LANDSCAPE ECOLOGY in AGROECOSYSTEMS MANAGEMENT - CHAPTER 6 pdf

Crow CONTENTS Buffers as Critical Components of Managed Landscapes Riparian Ecosystem Functions in Agricultural Landscapes Purposes of Riparian Ecosystem Buffers Making Buffers Sustainab

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CHAPTER 6

Implementation of Riparian Buffer Systems for Landscape Management

Richard Lowrance and Susan R Crow

CONTENTS

Buffers as Critical Components of Managed Landscapes Riparian Ecosystem Functions in Agricultural Landscapes Purposes of Riparian Ecosystem Buffers

Making Buffers Sustainable at Multiple Scales in Agricultural Landscapes USDA Programs for Riparian Ecosystem Buffers

Simple Decision Tree for Riparian Ecosystem Decision Making Summary

References

BUFFERS AS CRITICAL COMPONENTS

OF MANAGED LANDSCAPES

In recent years efforts have been made to incorporate a landscape perspective into U.S national policy initiatives for land management Supporting these efforts, landscape ecology has been regarded as an effective paradigm for organizing and evaluating various approaches to land management In part, this reflects landscape ecology’s focus on applying principles derived from studying landscape elements, their interactions and changes over time, to solving practical problems in the real world A fundamental premise of landscape ecology is that the pattern of component ecosystems or landscape elements affects ecological processes Study of landscape pattern focuses on three basic landscape characteristics: structure, the spatial rela-tionships among distinct ecosystems or landscape elements; function, the interactions

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or flows of energy, materials, and species among ecosystems; and changes in land-scape structure and function over time

Within this context, landscape structure may be described as a mosaic of three elements: patch, corridor, and matrix (Forman and Godron 1986) A patch is a unit

of a landscape represented by discrete areas or periods of relative homogeneity in environmental conditions, and is perceived by organisms or relevant ecological phenomenon of interest as bounded by discontinuity in environmental character Patches are dynamic, occurring at a variety of spatial and temporal scales Thus, a landscape is composed of a hierarchy of patch mosaics across a range of scales Corridors are narrow landscape elements differing from their surroundings, distin-guished by their linear spatial configuration in which corridor width and sinuosity are important characteristics The landscape matrix is the most extensive and con-nected of the elements composing a landscape and, therefore, is functionally the most dominant landscape element The overall pattern of landscape elements may include networks in which patch number and configuration, corridor connectivity, and boundary shapes affect flows and interactions of energy, matter, and species which is particularly relevant to riparian landscapes

It is important to note that landscape extent, as well as the area associated with each landscape element, is dependent on the organism or ecological phenomenon

of interest Therefore, there is no single definition of landscape Rather, it is necessary

to define a landscape by delineating the land area of interacting landscape elements relevant to a particular organism or phenomenon There are at least two important implications of defining landscape in this way The first is that scale and context are critical concerns for landscape description and management For example, in char-acterizing a particular landscape, is forest the landscape matrix or is forest an element within an agricultural matrix? A second important implication relates to the degree

to which the landscape may be characterized as an open or closed system If a watershed is defined as the landscape of interest, from a hydrologic perspective at least, the landscape may be considered a relatively closed system in which few outputs leave the system On the other hand, the system may be characterized as relatively open in terms of species movement through the system These implications are particularly relevant to the management of landscapes containing typical agri-cultural production systems because of the significant inputs and outputs associated with these systems

Landscapes and elements within landscapes may be more or less intensely managed In an agricultural landscape, for example, upland areas with fertile soils are intensely manipulated, whereas lowlands with hydric soils and streams may not

be directly manipulated (though certainly they may receive significant inputs as a result of management of adjacent areas) In these intensely managed landscapes, some landscape elements may be regarded as critical to mitigate exports of other landscape components and thereby support the overall healthy functioning of the landscape system Landscape corridors are recognized as major structural and func-tional components of the landscape, necessary to ecological regulatory processes such as animal dispersal, habitat for nongame species, prevention of soil and wind erosion, and integrated pest management (Barrett 1992) In addition, these landscape elements may serve to mediate or buffer the effects of outputs from other landscape

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components In this context, riparian environments are critical components in land-scapes managed for agricultural production, as well as other intensively managed landscapes

