One of the basic conclusions of the recently-released Millennium Ecosystem Assessment (2005) is the recognition that human well-being is inextricably linked to the health of ecosystems that provide essential services (i.e. supporting, provisioning, regulating and cultural). Since much of the world's surface consists of managed or artificial ecosystems (agriculture and forestry), agroecosystems also impact the provisioning and regulating functions, both in positive and negative ways.

Agroforests, particularly riparian buffers and windbreaks, are artificial but relatively diverse, managed agroecosystems which provide goods and services for both landowners and downstream inhabitants. Riparian buffers have the greatest potential impact o­n the downstream environment of any agroforestry practice.

Multiple rows of trees and shrubs, as well as a native grass strip, combine in a riparian buffer to protect Bear Creek in Story County, Iowa. The buffer is a nationally designated demonstration area for riparian buffers. (Photo courtesy USDA NRCS).

Water Quality Trading

Riparian buffers have the potential to mitigate nonpoint source (NPS) pollution, and as such play an important role in water quality (WQ) trading within a watershed. Water quality trading is possible when pollution sources in a watershed face very different costs to control the same pollutant, e.g. excess N or P that can degrade rivers and lakes. Trading programs allow point source facilities, e.g. industrial or municipal water users, facing higher pollution control costs to meet their regulatory obligations by purchasing pollution reduction credits from another source, e.g. farmers and ranchers as nonpoint sources, that have lower control costs, thus achieving the same water quality improvement at lower total cost (US EPA 2004).

According to a recent review of WQ trading in the US, more than 40 programs have been initiated covering 20 different states (Breetz et al. 2004). All but nine of the initiatives are set up for trading between point and nonpoint (i.e. landowners) sources. The target pollutants in 30 of the programs are N and/or P. Tradeable credits are created when NPS contracts (farmers) implement best management practices (BMPs) that reduce the amount of excess nutrients that flow from their farm fields to adjacent water bodies. At least o­ne trading program has been established in Canada (O'Grady and Wilson 2002).

However, despite many program start-ups, point-nonpoint trading involving farmers utilizing BMPs has not materialized (King 2005). Although WQ trading was mooted as an alternative to strict regulation, it is exactly the lack of strict limits o­n pollution discharge (e.g. TMDLs) and lack of enforcement penalties that are discouraging market activity. If the cost of non-compliance for industrial polluters is lower than compliance, there is no incentive to buy credits. o­n the supply side, farmers may fear increased government regulation of farming activities if, as assumed, their costs per unit of pollution control are lower than other discharges, making tighter restrictions o­n agricultural pollutant management appear to be a more cost effective strategy for government (King, 2005).

Beyond the legal and political considerations, there are several technical issues that could affect farmer's willingness to establish riparian buffers in order to receive payments under a WQ trading program. They are briefly examined below.

Design Flexibility

The first step to creating a riparian buffer for verifiable and tradeable pollutant reduction is its design and location. There have been many experimental studies of the pollution control functions of buffers, e.g. trapping sediment and absorbing dissolved nutrients and pesticides, that have different structure, species composition, width, location, etc. (Some recent reviews include Parkyn 2004; Schultz et al. 2004; and Wenger 1999.)

Most of the trading initiatives reviewed by Breetz et al. (2004) refer to the USDA Natural Resources Conservation Service practice standard for riparian forest buffers (USDA NRCS 2003) or the state modification thereof. This general standard specifies a protected zone of trees and shrubs adjacent to the stream plus up to two additional zones of vegetation intended to reduce excess sediment, nutrients and pesticides in surface runoff and shallow groundwater. Some states have modified the standard; for example, the Iowa state standard specifies a two-zone buffer of trees & shrubs (preferably native) closest to the stream and an outer zone of stiff-stemmed grasses (USDA NRCS, 1999). State NRCS standards and literature surveys have helped to inform state water regulatory agencies in the development of their own guidelines for buffers to be used for WQ trading; e.g., North Carolina Division of Water Quality riparian buffer rules for the Tar-Pamlico River Basin (NC Admin. Code 2000).

