Riparian Buffer Systems in Crop and Rangelands

Abstract

Riparian ecosystems occupy a narrow belt of land along streams and around lakes and wetlands and are characterized by plant and animal communities that are dependent on close proximity to water. These ecotones function as buffer zones for materials moving from the uplands toward the surface water. They control stream morphology and ecology and also maintain landscape biodiversity by providing diverse habitats and corridors for animals and plants. Most of the riparian zones in the Midwestern agroecosystems and arid and semi-arid western rangelands have been extensively impacted by agricultural cropping and grazing activities. These impacts have generally decreased water quality, impaired riparian and instream biodiversity, increased water quantity, and modified the timing of streamflow. Riparian zones are generally resilient because of their moist, moderate and fertile environments. With proper management, this resiliency can be sustained. Proper management should include construction or restoration of multi-species buffer strips and deferred or rotational grazing or exclusion of livestock. Several riparian zone restoration and management strategies are discussed.

Introduction

Riparian zones lie between aquatic and upland ecosystems in landscapes and play a critical role in the hydrology of watersheds (Smith 1992; Kira 1988; Lowrance et al. 1985a; Lowrance et al. 1984a). Because of their landscape position and their more frequent natural disturbance, riparian zones contain sharp biological and physical gradients. This results in a plant community that often contains a mosaic of age classes of upland species and species adapted to abundant water (Anderson and Masters 1992; Gregory et al. 1991). The typically long and narrow nature, along with the unique physical and biological processes, allow riparian zones to act as "strategic" buffers between upland and aquatic ecosystems (Osborne and Kovacic 1993; Nutter and Gaskin 1989; Lowrance et al. 1985b). Although a riparian zone may occupy as little as one percent of the land area in the arid watersheds of the west, these ecosystems are among the most productive in the landscape (Chaney et al. 1990). This paper will describe the important riparian ecosystem functions, present conditions of riparian zones in Midwestern agroecosystems and semi-arid and arid rangelands, and strategies for their restoration and management. Strategies discussed include the multi-species riparian buffer strip management system and methods of seasonal, deferred and rotational grazing.

Riparian Zone Functions

Riparian zones provide important links between the terrestrial upland ecosystems and aquatic stream or lake ecosystems (Osborne and Kovacic 1993; Franklin 1992; Elmore 1992; Gregory et al. 1991; Welsch 1991; Lant and Roberts 1990). Some of the most important functions in agricultural and grazing landscapes include filtering and retaining sediment, immobilizing, storing, and transforming chemical inputs from uplands, maintaining streambank stability, modifying stream environments, and providing water storage and recharge of subsurface aquifers.

Sediment Filtering and Retention

About 1.4 mt of sediment are delivered to surface waters in the US every year (Welsch 1991). Croplands account for 38 percent of this sediment while pastures and rangelands account for 26 percent (Welsch 1991). Excess sediment impairs aquatic life, clogs stream channels, reduces reservoir flood storage and contaminates water supplies. Riparian forest and grass communities can filter up to 90 percent of the sediment entering them from the uplands. The vertical structure of the standing plants and the organic litter provide frictional surfaces which slows water flow causing the sediment to be deposited (Magette et al. 1989; Dillaha et al. 1989; Cooper et al. 1987; Lowrance et al. 1986, 1988; Peterjohn and Correll, 1984; Brinson et al. 1981; Mahoney and Erman 1984). High infiltration rates of undisturbed riparian zone soils allow finer sediments and associated nutrients to enter into the soil before reaching the stream. As a result, as much as 80 percent of the phosphorus adsorbed to sediment particles can be filtered from surface runoff by forested riparian buffer zones (Welsch 1991). However, riparian zones are effective for sediment retention only if surface flow through them is maintained as sheet flow. Concentrated channel flow can destroy the continuity of the filter strip.