Riparian environments include those ecosystems adjacent to rivers and streams

In addition to extensive floodplains, riparian zones include narrow strips along downcutting rivers, islands, and channel landforms (Malanson 1993) These riparian systems represent ecotones between terrestrial and aquatic ecosystems and are char-acterized as landscape corridors in which lateral water flow is the main force organizing and regulating the function of riparian wetlands and their role in the landscape These unique environments are of particular interest to land managers and policy makers because of the indirect, but critically important, economic benefit they serve in maintaining water quality In particular, studies in agricultural land-scapes have established the effectiveness of riparian vegetation in filtering agricul-tural nutrients and trapping agriculagricul-tural sediments U.S agriculagricul-tural policy has designated riparian buffer zones as eligible for inclusion in the U.S Conservation Reserve Program, allowing farmers to be paid not to farm these environmentally sensitive lands if forest vegetation is regenerated either through planting or succession Agricultural production systems are inherently “leaky” for nutrients, sediment, and agricultural chemicals The degree of “leakiness” is generally measured by comparing agricultural systems to nonagricultural systems, typically forested land-scapes in humid regions The leakiness is caused by numerous interacting factors, including (1) tillage to produce seedbeds and control weeds; (2) application of nutrients from fertilizer and manure; (3) application of xenobiotics to control pests; (4) hydrologic modifications of drainage systems to remove excess water; and (5) application of water to the root zone to relieve soil moisture deficits Buffer zones, established as landscape corridors in agricultural systems and existing as integral landscape elements in agricultural watersheds and other landscapes, primarily have been implemented to control the unplanned leaks from agricultural management systems before they impact adjacent terrestrial, wetland, and aquatic ecosystems (Lowrance et al 1997) In addition, as discussed above, buffers are needed in agri-cultural landscapes to serve other landscape functions such as wildlife habitat, visual screens, and as setbacks from vulnerable ecosystems

In addition to the more conventional view of buffers as primarily linear ecosys-tems or ecotones juxtaposed between vulnerable ecosysecosys-tems and an agricultural production system, it is also useful to consider agriculture and buffer systems in a more general way Applying the concept of a buffer as any less intensively managed portion of a landscape producing fewer external outputs than other landscape com-ponents, agriculture may serve as a buffer For instance, in urbanizing landscapes, agriculture may serve as a transition between high density developments with a significant percentage of impervious surfaces and nearby naturally functioning eco-systems This is the case for many areas at the edge of urban sprawl and is one of the primary motivations for preserving agricultural lands for open space The empha-sis at the edge of urban areas should be on preserving agricultural land that can contribute to food supply for the urban centers Conversely, when agriculture is the most intensive land use in a landscape, the nonagricultural landscape elements serve

as parts of the general buffer system supporting sustainability of the entire landscape

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This situation is especially true in ground water-dominated landscapes where most lands are equally likely to provide recharge to ground water In these sorts of systems, water quality problems from agricultural production systems often may be avoided

by having a substantial part of the landscape in nonagricultural uses

RIPARIAN ECOSYSTEM FUNCTIONS

IN AGRICULTURAL LANDSCAPES

Riparian ecosystems originally referred to streamside or riverside ecosystems that developed due to the influence of the stream itself The definition has now been expanded to include almost any land adjacent to a freshwater system, including ponds, lakes, ditches, and canals As has been documented in numerous reports, riparian ecosystems control many of the interactions among terrestrial and aquatic ecosystems Most receive subsidies in the form of water and nutrients from adjacent uplands Many are flooded periodically, but whether the flood acts as a subsidy or not is a function of how often it is flooded and the severity of the flood In areas of infrequent scouring floods, the flood acts less as a subsidy and more as a perturbation

to produce new flow channels and to reset both soil and vegetation succession of the systems Although riparian systems are usually considered primarily as horizon-tally connected to the stream and upslope ecosystems, there are also important vertical connections with the hyporheic zone beneath the stream This point is of particular importance in regions where the hyporheic zone consists of gravel and coarser materials and where there are large interstitial spaces for water and organisms Although typically referred to as ecosystems, riparian zones actually form eco-tones between terrestrial and aquatic ecosystems They may or may not be wetlands, depending on both frequency and duration of flooding and hillslope hydrology Floodplain wetlands tend to develop in the riparian zone along larger streams and rivers, while seepage wetlands tend to develop at the base of hillslopes along smaller streams As a result of their position in the landscape, riparian ecosystems often serve as natural buffer zones Most riparian ecosystems serving as buffers in agri-cultural landscapes are functioning essentially as natural buffers with very little management

