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5.      Summary

Riparian management can be viewed as a last line of defence for attenuating contaminants before entering the stream. Fencing stock out of streams and retiring riparian margins from agricultural land use are also particularly important practices to improve stream water quality. Buffer zones can filter contaminants and sediments from overland flow by increasing the infiltration into soil, intercepting particulates, and removing soluble nutrients by plant uptake and denitrification.

Riparian management can take various forms, summarised below:

• Grass Filter Strips: Fenced strip of rank paddock grasses to filter nutrients and sediment.
• Headwater or riparian wetlands: Fenced wetlands as hotspots for nutrient removal.
• Rotational grazing: Filter strips with varied stock grazing practices, such as occasional light grazing by sheep.
• Forested or planted native trees: a buffer of native trees to return ecological function to the stream and provide water quality benefits.
• 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.
• 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 at outside of that and the outer edge beside agricultural land would be a grass filter strip.

The ability of buffer zones to attenuate pollutants will depend upon the mechanisms by which these pollutants reach surface waters. Three main transport processes can occur:

• direct pollution (e.g., stock access to streams, bank erosion);
• surface runoff;
• subsurface flow and drainage.

Buffer zones can be effective at removing nutrient and sediment inputs to streams by restricting the direct use of land beside the stream and by processing water that has been transported into the riparian zone. The mechanisms of contaminant removal in buffer zones differ according to characteristics of the hydrology, soils, and vegetation as well as the mode of transport to streams.

Buffer zones where stock have been excluded and where long grass or natural vegetation has been allowed to develop, or been planted, can reduce diffuse pollutant transport from agricultural land by:

• direct removal of stock trampling and faecal inputs;
• enhanced infiltration by riparian soils which reduces surface runoff thereby aiding the deposition of particulates (sediment and particulate nutrients);
• reduction of surface flow velocities from increased hydraulic roughness of the vegetation in the buffer (sediment and particulate nutrients);
• physical filtering effect of dense vegetation (sediment and particulate nutrients).
• denitrification (dissolved N);
• plant uptake (dissolved nutrients).

The consensus in the literature is that grass buffer strips are effective at filtering sediment and sediment-associated pollutants (particulate P and N) from surface runoff. However they are less effective in removing soluble nutrients such as nitrate, ammonia, and dissolved P. Nitrate removal from subsurface flows is considered to be greater in forested buffers, partly through uptake by plants (Fennessy & Cronk 1997, Martin et al. 1999). However, the main mechanism by which nitrate is removed from groundwater is thought to be biological denitrification (a microbial process whereby nitrate is converted to gaseous forms of nitrogen and returned to the atmosphere).

What type of buffer and how wide for optimal nutrient and sediment attenuation?

Studies comparing multiple width buffers in the same location have shown that sediment and total phosphorus removal rates (between 53 and 98%) increase with increasing buffer width (4.6 m to 27m). Many researchers report substantial sediment removal within a few metres of the upslope boundary (Barling & Moore 1994, Fennessy & Cronk 1997). Grass filter strips in particular have been shown to be very effective at trapping sediment particles. Much of the larger particles of sediment may be removed in 5 m of grass buffer, but finer particles may require up to 10 m (Gharabaghi et al. 2002).

The width required to optimise nutrient removal has been debated with little systematic study of the issue. Fennessy and Cronk (1997) reviewed studies of RBZ effectiveness for the removal of contaminants, particularly soluble nitrate: Nitrate removal rates of almost 100% were measured in buffers 20-30 m wide, while buffers of 10 m width achieved over 70% retention of N. Many of the buffers in this study were forested, and N uptake by plants and denitrification were believed to have been an important factor in removing soluble N. Saturated riparian wetlands have been shown to be highly effective at attenuating (denitrifying) N, although this process is strongly dependent upon hydrological residence time.

Because of the different modes of particulate and dissolved contaminant transport, multi-tier or combination buffers are often advocated. For water quality benefits, a narrow combination buffer consisting of 5 m of grass filter strip and a 1 m wide row of deciduous trees has been shown to reduce nitrate in subsurface flows beneath cropland in Italy (Borin & Bigon 2002). The single row of trees may also provide some shade to the stream, but is unlikely to achieve terrestrial or aquatic habitat benefits.

Combination buffer systems in the USA often consist of an upslope grass buffer, a managed forest zone and an undisturbed forest zone next to the stream. Hubbard & Lowrance (1997) studied the nitrate removal from shallow groundwater where the forest zone was either mature forest, clear cut, or selectively thinned. All three forest management treatments were effective in assimilating nitrate and there were no differences between treatments.