State agencies which set the rules for riparian buffers for use in WQ trading should not try to impose "one-size-fits-all" prescriptions, but rather give general guidance for the design of buffers that, based o­n controlled studies and farmer's experience, will have a high probability of adequate function over the range of expected nutrient loadings and disturbance events (e.g. heavy storms or flooding). Studies at the Bear Creek watershed in Iowa show that different combinations of trees, shrubs and grass can achieve significant pollutant reductions, and suggest that it is important to allow more flexibility in the design of riparian buffers (Schultz et al. 2004).

Standardized Monitoring Techniques

Government cost-share programs like the Conservation Reserve Enhancement Program (CREP) that subsidize riparian buffer establishment and maintenance do not require o­n-going monitoring to verify that buffers are in fact operating effectively to reduce pollution. Nutrient trading, o­n the other hand, requires two stages of verification: first, to establish a baseline for the target pollutant before corrective measures (BMPs) are put into place, and second, to measure the amount of pollutant reduction achieved after BMPs are adopted (US EPA 2004).

Examination of WQ trading initiatives cited by Breetz et al. (2004) shows a wide discrepancy among those programs which have published monitoring plans. These plans may call for, e.g., 1) annual inspection of BMPs by state agencies or local conservation districts, 2) periodic sampling to measure survival and health of species planted in the buffer, 3) random audits of 10% of NPS contracts in the watershed, 4) no systematic monitoring of BMPs or water quality due to the expense and long-term work required, or 5) site-specific monitoring of 5-10% of NPS contracts and continuous water sampling at different locations o­n sub-watershed scale.

The challenge is to develop verification techniques which measure levels of the target pollutant o­n a sub-watershed scale, identify possible "hot spots" for closer inspection, do not unduly burden individual landowners, and are cost effective for state or local agencies to conduct. A worthwhile goal at the national level may be to try to develop recommendations for standardized protocols to monitor nonpoint sources. Perhaps this could be similar to existing criteria and indicators of "proper functioning condition" (PFC) that are used to evaluate the health of natural riparian areas (Prichard et al. 1998).

Flexible Management Guidelines

Riparian buffers can not be left o­n their own after planting; long-term management is required to maintain their desired functions to reduce agricultural NPS pollution (Schultz et al. 2004). Some of the necessary practices include mowing in the first few years after planting in both grass and tree zones, control of weeds (especially invasive perennials), and animal damage control. Periodic removal of vegetation through grazing or mowing and tree harvest are necessary to stimulate new growth that will absorb nutrients from runoff and shallow groundwater (Schultz et al. 2004).

Riparian buffers as an agroforestry practice are managed not o­nly to maintain pollution control functions (and earn tradeable credits), but also to produce economic crops to supplement farm income. Harvesting of commercial tree species, specialty-crop shrubs (e.g. ornamental or edible) and controlled grazing are encouraged by the national NRCS practice standard in the outer zone(s) of the riparian buffer as long as they do not compromise its intended function (USDA NRCS 2003). A few studies have examined the effects of tree harvesting o­n buffer function (e.g., Lowrance et al. 2000; Sheridan et al. 1999).

Some regulatory agencies have adopted guidelines for forest harvesting activities within agricultural riparian buffers that are part of WQ trading programs, for example in Tar-Pamlico River basin of North Carolina (North Carolina Admin. Code, 2000). State forest practice regulations that govern riparian zone management in forests may not be appropriate for farmland riparian buffers. Management guidelines adopted by state agencies to govern buffers used in trading programs should be flexible and allow for different levels of management intensity, including commercial harvest of both tree and shrub products.

Future of Water Quality Trading

The promise of WQ trading as a cost effective means of water pollution control has stimulated much progress o­n developing program infrastructure in many watersheds in the US and Canada. However, the low number of actual trades shows that the promise is yet to be fulfilled.

Encouragement of trading between point source buyers and NPS sellers (farmers) may depend largely o­n the political will of lawmakers to impose strict and enforceable limits o­n water pollution discharges by both groups. Toward that end, states should follow through with the establishment of total maximum daily loads (TMDLs) for individual watersheds, as required by the Clean Water Act of 1972. (King 2005).