Longevity of sediment trapping ability varies between forest and grass communities. Cooper et al. (1987) and Lowrance et al. (1988) suggest that forest riparian buffers can filter sediments over long periods whereas Magette et al. (1989) and Dillaha et al. (1989) indicate that grass buffer strips may have short sediment filtering lives. If cool season, short grasses are replaced by native, tall prairie grasses, grass buffer strips have a longer sediment trapping life span (Schultz et al. unpublished data). In either case, sediment accumulation along the edges of any riparian buffer strip will have to be periodically renovated and areas of concentrated flow will have to be modified. Filtration of sediment from flood flows will also build streambanks and can create wet meadows and floodplain ecosystems (Chaney et al. 1990).

Nutrient and Chemical Processing

A growing body of evidence indicates that vegetated riparian zones can be effective at immobilizing, storing, and transforming chemical inputs from uplands. One of the major problems associated with agricultural production in the US is movement of fertilizers and pesticides from the uplands into the surface waters of the landscape (Knox and Moody, 1991; Lant and Roberts 1990; Felsot, 1988). Nitrogen is one of the most pervasive of the chemical non-point source (NPS) pollutants. Croplands contribute 43 percent of the annual nitrogen input to surface waters while pasture and rangelands contribute 25 percent (Welsch 1991).

Riparian forests and grass communities reduce nitrogen by 40-100 and 10-60 percent, respectively (Petersen et al. 1992; Osborne and Kovacic 1993). The methods of chemical removal in riparian systems include plant and microbial uptake and immobilization, microbial transformation in surface and groundwater and adsorption to soil and organic matter particles. The effectiveness of these processes will depend on the age and condition of the vegetation, soil characteristics such as porosity, aeration, and organic matter content, the depth to shallow groundwater and the rate with which surface and subsurface waters move through the buffer strip (Groffman et al. 1992; Lowrance 1992).

Plants can assimilate and immobilize nutrients such as nitrogen (N) and phosphorus (P) as well as heavy metals and pesticides. However, to be effective at removing these chemicals, plants must have access to high water tables or there must be sufficient unsaturated flow (Ehrenfield 1987). Plants will also not remove chemicals from water which is moving too rapidly over the surface or as preferential flow through macropores. Correll et al. (1994) and Schultz et al. (unpublished data) have observed that nitrate is not effectively reduced in coarse textured soils under high flow events when much of the annual N loading of the buffer zone might be taking place. In addition, riparian vegetation will be an effective sink only as long as the plants are actively accumulating biomass. Once annual biomass production is equal to or less than litterfall, there will be no new addition to the standing biomass sink. Plants must be harvested before that time if they are to remain viable agrichemical sinks. However, release of pollutants by litter decomposition may be beneficial if the vegetation removed the nutrients from the groundwater, where the potential for transformation to harmless by-products is often quite low (Groffman et al. 1992; Lowrance 1992).

Microbial processes are also important in reducing NPS pollution in the landscape. Microbe will assimilate and immobilize NPS pollutants but their rapid turnover and relatively small biomass may make this a minor sink. Microbes may also degrade many organic compounds such as pesticides. However, the metabolic breakdown of these organic compounds is dependent on readily available organic matter in the soil (National Research Council 1993).

Under anaerobic conditions microbes can denitrify nitrate into harmless nitrogen gas. This process has been found to occur in surface soils of riparian forests (Haycock and Pinay 1993; Jordan et al. 1993; Groffmann et al. 1992; Ambus and Lowrance 1991; Corell and Weber 1989; Jacobs and Gilliam 1985; Lowrance et al. 1984B; Peterjohn and Correll 1984) and seems to be dependent on the availability of carbon (Starr and Gillham 1993; Obenhuber and Lowrance 1991; Parkin and Meisinger 1989; Slater and Capone 1987: Smith and Duff 1988; Trudell et al. 1986). Wider vegetated buffer strips are usually more efficient at removing nutrients (Petersen et al. 1992). However, the long-term nutrient removal effectiveness of buffer strips is not known (Hanson et al. 1994; Osborne and Kovacic 1993). Wetlands that may be an integral part of integrated riparian management systems are highly efficient at denitrification because of their large quantities of organic sediments and decaying plant material (Crumpton et al. 1993).