Most riparian buffers that have been studied, and that form the basis for our knowledge of buffer system functions, are naturally occurring ecosystems that devel-oped over time as forests were converted to agricultural landscapes (Jacobs and Gilliam 1986, Lowrance et al 1984, Peterjohn and Correll 1984) The water quality functions of these naturally occurring buffers have been summarized in numerous reviews and are represented from most to least general in Figure 6.1 (Lowrance et al 1997) We have just recently begun to consider both the management and re-establishment of riparian buffers where they are not functioning adequately or where they do not exist (Schultz et al 1995, Vellidis et al 1993) The design of riparian buffer systems for agricultural landscapes is in its infancy Although it is theoretically possible to design buffers that have a very high capacity to filter pollutants, it is not clear that these buffers will maintain this function long term

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PURPOSES OF RIPARIAN ECOSYSTEM BUFFERS

Whether we realize it or not, we expect and receive numerous goods and services from natural riparian ecosystems We do not yet know which of these goods and services may be sustained from restored or established riparian ecosystems Natural riparian ecosystems provide, at least, the following goods and services: retain sed-iment; retain dust and windblown particles, including plant propagules; retain and transform nutrients; retain and transform toxics; shade streams and margins of rivers and lakes; provide leaf litter and woody debris as energy sources; provide woody debris as substrate and channel structures; provide sources of biodiversity; increase micro-topographic variability; provide carbon sinks; provide energy sinks, especially for moving wind or water; provide sources of plant propagules; provide terrestrial wildlife habitat; provide products such as wood and forage; and provide recreation, hunting, and landscape aesthetics

As fundamental components of agricultural landscapes, riparian buffers mitigate off-site effects of production systems, including increased nutrient, sediment, and pesticide loading Agricultural landscapes, especially annual crops, decrease perma-nent cover, wildlife habitat, and biodiversity, and they create undesirable habitats for noncrop species Buffers protect critical areas, including wetlands, recharge areas, aquatic ecosystems, and endangered species habitat, and link landscape components, providing corridors between critical areas and less disturbed areas Buffers also may decrease the intensity of agricultural production at the edges of critical areas Ripar-ian buffers provide specialized features for human use in managed landscapes, including recreation, woodlots, and aesthetic experiences

In a sense, riparian buffers serve as insurance to protect critical areas from both unexpected and expected external effects of production systems The expected effects

of production systems depend largely on the intensity of production and the extent

to which production systems dominate more natural systems As noted above, if the landscape matrix becomes agriculture, there are generally more expected effects from the production system than if the matrix retains more of the character of natural systems with agriculture located in the landscape on the most productive soils

In countries practicing modern production agriculture, emphasizing water and environmental quality best management practices, some have questioned why buffers

Figure 6.1 Most general to least general water quality functions.

Remove dissolved phosphorus Remove nitrate from groundwater Remove sediment & sediment bound chemicals Control Aquatic Environment

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would be needed for specific purposes such as nitrate removal It is reasoned that

we now have the ability to keep chemicals applied where they belong in the landscape

to enhance crop production, and if we do that buffers are not needed to control these chemicals In short, we are capable of controlling crop production chemicals on site; therefore, buffers are not required for their management There is no doubt that keeping chemicals where they belong, in the root zone or in plants, is the best way

to achieve water quality goals in agricultural landscapes At doubt is our ability to

do this with the consistency needed to protect the environment

At this point, what are expected effects and what are unexpected effects become relevant questions As long as we continue to till the soil, and use large amounts of nutrients and xenobiotic chemicals, and cannot control the weather, conservative management practice requires us to assume that the expected effects of agriculture will be soil, nutrient, and chemical movement significantly beyond both the back-ground from natural ecosystems and the ability of aquatic ecosystems to assimilate and process Given these conditions, implementing a combination of buffer systems and proper field management will provide both the best regional environmental quality and ecologically sustainable agriculture

MAKING BUFFERS SUSTAINABLE AT MULTIPLE SCALES

IN AGRICULTURAL LANDSCAPES

The goal of establishing buffer systems integrated with conservation practices

in agricultural production areas is to ensure sustainable, productive landscapes Buffers mainly contribute to ecological sustainability (Lowrance et al 1986) but may also contribute to economic sustainability Three biophysical factors influence the ability of buffers to contribute to ecological sustainability: size, time, and man-agement The ecological sustainability of the buffer systems, and in turn the eco-logical sustainability of the agricultural landscape, ultimately will be governed by properties of the buffer systems interacting with properties of the production system Given the linear nature of riparian ecosystems, size involves both the linear extent along a stream and the width of the buffer relative to specific pollutant sources Size matters in riparian ecosystems Although it is clear that a continuous buffer is best,

