In general, the extent and quality of stream vegetation has been shown to have a significant effect upon the water quality in the receiving environment (Becker et al., 2001; ARC, 2004; Rutherford et al., 1999; Allibone et al., 2001).
Riparian buffers act as biological filters between catchments and receiving environments, intercepting a significant proportion of groundwater nutrients. Stormwater runoff is slowed and filtered, with direct uptake and transformation of contaminants by plants. Vegetation and humus layers attenuate significant volumes of water, promoting infiltration into the soil and releasing it over a longer time period to contribute to stream base flows and to support riparian vegetation.
The 'edge effects' of sun, wind and pest invasion can extend over 40 m into mature vegetation (Davies- Colley et al., 2000; Young & Mitchell, 1994). A 10 m width planted on each stream bank is the minimum recommended by Auckland Council (Becker et al., 2001). Wider strips of 20 m or more are encouraged, particularly for larger rivers (Parkyn et al., 2000).
While the benefits associated with re-vegetating stream-side margins are well documented, the literature is inconclusive in terms of the necessary width of riparian buffer zones. However, it seems that the greater the width the more obvious the benefits to stream health. Parkyn et al. (2000) made the following observations about three selected riparian widths:
- 5-6 m: Ongoing maintenance is required to keep a buffer of this width free of weeds, and the natural regeneration of indigenous species is limited. This width should only be used on very small watercourses or where there is no other option.
- 10 m: Indigenous vegetation succession is allowed for, and relatively low maintenance is required. The outer edge (i.e. the edge furthest from the stream) is likely to suffer from long-term weed infestations, which could have the potential to spread to the interior wherever canopy gaps occur. This width should be used as a general guideline for a minimum buffer width.
- 20+ m: It is highly likely that a buffer strip of this width would support self-sustaining indigenous vegetation with few maintenance requirements and is recommended to provide long-term benefits to aquatic and terrestrial biota.
In terms of aquatic habitat, buffers less than 10 m in width do not necessarily protect algal, macro-invertebrate or fish biomass and diversity. However, buffers wider than 30 m have provided observable protection (Davies & Nelson, 1994). Invertebrate communities in particular are strongly linked to temperature, suggesting that canopy closure and protection of headwaters is required for in-stream diversity (Parkyn, 2004).
There is a direct correlation between buffer width and contaminant removal rates. Nitrate removal of 100% has been recorded in buffers between 20 m and 30 m in width, and removal rates of over 70% where the buffers were 10 m wide (Fennessy & Cronk, 1997). However, the effectiveness of these buffers is greatly influenced by site-specific factors such as slope, soil composition and drainage patterns.
Collier et al. (1995) provide practical guidelines to calculate the optimal filter strip width in agricultural systems based on a CREAMS model (Chemical, Runoff and Erosion from Agricultural Management Systems). This model suggests a wider buffer zone is required for a greater slope length and angle, a larger clay fraction in the soil, and a lower soil drainage capacity.
Riparian planting design
The Auckland Regional Council Technical Publication TP148 Riparian Management Guideline (Becker et al., 2001) provides a step-by-step process for riparian planting. This includes a planting guide for a range of stream environments. In some instances, buffers comprising sedges and grasses can be very effective at stormwater management due to the following (MfE, 2001):
- They typically form a dense cover over the ground which slows down the passage of water. The effect of slowing down runoff on the floodplain needs to be considered when choosing locations for riparian planting.
- Their many fine leaves are ideal filters, reducing the velocity of water and encouraging the settling of solids.
- They grow well in saturated soils and can tolerate periods of immersion.
- They can tolerate and grow through accumulated sediment.
- They can be tolerant of dry periods.
- They are generally tolerant of both low and high fertility.
- They tend to accumulate organic matter and help create anaerobic conditions, which are important steps in lowering nitrogen levels.
Trees and shrubs are less able to intercept and filter out contaminants in overland flows since they do not have dense foliage at ground level. However, there is some evidence that thick layers of forest floor organic matter (humus) can be very effective as a contaminant filter. Forest buffers provide for greater nitrate removal from groundwater flows, partly through uptake by plants (Martin et al., 1999). Collier et al. (1995) also observed that a single line of trees can provide 80% shade to streams once they have achieved canopy closure.
Parkyn (2004) noted that due to the different contaminant forms and entry points for stormwater, a combination of grass/sedge/rushes and tree/shrub buffer types provides the most effective outcome. Shade-tolerant reeds, rushes and sedges could also be considered.
In rural areas, there are a number of buffer zone management options that work alongside agricultural practices. Literature on the subject suggests unmown grass buffer strips are effective at filtering sediment and associated contaminants from surface runoff (Martin et al., 1999). Grass filter strips can be grazed on a rotational basis during dry periods to provide new growth and to uptake nutrients such as phosphorus. It may be possible to separate these grass filter strips from other pasture using electric wires to exclude cattle but allow sheep access. Combination buffers can provide an upslope grass buffer with appropriately spaced trees as a wood lot, with an undisturbed buffer zone next to the stream.
The Ministry for the Environment (MfE, 2001) provides specific recommendations for the harvest of productive tree species along riparian areas and dictates the most appropriate location, timing, techniques and extent of harvesting practices.
The diversity of stream environments is a function of their hydrology, geology and climatic conditions. Habitats vary along gradients from upland to lowland environments and from floodplains to the base of the stream (groundwater zone). Enhancing riparian buffers provides excellent opportunities to improve both aquatic and terrestrial ecology including the following:
- Shade from riparian vegetation regulates water temperatures and improves dissolved oxygen levels in streams, allowing more sensitive species to thrive. Shade also reduces light levels to prevent nuisance growth of algae.
- Trees and shrubs provide inputs of leaves, wood, and insects as sources of food for fish and aquatic invertebrates.
- Woody debris provides in-stream heterogeneity, habitat diversity and cover. Bullies spawn on logs on the stream bed.
- Tree roots provide bank stabilisation benefits, as well as opportunities for overhanging banks and fish refuge.
- Low vegetation, such as sedges, grasses and rushes, provide spawning opportunities for galaxiids that lay their eggs at the base of these plants at the water's edge.
- Riparian vegetation provides habitat for terrestrial species such as birds, lizards and insects. Sheltered conditions afforded by trees and shrubs are necessary to support the terrestrial adult (reproductive) phase of many aquatic insects.
Forested stream profile
One factor that needs to be kept in mind for restored riparian buffers is the likelihood that the stream channel will widen due to an increase in shading and subsequent loss of grasses along the stream bank. Parkyn et al. (2001) suggest that bank erosion would peak about 25 years after planting, with sediment yields being approximately double the amount expected under herbaceous/grass ground cover. Following this, bank erosion and sediment yield would be expected to decline, reaching low levels when the stream has widened to its natural forest morphology by about 35-40 years after riparian planting.
To prevent additional sediment entering stream environments it may be necessary to re-profile streams based on post-development forested morphology. Erosion may also be prevented through maintaining a balance of shade trees and low-growing littoral vegetation or by utilising shade-tolerant understorey species.