Streambank Stability

When riparian vegetation is drastically modified or removed, streambanks become unstable and collapse, resulting in changes in channel width and structure (Fleischner 1994; Elmore 1992; Armour et al. 1991; Platts 1989). The woody and fibrous roots of plants growing on the streambank provide strength to hold the streambank in place. Plant roots increase soil stability by mechanically reinforcing soil and by reducing the weight of soil through evapotranspiration (Waldron and Dakessian 1982). Deeper rooted plants extract more water from greater soil depths than shallow rooted plants. Woody plant roots provide superior soil stabilization when compared to herbaceous plants because of their deeper rooting habit and their larger roots (Waldron et al. 1983). Woody roots provide protection against the hydraulic pressures of high flows while fibrous roots bind the finer soil particles (Elmore 1992). Tall grass prairie species are more effective than short cool season grasses at providing streambank stability because of their deeper fibrous root systems. There can be up to nine times more roots in the top 45 cm of soil and up to five times more at 100 cm depth for prairie grass species than for cool season species (Schultz et al. 1995).

Instream Environment

Loss or modification of riparian vegetation is one of the major reasons for the reduced quality of the aquatic environment throughout the United States (Fleischner 1994; Sweeney 1992; Menzel 1983). Riparian vegetation controls the quantity and quality of solar radiation reaching the water surface in lower order streams and thus influences autochthonous production and water temperature (Gregory et al. 1991; Sweeney 1992; Sinokrot and Stefan 1993). Organic matter input into the stream from riparian vegetation is an important energy source for aquatic organisms. Differences in quality and quantity of organic matter inputs between conifer and deciduous forests and between forests and grasslands often determine the structure of the invertebrate populations in the stream (Bilby and Bisson 1992; Gregory et al. 1991; Gurtz et al. 1988; Oliver and Hinckley 1987; Behmer and Hawkins 1986). Finally, large woody debris in the stream channel influences the physical structure of the channel by controlling the distribution of pools which store and detain sediments and riffles which oxygenate the water (Sweeney 1992; Gregory et al. 1991; Bisson et al. (1987). The more riparian zones can perform these "natural" functions the more diverse, productive, and resilient the instream ecosystem will be (Franklin 1992).

Water Storage and Groundwater Recharge

Vegetated riparian zones function to slow flood flow which allows water to spread and soak into the soil thereby recharging local groundwater and extending the baseflow through the summer season (Elmore 1992; Wissmar and Swanson 1990). In the West many streamside aquifers go dry later in the season because of poor livestock management on riparian zones (Elmore 1992). In the Cornbelt states of the U.S., channelization and tile drainage lower watertables to reduce the chance of out-of-channel flood flows in the riparian zone (Menzel 1983).

Riparian ecosystems are also important travel corridors for both animals and plants. They provide lush and diverse habitat for wildlife and because of their rich, moist microenvironments they are often the source of both upland and bottomland plants species in the landscape especially after upland perturbations (Naiman et al. 1993; Gregory et al. 1991).

The functions and processes of long, narrow riparian zones are extremely important to sustaining quality agricultural landscapes. These narrow ecosystems intercept and process nutrients, sediment and organic matter, which originates from the adjacent land. If these materials reach the stream they reduce water quality and their loss from the uplands reduces productivity. Because of the importance of these riparian ecosystems in cropland and rangeland ecosystems, effective methods for saving, restoring and managing riparian zones must be developed (National Research Council 1993).

Present Condition of Cropland and Rangeland Riparian Zones

Midwestern Cropland

The highly productive crop production regions of the midwest are a mosaic of crop and pasture lands, human habitations and small remnants of native prairie, wetland, and forest ecosystems. Most of the natural ecosystems have been converted to intensively managed agroecosystems in the twelve states ranging from Ohio to the eastern portions of the Dakotas, Nebraska, and Kansas, and from the southern portions of the Lake States to the northern half of Missouri. In Iowa, for example, 99% of the prairie and wetland area and more than 80% of the forest area have been converted to other uses (Bishop and van der Valk, 1982; Thomson and Hertel, 1981). Ohio, Indiana, Illinois and Missouri drained more than 85 percent of their wetlands by the mid 1980's (Dahl et al. 1991). In most of the midwest region less than 20 percent of the natural prairie, forest, wetland and riparian ecosytems still exist (Burkart et al. 1994). In a typical watershed in central Iowa about 50% of the total length of stream channel may be cultivated with corn and/or soybeans to the bank edge. Another 30% of the length may be in pasture, most of which is overgrazed (Bercovici, 1994). Annual soil erosion is greater than 6.7 Mg/ha in much of the central part of this region and in some areas is greater than 11.2 Mg/ha despite that fact that many of these same areas have over 50 percent of the land in upland conservation practices (Burkhart et al. 1994). Because they have little other perceived value, many kilometers of Midwestern riparian zones have livestock fenced into them as a management practice. Livestock under these conditions do extensive damage to the stream channel, the streambanks and the riparian zone.

