1. Introduction
1.1 Purpose of the review
The purpose of this report is to review and summarise published research on the efficiency and management of riparian buffer zones (RBZ) with respect to the attenuation of sediment and nutrients, and biodiversity enhancement. While there have been numerous studies on the efficiency of RBZ with respect to sediment and nutrients, many of these studies have been small-scale and site-specific. Therefore, a review of these studies needs to consider an assessment of the catchment scale factors that influence the effectiveness of RBZ in attenuating catchment loads.
1.2 What are riparian zones?
The riparian zone generally encompasses the vegetated strip of land that extends along streams and rivers and is therefore the interface between terrestrial and aquatic ecosystems (Gregory et al. 1991, Martin et al. 1999; Fig. 1). In addition to streams and rivers, the definition of riparian zones in the literature often includes the banks of lakes, reservoirs and wetlands.
Figure 1: The riparian zone is the land beside the stream that interacts with (1) runoff from hillslopes and (2) streamwater when this overflows into the floodplain. The vegetated riparian zone can affect the stream by intercepting runoff, and thereby improving water quality, by providing shade, leaf matter and wood, and stabilising stream banks.
1.3 Riparian buffer zone functions
Riparian buffer zones are often advocated as environmental management tools for reducing impacts of land use activities on aquatic resources. The buffer zone, area, or strip is generally regarded as the strip of land that separates an upland or hillslope area from streams, lakes or wetlands. Land use activity is modified in this zone to prevent adverse effects on the water quality, biota and habitat within the watercourse. Buffer zones or strips have also been variously labelled as Stream Protection Zones (SPZ), Streamside Management Zones (SMZ), or Riparian Management Zones (RMZ). In agricultural landscapes, buffer zones often consist of a fenced area alongside streams that stock are excluded from and this may be left as a grassy sward, or planted with woody vegetation. In forestry systems, a buffer zone is generally one of production trees left beside the stream when the surrounding area is harvested.
Riparian management can take various forms, some of which are summarised below:
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Grass Filter Strips: Fenced strip of rank paddock grasses to filter nutrients and sediment.
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Headwater or riparian wetlands: Fenced wetlands as hotspots for nutrient removal
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Rotational Grazing: Filter strips with varied stock grazing practices, such as occasional light grazing by sheep.
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Forested or planted native trees: a buffer of native trees to return ecological function to the stream and provide water quality benefits.
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Production trees or plants: a buffer of forestry trees left unharvested along stream banks, or production trees that are planted in riparian zones for selective harvesting with minimal disturbance (e.g., Tasmanian blackwoods) Plants such as flax for weaving, or fruit and nut trees, or high value native tree species that can be selectively harvested may also provide ecological function and a mechanism to remove nutrients such as phosphorus from the riparian zone.
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Multi-tier system: a combination of buffers where native forest trees may be used beside the stream to enhance ecological function and biodiversity, a buffer of production trees may occur outside of that and at the outer edge beside agricultural land a grass filter strip may be used.
Riparian buffer zones are used as a management tool to perform many functions (Appendix 1, Fig. 2) including stabilising channels, preventing stock access to waterways, filtering sediment and other particulates (including nutrients and microbes attached to sediment), removing soluble nutrients, and providing terrestrial and aquatic habitat. In addition, wet riparian soils, generally rich in carbon and low in oxygen, can promote a significant loss of N through denitrification. Riparian vegetation can also provide corridors for the movement of native fauna and flora between geographically separate areas, although the spread of weed species can also be facilitated in this way.
Riparian zones are commonly areas with heterogeneous vegetation and soils and therefore provide a diverse habitat for terrestrial and semi-aquatic organisms (birds, insects, amphibians and plants) (Boothroyd & Langer 1999). Vegetation in the riparian zone can influence water flow, both surface and subsurface (through root systems) and has direct effects on stream functioning. Forest vegetation in particular can shade streams and lower stream temperatures. High light levels from deforestation around streams leads to increases in algae and in-stream primary production, and changes to invertebrate community composition. Stream temperature has a direct impact on aquatic species as most metabolic processes are accelerated with increasing temperature and many fish and invertebrate species have thermal tolerances that can be exceeded in unshaded streams (Quinn et al. 1994, Martin et al. 1999). Trees provide organic matter inputs in the form of leaves and woody debris, creating a diversity of food resources and habitats for in-stream fauna. Terrestrial insects may also be attracted to vegetated riparian zones and become a valuable food source for fish when they fall into the stream (Barling & Moore 1994).
Figure 2: (A) Inputs of direct and diffuse sources of contaminants to pasture streams; (B) implementation of riparian management through fencing allows infiltration, denitrification and filtering of contaminants from flows (except for deep sub-surface flow), and planting provides additional ecological benefits
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