it is not clear that there is an optimum buffer width, given the opportunity costs associated with buffers relative to other more economically important land uses Simulation studies using the Riparian Ecosystem Management Model have shown that most sediment and nutrient removal occurs in about 15m riparian buffer systems

in the U.S Coastal Plain (Williams, et al 2000) The generality of these results needs to be tested in other regions In the absence of government subsidy, optimum buffer width, although lacking more specific definition, is the minimum width that provides a desired control of pollutant externalities (both a societal and local benefit) and provides other amenities desired by the landowner (primarily a local benefit) Time required to establish buffer function also is variable, but it is likely to be short in most areas In areas of minor modification, such as a cut-over riparian forest, many of the soil and hydrologic functions are still in place and vegetation will often re-establish rapidly (Vellidis et al 1993) In areas of major modification, such as

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streamside fields, establishing a buffer will be a very large change in chemical loading, tillage, etc., and establishment of buffer functions should be rapid relative

to the agricultural uses Even in areas of surface drainage with ditches, providing coarse woody debris to the stream in the first few years of buffer establishment should have effects on flow regimes The three agricultural scenarios most likely to need longer time to re-establish buffer functions are areas with extensive subsurface drainage, areas with intensive streamside use by domestic animals, and steep areas with eroded soils

Managing riparian buffers for multiple benefits is a new concept in modern production agriculture Although one approach to buffer design for pollution control allows intensive management to be substituted for size, this may not be a realistic approach in practice Although many farmers may spend noncritical times (especially winter) managing buffer areas for multiple benefits (especially as wood lots and wildlife habitat), managing buffers for pollution control during the growing season will not be a high priority for most farmers Active growing-season buffer manage-ment is probably not a realistic expectation for most modern farmers Therefore, buffers should be largely self-regulating, be highly resistant to failure during the growing season, and be suitable for largely nongrowing season management

To date, riparian buffers have been implemented largely on a field-by-field basis Future efforts should establish and evaluate riparian buffers at multiple spatial scales

At the field scale, it is possible to adjust inputs and management to gain desired outputs Where there are likely to be greater outputs from a given field, for instance

an area in intensive vegetable or animal production, the farmer may implement a larger riparian buffer to interact with in-field buffers such as contour filter strips or grass waterways At the farm-scale, the goal should be to integrate production components where possible — rotations, animal manures, agroforestry — to buffer critical areas on and adjacent to the farm At the landscape scale, practices should

be coordinated to buffer and protect critical areas and link habitat resources with buffer systems serving as landscape corridors In most cases, it will be possible to restore critical areas only through such coordinated efforts

USDA PROGRAMS FOR RIPARIAN ECOSYSTEM BUFFERS

Riparian buffers are an important part of the USDA Conservation Buffer Initiative (CBI), intended to have 2 million miles of total buffers installed by the year 2002 Although the CBI includes many types of buffers, the focus of CBI financial incentive programs has been to provide financial support for long-term riparian forest buffers through the USDA Conservation Reserve Program (CRP) and the Conservation Reserve Enhancement Programs carried out as state/USDA partnerships Riparian forest buffers are the primary practice implemented under the Continuous Sign-up

of the CRP (CS-CRP) which allows landowners to offer eligible acreage at any time Eligible acreage includes cropland along streams with a cropping history in 3 out

of the 5 most recent years or pastureland that is along streams (FSA 1997) As with other CRP programs, the CS-CRP provides either 10- or 15-year annual rental contracts between USDA and landowners Unlike other CRP programs, CS-CRP for

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riparian forest buffers provides a one-time sign-up bonus and increased cost-share for installation of the practice With the extra incentives, there is very little initial cost to the landowner

A comparison of the use of riparian forest buffers under CS-CRP in two states provides insights into both the need for riparian forest buffers and the effectiveness

of USDA financial incentives for them Iowa has extensive acreage of cropland along streams and rental rates for CS-CRP land is from $150 to $175 per acre per year Georgia has much less row cropland along streams, more pastureland along streams, and lower rental rates for riparian lands, typically $40 to $50 per acre per year Rental rates for both states are based on soil productivity for typical riparian soils The difference in rental rates is reflected in dramatic differences in both the overall use

of riparian forest buffers under CS-CRP and the distribution of new riparian forest buffers under CS-CRP within the two states From October, 1999 through April,