Modern product-oriented agriculture has put this midwestern agroecosystem at risk. The production-oriented function of this landscape has produced unintended and undesirable environmental consequences. These include loss of biodiversity, detrimental alteration of waterways and groundwater aquifers and loss of significant portions of the productive topsoil resulting in greater need for fertilizer and energy inputs. Non-point source pollution has become so pervasive because of rapid surface and subsurface water movement and reduced soil residence time of agrichemicals. It is now apparent that upland conservation practices alone are not effective in reducing NPS pollution (Burkhart et al. 1994; National Research Council 1993). Field and landscape buffers, including riparian buffer zones, are also needed to develop a sustainable agroecosystem with improved soil and water quality (Castelle et al. 1994; National Research Council 1993). However, major issues about buffer strip efficiency and design must be clarified before they can be effectively implemented. These issues include: plant species selection and efficiency; optimal widths for various buffer strips; longevity of the buffer zones as nutrient and sediment sinks; criteria for identifying riparian zones in need of buffers; and criteria for long-term management of buffer strips (Castelle et al. 1994; National Research Council 1993; Osborne and Kovacic 1993).

Western Rangeland

Recent reviews by Fleischner (1994), Kauffman and Krueger (1984), Elmore (1992) and Chaney et al. (1990, 1993) identify livestock grazing as having dramatically changed riparian zones in the rangelands of the west. The changes by livestock have been so great and cover so much of the western landscape that it is even difficult to determine what the natural vegetation was or what the effects of livestock grazing has been (Fleischner 1994). Riparian zones in the semi-arid and arid West are probably even more important to the overall landscape than they are in the cropland of the Midwest. While they occupy less than one percent of the landscape they are the most productive and biodiverse ecosystem in that landscape. More than 75 percent of the wildlife in many of these watersheds depends on the riparian zone for existance (Chaney et al. 1990). Riparian ecosystems in the arid and semi-arid west also function to filter sediment, stabilize streambanks, store water and recharge subsurface aquifers (Fleischner 1994; Elmore 1992; Chaney et al. 1990). Excluding isolated examples, the condition of the riparian zones throughout the semi-arid and arid west are the worst they have been in history (Chaney et al. 1990).

Livestock tend to congregate in the riparian zones where there is succulent vegetation, shade, and water. In the process they compact the soil and destroy the bank by climbing into and out of the stream. Livestock will also rub, trample, and browse the vegetation, and relieve themselves directly in the stream. This results in the widening of the stream channel, decreasing average stream depth and increasing average stream temperature, and sediment and nutrient loads. Alterations in the timing and volume of streamflow and lowering of the local water table also occur (Kauffman and Krueger 1984; Platts 1981). These activities along with the lack of management strategies unique to riparian zones are responsible for the poor condition of these riparian ecosystems (Armour et al. 1991). Many kilometers of Midwestern riparian zones have also suffered the same fate.

In summary, riparian zones in crop and rangeland landscapes are presently in poor condition. However, these ecosystems are among the most resilient in the landscape because of their moist, fertile and microclimatically less extreme conditions and therefore should respond well to mangement and restoration activities. Research should be accelerated to develop design and management standards for landscape buffer zones in the crop and range landscapes (National Research Council 1993; Armour et al. 1991). It is especially important to understand the dynamics of riparian zone processes, to describe the impact of good management of riparian habitats on all natural resources and to develop predictive methods to determine optimal widths and management intensities needed to accomplish specific soil and water quality objectives.