % of the total land area) has been cleared and converted to pastoral agriculture. Agriculture is now the dominant land use in most of the middle to lower catchments of New Zealand rivers. This pastoral development has had profound impacts on water quality, aquatic habitats and macroinvertebrate communities. Riparian restoration has been occurring in New Zealand for over 30 years in an effort to minimise the impact on aquatic ecosystems by buffering streams from surrounding land use. Despite the extent of riparian restoration occurring in New Zealand, little monitoring or evaluation has been undertaken to determine whether planting efforts are achieving their aims. This thesis evaluates the impact of riparian plantings on water quality using a case study on lowland streams in the Lake Ellesmere catchment. A paired catchment design on four river reaches was used to compare restored riparian buffers with control sites upstream. Chemical water quality sampling was used in conjunction with a macroinvertebrate community assessment to provide a comprehensive assessment of water quality. Riparian restoration was found to have a positive effect on water quality in terms of increasing dissolved oxygen and decreasing turbidity. However, the four plantings that were studied all fail to meet the recommended minimum width of 10 m. This may have limited their effectiveness in protecting water quality, as seen by an increase in conductivity at planted sites, and no changes in other chemical and microbiological factors. Mixed responses were seen in invertebrate community composition, and it is likely that bed substrate, which was unmatched between some paired sites, had a large effect on species present. This research suggests that even narrow planted buffer strips may be effective in improving some water quality variables, and even when no baseline data has been established prior to restoration, monitoring can demonstrate the effectiveness of riparian restoration. When planning restoration efforts and in the evaluation of their effectiveness, a number of factors need to be considered. This most importantly includes the length and width of buffer strip, time since retirement, stream shade, stream flow and sources of invertebrate colonisers. Ultimately, the effectiveness of riparian planting in protecting water quality requires more planning and a significant monitoring effort in addition to the planting of a stream reach. Finally, it is important to remember that riparian restoration is a long-term task, and the beneficial results it provides will take some time to become apparent. Expectations of landowners and community groups need to be managed, and measurable goals set over a period of years to decades.