2001, over 22,500 acres of riparian forest buffer were installed under CS-CRP in Iowa (USDA-NRCS-PRMS 2001) In contrast, only 1586 acres were installed under CS-CRP in the same time period in Georgia (USDA-NRCS-PRMS 2001)

The distribution of CS-CRP riparian forest buffers in the two states is very different also In Iowa, CS-CRP riparian forest buffers are being installed in the majority of counties Although counties in the more humid eastern part of the state have higher acreages, the practice is widespread under CS-CRP (Figure 6.2) In Georgia, the practice is much less widespread geographically with the limited acre-age concentrated in a few counties (Figure 6.2) Of the total CS-CRP riparian forest buffer acreage in Georgia, 70% has been installed in five counties (out of 163 counties total) with 38% in one county

Although the CS-CRP recognizes the value of riparian buffers by providing for continuous sign-up, the rental rates are tied to the agricultural productivity of the land based on the soil type This is a relatively successful approach in cases where riparian soils are supporting productive croplands because the rental payment is relatively high However, where riparian soils are supporting pastures, and the soil productivity is generally low, the program has not been as successful in re-estab-lishing riparian buffers Yet many streams in Georgia are impaired due to low dissolved oxygen and high fecal coliform levels (SRWMD, 2000) Although cause and effect are not well understood, both low dissolved oxygen and high fecal coliform may be related to streamside pastures These conditions indicate that some important benefits to society are not being realized in CS-CRP riparian forest buffers

on pasturelands, and they suggest that adjusted rental rates might better capture the overall societal values of riparian forest buffers At this point, one of the most pressing concerns for CS-CRP is to acquire more riparian pastureland for permanent riparian forest buffers

SIMPLE DECISION TREE FOR RIPARIAN ECOSYSTEM

DECISION MAKING

The functions of riparian ecosystems relate to their current conditions, as well

as to societal goals and landowner goals A few simple decision trees will be used

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here to illustrate possible interactions between riparian ecosystem conditions and human goals to guide buffer design at a site or field scale

If riparian systems are largely intact, the goal will be to use and enhance existing riparian ecosystems (Figure 6.3) Unfortunately, incentives generally are not avail-able to preserve existing riparian ecosystems A few regulations exist, especially for wetlands, but programs are needed to reward landowners for maintaining riparian systems In many cases, riparian systems could be enhanced for multiple purposes via species enrichment, restocking, etc However, very limited financial incentives are available for these practices

If current conditions include degraded riparian systems with minor hydrologic modification, controlling nonpoint source pollution and restoring aquatic ecosystems should be relatively straightforward (Figure 6.4) Controlling nonpoint source pol-lution probably could be accomplished with any perennial vegetation; however, restoring aquatic ecosystems will require native vegetation Although USDA pro-grams have a default minimum buffer width of 15 m, this minimum is not well

Figure 6.2 Comparison of Georgia and Iowa riparian forest buffer acres enrolled in the USDA

Conservation Reserve Program from October 1999 to April 2001.

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tested Practices implemented beyond the minimum distance for cost-share programs such as CS-CRP will depend on landowner objectives

Given the same existing situation described above — degraded riparian systems and minor hydromodifications — restoring aquatic ecosystems and terrestrial wild-life will require a wider buffer (Figure 6.5) The default minimum distance for restoring aquatic ecosystems is only 5 m for USDA approved buffers, but, if terres-trial habitat is a secondary goal, up to 100 m of perennial native vegetation is likely

Figure 6.3 Simple decision tree for riparian ecosystem selection — natural riparian

ecosys-tems in place.

Figure 6.4 Simple decision tree for riparian ecosystem selection — degraded riparian

eco-systems, minor hydromodification.

Figure 6.5 Simple decision tree for riparian ecosystem selection — degraded riparian

eco-systems, minor hydromodification.

Preserve through incentives and/or regulations

Enhance through species enrichment, restocking, etc.

Use Existing Riparian Ecosystems

Primary Goal: Control Nonpoint Source Pollution

Secondary Goal: Restore Aquatic Ecosystems

Determine minimum

distance default=15 m

Determine Landowner preference of vegetation

Restore native vegetation for secondary objective Establish productive system of perennial vegetation

Primary Goal: Control Nonpoint Source Pollution

Secondary Goal: Restore Aquatic Ecosystems

Determine minimum

distance default=5 m

Determine Native vegetation

Restore much wider buffer for secondary goal Establish native perennial vegetation

Primary Goal: Restore Aquatic Ecosystems

Secondary Goal: Terrestrial wildlife

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