When and if all the legal and technical pre-requisites for active WQ trading come into place, it could be an important incentive for riparian buffer planting in the many watersheds around the US where nutrient and sediment runoff from farm fields are a problem. If farmers have flexibility to design and manage their buffers, efficient monitoring systems can be devised, and the income from credits is sufficiently attractive, this will help gain wider acceptance of buffers among the farming community. Buffers managed for both conservation and production will then become a more common practice.

References

Breetz, Hanna L. et al. 2004. Water Quality Trading and Offset Initiatives in the U.S.: A Comprehensive Survey, Dartmouth College Rockefeller Center, Hanover, NH, 337 p., Accessed at http://www.dartmouth.edu/~kfv/waterqualitytradingdatabase.pdf.

King, Dennis M.. 2005. Crunch time for water quality trading, Choices 20(1):71-75, Accessed at http://www.choicesmagazine.org/2005-1/2005-1.pdf

Lowrance, R, Hubbard, RK, and Williams, RG. 2000. Effects of a Managed Three Zone Riparian Buffer System o­n Shallow Groundwater Quality in the Southeastern Coastal Plain, J. Soil Water Conserv. 55:212-220

Millennium Ecosystem Assessment. 2005. Ecosystems and Human Well-being: Synthesis, Island Press, Washington, DC, 138 p. (http://www.millenniumassessment.org)

North Carolina Administrative Code. 2000 (effective date). Tar-Pamlico River Basin: Nutrient Sensitive Waters Management Strategy: Protection and Maintenance of Existing Riparian Buffers, 15A NCAC 02B .0259, Raleigh, NC, Accessed at http://h2o.enr.state.nc.us/nps/tarpam.htm

O'Grady, Dennis and Wilson, Mary. 2002. Phosphorus Trading in the South Nation River Watershed, o­ntario, Canada, Accessed at http://www.envtn.org/wqt/programs/ontario.PDF.

Parkyn, S. 2004. Review of Riparian Buffer Zone Effectiveness, MAF Technical Paper No. 2004/05, Ministry of Agriculture and Forestry, Wellington, NZ, Accessed at http://www.maf.govt.nz/publications

Prichard, Don et al. 1998. Riparian Area Management: Process for Assessing Proper Functioning Condition , Technical Reference 1737-9, USDI Bureau of Land Management, Denver, CO, Accessed at http://www.blm.gov/riparian/PDF/1737-9.pdf

Schultz, RC, Isenhart, TM, Simpkins, WW and Colletti, JP. 2004. Riparian forest buffers in agroecosystems: lessons learned from the Bear Creek Watershed, central Iowa, USA, in Nair, Rao and Buck, eds., New Vistas in Agroforestry, Kluwer Academic Publishers, Dordrecht, pp. 35-50.

Sheridan, JM, Lowrance, R, and Bosch, D D. 1999. Management effects o­n runoff and sediment transport in riparian forest buffers, Trans. ASAE 42(1): 55-64, Accessed at http://sacs.cpes.peachnet.edu/sewrl/bosch/1999%20Sheridan%20TASAE%2042(1)%20Mgt%20effects%20on%20runoff%20and%20sediment.pdf

US EPA. 2004. Water Quality Trading Assessment Handbook, Document EPA-841-B-04-001, Environmental Protection Agency, Washington, DC, 120 p., Accessed at http://www.epa.gov/owow/watershed/trading/handbook/

USDA Natural Resources Conservation Service. 1999. Forest riparian buffer conservation practice #391-Iowa, Natural Resources Conservation Service, Des Moines, IA, Accessed at http://polk-swcd.org/eFOTG%20pages/riparianbuff.pdf

USDA Natural Resources Conservation Service. 2003. Forest riparian buffer conservation practice #391, US Dept of Agriculture, Washington, DC, Accessed at ftp://ftp-fc.sc.egov.usda.gov/NHQ/practice-standards/standards/391.pdf

Wenger, S.. 1999. A review of the scientific literature o­n riparian buffer width, extent, and vegetation, Inst. of Ecol., Univ. of Georgia, Athens, GA.

By Miles Merwin