Harvesting production trees or plants, or fruit and nuts from trees in riparian zones can provide a mechanism where P can be removed from the riparian zone. Phosphorus accumulates in riparian soils and can be taken up by plants but there is no process similar to denitrification that removes P to the atmosphere. Therefore, buffer zones could potentially become saturated and their ability to trap P may decline with age unless sediments or organic matter are removed from the buffer zone (Barling & Moore 1994). Examples in addition to production forestry include indigenous systems of tropical agroforestry where non-timber products (fruits, nuts and ornamentals) can be harvested (Robles-Diaz-de-León & Kangas 1999). There may be scope in New Zealand to use riparian buffers as zones for flax harvesting, medicinal plant growth, manuka honey, etc.

The optimal width required for nutrient and sediment removal can be highly variable and Auckland Regional Council have suggested an alternative approach based on the width needed to develop a self-sustaining buffer of native vegetation. Parkyn et al. (2000) recommended a buffer width of 10-20 m as the minimum necessary for the development of sustainable indigenous vegetation with minimal weed control, and to achieve many aquatic functions.

Other factors affecting buffer zone effectiveness for nutrient and sediment attenuation

The effectiveness of grass buffer strips as filters for nutrients and sediment is less in steep hilly terrain than rolling land, as overland flow is concentrated in channelised natural drainage-ways giving rise to high flow velocities. As a result buffer effectiveness is minimal, or at best, patchy along the stream length. Grass buffers may need to extend further inland following a drainage way, resulting in a non-uniform buffer width along the length of the stream. Similarly, many review articles of buffer zone studies conclude that buffers need to be wider when the slope is steep, generally to give more time for the velocity of surface runoff to decrease (Barling & Moore 1994, Collier et al. 1995).

Soil drainage properties can influence RBZ performance. Free draining soils minimise the generation of surface runoff, both on the hillside and within a buffer, thus reducing sediment and particulate nutrient delivery to the buffer. In regions with deeper soils (i.e., aquiclude or bedrock 10-30 m below surface) or where water drains into aquifers or large rivers, the removal potential of RBZ is expected to be low for soluble nutrients, as the subsurface hydrological pathways may bypass the root zone of buffers (i.e., zone of uptake and denitrification). Artificial subsurface drainage can also bypass the riparian zone and deliver nutrients directly to streams.

Buffer zones may have a limited life span where they can continue to be effective for contaminant removal. For example they may become saturated with P, pore spaces in soils may clog with sediments, or dissolved nutrient uptake by plants may be greatest during early growth phases and decline as vegetation matures. Some researchers suggest that these factors need to be taken into account and widths may need to be larger than early stage studies suggest. Methods to remove P could include selective harvesting for wood or fruits as mentioned earlier, or in the case of grass buffers, light grazing with sheep for a short time during summer may be acceptable providing that temporary fences are used to keep stock out of the stream. Alternatively, the strip could be mown for haymaking.

Because the effectiveness of buffers can be greatly affected by design and site-specific factors such as slope, clay content of the soil, drainage patterns, etc. DoC and NIWA published a set of guidelines (Collier et al. 1995) that provided practical measures to improve the design of RBZ to manage bank stability, light climate, water temperature, carbon supply, habitat diversity, flood flows, and contaminants. For contaminants, the guidelines can be used to calculate the optimal filter strip width for attenuating overland flow. These calculations were based on the modified CREAMS model (Chemical, Runoff, and Erosion from Agricultural Management Systems), and they require information on topography, slope, soil types for drainage and clay content categories, and hillslope length. Generally, buffer widths will need to widen as the slope length, angle and clay content of the adjacent land increase and as soil drainage decreases. For nitrate removal in subsurface flow, the guidelines recommend protection of existing riparian wetlands, based on their proven effectiveness for nitrate removal.

Biodiversity

The key to improving biodiversity in streams and riparian zones is habitat diversity and connectivity. The greatest improvements in habitat diversity are likely to occur when riparian management involves planted trees or remnant forest.

Riparian planting effects on stream habitat for aquatic biota include:

• provision of woody debris as trees fall into streams over the long term, providing habitat diversity and cover for aquatic invertebrates and fish;
• increased shade and provision of terrestrial food sources (fallen leaves etc.) as riparian plants grow so that levels of instream productivity and trophic pathways resemble the natural state;
• reduced erosion and inputs of fine sediment from (1) exclusion of livestock, leading to an improvement in streambed and bank habitat and (2) interception of hillslope sediment over the long term, and (3) tree roots that stabilise the stream banks and provide habitat;
• reduced water temperatures if sufficient lengths of upstream shade exist, and lower air temperatures and humidities, and less wind exposure, in the riparian zone where the adult stages of some aquatic insects spend part of their lives and some native fish lay their eggs (banded kokopu, short-jawed kokopu).

The buffer width required to achieve improvements in aquatic biodiversity is uncertain and variable between studies. Few studies have the luxury of experimentally testing mature buffer widths (i.e., with replication and under similar physical conditions), rather it is a case of looking at whatever existing buffers are available.

In Australia, Davies & Nelson (1994) found that small buffers (<10 m wide), retained after forest harvesting, did not significantly protect streams from changes in algal, macroinvertebrate and fish biomass and diversity. Buffer widths of >30 m appeared to provide protection from short-term impacts in a variety of forest types and geomorphology. However stream temperatures were only increased when buffer widths were below 10 m. The buffer width required to decrease stream temperatures may be less than that required to provide a microclimate similar to forested conditions. A single line of trees can provide about 80% shade to streams when the trees have grown tall enough to achieve canopy closure (Collier et al. 1995). Five and 30m wide riparian buffers of native forest reduced the median daily maximum air temperatures by 3.25 and 3.42ºC, respectively compared with a clearcut area downstream of the site (Meleason & Quinn 2004), indicating that narrow buffers can maintain cool riparian air temperatures. The buffer widths of Coromandel forestry sites studied by Quinn et al. 2004) ranged from 8-27m and supported stream invertebrate communities similar to those in native or mature plantation forest.

Parkyn et al. (2003) studied a number of riparian restoration schemes in the Waikato region to determine whether riparian management was achieving improvements in stream health. Significant changes to macroinvertebrate communities towards “clean water” or “native” communities did not occur at most of the sites over the time-scales that were measured in this study. The lack of improvement in QMCI scores and taxa richness may indicate (1) a lack of source areas of colonists, (2) lack of suitable microclimate for adult invertebrates, (3) time-scales of recovery are large, or (4) that buffers were not achieving habitat goals. However, one stream with a wide buffer of > 50 m, 25 year old plantings, and the whole stream length planted did show significant improvement in invertebrate communities compared to a nearby pasture stream. Improvement in invertebrate communities appeared to be most strongly linked to decreases in temperature suggesting that restoration of in-stream communities would only occur after canopy closure and after protection of headwater tributaries. This was particularly evident in lowland streams where catchment influences had a greater impact than local riparian influences.

Biodiversity in streams with riparian plantings may be affected by being in a “transitional” state. As plantings mature and shade is introduced and water quality changes occur, some species characteristic of pasture streams may be lost, while there is a time lag before the buffer zone matures or until connectivity of riparian patches allows recolonisation of native forest species to occur.

Catchment scale issues

Plantings, especially through provision of shade, aim to restore the ecological function of streams. However, shade can result in a widening of stream channels with a subsequent loss of sediment downstream, and also reduce nutrient attenuation within a given stream reach as instream plants are shaded out. These issues may become a problem when riparian management is implemented in a piecemeal fashion and where there are sensitive lakes or estuaries downstream. It is therefore important that the linkages within the whole catchment are considered when designing riparian management schemes and best management practice in most cases would be to begin planting from the headwaters and continue downstream.

The success of riparian management in terms of biodiversity can also be linked to the sources of recolonists within the wider catchment. Often remnant blocks of native forest exist in headwaters, so planting from the headwaters down the catchment would also increase the chances of recolonisation and improved biodiversity. An understanding of the connection between patches and dispersal potential of biota would also aid predictions of biodiversity improvements and help avoid unrealistic expectations.

An understanding of hydrological pathways is critical to determining buffer strip effectiveness. For example, the key nitrate pathway upon porous pumice soils in the central North Island of New Zealand is vertically down to groundwater that may take several decades to emerge into surface waters. In this example, the nitrate predominantly bypasses riparian vegetation and is difficult to mitigate using conventional riparian management (Howard-Williams and Pickmere 1999).

Because of the link between streams and their catchments, improved land management together with riparian management are required to achieve improvements in water quality and stream habitat. Examples of improved land management include: avoiding overstocking and pugging of soils, retiring steep and erosion-prone land, protecting wetlands which are sites of denitrification, diverting road and track runoff which can be a concentrated source of effluent and sediments, ploughing in directions parallel to the stream, and avoiding fertiliser application directly to streams or when the water table is high or heavy rain is likely. There will also be many other land use specific practices that will be important to consider in conjunction with riparian management.

Research for the future will be most effective if it addresses catchment scale issues such as these. Riparian management options will need to be designed with the hydrological pathways, soil drainage, and topography of the catchment in mind and targeted to areas where the most benefit can be achieved. Assessment of catchment and regional hydrology, such as soil drainage profiles and mapping of wetlands as hotspots for denitrification, as well as grouping streams and riparian zones into classes according to their potential effectiveness for water quality and biodiversity goals, are approaches that could assist resource managers with these issues.

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